Sunday, November 30, 2014

Encounter With Polar Vortex On The Horizon in Early-Mid December

It now appears much of the United States will undergo its first encounter with the polar vortex in the second week of December.

Tropical Tidbits
The image above shows 500mb height anomalies over the Western Pacific on the evening of December 1st. In this image, we see what is very likely a lobe of the polar vortex passing just north of Japan, occluding and cutting north as it does so. Minimum values here at the 500mb level in this image, off the GFS ensembles, appear to drop close to 490 dam, an indication of either an incredibly strong upper level low, or the more-likely polar vortex lobe.
This situation appears similar to the case we had last winter, where we saw a very strong upper level low crash into Japan to round out the year 2013, which then led to incredibly cold weather in the first week of the new year.
ESRL
December 27, 2013
If we recall the Typhoon Rule, which states weather phenomena occurring over East Asia is reciprocated in the United States about 6-10 days later, we could expect a bout of  substantial cold here at home in the December 7-11 timeframe. Should this verify, which is becoming increasingly likely given model consistency, the Central and parts of the East US may expect a bout of intense Arctic air to hit in the middle of December.

But it doesn't end there...


Tropical Tidbits
The image above once again shows 500mb height anomalies from the GFS ensembles, now valid on December 6th. Notice that we still see below-normal heights over Japan. This means Japan has been under a strong upper level low's influence for nearly a week, and still going. Building off of the first graphic, it's quite possible that the United States may also undergo a harsh beating from cold weather for nearly a week straight. The degree of this cold air is still under examination, but if it's anything like the upper level low expected to trek north of Japan to open December, watch out.

To summarize:

- Japan looks to undergo severe cold weather to start off December, likely leading to a prolonged Arctic air intrusion in the US in the middle of December.

Andrew

Friday, November 28, 2014

Polar Vortex Split Occurring; Arctic Outbreak Possible in Mid-December

The stratospheric polar vortex has experienced a split as a result of its second interruption by warm air this season, and it appears some cold weather may be on the way as a result.

CPC
The chart above shows heat flux values over the 2014 calendar year, from January 1st to present day. In this graph, shifting our attention to November and December, we find that flux values have skyrocketed to near-record levels, only now beginning to drop down a bit. According to the Climate Prediction Center (CPC), strong episodes of eddy heat flux events to the poles may result in a Sudden Stratospheric Warming (SSW) event. You may recall that SSW events occur when wind direction in the stratosphere reverses from low pressure-favoring winds to high pressure-favoring winds. This decimates the polar vortex, and usually unleashes near-record intensity cold weather on whoever is in the line of fire about 2-4 weeks after the SSW event occurs. Considering we are seeing an eddy heat flux episode of this magnitude, it's no surprise the polar vortex is undergoing some serious damage.

FU-Berlin
The ECMWF forecast of height contours and temperatures (shaded) about 96 hours from now, at the 50 millibar level, shows a Wave 2 stratospheric intrusion event. For some clarification, let's remember that the stratosphere exists primarily from the 1 millibar to the 100 millibar level. Therefore, I'm using the 50mb level as a happy medium, per se, so we don't receive any tropospheric influences that may occur at the 100mb level, but also keep away from vortex events in the far upper stratosphere that will ultimately have little to no effect on the troposphere.
As for the Wave 2 event, stratospheric polar vortex split events are classified into a few 'Wave' numbers. Take a look at this image to see the difference between a Wave 1 and Wave 2 event.

NASA
This image here looks complicated to those who may not be as enthusiastic about the weather as others, so we will focus on the middle column. In that middle column, we essentially are seeing areas of low pressure in orange, and areas of higher pressure in blue. On the top row, a normal polar vortex form is displayed.

Wave 1 events indicate a very strong warming event or body of high pressure is forcing the polar vortex off the North Pole, while still keeping it in one piece.
Wave 2 events are seen on the bottom row, where a weaker, but still intense warming/high pressure event occurs, but now splits the vortex into two vortices.

Using that knowledge back on the ECMWF image, we can confirm that a Wave-2 stratospheric split has occurred.

Now, bearing in mind that it usually takes 2-4 weeks for stratospheric intrusions to result in cold air outbreaks on the surface, we flash forward to a potential chilly period in mid-December.

