Stratospheric Polar Vortex to Come Under Attack

The stratospheric polar vortex looks to be coming under attack in the next week or two.

The ECMWF forecast for Wave 2 activity shows highly anomalous values arising in the far upper stratosphere, mainly extending from the top of the stratosphere (1mb) to the middle-ish part of the stratosphere, ending up at the 20 or 30 millibar area. It is easy to see the attack that the stratosphere is undergoing, with the highly active Wave 2 values defining the polar vortex being strained in an attempt to be split into two vortices. There are two major types of wave activity- Wave 1 and Wave 2. In Wave 1 activity scenarios, the stratosphere undergoes distress as a major ridge attempts to displace the polar vortex from the Arctic, while keeping it fully intact. With Wave 2 activity, the polar vortex is experiencing ridging across the upper latitudes, making the vortex want to split into two vortices. These vortices can then shift to lower latitudes and bring extreme cold weather to the regions affected. In this specific case, it isn't looking like this projected polar vortex intrusion will do the job of splitting up the vortex.

This image, showing multiple wind speed and flux forecasts from the ECMWF, sums up the idea that while this will be a rather intense attempt by the stratosphere to break up the polar vortex, it won't actually finish the job. On the top panel, we have the projected wind speed of the top of the stratosphere, at the 1 millibar level (where this big stratospheric intrusion will be centered). The wind speed is projected to actually increase over the next 10 days, meaning the polar vortex will strengthen. We would want to see a steep drop-off in wind speeds in order to identify that this stratospheric event would actually be breaking up the vortex. The second panel shows wind speeds at the 10 millibar level (blue) and 30 millibar level (red). Again, we would want to see dropping wind speeds to indicate a weakening vortex, but as that indication has not been given, I do not anticipate a splitting vortex. The third panel shows the sharp spike in Wave 2 geopotential activity, which tells us about that incoming stratospheric intrusion, and this is confirmed with the increasingly-elongated EPV flux arrows on the bottom panel. If we see the arrows get larger, the attack on the polar vortex would intensify. But because the ECMWF can't show forecasts beyond 10 days, we don't know how intense the event actually gets.

In the long range, the GFS model sees things getting a little more interesting. This model allows a large strip of warm temperatures to surge into the Arctic via Eurasia, which does appear to be going up against the polar vortex (shown in the blues and purples). Now, the accuracy of the GFS in the long range is pretty low, so this forecast is pretty dicey to begin with. However, going off of the ECMWF beginning a pretty solid stratospheric attack on the polar vortex, the idea of this attack continuing and even intensifying isn't that far fetched as the Wave 2 geopotential height anomalies increase per the ECMWF. So, while this forecast above may not verify as it is shown above, something similar in terms of a continued attack on the polar vortex is possible.

Andrew

December 19-23 Significant Snowstorm & Potential Ice Storm Discussion

This is a new discussion about the impending significant snowstorm and potential ice storm for a wide swath of the US.

The two pieces of energy that are associated with this system are circled in black. We see a main piece of energy located just offshore the Californian coast, with the second piece of energy just scraping the Aleutian Islands.

There are a few things to discuss here.

I took a look back at the December 14th 0z GFS model forecast and compared it to the same image you see above, which is an analysis image of mid-level vorticity values as of 12z today. When I compared the December 14th image, valid for 6 PM CST tonight to today's 12z GFS forecast for 6 PM tonight, I found a couple of interesting tidbits. For one, the energy is a digging south in the Southwest a bit more than what the December 14th forecast had shown. This is good for those wanting a southern trend in the storm, as it helps prevent a more northern solution. Additionally, and this is probably a more important factor, that energy currently skirting the Aleutian Islands is faster in today's 12z GFS than December 14th's forecast. The faster this northern portion of energy goes into the US, the better chances are for this storm being more impressive than recent forecasts have projected it to be.

Secondly, we have reached what I believe is a crucial point in model forecasts. As the storm system offshore California finally drops down into the Southwest, the troubles models have been having with the speed and intensity of said energy dropping into California will be shed. This means that the upcoming 0z model suite should be forming something that may resemble a consensus of this winter storm. Currently, said consensus would bring heavy snow from portions of Kansas, through southeast Iowa and into Michigan. However, the recent southward trend in the models could continue as some have indicated, and if this is true, expect that heavy snow axis to shift south as well.

