Significant Stratospheric Warming Event Ongoing

A significant stratospheric warming event, or SSW event, is currently ongoing across the lower stratosphere.

The above image shows 70mb values from 2011 and 2012. The green dashed line displays average stratospheric values for that time of year. If we look at the most recently observed line in the stratosphere, we see that significant warming has occurred, with a sharp spike in temperatures now being observed.

While the stratosphere is so high up in the atmosphere, it does have significant implications on our weather now. What the warming stratosphere does is it weakens the polar vortex, which, when the vortex is strong, can restrict cold air from the US. Now that a SSW is occurring, the polar vortex is weakening, and a stronger push of cold air south should begin. However, the presence of the ongoing ridge over the Bering Sea (and the ECMWF's idea of said polar vortex restrengthening over Canada) tells me to be cautious of any incoming cold weather calls.

Andrew

Christmas 2012 May Be Cold, Snowy For Some

Image from WeatherBell Models.

The long range CFS model is forecasting a potentially cold and snowy Christmas week, as we enter the final days of November and launch into December. The above image shows the CFS model forecast for the December 21- December 26 timeframe, which does include Christmas. The CFS is projecting that a very strong cold blast will break into the nation, bringing below-average temperature anomalies to near double-digits, especially near the Ohio Valley. The warm temperature anomaly in the West is supportive of a positive PNA pattern, so such a solution as pictured above does not seem that far-fetched by looking solely at this image.

Image from WeatherBell Models.

This is the forecast snow depth for the same time period. The image shows 4 ensemble members forecasting the maximum snow depth by the December 21 - December 26 period. I might as well give a synopsis of my own on this forecast before you get too excited. First off, I highly doubt that San Antonio Texas will be under 4-6 inches of snow on Christmas, so I prefer to discount the upper-right corner image. Also, the idea of 1 foot of solid snow from the Plains, Midwest, Southeast, Northeast and Great Lakes is not that realistic. The most realistic forecast I see on here is on the top-left, where over a foot of snow is coating the Mid-Atlantic, with more in the Northeast.

So, the CFS v2 is projecting a cold and snowy Christmas. However, I don't trust the CFS model, as it does not have the best track record, but it certainly is interesting to see!

Andrew

December 1-2 Winter Weather Event - Northeast

Property of WeatherBell Models.

This is the snow depth forecast for the upcoming winter weather event for the Northeast. The above image is the total accumulated snowfall until December 2nd, and, as you can see, the Northeast gathers some accumulation, with totals ranging from under an inch right along the coast to as much as 6-7 inches in northeast New York. Vermont and New Hampshire also look to get a few inches of accumulation, with the most snow being right near the border.

The system doesn't look to be too strong, so the event will not last that long. The snow will likely begin around the early morning hours of the 1st, so no significant travel impacts are expected. However, it appears possible that airports in the region may have some slight delays. The early rush hour may start off slow, but generally no troubles.

Andrew

November 29-30 Winter Weather Event - North Plains

A snow event looks to take place over the Northern Plains on November 30th. The above image, courtesy WeatherBell Models, shows radar reflectivity forecasts from the NAM model. However, this is a reflectivity 4-hour streak, meaning it combines a 4 hour span of radar return forecasts into one image. This image shows where the heaviest snow will strike, which appears to be North Dakota.

Some snow may also fall in Minnesota and Wisconsin , but the main event will definitely be in North Dakota. I anticipate snow amounts to add up to 2-5 inches, centered in northeast North Dakota. The central part of the state will receive a messy slush, as a rain-to-snow event occurs with this system. Devils Lake and Grand Forks are most likely to see the best accumulation (the former location over the latter), while Jamestown and areas in the northern part of James River will see a more rain-sided event with snowflakes mixed in the further east you go.

This event is likely to start in the middle of rush hour as rain and transition to snow by the end of the rush hour period, approximately 6-7 PM central time. Snow will fall into the very early hours of the morning, and end by the 10 AM central hour on November 30th.