CPC
In the long range forecast from the Climate Prediction Center, the Arctic Oscillation (AO) looks to turn negative in about two weeks, right when the timeframe for any stratospheric consequences opens up. Many signals support the warm start to December ending around the middle of the month, and this stratospheric event only strengthens this potential.

To summarize:

- A stratospheric intrusion may result in a cold air outbreak in the middle of December.

Andrew

Tuesday, November 25, 2014

December 2-6 Potential Winter Storm

Another winter storm threat is on the horizon.

Meteostar
The image above shows forecasted 500mb vorticity values with superimposed 500mb height contours over Japan for November 26th, off the GFS model. In this graphic, we see a depression in the contours over Japan, as well as a swath of red positive vorticity values,  indicating the presence of a storm system. Using the Typhoon Rule, which states weather phenomena in Japan is reciprocated in the United States about 6-10 days later, we might expect a storm system to trek across the US in a December 2-6 timeframe.

PSU
The ECMWF model is already hinting at this potential storm, as the above frame shows a positively-tilted trough entering the Central Midwest on December 4th and 5th. The extent and intensity of snowfall remains in question, but for now, the possibility is certainly present.

Andrew

Monday, November 24, 2014

Thanksgiving Potential Coastal Snowstorm

It looks like the Northeast will see a pretty white Thanksgiving this year, as a coastal storm will lay down plowable snowfall.

Instant Weather Maps
The image above shows accumulated snowfall from the GFS model, valid over the next five days. We see a swath of snowfall across the Northeast region, primarily coastal areas, as the storm will be located relatively offshore, restricting snowfall. This forecast lays down a general 3-5" of snow for many on the coast, with spots of 6" and slightly above for a handful of areas. Given that this is November, warm air immediately near the coast will be acting to put a damper on any intense snow activities, something that will have to wait until later in the season.

Instant Weather Maps
The NAM model, a shorter-range forecast, puts down a similar 3-5" swath of snow, but enhances the coastal areas with 6-8" of snow, as the pink area shows. Far southeastern parts of Maine even look to see a shot at near 10" of snowfall, if not a bit more.

IWM
Peak storm time looks to be on November 26th, intensifying in New England that evening. Regardless of how this storm ends up, millions in the East US will have to contend with a very messy Thanksgiving commute.

Andrew

Sunday, November 23, 2014

November 24th Potential Accumulating Snow

This is a regional discussion for accumulating snow potential in the Great Lakes. Please note these types of regional discussions will become more prominent this winter.

Model guidance supports a swath of potentially plowable snowfall in the Great Lakes on November 24th, into the 25th.

Instant Weather Maps
The image above shows total snowfall from this morning (Sunday) to Tuesday morning. In this forecast, from the NAM model, we can define a swath of 3-6" of snow from Iowa to Michigan in the light blue to dark blue colors, with amounts over 6" (and even approaching the 10" mark) depicted in the pink colors.

This snowfall potential comes from a storm system currently placed in the southern Plains, forecasted to move north in the next day or so. As this happens, it is expected that a band of precipitation on the backside of this storm will form, laying down accumulating snow in parts of the country. The question is, where could this occur? Right now, the NAM model shows the brunt of the snowfall hitting the Wisconsin/Illinois border, but as of its most recent 18z run, snowfall has been shunted to eastern Wisconsin and Michigan. Let's see what the GFS model says.

Instant Weather Maps
The GFS model supports a swath of snowfall hitting primarily Northern Illinois, as well as southern Wisconsin and eastern Minnesota. Amounts would be maximized near Chicago, where the legend tells us 4-5" would be predicted to fall. Lesser totals nearing 4" would then be forecasted in the Northwoods region. We are awaiting word from the 18z GFS model run as I type this to see if we can build any bit of consistency. Why do we need consistency? If you haven't noticed yet, we currently have three completely different solutions:

• Solution 1 (12z NAM in top image): Snowfall strikes the Lower Great Lakes. Rockford, IL into south-central Wisconsin sees the heaviest totals (over 6").

• Solution 2 (18z NAM, not shown): Snowfall fails to fall anywhere, save for Michigan and eastern Wisconsin. Amounts totaling 3-5" in most places.

