FOX44 - Burlington / Plattsburgh News, Weather & Sports11.20.11 More Info On Thanksgiving-Eve Storm

11.20.11 More Info On Thanksgiving-Eve Storm

Early winter season forecasting can be difficult, especially when strong storms roll through.  Coming up this Wednesday, a very strong low pressure system will move northeast through New England.

Here is the latest with the storm. 

Each of those diamonds and crosses are the potential positions of the storm.  As a forecaster, this is where the uncertainty comes into play.  Each of the pink and blue markers is a different weather model run.  So as you can see, there are several scenarios that could develop.  The storm could be north-biased and travel along those markers on the northern edge.  Another example would be the timing.  Notice the markers are also displaced east and west.  To me that shows a difference in how fast the storm will be traveling.  Timing can play a role in this storm with regards to what falls from the sky (snow, rain, sleet).

With this, I like to take the middle line.  That is what the black line is on the map.  This is a pretty northerly track.  When I look at this, I know it's going to be a mixed precipitation event (at best) or just an all-rain event.  Since the center of the storm will pass so close to our south, a lot of warmer air will get pulled north into our region.  That will prevent it from being an all-snow event.  If this low track was through Long Island and across Cape Cod, it would be an all-snow event (given the same atmospheric conditions).

This is a picture of the 850 mb temperatures at 12 z Wednesday.....wait what?! Okay simply, this is a snapshot of temperatures about 5,000 feet above ground at 7 a.m. Wednesday.  All of the lines are differing weather models.  Again, the black line is the middle ground for all of these different opinions.  A quick and general rule to winter forecasting is to look at the 850-line, which is all of these lines on the map.  Generally north of the line you can assume mostly snow, and south of the line you can assume mostly rain.  Along the line is where it gets messy.  Also notice there are many lines that cover our entire area!!  (I love clear cut forecasts, but these challenging ones are kind-of fun too). 

At this time the storm will be approaching to our south and heavy rain/snow/sleet will be falling.  Looking at the picture above, northern areas will have the best chance of getting snow and southern areas will get rain. 

Here's an example of the early-morning hours from Montpelier.

This is just a forecast for Montpelier, VT at midnight Wednesday.  The two blue dots on the left show light snow.  The red and green lines are temperature and dew point respectively.  The dotted blue line just to their right is the 0 C line, or the freezing line.  When the red line is to the left like this, you can forecast snow.  This model run is showing a frozen layer through the entire atmosphere.  So I'm expecting the precip to start as snow in almost all the areas, then change to rain.

This is six hours later at 6 a.m. local time in Montpelier, on Wednesday.  Notice the blue dots have changed to green.  That's the BUFKIT hinting at rain instead of snow.  This shows a surface temperature of 34 degrees and a temp > 32 between 3,000 and 7,000 feet.  From 2,500 to the surface it's a mix of below and above freezing.  As a forecaster, I would add either wet snowflakes or ice pellets (sleet) to this, given the back-and-forth of temperatures (around 32F).  But this shows that warmer air coming in at the higher levels as the storm approaches, like I mentioned before. 

With this storm the warmer air is going to be coming in at higher altitudes first.  That means precip will start as snow, then mix to a brief sleet period, and then all rain.  During the day Wednesday almost all areas turn to rain by dawn and remain rain for several hours.  I expect a switch back to snow in the afternoon as cold air gets wrapped around the storm.  In terms of snow totals, I think northeast Vermont will get 2-4" of snow before the change to rain Wednesday.  Northern Vermont will get 1-3" before the change over to rain.  Central Vermont and New Hampshire will likely get 1-2", then a lot of rain.  Southern Vermont will get near nothing for snow and a lot of rain. Rainfall amounts look to approach 1" or more for many of us Wednesday.  Behind the storm we may see 1-2" snow in central and northern areas that will end by midnight Thursday.

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