19 November 2010

The Trip To Grandma’s House Is Looking Cold!

Posted by Dan Satterfield

Next Wednesday will be the busiest travel day of the year. This means a lot of questions for weather forecasters like myself.

So here’s a bit of what we are looking at as we head into the weekend before Thanksgiving.

The surface charts show the coldest air of the season has settled into Alberta and Saskatchewan. At Noon today (Central time) it was -19C (-2°F) in Saskatoon! The surface chart below is in Fahrenheit. The red numbers are air temperature.

Surface map at Noon Friday centered on Alberta and Sask. in Canada. Click the image for full size.

This is true Arctic air and all of the long ranger numerical weather prediction (NWP) models are moving it slowly southward.

The problem with this type of airmass is that cold Arctic air is heavy and shallow. It hugs the ground.  I’ve seen an Arctic airmass so thin that it was -2C at the surface with freezing rain, while people who lived at the top of an apartment block saw 10C on their window thermometers!

The NWP models have a tough time handling it well. The vertical resolution of the models is just not fine enough. Faster computers mean NWP models do a much better job than even a decade ago, but a cold outbreak usually means a tough forecast.

Now you know why true polar air makes weather forecasters really jumpy.

Look at what the long range NWP guidance is showing by sunset on Thanksgiving Day.

Forecast of winds and temperatures just above the surface for Noon CDT Thursday. Below freezing temps are nosing into Arkansas! Image from NCAR FIM Model.

Yes it’s cold, but look at the winds blowing across the Great Lakes. That water is still (relatively) warm. Water temperatures off of Erie PA. are near 10C (50°F). True arctic air that cold will convert that moist air over the lakes into lake effect snow bands. Areas downstream of the lakes could see some very heavy snows.

The rest of the country looks to be either mild and wet or cold and dry. The Northeast corner of the country will see rain Thursday, followed by much colder air with gusty winds. Lake affect snows may affect much of Western New York and other areas in the lee of the Great Lakes.

One last note. Forecast uncertainty can be broken down into two main categories. First, is will an event happen. For instance, will the high reach 50 degrees at a certain location. Secondly, is the timing. When will it reach 50 degrees? It’s the timing that seems to be the greater uncertainty here. The cold is coming, but how fast will it drop southward is questionable.

The Thanksgiving forecast for Canada is easy. I can tell you EXACTLY what kind of weather to expect. How? Thanksgiving Day in Canada was the 11th of October.