Monthly Archives: January 2018

Updates

A Difficult Forecast

After waking up this morning to very light snow, the question that begs is “what gives?”  Last night’s 1 AM run of the NAM shows this snow this morning and suggests that very light snow may continue through the day around Philadelphia.

BUT, the GFS is in a different universe and has no snow here until  later Tuesday night.   The QPF values also differ greatly, with the NAM maintaining a total QPF of about 0.25 for the event, while the GFS is in the 0.09 territory.  So big differences in timing and QPF values.

To add to the uncertainty, the NAM shows a tongue of warm air at the surface moving into Philadelphia  and just south later in the day and the evening, keeping accumulations low despite the higher QPF.

Since the GFS is already wrong, we’ll hang our hats on the NAM.

So very light intermittent snow possible during the day, increasing in intensity this evening, but surface temperature increases in the immediate Philadelphia area may significantly reduce accumulations.   Still a 1-3 inch snow best guess in Philadelphia, with a lean towards the lower range.  North and west of the city may have  3-4 inches.   This is a lower than usual confidence forecast. 

The other change noted is a reversal from yesterday- the snow essentially ends earlier Wednesday morning.

Too busy at work to give detailed updates during the day today, but I’ll send out a quick update if I get some time.

 

Winter Weather

Snow Forecast Update

The latest NAM model data has come in. Here are the trends—

The snow starts later in PHL, closer to midnight Tuesday.  QPF values are increasing, now 0.33 inches water, somewhat more north and west of the city. The snow continues into Wednesday morning. This translates into 3-4 inches.

So weak frontal passage seems to be evolving into more than a nuisance snowfall.  Expect further changes.

11 pm update:  The GFS has less QPF in the immediate PHL area, more north and west. For now, we’ll average the two models.  2-4  inches of snow is a best guess for now.

Winter Weather

Winter Weather Update- Monday

How things change! A frontal passage with limited moisture and little development now appears to be somewhat more substantial.  QPF values for the NAM and GFS have increased closer to 0.20 inches water for PHL with greater amounts far NW of Philadelphia.

The latest models are beginning to suggest that a low pressure system might form off the coast, close enough to enhance the snowfall amounts Tuesday afternoon thru Tuesday night.  There’s been a wide range of model solutions with this frontal passage — most have either not had any development, or they had development far off the coast.  The Navy NAVGEM and the Canadian model were showing more precipitation yesterday, but I discounted both models, since the GFS and NAM were showing no development.

The experimental statistical models have consistently shown a low chance of snow through the Tuesday-Wednesday time period.

So the minor snow for Tuesday into Wednesday is beginning to appear that it might be more substantial.  Looking like 2-3 inches now, but too early to be that precise.   Stay tuned!

7 PM Monday-Further Complications to the forecast. This afternoon’s models show surface temperatures at or above freezing during the evening hours of Tuesday, not going below freezing until after midnight.  Some initial snow will melt and not add to accumulations in the immediate PHL area.