Here's the most recent ECMWF snowfall map, showing the predicted location of that heavy snow axis. Model guidance in general has been lighter with snowfall amounts, minus the ECMWF. Despite that discrepancy, I do believe that the strength and pure amount of moisture the system is pulling up should see amounts close to the ECMWF verifying. I really can't see locations in the thick of an all-snow event from this system not exceeding 6-8", even though some guidance systems want to place the threshold at 6". I definitely believe we should watch the models for future shifts to the south; I don't really expect a northward shift at this point. Either the snow shifts south, or we see the current idea verify. Not that enthusiastic about a northward shift from here on out.

As far as this ice storm potential, the exact details are pretty tricky to nail down. A primary factor is the track of the storm still relatively unknown, and the location/intensity of any potential freezing rain/sleet will be determined by the storm track. Regardless of who gets hit with any freezing rain, I think it will be a decent icing event, possibly with decent accumulations as well. I would watch out for changing freezing rain forecasts, as the ingredients needed for accumulating freezing rain are rather tricky to pinpoint, especially with a system as dynamic with this one. One thing is clear- there is certainly potential for an accumulating ice event.

I'll probably be ready to issue my next outlook tomorrow, when the model consensus should either have formed or be forming. Until then, here is a summarization of my thoughts.

-A significant winter storm is probable for the United States, with effects including significant snows and a potential icing event.
-Heavy rains are likely in many parts of the Ohio Valley and Midwest.
-Models should begin to get a substantial handle on this system tomorrow, with the first hints of a model consensus appearing tonight.
-A southward tick in the track with upcoming model runs is not out of the question.

Please do not ask for a specific location getting affected by this storm- this is a very fluid situation, and what you see here is about all I'm willing to say with confidence right now.

Andrew

December 19-22 Significant Snowstorm & Ice Storm Update

This post will address the current situation of the model guidance, as well as what I think will be happening.

The ECMWF model, which previously dumped a wide plethora of snow in excess of one foot across the Lower Great Lakes, suddenly flipped over to the GFS' solution in an unexpected turn of events. The European model had been forecasting this solution for five consecutive runs until this afternoon, when it decided to take the snows further north in accordance with shifting the low pressure system itself further north. The GFS and GGEM had also taken this system further north with today's afternoon runs, meaning a consensus appears to be forming.

I have a few thoughts on this. For one, with a storm this strong, a northern shift to the storm was bound to occur, though one could argue it was made a while ago when the GFS came north after making its far-south forecast a few days ago. Strong storms naturally have the tendency to overachieve to the north, so to say, in that models may underestimate the northward motion of the storm. I find that to be a very plausible scenario, and it is possible this ends up being the final track to the storm.

Another possibility with this system is that it is not being handled correctly. This would arise due to the lack of this system being in the United States, where the radiosonde (instruments attached to the big weather balloons) can collect crucial data that then goes into the models, and enhances their forecast's credibility. Our system is not in the US yet- it is currently skirting the west coast of Canada. It is possible that the track will change again over the next 24-48 hours before it finally comes ashore. When it does come ashore, expect to see a model consensus quickly materialize, whether it be the one depicted above by the ECMWF, or a further south one.

One thing is for sure, however- this storm will be hauling loads of moisture northward. As the relative humidity forecast chart from the GFS shows, there will be immense amounts of moisture being pulled north for this storm. Regardless of where the storm ends up tracking, chances are good that someone will receive significant snow, while others receive a significant ice storm, and still others get flooding rains. This system will be one of the more powerful ones we have seen in recent years, and its impacts will not be as weak as some modeling systems have previously been predicting them to be.

