Update Mon 02/27 @ 8:15 PM — The heavier rain will be ending before daybreak. Tuesday morning will have very low clouds, areas of fog and some widely scattered sprinkles as a weak trough remains over central PA and Maryland.
The low clouds gradually lift during the afternoon, but it may be mid (to late) afternoon before things break up for some sun.
Mon 8:15PM Forecast Review — It’s interesting that the weather outside is unfolding closer to yesterday’s models than this morning’s models.
Examples : The rain started around 4-5 PM as forecast yesterday but today’s models had forecast a later start time.
Example: The transition to sleet and snow is further south, almost as predicted by yesterday’s models, but today’s models had all rain in the city and just outside.
If we backtrack to yesterday’s forecast, it will changeover to rain by later this evening.
Updated Monday Forecast
Update Mon 02/27 @ 9:11 AM — Current models have decreased the extent and duration of the sleet this evening.
Clouds increase by 1 PM and lower and thicken during the afternoon.
Rain starts between 6 and 7 PM in the city, but areas near Yardley/Trenton, it may start as light snow.
It’s looking like it stays rain in the city and immediate suburbs, but northwest, it transitions to a mix of sleet and rain around 9 PM. Here’s where the mix occurs according the the HRDPS—
It then moves back to all rain except areas near Quakertown and Allentown and north where a mix lingers longer before changing to all rain after midnight. Temps remain above freezing here.
Update Mon 02/27 @ 10:20 AM —It looks like much of Tuesday through early afternoon will be very cloudy with sprinkles in the morning.
Previously Posted Sun 9:10 PM —
Monday Evening Sleet Rain mix
The rapidly changing weather pattern will produce yet another change Monday evening.
Low pressure will bring clouds Monday afternoon, then precipitation Monday late afternoon, starting as early as 4 PM but possibly as late as 6 PM.
Due to the initial rain falling through very dry air, the onset of the rain is expected to result in evaporative cooling.
This evaporation cooling will drop the air temperature to the “wet bulb temperature”, which will be below freezing in such a dry column of air.
So precipitation may start as rain, but this dynamic evaporative cooling process will cause the precipitation to change to sleet as the rain falls and freezes falling through this colder lower layer of air.
By 11 PM to midnight, Philadelphia and the immediate surrounding areas should change back to all rain.
Here’s the NBM by 3 AM —
Some accumulation of sleet possible in the immediate PHL area during the evening but it should all melt with the considerable additional rain expected, so no accumulation around here by morning.
These dynamic temperature situations sometimes causes surprises, but there appears to be good model agreement with the current forecast.
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—
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.
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—
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.
Here’s tonight’s 00z NBM model—
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.
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—
This afternoon’s high resolution HRRR has this forecast—
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—
I’m providing this morning’s NAM forecast again for easy comparison—
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?)
Update Fri @ 10:06 AM — FYI, the light snow this morning was predicted yesterday, although the amounts appear to be more than forecast.
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—
The model blend (NBM model) 12z —
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.
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—
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—
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.
Previously 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!)
Regular readers know my motto, “Never Ignore the NAM”. And it’s time not to ignore it.
Here’s the latest NAM Model forecast–
Here’s the latest GFS which has just joined the rest of the models with significant snow here—
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:
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.
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
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—
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: