Daily Archives: Monday, January 13, 2020 8:46 pm

Basic Weather Facts

SOME INFO ABOUT 5 DAY FORECASTS

Let’s say you read this blog and watched the local 6 pm evening news/weather on TV.  There’s some snow possible on Saturday, five days (120+ hours) in the future.

You’re thinking…I want more information about this possible storm.  If I watch the 10 or 11 PM news/weather tonight, will I get more insight or an updated forecast for Saturday?

The answer is no.

Let me explain—

When it comes to forecasts for the 120+ hour time frame,  there are really only three models that go that distance into the future: the GFS run four times a day, the Canadian Global CMC,  and the European ECMWF.  The Canadian and ECMWF are only run twice a day.

A new GFS 120 + hour forecast will become available about 11:10 PM,  the new  Canadian about 11:25 PM and the ECMWF around midnight.

Add some time to review the data and an updated 120 hour+ forecast simply isn’t available for the late 10 or 11 PM news/weather.

Yes, there are other important statistical “ensemble” models based on these three, but their data is available even later!

(Another good model, the German ICON model, just goes to 120 hours.  The U.K.’S UKMET and the Navy NAVGEM are rarely worth looking at.)

So, when looking at long range forecasts (five days or more) where one wants to  consider more than just the GFS, meaningful forecast updates more frequent than every 12 hours just aren’t possible.

Winter Weather

SO, MAYBE OUR FIRST SNOWFALL THIS SATURDAY?

A low pressure system over the Great Lakes Friday night into Saturday is expected to slowly move to the east as a secondary coastal low develops north of us along the New England Coast.

The possibility of this setup has been showing in the models for almost a week and until today, was expected to start Friday night and quickly changeover to rain Saturday.

As is seemingly always the case, things have changed with the last two model runs of the GFS, Canadian and ECMWF.

Here’s this morning’s  model forecast—

GFS PTYPE (precipitation type) forecast for Saturday noon. Violet-snow, pink-sleet, red-freezing rain, green- rain.   White line is 32º degree surface temp line.

Snow starts before daybreak Saturday and changes to rain during the afternoon.  Hard to believe, but the current GFS shows 4.7 inches of snow (Fort Washington) before a changeover to rain by 3 PM.

We all know that this is going to change by the time Saturday rolls around, but I think it’s useful to see what the current forecast trends are.  Stay tuned.