Update Sat @ 9:51 AM — Our neck of the woods has about 5 inches of snow, just below the low range of last night’s NBM forecast for here (5.9″)
Additional snow until noon is expected. Here’s the current radar loop—
Nexrad Radar 9:32 AM loop, courtesy of https://.weathertap.com (Click on image for a larger view.)
Of interest is the water vapor image which shows a possible low pressure center developing near the coast of NJ in response to the upper low passing through.
Upper air low center developing some circulation off the southern coast of NJ at 9:30 AM (Click on image for a larger view.)
This will likely provide some late enhancement of snow towards its final taper.
Update Fri @ 10:54 PM — Tonight’s GFS just came out. Consistent with the range of the NBM model just posted, a little higher accumulation in western sections.
Update Fri @ 10:13 PM — Tonight’s models are fairly consistent with the afternoon runs. Several models have moved a bit lower with snow totals (specifically the NAM, Canadian GEM and ICON) and some have moved higher (RAP, HRRR)
The current developing storm is fairly consist with the models, based on the latest water vapor imagery—
WV image with superimposed potential vorticity. (Click on image for a larger view.)
Again, I’m going to lean on the NBM model (model blend) which is designed to do all the hard work of coming up with a composite snow total.
NBM 00z percentile range snow totals for this storm. I’m leaning towards the lower number, especially from Philadelphia westward. (Click on image for a larger view.)
My present inclination, based on the NAM trend, is to lean towards the lower range number in the NBM from Philadelphia and westward.
Update Fri @ 5:25 PM — Today’s snowfall was more widespread and lasted longer than previously forecast. Does that bode well for the rest of the forecast?
The afternoon models have become available. Both the GFS and NAM are in a similar range as this morning, but with a trend downward of about 0.5″-1′ overall in Philadelphia and surrounding areas. That can simply be model variation. Snow totals are posted below.
I did notice that the water vapor image and the GFS/NAM model vorticity do not superimpose very well, suggesting some modeling error. I guess we’ll find out.
Water Vapor and forecast Potential Vorticity . Are the models too far west with their forecast? I guess we’ll see. (Click on image for a larger view.)
Snow starts this evening and lasts through the morning in the Philadelphia area. Ends by noon or so. Longer at the shore.
The model blend (NBM) is designed to do all the heavy lifting and here’s its current forecast—
NBM Model Thur 19z Average (mean) snow totals by 2 PM Saturday (Click on image for a larger view.)
This afternoon’s high resolution HRRR has this forecast—
HRRR 18z snow totals by 1 PM Saturday (Click on image for a larger view.)
I’m probably going to stay with this forecast. This evening’s models run the risk of not necessarily being more accurate due to lack of adequate model spin-up time. But I will chime in if there are major changes.
Update Fri @ 11:02 AM — The latest GFS just became available. As is almost always the case, its totals are somewhat less than the NAM—
GFS 12z snow totals White contours are 1 inch increments. (Click on image for a larger view.)
I’m providing this morning’s NAM forecast again for easy comparison—
NAM 12z (Click on image for a larger view.)
My rule of thumb is to average the GFS and NAM for a best forecast.
Food for thought. The latest HIRESW-ARW 2.5 KM (high resolution)— (possibly too high?)
HIRESW-ARW 2.5KM high resolution. White contours are 1 inch increments. (Click on image for a larger view.)
Update Fri @ 10:06 AM — FYI, the light snow this morning was predicted yesterday, although the amounts appear to be more than forecast.
MRMS RADAR 9:54 AM (Click on image for a larger view.)
The new 12z models are beginning to come in. (The 12z models, along with the 00z models, are important because they are the two daily model runs that include fresh directmeasurementupper air data from weather balloons launched globally.)
The trend is for an increase in snowfall with snow lingering into noontime Saturday in Philadelphia and later at the NJ shore.
Latest NAM model just available—
NAM 12z snow totals 1 PM Saturday. (Click on image for a larger view.)
The model blend (NBM model) 12z —
NBM Model blend with median snow totals as white contours in 2″ increments. The ranges (in smaller type) show the 25th percentile and 74th percentile ranges for that area. The 75th percentile means that 75% of the models are at or below that amount. It’s tricky. (Click on image for a larger view.)
Things seem to be coming together, based on the latest Water Vapor image and superimposed NAM/GFS potential vorticity (see explanation below)—
Last night's (Thurs) Water Vapor with Potential Vorticity
Friday Morning Water Vapor showing Potential Vorticity
Update Thu @ 11:16 PM — Quick update. Latest GFS has increased snow totals almost as much as NAM.
GFS 00z Contours are 1″ increments (Click on image for a larger view.)NAM 00z Snow totals (Click on image for a larger view.)
Update Thu @ 9:55 PM — One way of deciding if the models are on the right track is to see how their forecasts, in the short term, line up with actual observed conditions.
It turns out that a specific model derived parameter, “potential vorticity” corresponds very well with with warmer (darker) areas on satellite water vapor images.
Here’s tonight’s recent water vapor image with superimposed potential vorticity from this afternoon’s NAM and GFS forecasts. It looks pretty good, but there are some areas that are a bit off, designated as “?” below—
Major areas of potential vorticity (1) and (2) will combine and develop deep low pressure in area (X). There are a few areas, designated as (?), which are not perfectly forecast by this afternoon’s NAM/GFS models. That’s why forecasts need to change; models can’t exactly capture the future state of the atmosphere, but they can come close. (Click on image for a larger view.)