• Solution 3 (12z GFS, image above): Snowfall impacts North Illinois primarily, laying down just under half a foot of snow.

You can track the latest updates on this event on our Facebook Page, which you can find on the 'Social Media & Contact Info' tab on the right sidebar.

Andrew

Friday, November 21, 2014

Thanksgiving Potentially Strong Storm System

I'm still watching for the threat of a Thanksgiving winter storm, this time with renewed caution concerning its evolution. Please read here for the discussion on how this potential came about in the first place.

PSU
We'll begin with the GFS model forecast for November 27th. The top left panel shows 500mb vorticity values in shaded colors, as well as 500mb height contours superimposed. The top-left image depicts 1000-500mb thickness values (people north of the red dashed '540' line generally can expect snow) as well as sea level pressure. The bottom-left panel gives an indication of low-level relative humidity, basically a measure of moisture in the air, while forecasted precipitation is on the bottom right in conjunction with 850mb temperature values. Glancing over this image, we see a significant storm evolving in the East US, with the combined energy of a trough in the southern jet stream, as well as a potent clipper system dropping down from Canada. These energies combine into one strong trough, traveling northeast and dropping intense snowfall along the East Coast. I'll provide analysis of this scenario after the GEM model, what I'm doing now is explaining what it shows.

PSU
The GEM model is different. Using the same definitions for each panel in this graphic as the GFS model, we see the storm is separated into two pieces of energy. We see our primary southern-stream storm in the Southeast US with the expansive precipitation shield, but that clipper is now located to the northwest, not conjoined with the Southeast-US storm. As a result, we see accumulating snow strike the Midwest and Plains, putting down precipitation amounts as follows.

Meteocentre
This image shows snowfall accumulation in its liquid-equivalent from right now to 10 days from today, but the clipper's snowfall is shown by the swath of greens and light yellows stretching from the Dakota into Indiana and Ohio. Doing a quick conversion tells us 10 millimeters is equal to 0.39 inches, 15mm is 0.59 inches, and so forth. At face value, the GEM would be kicking out a good 4-9" snowstorm from this clipper, with the highest amounts in those yellow shadings. With higher ratios taken into account, we would probably be facing a 5-10"+ snowstorm. Since the GEM is notorious for exaggerating snowfall amounts, this isn't something to hang your hat on. However, it does give breadth to the idea of two solutions to this storm.

For our model analysis, I want to focus on the problems with the GFS model. I've mentioned a handful of times that the GFS model (and most models in general) are prone to a progressive bias. This means that forecasts will move storms along quicker than they should, which might be leading to that merging of the clipper and southern-stream storm on the GFS. Dropping/correcting this bias might lead to a forecast not unlike the GEM model.

For now, purely due to uncertainty, I'm not willing to side with one model over the other. However, keeping in mind this bias which does appear to be showing itself in the GFS model (for now), a solution similar to the GEM forecast might be expected, something that could easily change in coming updates.

Andrew


Wednesday, November 19, 2014

November 23-25 Significant Storm System

A strong storm system looks to impact the US in the November 23-25 period.

Tropical Tidbits
The image above shows the temperature forecast for 5,000 feet off the ground, as forecasted by the ECMWF model, valid on November 24th. In this image, we see a storm system of 973 millibar strength shooting northward into Wisconsin, surrounded by above-freezing air temperatures on all sides. Pure observation of this chart tells us there won't be that much of a snowy side, but given the potential for model guidance to cool down as the forecast grows colder, or more likely yet, the precipitation shield does extend into the cold sector, snowfall may still occur. This snow would not be significant, at least according to this forecast, but the storm itself would be.

Tropical Tidbits
The GFS model gives a very similar story to the ECMWF. We see a 973 millibar low pressure system over the Upper Peninsula of Michigan on November 24th, but now with significantly more cold air on the western fringe of the storm. This appears to have happened as the storm wrapped itself up and occluded, pulling all that Arctic air to the south (this will have significant implications for the Thanksgiving storm, which we will discuss tomorrow (Thursday) afternoon).

Tropical Tidbits
That GFS forecast does lay down some hefty snows in the Upper Midwest, where amounts of 6-12" may be seen. The heaviest snow appears in northern Minnesota into Canada, where amounts closer to the 2' mark may be anticipated. However, it remains to be seen if this solution will win out against the ECMWF, or vice versa.

