Mon 08:32 PM Forecast Review —We had more snow than forecast.  The models (with the exception of the Canadian GEM model) underestimated the QPF and didn’t fully take into account the cold upper level atmospheric temperatures, which resulted in more snow and less rain. The NAM erroneously hastened the transition to rain, despite having the QPF somewhat in-range.   

The Canadian “Regional GEM” model is one model I always look at, but it it isn’t necessarily a model that is more often accurate.  I’ve seen it over-estimate snow totals many times.   Last night’s GEM had these higher snow totals, but as I said, “Not sure what to make of it” 

Here are the NWS official snow totals:Here’s the graphic posted yesterday afternoon from the GEM model—

 

Here’s the GEM model that I looked at 11 PM last night but I didn’t know what to do with—

 

 

Update— Tonight’s Canadian model continues to forecast 4-5 inches of snow in central parts of Montgomery county.  Not sure what to make of it. 

Tonight’s latest models (HRRR, RAP, NAM)  shows the following changes— snow changeover a bit delayed northwest of the city, although snow totals are pretty much the same.  (It might be snowing an hour longer, but it will be increasingly difficult for accumulating snow.)

Dark, paved roadway surfaces will likely see less accumulation than grassy surfaces.

Snow starts about 9-11 AM from west to east and changes to rain from southeast to northwest by 2 PM.    Precip ends 3-4 PM.

The differences with this storm and recent previous storms  were outlined in an earlier post:

•  The models have been consistent with total water falling (QPF) will only be 0.3-0.4″ water equivalent or less in the immediate PHL area, in  contrast to much greater amounts in previous storms.
• The thermal structure of this storm is vastly different.   This storm will have southerly winds at the surface and surface temperatures will be at or above freezing for most of the area once things get going.   (Previous storms had northeasterly or easterly winds at the surface.)
• In contrast, upper air temperatures will be colder northwest of the city, supporting snow formation but the surface temperatures will go against accumulation.
• We’re into the final part of February.  The solar angle is higher and “insolation” effects through clouds are more likely to interfere with and cause melting on dark asphalt road surfaces when the precip occurs during the daytime hours.

Things can be summarized with this graphic slide show:

Forecast Updated Sun 5:36 PM — This afternoon’s NAM has backed off on some of the precip and I think the earlier RAP model’s higher snow totals are too high. Either way, lower snow accumulations are expected on roads and dark asphalt. While the NAM still has about 0.4-0.5 water (QPF) falling, temperatures are above freezing by noon. It’s going to tough to accumulate much snow on paved surfaces.

This is a fast moving disturbance. Precip starts 9-11 AM and tapers off around 3 PM. Any snow comes as a heavy slug early.

This afternoon’s (18z) HRRR is very similar to its model run earlier this morning, so here’s the same graphic posted earlier, which are the likely snow totals for Monday—

Just to share how crazy numerical weather prediction models can be, here’s the very latest Canadian GEM regional model—

Forecast Updated Sun 12:13 PM — This morning’s models have become available.  Here are the trends:  the QPF has increased with the NAM and RAP models have total water equivalents as high as 0.6″ water. 

The temperatures at the surface continue to be at or above freezing, but the upper atmosphere maintains its cold, allowing precip to fall as snow in areas north of the west and north.  Precip still starts 9-10 AM and tapers off around 3 PM. 

Latest RAP  (Rapid Refresh) Model—

Predominant Precip by 2 PM—

from earlier Sunday morning..

Monday’s wintry weather is almost within a 24 hour range and it’s worth trying to take a stab at the snow totals.

As mentioned in my previous update, the trend has been for the precipitation to start from west to east about 9:00-11 AM Monday.   

This storm will be vastly different from the several previous  storms—

•  The models have been consistent with total water falling (QPF) will only be 0.3″ water equivalent or less in the immediate PHL area, in  contrast to much greater amounts in previous storms.
• The thermal structure of this storm is vastly different.   This storm will have southerly winds at the surface and surface temperatures will be at or above freezing for most of the area once things get going.   (Previous storms had northeasterly or easterly winds at the surface.)
• In contrast, upper air temperatures will be colder northwest of the city, supporting snow formation but the surface temperatures will go against accumulation.
• We’re into the final part of February.  The solar angle is higher and “insolation” effects through clouds are more likely to interfere with and cause melting on dark asphalt road surfaces when the precip occurs during the daytime hours.

Whatever starts as snow will mix with or change to rain,  as depicted by this graphic with snow totals, especially in the green shading and south of the magenta line—

The latest HRRR shows these snow totals before a mix and change to rain in most areas.  This only shows snow totals.   (Much of the area will eventually become mostly rain, as depicted in the SREF above) —

The Model Blend (NBM) is similar to the above forecast, as is the German ICON model.

I’ll update this evening with the latest models.

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. 

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—

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

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—

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—

 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.