Monthly Archives: January 2020

Winter Weather

SATURDAY STORM UPDATE

Update Tues 07:41 PM— Today’s GFS, GEFS, CMC and ECMWF have become available and continue with the trends discussed this morning:

Snow starts mid morning Saturday instead of before daybreak.  A changeover from snow to rain occurs between 3 and 5 PM Saturday.

The GFS is maintaining 4 inches just west of the city before being tamped down with the rain late in the afternoon. The GEFS (Statistical “Ensemble”) version of the GFS has 2.3  inches.  The Canadian CMC GDPS has 1.8 inches and the European ECMWF has less than an inch. All have a  mix and change to rain 3-5 PM.

It’s important to keep in mind that the models have over-predicted snow accumulation for every storm so far this season.  The storm is still 100+  hours in the future.  We probably can’t make definitive snow forecasts until Friday.  This will be a minor snow at best.

GEFS Snow Depth Forecast for 7 PM Saturday

 

Update Tue 08:53 AM — The EKDMOS is great for temperature forecasts, but it also gives a good indication of the degree of uncertainty in the coming days’ forecasts.  Notice the increase in “spread” between possible maximum temperatures on Saturday- Sunday (1/18, 1/19) compared to the two days prior—

EKDMOS max Temperature forecast

…From earlier Tues:

Last night’s GFS, GEFS, CMC and ECMWF have become available. Here are the current trends with the storm forecast for Saturday—

All models are showing the snow to start somewhat later now.  Instead of starting before daybreak, the models have it starting about 9-10 AM Saturday. With the later start and daytime warming, warmer air moves in somewhat earlier.

A changeover to a mix may occur sooner around the city and immediate suburbs.   That said, the GFS is still showing 4.2 inches (Ft Washington and KLOM- Blue Bell) Canadian 2.6 and ECMWF 1.2 inches snow before mixing with sleet and rain.  The Canadian has a burst of heavy precip over us late afternoon, resulting in snow falling through dynamic cooling, despite the fact that surface temps will have moved above 32º.

A changeover to rain is expected between 4-6 PM Saturday and precip should end by midnight.  It doesn’t look like cold air rushes in immediately behind this storm.

These numbers are provided simply as a guide.  It’s important to know that this doesn’t appear to be a major storm for the PHL area. Accurate Snow Forecasts more than 24 hours in advance are almost impossible to do in this area.

Areas north and west of Allentown/Reading will have several inches.

 

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.