Guidance does agree that there will be potentially significant icing involved with this storm. The GEM model above shows freezing rain accumulations for this storm in millimeters, and if you look closely at the pink dot on the right, it shows up as 100 millimeters of freezing rain. That is equivalent to just a few hairs under 4.00" of freezing rain. Now, I don't think this will be a 4.00" freezing rain event. The system is powerful enough, however, to produce a significant ice storm. We won't know specific accumulations for the freezing rain portion of the storm, but I would hazard a guess that over 1.00" of freezing rain is definitely within the realm of possibility.

Here's my last thought: I believe a southward shift is in the picture for this storm yet again. Why? Taking a look at model bias' yet again, as I have throughout this system, it was determined by the NCEP that energy in the Gulf of Alaska moves too slow across all models. Additionally, models have issues in not moving Arctic air and Arctic cold fronts south quickly enough to the east of the Rockies. Considering these two model bias', I do think there is wiggle room for another shift southward, and would not be surprised to see such a shift unfold. If we did see the energy in the Gulf of Alaska move faster than it is projected to (considered at this time to be a possibility), the system could very well return to what the ECMWF had previously been showing in terms of track, and possibly heavy snow as well. I'm not completely confident in these ensembles, so the chance of this happening is consequentially in question.

I would watch out for a southward model correction in future runs, in the event that we see the energy along the Canadian coast actually move faster than it is projected to. At the same time, however, the longer this new model consensus goes on for, the better chance it has of working out.

I'm not confident enough to release a second outlook, but here's a summary of my thoughts.
-We are looking at a significant storm system hitting a large part of the nation.
-A significant snow, ice and rain event is possible/probable with this storm system.
-There is potential for models to correct southward once again, though the degree of correction and probability of said correction is unknown at this time.

Andrew

December 20-23 Significant Winter Storm - First Outlook & Discussion

This is my first preliminary outlook for what is shaping up to be a significant winter storm over the upcoming weekend for a large swath of the country. A model discussion is included in today's post, and that's what we will begin with.

We're going to start off with the ECMWF model's projection, since it is the one that seems to be setting the trend for this event. In the image above, valid for Sunday morning, we see a strong low pressure system emerging over the southern Midwest and Ohio Valley. This chart from Meteocentre calculates the minimum central pressure to be at 996 millibars, making this a pretty strong storm. The intensity is no surprise, as I've been illustrating since December 11th that a very strong storm over East Asia will provide a base for a strong storm in the United States. The ECMWF has this system follow a track extending from the Southern Plains through southeast Missouri before continuing on through the Ohio Valley and eventually leaving the United States. I believe we will eventually see something very similar to this track, maybe shifted a bit north or south by the time the storm arrives. My reasoning?

The Earth System Research Laboratory (ESRL) puts out almost-daily teleconnection forecasts, and I have screenshotted two of them above. We see the forecast for the Pacific-North American index (PNA) on the left, with the North Atlantic Oscillation (NAO) forecast on the right. Matching up the index with the date on the bottom legends of each panel, we see that the PNA is projected to be negative (albeit rising) during this timeframe, with a strong positive NAO.

During a negative NAO, we tend to see big high pressure form across the Gulf of Alaska. Because we have a big rise in height anomalies over that body of water, there is a response of a dropping in height anomalies over the West Coast, meaning stormy weather forms in that region, as the graphic above from NCSU illustrates. Thanks to that stormy weather in the West, high pressure likes to form in the Southeast, and the two arrows (which illustrate the wind pattern in the area) combine over the Plains to show the storm track of systems during a negative PNA. Now usually, negative PNA storms like to go up into the Plains because the Southeast ridge is usually very strong. In this case, however, the positive NAO helps to keep the overall atmospheric pattern very progressive, meaning we don't see any persistent high or low pressure systems in any given area. As a result, that Southeast ridge is not given the opportunity to develop into a massive high pressure system. So, with this particular storm, I expect the system to move north due to that Southeast ridge (which will be slightly suppressed), and then take a track favoring snowfall from the east-central Plains through the Midwest and Lower Great Lakes and into a portion of the Ohio Valley, possibly even into the Northeast (though that is TBD).

