Philadelphia Weather Outlook

THIS WEEK’S WEATHER

Light Snow Possible before Daybreak Wednesday

Update Tue 01/31 @ 4:32 PM — Just a quick update…latest ECMWF and NAM-NEST are forecasting very light snow between 3 AM and 6 AM Wednesday morning. A light dusting possible less than 0.20 inches, especially in South Jersey.

NAM-NEST snow-rain at 6 AM (Click on image for a larger view.)

Who Predicted This Wet Snow?

Update Tue 01/31 @ 3:20 PM — So, I’ve been “off-duty” the past two days and I haven’t done a daily forecast, just a jet stream over-view yesterday. But I’m pretty sure I heard the forecast on the radio— ”some showers possible in the morning’. That was about it.

We’ve had very light, wet, non-accumulating snow falling for the past 4-5 hours.

So folks, this is going to have to be a Retroactive Forensic Forecast to answer the question, “what the heck is going on??”

Basically a cold front went through overnight, as had been forecast. Looking back at last night’s models, several were hinting for the front to stall just to our south and several waves of low pressure have apparently formed along the stalling front. That said, this morning’s HRRR had light rain, not light wet snow, and ending earlier than it has.


This morning’s GFS was not capturing this afternoon’s wet snow in its forecast, although the wavy nature of the thermal contours is apparent—

Today’s 12z GFS shows the 800 and 900mb freezing temperatures (blue and violet) are quite wavy, suggesting the weak waves of low pressure. The isobars just south of us are all “baggy” which also shows the very weak low pressure waves. Also notice the 32º surface temperature line (faint white) is north of us. It doesn’t have to be freezing outside to have snow. (Click on image for a larger view.)

I’ll have to check to see if there are any more surprises in store for us. Stay tuned.

Transitioning Jet Flow

#weather #paweather #wx #pawx #philadelphia

Useful Weather Link from the NWS—

The NWS has an Impact Based Decision Support Services (IDSS) website that provides detailed and up to date hour-by-hour weather information and forecasts. There’s a button that allows you to save a specific location URL to your web browser Favorites or Bookmarks.

Excellent! Better than most weather mobile apps!

A split jet flow, flat in our area, will transition to a trough by Friday. Most systems will pass by to our south and to our north through Friday.

Current Water Vapor Image shows jet positions—

Monday Morning Satellite Water Vapor image (mid level) shows two jet streams and a flat configuration over our area on Monday morning. (Click on image for a larger view.)

By Wednesday, things begin to change. Here’s the NAEFS model showing more detail at jet stream wind level as it transitions on Wednesday —

NAEFS Wednesday 250 mb (jet stream level) winds Strong jet streak over Pennsylvania (white color code) (Click on image for a larger view.)

By Friday, we’re under an upper level trough with cold air—

NAEFS 250 mb wind Friday forecast shows full transition into trough over our area. (Click on image for a larger view.)

No large storms expected to form here this week and no snowstorms expected here within in the next 10 days.