Tuesday, December 30, 2014

January 3-5 Potential Winter Storm & Ice Storm Threat

The tables have turned (yet again) on the forecast models, as guidance is favoring a northward track for this storm system. Additionally, some guidance is seeing a major ice storm threat.

The image above shows observed 500mb vorticity values and height contours as of this morning. We can see our primary piece of energy located in the West, digging to the south and slightly to the west as it begins to close off in the Southwest. Interestingly enough, this will be the same piece of energy that will bring the rare snowfall to the area that news agencies have been reporting on. Eventually, this energy will eject into the Plains, and that's where the real action begins.

By 102 hours out, we see our primary piece of energy has moving into the Midwest, and we also see another piece of energy diving south from the North Plains to interact with the primary energy. This interaction is critical to the evolution of this storm. If the system coming south from Canada is delayed, or speeds up, this storm will take on an entirely different approach, which is why model guidance still retains some problems with sorting this storm out. Lately, guidance has agreed on this storm being a "cutter" event, where the wintry precipitation will impact the Great Lakes, Plains and Midwest (those of you who read my previous post know this as the track I did not side with), but beyond that consensus, there isn't a whole lot of agreement.

We'll analyze three different model forecasts in today's discussion.

Tropical Tidbits
The above image shows the latest GFS model's forecast of accumulated snowfall out to next Monday. We see the track this storm has taken in this projection, with the bullseye of the snow hitting northeastern Missouri. A heavy swath of snowfall extends northeast from that corner of Missouri, where an additional 6"+ of snow is forecasted through the Quad Cities, Chicago, Milwaukee, and into North Michigan. This solution has been one that has shown up rather consistently, particularly in recent runs of all guidance, as the trend has been to go north. However, as was stated above, model disagreements are still evident.

Tropical Tidbits
The above image shows the snowfall forecast off the GFS-Parallel model, the incoming upgrade to the current GFS model. This forecast has one band of snow dropping about 6" or 7" in much of northern Illinois, while what may be a second band delivers similar amounts in central Wisconsin. The intriguing part about this model is its poor verification, especially as of late. It remains to be seen how this model does with this storm, but so far, recent history suggests one ought to be wary of this model's forecasts.

Lastly, but certainly not least, we need to discuss this ice storm threat.

The above image shows forecasted precipitation types from the Canadian GGEM model, valid for the morning of January 3rd. We initially see a swath of moderate to heavy rain across the southern Midwest into the Ohio Valley, but areas north (particularly the western Great Lakes into Indiana and Ohio) are under the gun for a significant ice storm. Earlier projections, made not even 24 hours ago, favored freezing rain accumulation of 0.8" in Chicago, IL, though this model has since lowered those totals to a "better" 0.4" outlook of freezing rain. The GGEM model isn't the best at verification, either, so I wouldn't get hung up on this forecast. I would, however, keep an eye on forecasts for the potential for an ice event, should these ominous outlooks continue to surface.

To summarize:

- Model guidance is coming into agreement on a winter storm impacting the Plains, Midwest and Great Lakes.
- Some forecasts are predicting a major winter storm to hit the aforementioned regions, while other forecasts favor rain. Still others favor a miss to the south.
- There is some indication of an ice storm threat, but due to model uncertainty I wouldn't begin worrying yet.


Monday, December 29, 2014

January Long-Range Outlook: La Nina Pattern Expected to Dominate

This is the long-range outlook for the month of January, and is one of the more unfortunate posts I have to write. At this time, it does appear my winter forecast is in trouble, primarily with respect to the Eastern US, based on how January is shaping up. Let's jump right in.

Paul Roundy

The above image shows a long-range Hovmoller forecast, on a time-by-longitude scale. For this chart, we want to focus on the colors, where yellow indicates suppressed tropical convection, and blues represent enhanced tropical convection from an area encompassing 7.5º North latitude to 7.5º South latitude. The other item we want to watch for is the solid red lines, which indicate the presence of a Madden-Julian Oscillation (MJO) wave. These are almost always accompanied by enhanced tropical convection signals (the blue colors).

On this graphic, notice how we see an MJO wave moving from 135E at the start of the New Year, to about 170E before it dies out near the International Date Line at 180º. If you put this into terms of the Madden-Julian Oscillation, it means the MJO will be moving through Phases 4, 5 and 6 before dying out just before reaching Phase 7. There are a total eight phases in the MJO, and Phase 4-6 are classified as 'La Nina pattern' phases. Similarly, Phases 8-2 tend to be associated with 'El Nino pattern' phases; I personally see Phases 7 and 3 as 'transitional phases'. Therefore, we can determine that we will be entrenched in a primarily-La Nina pattern until this MJO wave dies out close to January 20th, and a new wave commences around ~65E longitude to close out January. Long-long range forecasts into February also take this second wave through La Nina phases, a bad signal for winter weather fans in the East.

Nicholas Schiraldi
The image above shows the GFS forecast for atmospheric angular momentum, or AAM over the next 16 days. Notice how the GFS takes the AAM well into low territories, near phases 1-4. These phases and general Low AAM pattern are highly characteristic of a La Nina-dominated weather pattern, and this jives well with the indications we have been receiving from tropical forcing, as was described above.

So, what exactly does a La Nina pattern entail?