The image above, from WeatherBell, shows mean sea level pressure anomaly forecasts from the ECMWF ensemble prediction system (EPS) across the nation for 144 hours, or 6 days out. We can clearly see the low pressure system in the Ohio Valley, and this is something the ECMWF Ensemble system has been hinting at for a while now. It has preferred this sort of track, and although I do not have snowfall figures from the ensemble mean, the similarity in track to the aforementioned 12z ECMWF run would tell me snowfall figures should be relatively similar. (For anyone interested, the ECMWF EPS Control snowfall forecast is directly below)

And now, for the moment you've all been waiting for, here is that 12z ECMWF snowfall forecast, using 10:1 ratios (where 10 inches of snow equals 1 inch of liquid)

Do I believe that this snowfall forecast will work out? Well, it's hard to tell. On one hand, the ECMWF does have a pretty good handle on this event to me in terms of consistency (compared to other models), and its idea for a significant snowfall jives well with the East Asian correlation saying that this will indeed be a significant system. On the other hand, we're still a ways away, meaning forecast models will most certainly change- the big question is if they change significantly, or just a bit. Based on the atmospheric set-up associated with this storm, I would think that this solution of snowfall placement would be within the realm of possibility.

Another possibility is that the storm actually goes further north, giving heavy snow to southern Wisconsin and a stripe of Michigan, as the most recent GEM model shows above. I believe this is a possibility, considering there shouldn't be a huge high pressure system in Canada to really suppress this storm to the south, but going against this idea would be the lack of a significant ridge in the Southeast to push it north, especially with the weakening -PNA (which really propels that Southeast ridge). In order to consider this solution a bit more, I would want to see more consistency with the GEM, as well as agreement from other modeling systems. However, judging by the overall pattern, as well as some model bias' I will discuss next, I'm not so sure that it's a real possibility.

Model guidance can be great, but it's not perfect. There are known bias' that have been outlined by various weather agencies, specifically the NCEP agency here in the US. These bias' should be able to help us determine what may happen that the models aren't clueing in on yet.

1. Tendency for models to hold energy in the Southwest for too long.
This is a rather well-known model bias, in that most guidance systems will end up keeping storm systems in the Southwest US for longer than they actually end up there. This affects our situation specifically in that solutions that are going for a further south projection for this system (see the GFS) may be incorrect. Should the energy eject from the Southwest quicker, it would seem to me that a more northern solution would be probable. I'm not exactly sure if that has been shed off by models like the ECMWF, GEM and ECMWF EPS yet, though it's just something to keep in mind.

2. Some models may close off low pressure systems too readily.
This is a model bias that really affects us here. Some medium range model guidance systems have the tendency to close off a low pressure system, meaning it will be cut off from the jet stream and just meander around until it is forced away by another low or high pressure system. In this case, model guidance generally agrees that our system will be closed off when it moves into the Southwest, and that's when model discrepancies begin to arise. Some guidance systems prefer to keep the system closed off for a while, which then results in a further south storm track, whereas other model systems don't keep the system closed off for so long, resulting in a northward track. In this case, I believe the GFS has kind of shed that bias it previously showed last week, and as a result we did see (and will probably continue to see) a northward movement in the storm track. As we move closer to the event, bear in mind these bias' become less and less involved in the models as they are shed away.

3. Arctic air will plunge southward quicker than models may indicate.
This is one that may or may not affect this system, as it's pretty apparent there will be precipitation mixing issues regardless of if this bias is in play. The NCEP indicates that medium and short range models will push cold air south to the lee of the Rockies slower than how it actually happens. In this case, one could argue that the cold air on the models is being underestimated, but as the cold air source in the Pacific (you weather junkies know it as the weakening negative East Pacific Oscillation) weakens during this storm system, I'm hesitant to count this bias as one to watch.

When I account for all the bias', the model trends, and some things I've been looking at over the past few days, I come up with this preliminary outlook.

To be quite honest, I'm feeling pretty good with the ECMWF operational and ECMWF EPS agreement. I feel that they do have the best handle on this system and the atmospheric pattern surrounding it, so my forecast was modeled after it. It should be noted that I did extend the significant snow area a bit north to account for any possible northward trends in this system. It should also be noted that there will be a sharp cutoff with snow amounts on both the northern and southern fringes, regardless of its track.