To summarize:

- Model guidance favors a very strong storm system moving into the Upper Midwest by the start of next workweek.
- Some model guidance favors heavy snow in the far northern US, while other guidance keeps this a rain/ possible severe storm event.

Andrew

Sunday, November 16, 2014

November 22-25 Potentially Significant Winter Storm

A powerful storm looks to present itself to the US around the November 22-25 timeframe.

Tropical Tidbits
Since this post will primarily be a model analysis, we'll begin with the ECMWF-Ensembles. This image shows the forecasted 500mb geopotential height values in the color shadings, with sea level pressure contours and high/low pressure demarcations superimposed. In this graphic, valid for the evening of November 24th, we see a storm system placed on the border of northeast Illinois into northwest Indiana. The minimum pressure is about 1003 millibars, which is a rather weak storm system. Despite its weakness, the mere presence of a storm on this graphic tells us there is some confidence in a storm occurring within this timeframe. Confidence is low to begin with, but it is there.

Tropical Tidbits
We now move on to the ECMWF model, also showing 500mb geopotential height values and SLP contours. For future reference, those two parameters will show up on all model graphics we analyze here today. The ECMWF model favors a very strong 987 millibar storm striking central Illinois on the morning of November 24th, with windy conditions overtaking the Ohio Valley and East US within the storm's warm sector. The sub-540 geopotential height values in the Midwest tell us that a widespread accumulating snow event would likely occur, particularly in the central and northern Plains into the Upper Midwest.

Tropical Tidbits
Pushing ahead, we now analyze the GFS ensembles forecast, valid here for the evening of November 24th. We find a weak low pressure system over central Lake Michigan, with a minimum central pressure reading of about 1005 millibars. Ensemble systems typically tend to be weaker than their operational counterparts (as observed with the ECMWF-Ensemble and ECMWF graphics above), since the ensembles take into account double-digit forecasts and average them all out. Regardless, the mere presence of a storm system is re-assuring to confidence.

Tropical Tidbits
Continuing on, we arrive at the GEM model forecast, valid on the evening of November 24th. The GEM, made by the Canadian meteorological service, shows a deep low pressure system of about 995 millibars right over Chicago, Illinois. This would support some stormy activity from the Southern Plains to the East Coast, but due to the retracted 540 line into the far Northern Plains, I'm not confident that this model is supporting a snowstorm for the Upper Midwest and Central/Northern Plains.

Tropical Tidbits
We now arrive at the GFS Ensembles, which paints an interesting picture for this storm. We see a storm system of minimum central pressure 1001 millibars placed just south of Chicago. This graphic, valid on the evening of November 24th, also draws the 540 line south into the Midwest and Central Plains. Again, since this is an ensemble forecast, the storm is not as strong as individual model forecasts. However, since it is showing up in the first place, confidence continues to rise in a substantial storm in this timeframe.

Tropical Tidbits
We've saved the best for last: The above graphic depicts the GFS-Parallel model forecast for this storm. We see a 979 millibar storm striking the northeast Illinois-southeast Wisconsin border, while 500mb geopotential height color shadings tell this storm to be a closed low. With very windy and cold conditions extending across the Midwest and Great Lakes, an accumulating snow event would likely be expected in the Upper Midwest and Northern Plains.


To summarize:

- Model guidance is confirming the possibility of a substantial storm in the November 22-25 timeframe.
- Some guidance supports a powerful storm system, possibly bringing both accumulating snow to the Upper Midwest, and severe weather to the East/Southeast.
- Confidence in this solution remains low due to the extended timeframe of this storm.

Additional updates will be posted in the coming week as more information becomes available.

Andrew

Saturday, November 15, 2014

Thanksgiving Potentially Significant Storm System

Model guidance is now hinting at this Thanksgiving storm system to impact the Central and East US.

PSU
Beginning with the ECMWF model, with this forecast going out to 10 days, we see a deep trough pushing into the Southern Plains, neutrally-tilted, as the isobars pushing due south show. Note how pressure tendencies have rotated to the southeast corner of the vort max, likely telling us that this trough will start its maturing phase into a negatively-tilted trough in the next few days, if this model forecast were to go out further. A better description of tilted troughs is below from theweatherprediction.com .

