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Sunday, November 20, 2011

Rainy Travel Day Wednesday. Please stop worrying about a snowstorm.

“How are you ignoring this possible storm Tues/Wed next week....isn't there a shot for snow??”

This is a tweet I received on Friday and I continue to hear messages about a potential snow storm for Wednesday. I’m not sure who was sounding the alarm on this one…but it wasn’t anyone I work with!

A storm moving in on Tuesday night into Wednesday will take a northerly track through central New York, Vermont and New Hampshire putting us in the “warm sector” of the storm. (See diagram below for a typical midlatitide cyclone.) Winds will blow out of the southwest funneling in milder air and keeping temps in the 50s and our precip as rain.
Rain will come down heavy at times Wednesday (adding 1"-2" to what is already the 5th wettest year on record). As the storm pulls away Wednesday night winds will shift out of the northwest (see diagram above again). Now the question is…will the rain move out before the cold air gets here? OR will the cold air rush in changing the rain over to a wintry mix?

The model consensus is that rain moves out before the bulk of the cold air gets here. However, I wouldn’t be surprised if some of the lingering showers Wednesday night mix in with a little wet snow above 1000 feet. Either way, rain is the main concern Wednesday as you head off to see friends and family for Thanksgiving. And Thanksgiving Day looks lovely with sunny skies and temps in the mid 40s.

NERD ALERT: This is why I’m not concerned about accumulating snow Wednesday

Below you’ll find the latest European model run for Wednesday at 1 PM. The colored shading represents QPF (quantitative precipitation forecasts) or in this case, 6 hour rainfall amounts. The dashed lines show 1000 -500 mb thickness. The thickness represents the distance between the 1000 and 500 mb pressure levels. The distance is mainly a function temperature. The greater the thickness, the warmer the air (because warm air takes up more volume than cold air. That’s why you need to add air to your tires when it gets cold.)
Notice how all the dashed lines are blue except the one marked “540”. That’s because the 540 line is often used to approximate the rain/snow line. This isn’t always the case but it’s a rough estimate. Notice the 540 line is well north of CT with rain falling across almost all of New England.

Now below you'll find the Euro (or ECMWF) at 7 PM. Now the 540 line has moved south of western Massachusetts and Vermont where rain is changing over to snow. The 540 is just west of CT. But by this time, most of the precipitation has already moved out. This is where we could see some wet snowflakes begin to mix in for the highest elevations.
But when the coldest air invades and the 540 line drops south of CT, all the precipitation has already moved out. See the Euro at 1 AM Thursday below.
Here’s the European’s snowfall forecast with accumulating snow in parts of MA up through northern New England where they spent more time in the cold sector of the storm. I’ll let you know if anything changes. But for now, rain is the main type of precip and for most of the state the only precip type on Wednesday.


doris carlson said...

You are one smart lady Rachel! Thanks for the clarification on our weather this husband is a builder and was hearing from some people that we could get snow. I prefer to consult with the experts on such matters! ;o)

Cooking with Tammi said...

Thank you Rachel.