The above image shows a general overview of the atmosphere during a La Nina-dominated pattern. During such a pattern, high pressure will take over the Gulf of Alaska and general Northeast Pacific regions, as the polar jet stream rides up into Alaska, before plummeting south into the Rockies. As a result, a general wet and cool pattern takes hold of western Canada and the North Plains. In the United States, high pressure and warm weather will take hold of the Southeast and East US, something we've been seeing, and ought to continue to see for the coming month. This ridge then provides an excellent 'blockade' that can force storm systems from the south to come north, and dump precipitation into the Great Lakes, Midwest, Plains, and even the Ohio Valley at times. That's why we see a 'wet' outline in the areas mentioned above (this outline is centered a bit to the east of where I believe this 'wet' pattern may set up).

Let's split this up into three portions, to better serve our forecasting purposes.

Early January
The early part of the month is likely to favor a cold Central and East US, as a lobe of the polar vortex ventures dangerously close to the United States. This time period is likely to feature multiple winter storm threats, which may affect the East US as well as the Central US. More examination of ridging in the Southeast will be needed, as far as knowing where the individual storms may track.

Middle January
The middle part of the month is likely to feature a warmer than normal East US, but long range model guidance is indicating this warmth may eventually push westward to encompass the Great Lakes and much of the Midwest. Time will tell on how this outlook unfolds, but a warm East and Central US may be a good bet, as the aforementioned MJO wave moves into phases unfavorable for warmth in those areas.

Late January
The closing part of the month is likely to feature a return to some cooler conditions, primarily in the eastern third of the country as a new MJO wave forms in phases favorable for a chill in the East. However, the final days of January and opening days of February could see warmth return to the East, if this new MJO wave moves quickly to La Nina phases.

The parts that would be included in the Summarization have been highlighted in red above.


Sunday, December 28, 2014

January 6-10 Potential Winter Storm

Model guidance is latching onto what could be our first shot at a strong winter storm, in the January 6-10 timeframe.

Tropical Tidbits
The above image shows 500mb geopotential height values in colored shadings, and MSLP values in contoured lines, across the West Pacific on New Years Eve. In this image, we see a strong low pressure crossing Japan, with a minimum central pressure value of 991 millibars, before sliding north and east towards the Aleutian Islands. If we apply the Typhoon Rule, which states that weather phenomena occurring in the West Pacific is reciprocated in the United States six to ten days later, we may extrapolate this to mean a strong storm could impact the US in a January 6-10 timeframe.

Other model guidance supports a weaker system moving over Japan, meaning we will need to monitor the region in coming days. If the storm crosses Japan at a weaker intensity, the storm here in the US would likely be weaker as well.

Purely for 'eye candy', here's the latest GFS projection of a storm in the East US on January 6th. This graphic isn't a forecast you should count on to verify.
To summarize:

- A storm system may impact the US between January 6-10.
- Storm strength is still uncertain, and all aspects exhibit very high uncertainty at this juncture.


January 3-5 Potential Major Winter Storm

** This post is dedicated to the lives potentially lost on the QZ8501 flight, reported missing last night. **

The potential for a winter storm in the January 3-5 period appears to be growing.

The above image shows 500mb vorticity values across the United States from the GFS model, valid for the afternoon of January 2nd. Here, we see a rather potent piece of energy pushing eastward across the southern Plains at a neutral tilt. This energy is the piece we will be watching to impact a significant portion of the Central and East US. As of now, there are two tracks: one takes the system north and lays down plowable snowfall in the Great Lakes (Madison, WI or Chicago, IL), while the other track goes into the Ohio Valley, putting down snow in that area, even into the Northeast.

I'm going to go through the different mechanisms for this storm, and why ingredients are here for both a north and south track.
Shown here is the mid-level wind speed chart from the GFS model, valid at the same time as the first image we looked at. Notice the streak of enhanced wind speeds rounding the base of the trough/storm system in the South Plains. This streak of strong air is a textbook signal for a strengthening storm system, as the storm will then attempt to attain a 'negative tilt', where those vorticity values in that top image will try and "push" in a southeast direction. This negative tilt indicates the storm has reached peak intensity, and is now a mature storm. Additionally, it allows the storm to curve northward, and qualify the northward track.

Tropical Tidbits
Here's the snowfall projection off of the most recent GFS model forecast, giving you a good idea for where snow would fall if this northern track were to verify. Northeast Iowa into much of southern and central Wisconsin would see amounts near or in excess of 6", with similar amounts nearing 12"+ in Michigan. The GFS has continued to trend north as time has gone on, and is now reaching 'outlier' status, as other model guidance comes in south.

Tropical Tidbits
Here's the Canadian GEM model forecasted 500mb geopotential height values (color) and MSLP values (contoured lines) for the afternoon of January 3rd. We see our storm system displaced well south of the GFS track, with this forecast leading the low pressure system over western Tennessee, eventually into the Mid-Atlantic, producing accumulating snow as it does so. The GFS-Parallel model, the successor to the current GFS model, is also siding with this GEM projection of a further south track. This southern track would deliver snows to portions of Indiana, Ohio, and into the Northeast, finally adding a bit of winter for those regions.

Now, it's time for a bit of analysis.

Let's use the same image now as we did to kick off this post. As we see the energy push into the Southern Plains, notice how the longwave trough over Canada seems to almost converge with that ridge in the Southeast, right over the Midwest. This tightening up/coming-together of contour lines is a process called 'confluence'. Confluence works by piling up air, as those contour lines press together. At the surface, this results in high pressure. My concern is that model guidance (the GFS model in particular) is not accurately accounting for this high pressure. The formation of high pressure right where the storm is supposed to track would usually see the storm forced southward, similar to what the GEM and GFS-Parallel are showing. 
Adding to this concern is that all model guidance has a known bias to be too slow with the progression of Arctic air southward. Lo and behold, we have a strong Arctic high pressure system on the heels of this storm system, and if models are retaining this bias, it's quite possible the storm ends up shunted to the south, giving snow to the Ohio Valley and Northeast.