The Weather Prediction
Given the freezing line / 540 thickness line in the top-right image being pulled all the way to the Southern Plains on that ECMWF graphic, as well as the expected negative tilt to this trough, the going ECMWF forecast would likey deliver severe weather to the Southeast, as well as some snow to those in the Great Lakes and southern Midwest, with exact areas to be determined.

Tropical Tidbits
Moving ahead to the GFS-Parallel forecast for the same November 24th timeframe as that ECMWF graphic, we see a very similar forecast. Once again, a strong trough is located in the Southern Plains, as shown by the deep negative height anomalies centered over Texas. If you guessed this trough was neutrally-tilted, you are correct! The height contours seem to be 'pushing' due south, which means it's neither positive or negatively-tilted. Unfortunately, I don't have access to the jet stream forecast for this timeframe from the GFS-Parallel model, which means we cannot tell if this trough is preparing to tilt negatively or not.

We don't have much to work with right now, since the storm's still about 10 days away, so here's a few graphics of precipitation forecasts from the GFS and GFS-Parallel model with this storm.

Tropical Tidbits

Tropical Tidbits
Andrew 

Wednesday, November 12, 2014

November 25-29 Potentially Significant Winter Storm

We are beginning to nail down how this potentially significant winter storm may evolve across the country in the days surrounding Thanksgiving.
For my humor and your reading pleasure, I've divided this post into multiple sections, each concerning a different timeframe of this storm.

I. The Instigator 

Let's begin with a refresher on how this potential has come about.

OPC
On the morning of November 8th, we saw the remnants of Typhoon Nuri reach peak strength via Ocean Prediction Center observation. As the chart shows here, the massive storm bottomed out at 924 millibars. This means the storm, located at about 170E and 55N, didn't break the record for strongest extratropical cyclone on record, but it certainly came close.
Why do we care about this storm? The Bering Sea Rule states that a strong storm that hits the Bering Sea can produce a consequential storm in the United States about 17-21 days later. This is the same for high pressure in the Bering Sea. You probably get what I'm getting at here with this observed strong storm in the Bering Sea, so let's keep reading...

NWS
The graphic above, produced by the National Weather Service in Alaska, shows the observed mean sea level pressure of the storm at that 924mb reading, while the NWS office measured it at 930mb on the chart on the left, where strongest extratropical cyclones in the North Pacific are depicted. For multiple reasons, including the possibility that past storms may have been stronger than that 924mb reading, this storm was not declared the strongest on record in the North Pacific. Despite this, it's quite clear this storm was a historically-strong one, relative to storms in the last 60 or so years.

NOAA
Purely for comparison purposes, the team at KOPN Weather identified a strong storm in the Bering Sea on April 7th, 2011, bottoming out at 936mb, that was about 10 degrees W of where this very strong storm was observed yesterday in the Bering Sea. If you recall what happened about 2-3 weeks after the date of April 7th, we saw a certain tornado outbreak strike the South US, devastating thousands across the country, and causing millions on millions of dollars of damage. If we look at where the resultant storm ended up in mid-late April, utilizing the Bering Sea Rule, we find the storm situated in the Ohio Valley.

WPC
Now, keeping in mind that this sort of correlation is a tough one to use at best, not to mention all the caveats associated with long range forecasting, we could theoretically juxtapose the remnants of Typhoon Nuri and this storm in the Bering Sea on April 7th to get an idea of where the consequential storm in the US may end up. Recalling that the storm in 2011 in the Bering Sea was at about 180 degrees longitude, and almost the exact same latitude as the one observed in the last day or two, we find the remnants of Nuri placed about 10 degrees west of that 2011 storm. If we take the location of that storm system in late April (pictured above) and move it west, like the remnants of Nuri were west of that 2011 Bering Sea storm, we end up with a map like this:

WPC
Continuing this correlation, just to see what would happen, we notice that the remnants of Typhoon Nuri are moving eastward (a bit northeast in the process) in the Bering Sea right now, slowly at that. If this storm somehow does end up in that potential location outlined above, and if enough cold air is available (this will be discussed later), a significant winter weather event may strike the Central Plains, Midwest, Great Lakes, and Ohio Valley. Similarly, if the correlation works out, a severe weather event may strike the South US. Confidence remains low, but the potential for this correlation to verify is on the rise.