Rather than ask how your location looks, please look at the map- there really isn't any additional information I can provide you if you ask for a certain location.

Andrew

December 19-23 Potential Significant Snowstorm & Ice Storm

I'm feeling pretty confident that there will be a significant winter storm, composed of heavy snow and accumulating ice, between the December 19-23 period.

The following text & images are from yesterday's post on this system. The end of this copied portion will be denoted as (End of December 15th post).

A pretty strong storm system is expected to move across Japan in 48 hours, after dumping multiple storm systems over that area in the past few days. This storm system is looking healthy, as defined by the GFS ensemble mid-level height anomaly forecast above over Japan. As I have mentioned on this blog, there is a 6-10 day correlation between weather that occurs in East Asia, and weather that occurs in the United States. Thus, with this forecast image, we arrive at the December 13th date. Go ahead 6-10 days, and we find the potential storm timeframe of December 19th to 23rd. The reason I believe it may be significant is due to the strength of this system on the image above. Unlike the upcoming weekend storm, which saw only a moderate system in East Asia 6-10 days prior, the current East Asian projection is for a strong storm to hit Japan, and long range model guidance senses that this system will also be a strong one for the US. (End December 11 post).

The teleconnection forecast for the next little while from the Earth System Research Laboratory (ESRL) above shows projections for the Pacific-North American index (PNA), the North Atlantic Oscillation (NAO), the West Pacific Oscillation (WPO), and the East Pacific Oscillation (EPO). For this discussion, we will focus on the PNA and the NAO.

The forecast for the Pacific North American index is negative over the time period for this storm system, even bottoming out to its minimum during when I anticipate this storm system to strike. So what does the atmosphere look like during a negative PNA?

The image above is what is referred to as a negative Pacific-North American index phase (or -PNA for short). The negative PNA results from ridging in the Gulf of Alaska that then produces storminess/troughiness in the Western US. In response to that West US storminess, we then see the development of high pressure in the Southeast. The jet stream in this situation takes storms from the West and runs them north and east, typically delivering snow to the same areas that have been getting snow recently (think Plains, Midwest, Great Lakes regions). An interesting point of interest is how the two arrows on the image above (from NCSU) merge in the Plains. That means that, if the timing is just right, two storm systems could combine (phase) and form a stronger, single storm. History suggests storms that encounter phasing do become anomalously stronger thereafter, and can produce significant wintry precipitation. I'm not implying this will be a phasing situation, just something interesting.

The next item to discuss is the projected positive phase of the North Atlantic Oscillation. During the positive NAO, we see stormy weather over Greenland, which shifts the jet stream so the weather pattern is not conducive to those big high pressure systems you might sometimes see over the Bering Sea or even the Western US. The positive NAO pattern is what is called 'progressive', meaning no atmospheric feature really sticks around in any single area, because the atmosphere wants to keep this moving, so to speak. That said, previous forecasts for this storm had the system going north-northeast from the southern Plains into the Dakotas and Upper Midwest. Because of that progressive pattern, I'm really doubting we see such a spike northward. Will we see northward movement? Yes, because the -PNA will supply that ridging in the Southeast. However, the progressive nature of the positive NAO should limit any NNE or even due-north movement by this storm. I would watch out for the storm to move northeast, or even ENE.

This relative measure of predictability map, from the NCEP, gives us a glimpse at what atmospheric conditions are likely to be unfolding by an assortment of colors, defined in likelihood by the legend at the bottom. In this case, we see that blues have a single or low double-digit percent chance of verifying, while oranges define close to a 65% or better chance of verification. This map is valid on the evening of December 20th, roughly a day or two before this storm is projected to strike. There are three things we have to look at with this graphic.

The first item we'll discuss in this image is the ridging over the southern portion of the Gulf of Alaska. This ridging is encased in solid oranges right over the heart of the big arcing in the waters west of California and the coast, signaling high confidence that this ridge will be present. If that ridge is present, then is essentially guarantees the set-up I described above. Why? Well, that ridge in the waters off the West Coast then creates West US troughing, which provokes high pressure in the Southeast, and suddenly we have a classic negative PNA pattern.

