Added Thu 12:32 PM — This update falls into the category of “too much information”  but the latest RAP model shows upper temperatures  (as reflected by the 1000-500 mb “thickness” lines -yellow exceeding “540”) and the 800 mb (6000ft temperatures-orange) have exceeded freezing. 

RAP model critical temperatures and 1000-500 mb “thickness” with MRMS radar at 12 noon . (Click on image for a larger view.)


The 800mb (orange lines) is above freezing (0.4ºC) now in Philadelphia and just north.   Superimposed is the radar at 12:17 PM with the heaviest precip moving up towards the northeast.

With the back edge of the heavy precip in sight on radar, that should do it for this storm.



Forecast Updated Thu 11:52 AM — Latest NBM  (National Model Blend) from 10 AM  (it’s run hourly and takes about 1.5 hours to process) shows the transition to a mix occurring right now at noon—

NBM hourly model from 10 AM showing transition occurring now (earlier than RAP and HRRR). Note the position of the 32ºF  wet bulb temperature isotherm (purple line)  which continues to move northward. Wavy white line is 32º surface temperature.  (Click on image for a larger view.)






Forecast Updated Thu 9:03 AM — Latest HRRR expected additional snow forecast for today.  (Whatever you had at 7 AM, add this amount)—

HRRR additional snowfall through 3 PM Thursday (starting at 7 AM) (My neck of the woods has already had at least 2 inches of the ~6 inches forecast going forward as of 9 AM)   (Click on image for a larger view.)
The change to sleet will occur about 2-3 PM according to this morning's NAM model.

added 9:44 AM— While the NBM model has a precipitation statistics problem, it does have a very useful  average wet bulb temperature parameter which corresponds well to the transition to sleet at 3 PM today—

NBM (12z) “wet bulb temperature” purple line average 610 -40000 meters above ground freezing 32º isotherm at 3 PM. Areas north of the line will be snow, areas south will mix with sleet and rain.  (Click on image for a larger view.)


I wanted to get this update out there.  I didn’t bet on the right horse last night (the NAM).   Last night’s forecast was based mostly on the HRRR which was a middle ground between last night’s NAM and the previous afternoon GFS.   When the evening GFS came out near 11 PM with its low QPF, I kept the HRRR forecast  but suggested near PHL it might be an inch or so less.   I went towards the GFS, not the NAM.  

The NAM is run every 6 hours.  The subsequent NAM (06z) was run at 1AM, available about 3 AM.  Here’s the forecast based on that forecast. Maybe I might have used it had I been awake—

NAM (06z) water to snow conversion at 1:12 ratio (Click on image for a larger view.)



 from the earlier Thurs 7:38 AM update…

So, there’s a lot more snow out there than had been predicted.  Last night’s update at 10:55 PM indicated the forecast might be  in trouble when the  GFS water QPF was 0.4” and the NAM was over 1.0”.  

The QPF is looking closer to that predicted by last night’s  NAM and RAP models.  (I had panned the RAP forecast and when the low QPF GFS came out, I had subtracted from the HRRR instead moving towards the NAM.   “Never ignore the NAM”.).

The precipitation has started earlier than most models had predicted.  The cold temperatures has made the snow ratio closer to 1:15  instead of the usual 1:10 or so.  So we have a trifecta of issues: timing, more QPF and a snow ratio that was too low. 

The 4 AM RAP model has snow falling until 4 PM; the changeover to sleet is delayed.  The new RAP, HRRR and NAM will be available soon.    I’m going to go back and look at the 1AM models and see which, if any, got this right.