Andrew

December 10-12 Long Range Potential Winter Storm

I am officially opening up the discussion on a potential December 10-12 storm system that has been in many of the last couple days of the GFS model forecast.

Top left: 500mb heights. Reds are low pressures, purples are high pressures.
Top right: Pressure values. High/Low pressure systems are outlined.
Bottom left: 700mb relative humidity. Higher humidity values are in darker greens.
Bottom right: Precipitation. Any values above the red line are snow, any values below the line indicate rain.

For quite some time now, the GFS model, as pictured above, has been projecting a strong storm system to develop in the Plains and move northeast and strengthen, in a path all too familiar to those that went through the 2011 Groundhog Day Blizzard. This particular image above is of the 12z GFS for December 11th, depicting a strong storm system in the top right image. Precipitation in the bottom left image is widespread, with much of the Midwest getting snow while the Southeast is hammered with rain and storms. But first, let's determine the likelihood of such an event even happening.

It should be noted that 9 of the last 11 GFS model forecasts have had this storm system- a good sign as far as reliability goes.

This is something called the Pacific-North American Index, or PNA. In positive phases, storms are directed at the Midwest, like the above example shows, and negative phases generally induce warmer spells over the East US and storms diverted to the Plains or Deep South. These two images are separate forecasts of the PNA, with the left image being from the ESRL/PSD agency and the right image from the NCEP, essentially the GFS Ensembles. Because the NCEP is just composed of the GFS Ensembles, I favor the ESRL/PSD ensemble forecasts. That agency has the PNA just as negative as the NCEP, but has the index spike around December 8-12 to only weak negative values, which would help out this storm system into taking the track the forecast at the top of this post is showing.

However, when I take a look at the individual 12z GFS Ensembles for this same timeframe, it occurs to me that sometimes, you can't trust forecasts like these. The ensembles have a strong ridge over the northeast Pacific and West Coast, typical of a positive PNA. While it is not necessarily a positive PNA, it is apparent to me that such a set-up could indeed support a storm of this magnitude and track.

Also enhancing my investigation into this particular storm is the West Pacific Oscillation (WPO) and East Pacific Oscillation (EPO). These two indices affect the Bering Sea, which is currently experiencing an Omega Block. An Omega block involves a high pressure forming in any given area. In response to the sudden increase in height anomalies (a.k.a. the formation of a high pressure system), a stormy pattern develops in the immediate vicinity of the high pressure system, forcing height anomalies down on either side of the high pressure system. This creates the Greek symbol Omega (if you were to look at the 500mb height chart), hence the name 'Omega Block'. The WPO/EPO, both of which affect this block which just happened to form in the Bering Sea, are looking to be on the move near this storm's potential occurrence.

The EPO looks to be more or less positive during the storm's potential arrival. Such a solution would mean that a disturbance would try to form over the Bering Sea, thus hurting the high pressure system stationed over that body of water. If we want cold weather to enter the nation, it is an absolute must that we get that Omega block out of the Bering Sea, and this positive EPO could be helped by the WPO. The WPO index is looking to be trending to weak negative territory by the middle of the second week of December, which is when we could expect this strong storm system to hit. This more positive trend to the WPO also enhances the atmosphere to attempt and create a stormy pattern in the place where this Bering Sea Omega Block is.

The 12z GFS Ensembles have the Omega block retrograding north into the Arctic Circle, which would weaken the polar vortex and help cold air flow south. If such a solution were to happen, I see little reason to why we should give up the second half of December.

All in all, keep your eyes peeled. I'm seeing changes in the teleconnections, and when you get changes in the teleconnections, it's not all that difficult to see a brief stormy pattern as the pattern shifts from one phase to another. If the WPO/EPO/PNA forecasts verify on their forecasts as shown above, there could indeed be potential for a storm like this to blow through. Encouraging signs are also emerging from ensemble and model forecasts, as shown above by the GFS/GFS Ensembles.

Andrew