Second, we see a swath of oranges extending from southern Canada into the Rockies. The troughing in the West from that negative PNA pattern tells us that the ensembles are sensing the trough to be our storm system in the December 19-23 timeframe. Like the Gulf of Alaska ridge, the oranges signify over a 60% confidence among the ensemble system with this storm system. Rising confidence among the ensembles for not only the storm itself, but for the NE Pacific ridge tells me that this storm is becoming even more likely.

Third, there is a wide body of oranges in the Southeast US. Going back to that negative PNA pattern, high pressure/ridging in the Southeast is expected, so it really isn't surprising that the ensemble set is going with a >60% chance of that ridge occurring. However, with the aforementioned progressive pattern thanks to the +NAO, it is unlikely that we see this ridge take over the East entirely. (End December 15 post). 

Now, I was running through model solutions and found something that makes me believe this will be a storm for the Midwest and Great Lakes, possibly not as far south as recent solutions have been indicating.

This screenshot shows mid-level vorticity forecasts from the GFS on the left and from the GEM on the right. Both are valid for Sunday, December 22 in the morning. Right off the bat, we notice significant differences among each model. The GFS on the left has closed off our impending storm system in the far Southwest, extending into Mexico, while the GEM has refrained from closing off the low and has instead propelled it eastward into the Plains and Midwest. As a result of the GFS' closed off solution, the storm system is cut off from the jet stream and meanders eastward into Mexico before finally being pushed northward again. It is this closed off low idea that creates the southern storm track (where snow is given to southern IL, IN, OH and into the Northeast), while the GEM's more progressive solution forms the northern track (snow falls across northern IL, IN, MI, OH into southern Ontario).

To be quite honest, I feel that the GEM has the right idea here, at least in not closing it off in the Southwest. In a recorded list of model bias', the NCEP agency notes that medium range models have a tendency to close off low pressure systems too readily. I feel that this is expressed in the GFS system on the left panel above, and in the latest ECMWF forecast. The GEM also closes off the low pressure system in the Southwest, but releases it from that closed position earlier, thus transferring the storm eastward quicker and going with the northern track. The difference between the GFS/ECMWF and the GEM is that the GFS/ECMWF hold the storm in the Southwest for a longer period of time than the GEM, and that leads me to the second model bias. I have mentioned on this blog that there is a bias that indicates models will hold storms in the Southwest for longer than they actually end up there. The GFS and ECMWF are perfect examples of this phenomenon (seen well in the GFS/GEM panel above), whereas the GEM does not keep the energy in the Southwest for such a long period. Based on those two bias', I do feel like the GEM has an accurate handle on this system.

And the interesting part is, it's not alone.

This dual-panel image shows mean sea level pressure (MSLP) values for Hour 168 from the GEM on the left, and the ECMWF Ensemble Prediction System (EPS) mean on the right. Thus, both images are valid for next Sunday morning. Notice the similar tracks in the MSLP values across the GEM and ECMWF EPS. Timing is a bit off, with the ECMWF EPS faster by a good 12 hours with this system moving into the Ohio Valley. However, the main point is evident in that the GEM and ECMWF EPS agree on this faster ejection of the system out of the Southwest. I feel that the GFS and ECMWF operational models are holding the energy in the Southwest for too long due to that bias, and while the GEM is also subjected to the same bias, it would appear that the model is circumventing it and is going with a quicker solution.

Here's the GEM snowfall and freezing rain projections, respectfully (Use Caution):

Measured in millimeters

I do think that one part of the model tendency to hold that energy in the Southwest might be due to the aforementioned negative PNA during this timeframe, but with the PNA gradually rising to positive territory around Christmas time, as well as the previously-discussed model bias', I doubt the storm would be able to stick around in the Southwest for as long as the ECMWF/GFS believe it will.

It's still a little early for any projected outlook due to model confusion, as well as a couple things I want to monitor over coming days to see if a southward shift shown in the GFS/ECMWF may be a valid possibility (though as I said, I'm not sold on it yet).

Andrew