II. The Japan Connection

Now that we know where this potential is coming from, let's start to use it to our advantage, in the form of the Typhoon Rule (click here for explanation on the rule).

Tropical Tidbits
The image above shows the ECMWF model's forecast of 500mb geopotential height anomalies over the West Pacific. Here, cool colors denote stormy and cold weather, while warm colors depict mild and generally quiet weather. If we take a look at this forecast graphic, valid on November 17th, we find a rather strong trough/storm system pushing eastward into Japan. We can see this trough by the depression of height contours, and associated blue shadings. As this trough pushes through Japan, it looks to close off, a phrase used to describe when those contour lines literally close off and make a circle, indicating a closed low. Until then, we see this negatively-tilted trough hitting Japan around the 17th and 18th. Extrapolating that out using the Typhoon Rule, we find the potential for a storm hitting the US around November 23rd to 28th, oddly enough right around that timeframe that we saw with the intense Bering Sea low.

Tropical Tidbits
Moving on to the next graphic, we find the GFS-Parallel model forecast for November 17th, again forecasting 500mb height anomalies. The GFS-Parallel is not the same as the regular GFS; this Parallel model is the new, enhanced version of the current GFS model, which is set to be 'retired' in the next few months, where the new GFS will take its place. Among new corrections are bias fixes and increased accuracy, etc. The difference from the ECMWF model to this GFS-Parallel forecast is the Parallel model closes off this trough a bit quicker than the ECMWF, as that circular contour line over Japan shows. At this point, it's not so much a question of if this storm will strike Asia, so much as it is what strength will it be and when will it close off. Regardless, the ECMWF and GFS-Parallel both support this storm threat.

Tropical Tidbits
To add in a bit of diversity, let's check out the ECMWF ensemble mean 500mb height anomaly forecast for November 18th. In this image, we see the average of all 52 - yes, as in fifty two separate ensemble members - forecasts favoring a non-closed trough over Japan. This is a bit surprising, as the guidance we went over above has this trough closing off over Japan or just after it leaves the "mainland" of the island nation. Despite this disagreement, which will no doubt be worked through as the time between now and November 17th/18th approaches, the ensembles agree with the other two models on this being a substantial storm crossing Japan, with a consequential storm in the US around November 23-28.

III. The Set-Up

We're now in the timeframe where we can get a view of model projected set-ups for the storm environment (with typical low-confidence, of course). Let's go through the projected set-ups.

Tropical Tidbits
The image above may seem confusing, but it's not that difficult to interpret once you get a feel for it. This image shows the GFS ensemble forecasted 500mb height anomalies, averaged out across the ~20 ensemble members, valid on November 26th. Let's first begin with the positive height anomalies in the West US into British Columbia. We see what are interpreted to be slightly above normal height anomalies, but if this forecast verifies, you'll see these anomalies increase to more extreme values as confidence among all ensemble members increases. For now, confidence is low, so the anomalies aren't as pronounced.
Those positive height anomalies in the West are enabling a positive Pacific-North American (+PNA) pattern to set up. In a positive PNA pattern, ridging in the West forces the jet stream south. If resultant ridging forms in the East, the jet stream then bends north, to enable frigidly cold air to strike the Central/East US, also driving the storm track through that area. This pattern across North America is a classic +PNA pattern, and should be treated as though the Great Lakes/Plains may see the brunt of this storm... initially.
My concern rests with that big upper level low stationed just west of Greenland. If that low becomes too strong and pushes too far south (which is a plausible result), the storm track may be suppressed, and the big winners could end up being the Ohio Valley and interior Northeast. This is something to watch closely in coming days. For now, due to how the Pacific appears to be controlling the pattern, I would favor a Midwest/Great Lakes impact, but let's keep analyzing.

Tropical Tidbits
This graphic is the same type of forecast chart as the one we just discussed, but now comes from the Canadian ensembles, and is valid for November 25th. Despite this slight time difference, note that trough in the Central US, which could be our storm (though the timeframe's a bit fast for my liking). Once again, we see a positive PNA pattern, somewhat suppressed by lackluster ridging in the West US, but compensated by intensified ridging along the East. This would bend the storm track in favor of the Midwest and Ohio Valley for any big snows, and the Gulf Coast could then see some severe weather. Again, this will all sort itself out in due time, and this is merely something to watch for now.