Tropical Tidbits
The image above shows the projection for 500mb geopotential height values (colored regions) and MSLP values (contour lines) for December 29th over the West Pacific. We see a low pressure system has formed on the southeastern coast of Japan, and is moving east-northeastward along the coast. As time progresses, this low will eventually shift out to sea and away from Japan.

Our Christmas Eve storm was expressed well by this Typhoon Rule application, and this storm looks rather similar to the track the storm took in Japan preceding the Christmas Eve event. The only difference here is this time around, the low pressure system drifts out to sea instead of cutting up the eastern coast of Japan. This is a huge red flag, and tells us that this storm may very well go further south and east than current model guidance is suggesting. Given the success this predictor has had in the past few years I've used it, there is reason to believe this system could go further south and east. 

For now, I'll set my sights on this storm tracking to the south, more in line with what the GEM/GFS-Parallel are showing. The GFS model, in addition to being too far north, may be too quick with the storm, so other guidance appears to be good to use at this time. The combination of potential model error, as well as expected high pressure issues, in addition to the Typhoon Rule, is too much for me to put faith in the northern solution. The GEM snowfall prediction is shown below (this event's snowfall would be through the upper Ohio Valley into the Northeast).

Tropical Tidbits

To summarize:

- Model guidance is now confirming the possibility of a storm in the January 3-5 timeframe.
- A southward track is expected at this time, with snow hitting the upper Ohio Valley and Northeast.
- Substantial uncertainty still exists.


Friday, December 26, 2014

January 3-5 Potential Winter Storm

It's looking like the next threat of a winter storm will arrive in time for the first few days of the New Year.

On December 31st, a strong trough is expected to drop into the West US and close off, as the circular 500mb geopotential height contours show above. This cyclone closing off in the West slowly drifts eastward, and that's the first thing we must watch for. A common model bias involves guidance closing off these troughs too quickly. For those located in the Plains and Midwest hoping for a snowstorm, you want this trough to close off quickly. A later close-off of the trough would result in a further south track, which might be in the cards of this is an instance of that model bias. For now, however, the trend has been to close off the energy earlier and earlier.

Tropical Tidbits
This image shows the projected 500mb vorticity values for January 3rd, the same type of chart as above. Off of the latest GFS model projection, we can see the energy now located in Indiana, taking on a negative tilt, as the contours seem to be "pushing" in a southeast-ward direction. When the system initially exits the Southwest, it will have a positive tilt. In order for those north of Kentucky to have a shot at a good snowstorm, this storm needs to attain a negative tilt rather quickly, as the GFS has done above.

Tropical Tidbits
The 500mb geopotential height anomaly chart off the ECMWF, valid for January 3rd, shows a similar situation as the GFS is predicting. We see our energy in southern Minnesota and northern Arkansas attaining a negative tilt. Although I don't have access to the smaller-interval charts from the ECMWF, it's not a stretch to assume this storm takes a similar path as the GFS model, as well as the Canadian GGEM model (not shown).

There are a few, rather significant items standing in the way of this being a 'likely' winter storm for the Midwest.

Tropical Tidbits
The image above shows the projection for 500mb geopotential height values (colored regions) and MSLP values (contour lines) for December 29th over the West Pacific. We see a low pressure system has formed on the southeastern coast of Japan, and is moving east-northeastward along the coast. As time progresses, this low will eventually shift out to sea and away from Japan.

Our Christmas Eve storm was expressed well by this Typhoon Rule application, and this storm looks rather similar to the track the storm took in Japan preceding the Christmas Eve event. The only difference here is this time around, the low pressure system drifts out to sea instead of cutting up the eastern coast of Japan. This is a huge red flag, and tells us that this storm may very well go further south and east than current model guidance is suggesting. Given the success this predictor has had in the past few years I've used it, there is reason to believe this system could go further south and east.

Tropical Tidbits
Also of concern is that massive Arctic high pressure system to the north in Canada, pressing southward into the United States. It is well known that model guidance has a bias to be too slow with the progression of Arctic high pressure systems southward. Applying this to our storm here, this means that model guidance might be forecasting this low pressure system to be too far north. Consequentially, this would result in the storm going further south and east.

When you shed all the more 'uncertain' parts to this forecast, you end up with a model consensus favoring a northward track, and the Typhoon Rule in favor of a southward track. Usually, I would go in favor of the Typhoon Rule and predict a further south track, but we have a strong streak of mid-level winds rounding the base of the trough on the latest GFS, which is a big signal for a storm to go negative-tilt, and in this case, go north. Add up all the model biases and predicted phasing of the subtropical & polar jet streams for this event, as well as a suspicious timeframe for the system in Japan, and there's too much uncertainty to decide on one particular track at this time.

To summarize:

- There is the potential for a winter storm to impact the Central and Eastern United States between January 3-5 (good confidence).
- There is too much uncertainty to tell whether this storm will hit the Midwest and Great Lakes, or stay south in the southern Ohio Valley.
- With good Arctic air already in place prior to this storm, accumulating snowfall is a good bet at this time.


January to Commence With Wintry Central US, Balmy East

The pattern to kick off January 2015 is looking cold for the central section of the country, while the East looks to bask in warmth.

Tropical Tidbits
The above image shows 500mb geopotential height anomalies, valid for December 28th, when we expect this pattern to start establishing itself. The atmosphere is quickly turning very La Nina-like, as we see atmospheric angular momentum values begin to circulate in anomalously low regions. This La Nina-like set-up will kick off with a strong ridge forming in the Gulf of Alaska, the primary indicator of a negative East Pacific Oscillation (EPO) pattern, which favors cold weather in the United States. Energy coming down the pipe from the Pacific will ride the jet stream and crash into the West US, setting up a nightmare scenario for the East US' cold weather fans.

Shown above is the typical jet stream alignment for a La Nina pattern. Notice the aforementioned ridging in the Gulf of Alaska, as well as the polar jet stream arcing over that ridge and then pushing southward into the West. Due to all the energy crashing into the West, we will see a ridge form over the East US, shown by the push north in the polar jet stream over the eastern CONUS in the above image. What ends up happening is a dream scenario for snowstorm fans in the Central US, and a nightmare scenario for all winter weather fans in the East US.

Energy that does crash into the West US will have the potential to become a substantial winter storm in the Plains and Midwest. This is due to what looks like phasing of the jet streams over the Central US. Notice how the polar jet stream shifts south from Canada and then mixes in with that other jet stream coming in from Mexico. This phenomenon supports potential strengthening of any storm systems that form and maintain their structure as they eject into the Plains, eventually progressing east-northeast into the Ohio Valley.

To summarize:

- A pattern change is currently in the works, and is expected to take hold in just a couple of days.
- This new pattern resembles a La Nina pattern.
- This pattern will favor cold and possibly snow in the Plains and Great Lakes, as well as very wet conditions in the West.
- Warmth and relative quiet will prevail to end 2014 and start the month of January in the East US.


Monday, December 22, 2014

Special Forecast Discussion - High-Impact Christmas Snowstorm

This is a special forecast discussion concerning the potential for a high-impact Christmas snowstorm.

Discussion prepared Monday (12/22/14), 22z.
Purpose of discussion: To evaluate the increasing threat of heavy snow immediately prior to Christmas.


The attached image shows 500mb vorticity values over the United States from the American GFS model, valid on Christmas Eve morning. Model projections see a strong trough anchored over the Central US beginning to tilt more neutrally, eventually negatively as the storm pushes east. As the storm strengthens and the trough continues to attempt a negative tilt, the low will shoot northward, bringing the potential for heavy snow across a portion of the western Great Lakes and Midwest. Recent model guidance has favored a slight speeding up in the storm's progression into the Central US, as well as a strengthening of the trough as a whole. This, in turn, leads to a more westward track.

Global model guidance is coming into agreement on this westward track, as ensemble projections finally line up with operational model projections (something that had been listed as a red flag in this morning's special discussion). The thin red lines show individual Canadian model ensemble members, while the blue lines depict individual ensemble members from the American model. The dark blue line signifies the ensemble control forecast off the American suite, while the dark red line does the same for the Canadian suite. All in all, it is clear that model guidance has converged on what may be our final solution. This solution is split into two potential tracks.

The first track is favored by most of the Canadian ensemble members and a slight majority of the American members, and brings the storm through western Indiana and western Michigan. This, in turn, creates an environment favorable for accumulating snow in north-central to north-east Illinois, as well as a bit in southeast Wisconsin. Accumulations on the order of 6" may be anywhere from isolated to widespread, depending on how the warm layer projected to be present a few thousand feet off the ground reacts.
The second track, agreed upon by the remainder of American ensemble members and high-resolution NAM & regional GEM model guidance, takes the low through eastern Illinois. This drops significant snows (on the order of 12"+) on western Illinois to southern Wisconsin, in addition to eastern Iowa. This solution is observed in the latest high-resolution NAM model run below:

I don't have an image to illustrate the first track I described, but you can cut totals above in half and shift the entire band southeast to end just north of Chicago to get a good idea of what that solution entails.

Caveats to Forecast
Model disagreement on track remains a primary concern, though this concern has been somewhat alleviated in recent model runs. Placement of the low to a westward extent is rapidly becoming an important caveat, as the two solutions described above show.

Current prediction from my end is we will see Solution 1/the first track verify, probably nudged west a bit. I feel high resolution guidance and other projections on the western edge of the envelope are too far displaced, especially considering this high resolution guidance is notorious for over-strengthening storms. Also can find reason to lower snow totals from high-res guidance, so that maximum amounts throughout the event may just touch the 10" or 12" benchmark; anything beyond that seems a bit too far-fetched for my liking at this time. NCEP control forecast on the large ensemble image above appears the most reasonable at this time.

Favored Track
My favored track at this time is represented by the pink NCEP ensemble control forecast below.

Next Update
Another update to this special forecast discussion is not currently required. If one is needed in the short term, the next update can be expected prior to 1:30 AM central time (0630z).


Special Forecast Discussion - Christmas Snowstorm Threat

This is a special forecast discussion concerning the Christmas snowstorm threat.

Discussion prepared Monday (12/22), 06z
Purpose of discussion: To analyze the Christmas snowstorm threat

Model guidance continues to have unresolved problems with the upcoming potential of a high-impact Christmas storm system. Solutions range from a major snowstorm impacting Chicago, to snow falling in central and southern Ohio.

The attached image shows the American GFS model interpretation for the set-up of this storm. In this depiction, we see a positively-tilted trough positioned in the Southern Plains, with ridging set up on either coastline of the United States. The previous expectation saw the northern piece of energy slide north and out of the picture, allowing the southern energy to become negatively-tilted, and turn into a significant storm. Since then, guidance has converged on stronger northern energy, and slower progression of the southern energy. These two factors combine to force the trough to tilt more positively, and restrict strengthening.

Consequentially, precipitation impacts continue to be reduced on incoming model runs, as both the southern energy weakens and slows down, with continued persistence of the strong northern energy. Thus, a strong snowstorm is not necessarily expected at this time. However, should a decent precipitation shield form on the western flank of the low, dynamic cooling aloft may permit accumulating snow in a swath of the Great Lakes. Exactly where this snow may fall is uncertain. For that, we turn to ensemble guidance.

Shown above is the ensemble spread track guidance from the government modeling agency. This graphic depicts American GFS model ensemble members, and their individual predictions for where this storm will go. For this discussion, we will only focus on the tracks leading from the Gulf of Mexico on northward. Analyzing this picture closely, you may see a brown line tracking north and east into western Ohio. That brown line is the GFS model itself; all the other green lines are ensemble members. Notice how the ensemble members are predominantly west of the operational model. It appears the ensembles are split into two groups: one that carries the storm in a similar track to the GFS operational model, and one that drags the storm through western Michigan, even through eastern Illinois. This means that we could see the GFS model correct westward with its track. It may very well not happen, as the trend has been for a weaker, slower trough, but it's worth keeping an eye on.

Briefly adding in other ensemble guidance, we see the CMC ensembles favor a track through eastern Indiana and western Ohio as of their morning forecast run (12z). Since then, the situation has changed as more energy has come onshore, but this westward idea remains something to monitor (as does an eastward correction). It is worth noting that those same 12z CMC ensembles see the storm as being stronger than projected, and just slightly to the west of this afternoon's actual 12z CMC model forecast, shown on that red line above.

Caveats to Forecast
Significant caveats exist with continued ensemble support for a westward shift in the track, but operational guidance's refusal to take such a step. Additional problems arise with significant timing variability among ensemble and operational guidance, as well as temperature profile variability.

The continued degradation of the trough by model guidance, as well as slowing progression and strengthening northern energy, ultimately leads me to believe a more eastern track will come to fruition. Ensemble guidance appears to foresee a westward shift in track, though I fail to see the basis for such a solution unless the southern energy ends up stronger and a bit more progressive than recently projected. Additional monitoring of both operational and ensemble guidance is needed, as continued disagreement among individual models, and even among their own ensembles, means only one solution can be right. I can see reason for trough ending up slightly stronger than advertised, and this could bring about a notable shift in track, and I am still struggling with continued ensemble support for a western shift in track, but for now it appears best to go with newly emerging model data.

Favored Track
My favored track as of this writing resembles a CMC/GFS (pink/red lines respectively) track, through western Ohio and central Kentucky. Confidence in this track is low.
Next Update
The next update to this special forecast discussion is not currently planned. If an update is needed, one will be provided prior to 11:00 AM Central Time on Monday, December 22.


Sunday, December 21, 2014

Christmas Potential High-Impact Winter Storm

Model guidance is still wreaking havoc on forecaster confidence, meaning this post will be laced with numerous caveats.

Instant Weather Maps
The image above shows the 12z GFS model forecast for snowfall totals from this Christmas storm. That thin swath of snow in northwest Indiana into western Michigan is the accumulation from this storm. That's a far cry from the projections of 6"+ earlier this week, even twenty-four hours ago.

This big change came about late last night, when the energy associated with this system finally crashed into North America, and the weather balloon network could take measurements of the system. This process, called 'sampling', as the weather balloons sample what the system is doing and how strong it is, gives weather models more confidence in a solution, as there are no weather balloon launches out in the Pacific.
At least, that's usually how things go. Late last night, when this first sampling took place, model guidance took a significant jump eastward and greatly weakened the overall storm, bringing about minimal snowfall accumulations. The theory among forecasters was that this would be the final solution.

But the drama appears far from over.

Instant Weather Maps
The above image shows the 18z GFS forecast for snow from this storm, the most recent forecast we have to work with. In this projection, the storm jumped west again, like model guidance was favoring about twenty-four hours ago. In this forecast, we see amounts generally of 3-6" draped across east Missouri into west and northern Illinois, with a sliver of southeast Wisconsin getting into the action.

Now's the time when I would give my personal analysis of what will happen here. But to be frank, I have no clue. This is one of the worst performances of model consistency I've seen in my five years of forecasting, if not the worst performance. I'm preparing to go about this storm on a now-casting basis, where the forecast is made as the storm actually happens.

For now, most of the population living in St. Louis on eastward will likely have to contend with travel difficulties, as this storm system still looks to be a strong one. Snow forecasts are useless at this juncture, as it's possible people could get a foot, or could get nothing. On top of that, location of this snow is nearly-impossible to predict at this point in time.


Friday, December 19, 2014

Christmas Eve Potentially Significant Snowstorm

Model guidance continues to struggle on the concept of a winter storm around Christmas Eve.

Tropical Tidbits
Click images to enlarge
The image above shows the ECMWF model's projection of 500mb geopotential height values (shaded colors) and mean sea level pressure values (contours) for the morning of Christmas Eve. On this image, we see a deepening low pressure system pushing northward into western Ohio, as a strong trough (seen by the blue colors) begins to negatively tilt (seen in this image as those blue colors "pointing" southeastward) and drag the storm northward.

This forecast solution is one of (quite literally) tens of ideas for this storm that have come out of recent model runs, so individual model projections are being weighted unusually low, due to unusually low confidence. Though I don't have the maps to confirm, this projection from the ECMWF would likely deliver a hefty snowstorm, with strong winds, to portions of the Upper Midwest and Great Lakes, particularly Michigan.

Tropical Tidbits
We now move on to the Canadian GEM model's projection, once again showing 500mb geopotential height values and MSLP contours, and once again valid for the morning of Christmas Eve. In this forecast, the storm system in question becomes negatively tilted quicker than the ECMWF model, which then leads to this more westward solution. Here, the GEM has the storm in central / west-central Indiana, instead of the ECMWF's west-Ohio idea.

Tropical Tidbits
As a consequence of this change, the GEM places a swath of accumulating snow in a north-south orientation, from eastern Wisconsin down through Illinois, barely scraping far western Kentucky. Additional snowfall hits Ohio and Indiana, but the amounts in Illinois are the most significant, where totals surpass the 6" benchmark. Recent model guidance has been looking more into this westward shift, but there's no consensus at this time.

Tropical Tidbits
The last model projection we can analyze, owing to an ongoing massive NOAA data outage, is the Parallel GFS model. Once again, the forecast shows 500mb geopotential height and MSLP values, and this projection is once again valid on the morning of Christmas Eve. This forecast is similar to that of the GEM model, in that we see quicker strengthening of the storm system, leading to a more westward solution.

It should be noted that ensemble guidance is a bit further east of these western tracks.

All in all, we have a very difficult forecast still evolving, which might not be resolved for another few days. For now, those across the entire Central and Eastern US should be prepared for potentially significant weather, especially if you are traveling immediately prior to, or after Christmas.

To summarize:

- Model guidance continues to suggest a powerful storm moving through the Central/Eastern US around Christmas Eve.
- Guidance remains very inconsistent, with little to no consensus currently built around a certain track.
- Regardless of where this storm tracks, significant and adverse weather remains possible, if not likely.
- Travel plans may need to be re-evaluated.


Wednesday, December 17, 2014

Christmas Eve Potentially Significant Winter Storm

I'm watching the increasing potential for a significant winter storm in the Central US, created by what could be one of the strongest low pressure systems in the last few years, if not longer.

Instant Weather Maps
The above image shows 500mb geopotential height contours over the United States from the prestigious ECMWF model, valid for December 23rd. In this image, we see two pieces of energy in the Central US. One is a closed low, placed in the Northern Plains, while the other is a deepening trough in the Southern Plains, shown by the depression in contour lines. In a situation similar to that I described in my December 10th post, we see this trough begin to lift northward into the Central US, but not before the "bomb" goes off...
Instant Weather Maps
Just 24 hours later, on the morning of Christmas Eve, we find that the two pieces of energy have combined, and the trough as a whole has now attained a negative tilt, indicating it has reached its mature phase. As a result, the storm undergoes rapid strengthening, very near the criteria of bombogenesis. Bombogenesis is a meteorological term used to describe the phenomenon when extratropical cyclones rapidly strengthen, and their minimum central pressure values decrease by 24 millibars in a 24 hour period. Switching between December 23rd and 24th, we find that pressure values from the ECMWF lower by about 19.3 millibars, only a little ways off from being a true 'bomb'. Even though this storm doesn't fit the criteria, the strengthening is nothing to shake a stick at.

This solution is a tricky one. I don't have access to pay-to-view weather model graphics (yet), so I cannot see the precipitation pattern from this system. However, from what others are discussing, it appears that not only will the nearly-due-northward movement of this system foul up the precipitation shield, but temperature profiles are above freezing in many spots that would otherwise see snow. This could be placed on model error, or it may be a legitimate forecast. For now, it's just too far out to tell one way or another.

Weather Online
What is interesting, however, is the difference between the ECMWF model and the ECMWF ensembles. The above image shows mean sea level pressure values, valid on Christmas Eve (the same time as the second image we discussed). By simple comparison, note how the ECMWF model places the center of this storm somewhere in southern Ohio, while the ECMWF ensembles put northeastern Indiana in the center of this cyclone.

A further east track of this storm system could result in more of a snow impact to the Great Lakes, instead of primarily wind-driven snow in the Midwest and Ohio Valley from current runs of the ECMWF. But as I said earlier, we are still quite a while away from nailing down these details.

To summarize:

- Model guidance is beginning to sniff out a very strong storm system impacting most of the Central and East US on Christmas Eve into Christmas Day.
- This would severely impact travel.
- Snow would be confined primarily to the north-central Great Lakes into Canada.
- Very high uncertainty still exists.


Tuesday, December 16, 2014

Atmospheric Trifecta Preparing to Deliver Cold, Snowy January

A trio of atmospheric signals are gearing up for what could be a rather cold, snowy January.

Research I completed last night showed significant (10"+) snowstorms in the Midwest are most favored under the negative phase of the Arctic Oscillation, the negative phase of the East Pacific Oscillation, the negative phase of the North Atlantic Oscillation, and the positive phase of the Pacific-North American index. We look to have at least three of these factors locking down the atmosphere to round out December and kick off 2015.

Tropical Tidbits
The image above shows the GFS ensembles' forecasted 500mb geopotential height anomalies over the Northern Hemisphere, valid on the evening of December 26th. On this graphic, we see a prevailing negative EPO, as highlighted in this graphic with ridging over the Gulf of Alaska. A negative NAO has also emerged just west of Greenland, combining with ridging in Eurasia to provoke a negative AO regime. I'm watching for a positive PNA signal, but as a ridge is passing over the region, I'm content on acknowledging a trifecta for now.

It doesn't end there, however...

Tropical Tidbits
By December 30th, we find more than a few big developments have taken place. We see our negative EPO ridge has blossomed into a massive swath of high pressure, now extending along the West Coast into Alaska... and then some. Ridging over Greenland still exists, paving the way for a continued negative NAO and negative AO tandem. As a result, we continue to observe below-normal anomalies in the East US, signaling a very cold period to end December and start January.

My thinking is we'll see those below-normal height anomalies slowly progress west, as a west-based negative NAO (where the ridge is displaced to the west of Greenland) typically favors cold weather more into the Central US. This would be amplified/supported by the strong -EPO ridge, slowly evolving into a +PNA signal, it appears.

Now, onto the snow...

Tropical Tidbits
The image above shows the GFS ensembles projection on December 28th, showing jet stream wind speed values. There are a few things to note here.
First off, check out the extended Pacific jet stream. We're seeing that occur in the very near future, and it's no coincidence that this extended jet stream into North America is happening at the same time some storm threats are arising. The extended jet allows for cyclogenesis in the Northern Pacific, where energy may be shunted east into the United States, leading to those winter storm chances.
Second, we see a notable subtropical jet stream (STJ), as depicted by the green colors extending from the northeast Pacific into Mexico and the South US. With a negative NAO regime in place, as well as an enhanced subtropical jet stream, it's possible the East Coast could get rocking and rolling with these storm chances.

As for the Central US, I alluded to the below normal height anomalies pushing east. As far as snowfall impacts, clipper systems look to make their return, as the strong ridge pushing into Alaska produces a sustained northwest flow pattern (where the winds are out of the northwest). This could lead to not only episodes of snow in the North US, but also lake effect snow episodes, as all Great Lakes are currently well below last year's ice levels at this same time.

To summarize:

- The atmosphere is preparing to shift from a generally mild December pattern to a rather harsh January pattern.
- Sustained cold weather is likely for most of the nation east of the Rockies.
- Enhanced chances of snowstorms will be seen both along the East Coast, into the Central US (depending on individual storm tracks, of course).
- Buckle up, things are about to get fun.


Saturday, December 13, 2014

December 19-21 Potential Winter Storm

Model guidance is supporting the risk of a winter storm in the December 19-21 timeframe.
I'm trying to decide how to start off this post, since simply going through model guidance won't work. I'll begin by explaining each graphic, doing a compare/contrast as we do so.

Tropical Tidbits
The latest GFS model has a trough pushing into the Midwest on December 20th, as the blue colors and depression of contour lines shows. This trough is beginning to close off after becoming negatively tilted, and is pushing northward as a result. The surface low takes a track through the Ohio Valley on this run, dropping appreciable snows from east Kansas into southern Michigan. North Missouri sees snow over 6" from this system.

In this overview of the pattern, we can diagnose a few items arguing for this more inland track, as future guidance I'll show you will depict an East US track. First of all, we have another strong trough dropping into the West US, trying to work its way southward. This will try and force a ridge to develop in the central and eastern Rocky Mountains, as we can already see above. However, as we see a deep upper level low over Greenland, this ridge won't be able to exert too much influence (that ULL over Greenland defines the positive phase of the North Atlantic Oscillation/NAO, notorious for keeping the jet stream very 'non-wavy')
What does make a ridge form, however, is the troughing in the Bering Sea into the Gulf of Alaska, a textbook positive East Pacific Oscillation (EPO) pattern, which will act to limit cold air reserves in Canada, but also try to direct storm systems northward. The latter influence is seen well in that ridge centered over New York, just east of the storm system in question. That same northward influence could happen along the East Coast, but for now, the GFS favors this solution.

Tropical Tidbits
Here's the ECMWF 500mb geopotential height anomaly forecast, valid at the same time as the GFS graphic. We see more than a couple significant differences here.

First and foremost, the storm system in question seems to be developing a second piece of energy, signified by that second dip in the contour lines along the Gulf Coast. Looking at the forecast following this timeframe. it looks as if that second piece of energy will act to pull the main trough east, and develop it into an East Coast system. This is far different from the GFS, which maintains a single trough.
Additionally, we see the entrance time of the second trough into Western North America has been slowed by about a day. This allows the ridge in the central and eastern Rockies to flourish well into Canada, where the ridge up there is also quite a bit stronger than its GFS counterpart. Consequentially, the storm system is suppressed to the south in the ECMWF forecast. Also, note the lack of a ridge to the east of the storm system in the ECMWF image, compared to the GFS. This, combined with the extra piece of energy along the Gulf Coast, appears to favor an East Coast solution. The solution results in this pattern, valid 24 hours after the image above:

Tropical Tidbits
Purely for comparison, here's the GFS snowfall forecast I mentioned earlier, expressing the solution in opposition to the ECMWF:

Instant Weather Maps
I've been looking back and forth between the ECMWF and GFS images I've shown above, trying to figure out which one I think is the most valid, and I can't decide.

On one hand, the ECMWF projection appears to be obeying the positive EPO signal, as exhibited by troughing along the west coast and a strong ridge in central Canada.

On the other hand, the GFS is doing well with the emergence of a ridge just east of the trough, possibly as a result of the negative PNA orientation out west, as the aforementioned second storm system drops into the West.

It will ultimately depend on the timing of when the trough drops into the West, just how strongly the atmosphere responds to the +EPO signal, and (of course) if that secondary piece of energy forms along the Gulf Coast. It's worth noting none of the model guidance is having any consistency with the ridge in Canada, purely by looking at run-by-run comparisons. Additionally, today's 12z ECMWF run (examined above) is the first one to have that secondary piece of energy develop to the south of the storm, something that does not bode well for any consistency that either the GFS or ECMWF may have built up. I'll pass on giving my opinion right now, because this is a truly grotesque set-up.

To summarize:

- A winter storm is possible for the Central or East US in the December 19-21 timeframe.
- Model guidance is expressing little to no consistency on a defined track for this storm.
- Cold air availability will eventually become a concern.
- Anomalously low confidence exists.


Thursday, December 11, 2014

Christmas Potentially Significant Winter Storm

I'm now watching the risk of a strong winter storm rise as we move towards Christmas.

Tropical Tidbits
The image above shows 500mb geopotential height values (colored shadings), with mean sea level pressure (MSLP) values superimposed. In this graphic, valid on December 16th, we see a pair of very strong storm systems, one on each side of Japan. The prognosis is that a strong system will push into the Sea of Japan (located west of the country), while a second storm system will develop in the southern part of the country and move northeast-ward, skirting the eastern fringe of Japan as it does so. It is this second storm that we need to keep a close eye on, and is the one we will be discussing here today.

Using the Typhoon Rule, which states that weather phenomena occurring in East Asia is recipricated in the US about 6-10 days later, we can extrapolate this December 16th date out and predict a storm in the United States in a December 22-26th period... right in the Christmas rush.

But we can expand on this quite a bit more. The storm will be shooting north along the eastern coast of Japan. This does have an impact on the expected storm in the US. As you might expect, it raises the chance of this consequential storm also moving northeast-ward rapidly, and from there, we come out with two prevalent/possible storm tracks:

- A Panhandle Hook storm, where the system shoots north from the Southern Plains. These storms are climatologically favored to bring heavy snow to cities in the east-central Plains and Lower Great Lakes. This scenario is a possibility, as that strong storm in the Sea of Japan would likely correlate to a strong North Plains cyclone. This would keep that body of low pressure east of Japan in an area close-by, as the storms would eventually phase (not to mention low pressure areas are attracted to other low pressure areas).

- An East Coast storm. Because this body of low pressure is forecasted to merely skirt the eastern side of Japan, this could be a plausible scenario. We won't know if either of these are correct until we have more model runs to access.

The graphic above only shows the GFS model view... let's head on over to the European model projection.

Tropical Tidbits
Wow! Can you see the change?

This graphic, showing the same parameters as the GFS image, and for the same timeframe, portrays that strong storm in the Sea of Japan, but now the second storm skirting eastern Japan is more inland. It hasn't shifted much, but it has shifted nonetheless.

What does this mean? It means it's time for East Coasters to throw in the towel.

Not really, but a more inland storm does favor an inland track when the storm comes around in the US. The interesting thing is, this more inland track is an idea. Here's why.

Recall that, whether you learned it or just know it through logic, low pressure areas will try to move towards areas with the least resistance, in this case the least high pressure. My theory here is that the storm in the Sea of Japan, the stronger of the two (shown on the GFS as 993mb, 980mb on the ECMWF), is trying to pull the storm skirting east Japan towards itself. Down the road, model guidance shows the second storm absorbing the stronger Sea of Japan storm, rather than vice versa, and that's also a possibility.
My point here is, there is the possibility of a phased storm.

For those who aren't as knowledgeable with weather lingo, a 'phased storm' is a storm system which is made up of previously-two or more pieces of energy. Typically, phased storms end up stronger than either of the first two pieces of energy were. I'm not holding my breath on this Christmas storm phasing, but it probably isn't a bad idea to keep it in the back of your mind.

Regardless of if this storm phases, remember that the storm on the east coast of Japan is projected to be below 1000 millibars, so it's likely to be a nice little storm in itself.

Tropical Tidbits
We've now confirmed that not only are looking at a storm in the Christmas time period, but model guidance has amped up that threat since yesterday. Now, we have to diagnose the weather pattern here at home in that December 22nd - 26th timeframe, to see if we can pull any hints out.

I've posted the image above from the GFS ensembles, showing 500mb geopotential heights on Christmas morning. Warm colors depict ridging/high pressure, usually indicative of warm and quiet weather. Similarly, blues indicate troughing/low pressure, accompanied by colder and stormier weather. We have more than a few things to talk about with the above graphic.

First and foremost, we're looking at the Pacific driving our pattern to round out December. Tropical activity in the Equatorial Pacific will be dying off in the next few days (more knowledgeable weather folks know this as the MJO weakening), which will shift the weather pattern 'responsibilities' to the North Pacific.

We look to have a positive Pacific-North American (PNA) index pattern in place for this event. We can observe this positive PNA as a ridge forming in the West US, which allows the jet stream to buckle in the Central US. Such a pattern is climatologically favorable for a Central US storm track. In addition, a positive East Pacific Oscillation (EPO) signal will begin showing up, as ridging overtakes Canada. This will be in part due to that positive PNA, but the further east you go, the more the EPO influence takes over. A positive EPO doesn't affect the storm track so much as it does temperatures (above normal in the North US). When we factor into account slight ridging along the Eastern Seaboard, we start to see that signal for a storm system in the Central US, favoring development in the Central US.
I'm a bit skeptical, however, Many Northeast weather buffs may know that winter storms are favored in the East when the North Atlantic Oscillation (NAO) changes phases from positive to negative, or vice versa. Guess what the forecast for the NAO is around this storm's timeframe:

We've got a dilemma on our hands, with some variables favoring a Central US storm, and others favoring an East US storm. So is the world of forecasting...

To summarize:

- A winter storm appears to be in the cards for December 22-26th, likely impacting Christmas travel plans.
- A second storm system may need to be watched for the Northern Plains.
- The primary threat here may become a storm favorable for heavy snow, either in the Central/East US (ideally the Ohio Valley/Midwest) or along the Eastern Seaboard.
- Rather high confidence in the threat of a storm in this timeframe, but low confidence in who will be most affected.