Tue 07:21 AM Forecast Review —The ‘very light possible snow’ last night didn’t happen.  The NBM over-predicted the snow.  Current temperatures are warmer than predicted by last night’s NAM and are closer in line with yesterday’s GFS. 

While the model’s PTYPE is still for sleet mid morning,  I think we can rule out freezing rain in Philadelphia and immediate suburbs.  A changeover to rain still expected.  The NAM did poorly with PTYPE, despite my preference for the model from past experience.   

from Monday night…..

Tuesday’s weather forecast continues to be complex regarding precipitation type (“PTYPE”)— snow, sleet, freezing rain and rain.

First, this will NOT be a snowstorm.

Tonight’s 00z models are just becoming available.  The model blend (NBM) has some very light snow possible tonight.  Some minor snow accumulation is possible before a changeover to sleet and freezing rain, according to the NBM—

The heavier precipitation arrives during the morning hours, sometime between 9 and noon as sleet, freezing rain, and rain. 

The complexity of the forecast revolves around the transition and the PTYPE.  The newest HRRR and NAM show heavier precipitation around noontime with some dynamic cooling. 

Surface temperatures for most of the area will be above freezing according to the HRRR, but below freezing according to the most recent NAM—

Here’s the somewhat warmer HRRR—

Just available, the HIRESW-MEM2 is very similar to the NAM

In these situations, I have historically gone with the NAM.  So a prolonged period of sleet is expected, with possible prolonged period of freezing rain in areas north of the city.

The HRRR changes everything to rain about 2- 3 PM, but the NAM maintains freezing rain north of the city until 3-5 PM.

Last night’s models left us in two camps with different predictions about the amount of sleet and rain that will mix in this evening, with the effect of reducing total snow accumulations. 

As explained in yesterday’s post, while the “main” model runs are done at 00z and 12z, many models are re-run six hours later at 06z and 18z.  At this time of the morning, we have the 06z (1AM) model runs to see what trends have developed.

Here are the trends—

• Snow starts between 12 and 2 PM today
• Change-over to sleet and rain 6 PM- 11 PM
• High gusty winds 40-50 mph this evening
• Change back to snow after midnight
• Snow ends 7-10 AM Thursday morning.

The NAM and NAM-NEST continue with significant sleet and less snow,  while several other models which had similar sleet forecasts (Canadian GDPS and RDPS) have more snow now.

At this point in the model cycle, the NBM (national model blend) 50 percentile forecast is the one to hang one’s hat on and the model’s statistical blend allows us to see the two different camps.

First, let’s look at the 25 Percentile—

The NBM 25 Percentile (meaning that only 25% of its model components have this forecast as its maximum snow depth—

The NBM 50 Percentile (half of its models are above this value, half are below) is most representative of the current GFS, HRRR, CMC and ECMWF forecasts.

This is most likely total snow accumulations to expect—

The above 50 percentile is the most likely snow totals to expect with this storm by Thursday morning. 

I’ll keep an eye on things. Stay tuned