Winter Weather

Important Snow Update- Wednesday AM

Wednesday 6PM Update: The afternoon model runs (1PM) usually show a reduction in QPF and that’s the case this afternoon.  Latest estimate- the transition to snow occurs about 4 AM and snow continues heavy until 10 AM, then light snow tapers off after noon.  Latest estimate for precipitation is 6-8 inches of snow in PHL.

Next Update about 9:15 PM

Wednesday 1 PM Update: Latest GFS has become available and it has  a significant amount of precipitation falling as snow between 4 AM and 10 AM on Thursday. Total QPF is over 1 inch water which could translate into 8-12 inches in the immediate PHL area, more north and west.  That means that both models have reverted back to yesterday morning model’s rather impressive snow potential.   All the factors cited of warm surface temperatures make exact accumulations difficult to predict, but this storm will likely be one for a snow blower, not a shovel.

The GFS and NAM are remarkably similar, which puts high confidence in this forecast. More interesting is that the GFS has higher QPF values than the NAM, something that is usually the other way around.

Will wait for the next model runs this evening, after 9:15 PM.

Quick Update- Latest NAM from this morning just in.  It has reverted back to yesterday’s scenario.   Precipitation continues into the morning hours due to greater intensification and it’s showing about 4 inches during the daytime hours alone.  Total accumulation for the immediate PHL area looks to be over 6 inches!  More north and west.

An interesting storm with many factors coming into play.  Won’t have a final handle until this evening’s data which becomes available after 9 PM

Philadelphia Area Weather Forecasts

Thursday Snow Update

Reviewing last night’s models,  here are the trends– Cold air in the upper atmosphere moves in sooner and supports snow sooner, however the models still have the lower atmosphere at 40 degrees around midnight, so accumulating snow likely won’t occur earlier.

Precipitation (as rain)  starts earlier, now  in the late evening Wednesday.  With the  earlier chilling of the upper atmosphere, the transition from rain to snow may occur earlier (about 2 AM), BUT it will be a wet snow and warmer temps at the surface will greatly reduce early accumulations, especially around PHL.

Accumulating snow may not get going until 4AM or later and the accumulating snow will be ending about 8 AM.  

With  colder air moving in earlier in the upper atmosphere, the NAM’s “precipitable water” is less and  QPF values have reduced  to about 0.50 inches water.

What this all means is that total snow accumulations are looking lower at this time, especially in the immediate PHL area.  I’m thinking 3-4 inches around here, greater accumulations in the north and west.  It will be an icy and heavy accumulation.

Competing aspects of temperature and precipitable water values will make this storm accumulation forecast lower than usual confidence, but I’m leaning on the low side.

We won’t really know until tonight’s models.   Stay tuned.

 

Winter Storm Outlook

Thursday Snow Update

Tonight’s NAM model data has become available.  Here are the trends– the NAM has reverted back to its forecast from a day ago. The storm is a bit less developed and a bit faster.  Total QPF values about 0.70 inches water with the heaviest snow ending about 8-9 AM Thursday.

Based on temperature profiles, the precipitation starts as rain or rain mixed with snow about midnight Wednesday and changes to snow sometime between 3-4 AM, then continues as snow until 8-9 AM.  Warm surface temperature before the storm will reduce snowfall accumulations. Best guess is about 4 inches in the immediate PHL area, up to 6-8 inches north and west, less in southern NJ.

This afternoon’s GFS had much higher QPF values and greater intensification of the storm. ( Tonight’s GFS model data won’t be available until later tonight.)

Will update tomorrow morning, but we won’t really have a handle on this storm until Wednesday evening’s model runs.

A quick 11 pm  update- the latest GFS is similar to the NAM – QPF similar (0.89)  and ending earlier  about 7-8 AM

 

Weather… and Other Things 'Up in the Air'