IV. The Storm

It's all been leading up to this, folks. While I don't trust individual model guidance to show what the storm will be like (since it changes from forecast to forecast; no consistency), I would like to show the GFS ensembles' thoughts.

WeatherOnline
What you see above is a 'cluster' forecast from the GFS ensembles, valid on November 26th (technically the evening of the 25th in our time zones), forecasting precipitation and sea level pressure values over North America. I'll show the description of 'cluster' modelling below from the Weather Prediction Center, then I'll try to interpret it for others who may not understand at first.

Sometimes the ensemble members tend to group themselves into two or more solutions.  For example in the image above the ensembles cluster in two solutions off the Pacific NW coast  of the U.S. (a trough south of the Aleutian Islands and a trough off the Pacific Northwest coast of the U.S.).
CLUSTERING is an automated method that identifies and extracts like members and derives output from these like solutions (of which there are different methods to identify clusters).
If you didn't understand that, let's go through an example. Recall that the GFS ensembles have around 20 members, each of which produces its own, different forecast. Let's say that on November 26th, a certain number of ensembles are showing a relatively similar forecast. The 'cluster' method combines these similar forecasts, and does the same with other, like forecasts into 'clusters' of ensemble members. This allows us to narrow down how many ensembles are showing what type of solution for a certain time frame.

Going back to the GFS ensemble image above, approximately 30% of ensemble members' forecasts for this timeframe show a solution like the one above. This solution means a very strong storm would push north and east across the Midwest, like that positive PNA pattern may induce. The result? Heavy snow likely in parts of the Midwest and Great Lakes, while the South may see severe weather.
At first, you may think 30% is not that high. And you'd be right. But for a forecast 324 hours out, about a third of the ensembles showing this sort of solution isn't a bad thing to see if you're hoping for snow in the Central US.

Here's a good representation of what a scenario like the one above may result in.
I'm going on record and saying this is not a forecast! Please don't treat it as such!

Representation of one possible track for this storm.
Again, this is not a forecast, and it shouldn't be treated as such.
To summarize:

- Model guidance continues to support the idea of a strong storm system in the United States around Thanksgiving.
- The set-up for this storm looks to favor a storm track over the Central/East US.
- One model's representation might show heavy snow for the Midwest and Great Lakes.
- Low confidence, high caveats remain present.

Andrew

Sunday, November 9, 2014

November 25-29 Potentially Significant Winter Storm

The storm system around the November 25-29/Thanksgiving timeframe continues to look like a significant storm system.

OPC
On the morning of November 8th, we saw the remnants of Typhoon Nuri reach peak strength via Ocean Prediction Center observation. As the chart shows here, the massive storm bottomed out at 924 millibars. This means the storm, located at about 170E and 55N, didn't break the record for strongest extratropical cyclone on record, but it certainly came close.

NWS
The graphic above, produced by the National Weather Service in Alaska, shows the observed mean sea level pressure of the storm at that 924mb reading, while the NWS office measured it at 930mb on the chart on the left, where strongest extratropical cyclones in the North Pacific are depicted. For multiple reasons, including the possibility that past storms may have been stronger than that 924mb reading, this storm was not declared the strongest on record in the North Pacific. Despite this, it's quite clear this storm was a historically-strong one, relative to storms in the last 60 or so years.

NOAA
Purely for comparison purposes, the team at KOPN Weather identified a strong storm in the Bering Sea on April 7th, 2011, bottoming out at 936mb, that was about 10 degrees W of where this very strong storm was observed yesterday in the Bering Sea. If you recall what happened about 2-3 weeks after the date of April 7th, we saw a certain tornado outbreak strike the South US, devastating thousands across the country, and causing millions on millions of dollars of damage. If we look at where the resultant storm ended up in mid-late April, utilizing the Bering Sea Rule, we find the storm situated in the Ohio Valley.

WPC
Now, keeping in mind that this sort of correlation is a tough one to use at best, not to mention all the caveats associated with long range forecasting, we could theoretically juxtapose the remnants of Typhoon Nuri and this storm in the Bering Sea on April 7th to get an idea of where the consequential storm in the US may end up. Recalling that the storm in 2011 in the Bering Sea was at about 180 degrees longitude, and almost the exact same latitude as the one observed in the last day or two, we find the remnants of Nuri placed about 10 degrees west of that 2011 storm. If we take the location of that storm system in late April (pictured above) and move it west, like the remnants of Nuri were west of that 2011 Bering Sea storm, we end up with a map like this:

WPC
Continuing this correlation, just to see what would happen, we notice that the remnants of Typhoon Nuri are moving eastward (a bit northeast in the process) in the Bering Sea right now, slowly at that. If this storm somehow does end up in that potential location outlined above, and if enough cold air is available (this will be discussed later), a significant winter weather event may strike the Central Plains, Midwest, Great Lakes, and Ohio Valley. Similarly, if the correlation works out, a severe weather event may strike the South US. Again, many caveats are associated with this method, and this should not be taken as "gospel", or at face value.

Tropical Tidbits
The above image shows temperature anomalies at the 850 millibar level (about 5,000 feet off the ground) over North America, as forecasted by the ECMWF ensembles ten days from today. In this image, we see a large swath of warmer than normal temperatures in the Bering Sea, with colder than normal conditions encompassing much of the United States and southern Canada. This looks to be a persistent pattern in coming days and weeks, as a large block of high pressure looks to set up shop directly over the Arctic, providing for a very cold period for North America. Extrapolating this to Thanksgiving, enough cold air should be in place for at least a modest threat of a significant snow event. Again, bear in mind long range caveats, but such a prognosis is favored right now.

To summarize:

- A potentially significant storm system still looks to evolve in the United States around Thanksgiving.
- Severe weather will be a possibility, namely in the South US.
- Significant snow will be a possibility, predominantly in the Central Plains, Midwest, Ohio Valley, and Great Lakes (for now).
- Thanksgiving travel may be severely hampered by this storm, if it does come to fruition as currently projected.

Andrew

November 8-12 Significant Snowstorm - Final Update

This is the Final Update for this significant November 8-12 snowstorm.

NWS
Winter storm watches (dark blue) and warnings (pink) have been hoisted across a swath of the Upper Midwest into the Northern Plains, as a strong storm system looks to push south into the US and create a potentially dangerous environment for those caught by surprise from this storm. Winter weather advisories (purple) have also been issued for areas where the storm will strike earlier, compared to those in the Midwest.

Tropical Tidbits
The image above shows the GFS model's forecasted total snowfall from this storm over the next 4 days. We can see how the storm is forecasted to lay down amounts near 12" in the Northern Plains, before unleashing on the Upper Midwest. The yellow swath shows amounts in excess of 18" of snow, while those brown-ish areas depict amounts close to two feet of snow. Going by this forecast, we would see the brunt of the storm strike central and northern Wisconsin, as well as the Upper Peninsula of Michigan. Significant amounts would still impact portions of Minnesota and the Dakotas, but Wisconsin would receive the heaviest totals.

Tropical Tidbits
We'll next look over the forecast from the GFS-Parallel model, the newer version of the GFS model which will replace the current model in coming months. This GFS-Parallel model is rumored to be more accurate in multiple aspects of its forecasts, and this storm looks to be its first real test. The GFS-Parallel shifts the bulk of the snow east, now slamming the Upper Peninsula of Michigan into south-central Canada with those two-foot totals. Amounts in Wisconsin would still near 24", but not as widespread as the current GFS model shows. Minnesota is also forecasted here to receive heavier snow, extending into the Dakotas as well.

Regardless of which model you're looking at, the picture is clear: this storm will bring very heavy, very early-season snow to the Upper Midwest and North Plains.

WPC
The Weather Prediction Center issues probabilistic forecasts for snowfall amounts in a given period. In the above image, we see the WPC's forecast for the likelihood of 12" of snow falling in a 24 hour period, centered around November 11th. The WPC places the likelihood at around 50%, a value that's likely to rise as we get closer to the storm's impact and confidence increases.

To summarize:

- A significant winter storm is forecasted to affect the North Plains and Upper Midwest.
- Amounts nearing 24" may be expected, especially in Wisconsin and the Upper Peninsula of Michigan.
- Preparations for this storm should begin now.

Andrew