Tonight’s models are becoming available. The general trend has been an increase in snow totals.
Here’s the latest NAM forecast Saturday 3 PM—
NAM 00z Snow forecast depth (Click on image for a larger view.)
I think the NAM might be a tad too high. (Tonight’s NAM-NEST is an inch or two below these amounts.) Ultimately I think we’re going to go with an average of the GFS and NAM.
The GFS is out about 11:05 PM. I may not be able to update until tomorrow morning.
Originally Posted Thu 5:32 PM —
The storm late Friday through Saturday time frame has entered into the range of the high resolution models.
Our weather Friday through Saturday will be influenced by a sharp upper air trough and a deep storm developing in the western Atlantic.
Everyone knows that there has been much uncertainty regarding the track and snow potential here around Philadelphia. The models have been back and forth regarding position of the storm and the size of the precipitation shield westward.
Here’s a summary (you’ll only find this here!)
Most recent snow forecast in blue column. THIS IS FOR A SINGLE LOCATION- BLUE BELL PA. YOUR MILEAGE MAY VARY. (Click on image for a larger view.)
Regular readers know my motto, “Never Ignore the NAM”. And it’s time not to ignore it.
Here’s the latest NAM Model forecast–
NAM 18z snow forecast (Click on image for a larger view.)
Here’s the latest GFS which has just joined the rest of the models with significant snow here—
GFS 18z forecast for 1 PM Saturday (Click on image for a larger view.)
I am going with an average of the NAM and GFS models at this timeas the most likely snow totals.
With the high resolution models coming into the mix, here’s the forecast:
Friday:
An upper air disturbance and cold front will move through. We’ll have light snow break out northwest of the city as early as late morning Friday. Light snow/flurries on and off, mostly north and west of the city. Accumulation a dusting to 1/2 inch. High 34º sd 1.7º NBM model Blue Bell.
Snow starts Friday evening.
Most of our snow with this storm will occur Friday night into early Saturday morning.
Saturday:
Cloudy, cold and WINDY. Snow tapering and ending in the morning but lingering at the NJ shore. High 23.2 sd 2.9º NBM model Blue Bell
Update Thu @ 11:13 AM — The new GFS model’s snow forecast just became available—
GFS 12z forecast Snow totals by 8 PM Saturday. (Click on image for a larger view.)
I’ll try to make sense of these giant differences later this afternoon. Right now I’m inclined to go with the GFS, but that could change.
Update Thu @ 10:17 AM — This morning’s early models are becoming available and their higher resolution forecast ranges are taking us to 1 PM or 7PM Saturday timeframe.
The dichotomy between the NAM and GFS still exists. Here’s the very latest NAM snow forecast—
NAM 12z snow forecast through Saturday 7PM (Click on image for a larger view.)
Despite the new NAM forecast,the other new models entering into the picture this morning are tending towards the much lower snow totals of the GFS.(about 1 inch in Philadelphia and less west)
The new GFS comes out just after 11 AM. I’ll take a look and update. Stay tuned.
Update Thu @ 8:43 AM — A quick update with last night’s 00z and 06z model trends. The GFS remains as the eastern outlier with the NAM back with a significant snowfall. European ECMWF still shows 4-6″.
Here’s current snow totals by model (for grid point: Blue Bell PA)
Current model snow forecasts by model for Blue Bell PA. (areas east will have more; this is only one point data)
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’s the GEM model that I looked at 11 PM last night but I didn’t know what to do with—
Last night’s Canadian GEM snow totals. (The GEM shows snowfall as snowfall rate in Kg2/m2) These numbers are correct. The 4-5″ mentioned in last night’s “update” was a quick, mental approximate conversion to inches of snow.)
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 formationbut 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:
RAP model 10 AM Precipitation moves in
NAM model 1 PM Changeover northwest of the city. (note 32º surface temp white line)
RAP model Snow totals 4 PM
RAP model snowfall on grassy surfaces. (Click on image for a larger view.)
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.
NAM 11 AM showing surface freezing line (32º) white line already north of the city. Wet snow falling northwest of the city.
NAM 1 PM — much of the area has changed to rain
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—
High Resolution Rapid Refresh (HRRR) current likely snow totals. (Click on image for a larger view.)
Just to share how crazy numerical weather prediction models can be, here’s the very latest Canadian GEM regional model—
CMC GEM Model Snow totals
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—
Rapid Refresh (RAP) model snow totals by 3 PM Monday (Click on image for a larger view.)
Predominant Precip by 2 PM—
RAP Model 2 PM PTYPE (green=rain) (violet=snow) (Click on image for a larger view.)
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 formationbut 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—
Short Range Ensemble Forecast (SREF) precip with precip type. Any area south of the magenta line (850-1000 mb thickness line) should have be a mix, mostly rain, by 1 PM. The PTYPE predominant rain is shaded in green. (Click on image for a larger view.)
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) —
High Resolution Rapid Refresh (HRRR) snow totals before a change to rain. These numbers will reduce after the change to rain where rain becomes the dominant precip type. (Click on image for a larger view.)
The Model Blend (NBM) is similar to the above forecast, as is the German ICON model.