Additional Update Fri @ 9:00 AM — Some of the early morning models just coming in. The model blend (NBM) has moved to a 1.7” snow total as a 50 percentile value. (Half of the models comprising the blend are above this amount, half are below.)

One thing to keep in mind is the snow will fall at the height of the day in March, with a strong solar insolation effect through clouds; the sun angle is high this time of year. ) With the ground warm from Friday, I think most of the accumulation will be on grassy surfaces, not dark asphalt roadways. Some of that will be also influenced by the precipitation rate; high rate, more accumulation on roadways.

According to the NBM, the rain will start as early as 3 AM and the transition to snow between 11 AM and noon, earlier west of Philadelphia. With the earlier start, it will end about 3-4 PM.

Update Fri @ 8:13 AM — Last night’s models have all shifted their forecast towards snow accumulations in the immediate PHL area with Saturday’s storm.

The NAM, NAM-NEST, GFS, ECMWF all show at least 1” of snow in the immediate PHL area. Several of the HIRESW models and the HRRR are predicting several inches!

The trend has clearly moved towards an earlier start and a colder storm. I will update later this afternoon.

Update Thu @ 10:45 PM — Tonight’s models continue with the trend of a faster moving storm with cold air entering earlier. The NAM-NEST has reverted to its earlier accumulating snow forecast of 1-2 inches of snow and the model blend NBM, ECMWF, HRRR and HIRESW also show accumulating snow in similar ranges.

The NAM and ICON show only a coating. So uncertainty remains with this storm. Too soon to invoke “never ignore the NAM”.

I’ll update tomorrow.

Update Thu @ 5:35 PM —This afternoon’s models have trended towards little or no accumulating snow in the general Philadelphia area.

The storm is expected to move more rapidly than previously forecast. The actual cold front will move through early, 9-10 AM with heavy rain and with a shift in the winds to the NW.

There has also has been a general trend for temperatures to drop below 32º a bit earlier, now sometime between noon and 2 PM.

Coinciding with that temperature drop will be a transition from rain, briefly to sleet, and then snow showers before ending about 6 PM. Still expect WINDY conditions.

Another change for Sunday— The ECMWF shows an upper air disturbance moving through Sunday afternoon with clouds and some snow showers possible. The GFS just has clouds.

Update Thu @ 10:06 AM — Saturday continues to look like a stormy day, with the coastal low forecast to deepen below 970 millibars (that’s hurricane level low pressure) by the time it gets to Maine.

Rain should begin about 6 AM Saturday morning. Right now, just a moderate rain is expected during the morning hours (less than 0.5″ water). The big issue will be wind and rapidly falling temperatures, with highs near 50º early morning falling below freezing by 2 PM and into the 20s by evening.

A changeover to some snow is expected between 1 and 2 PM. There’s a lot of model uncertainty regarding any snow accumulation with the SREF showing a mean snowfall (Blue Bell) of 2 inches but the range is anywhere from zero, a grassy coating, to over 2 inches. The RGEM and GFS are showing little to no accumulation. Too early to predict now, but rapid icing is possible on some surfaces.

Snow flurries end about 6 PM.

Here’s the latest NAM-NEST snow accumulation forecast which is on the high side of things—

High winds of 20 mph sustained with gusts to over 30 mph. The only good thing about the winds is that it might dry surfaces before too much icing occurs.

Sunday, first day of Daylight Saving Time, looks sunny and unseasonably cold. Windy. High just 37-38º

Previously Posted Wed 8:17 PM —

Generally moderating temperatures expected Thursday through early Saturday.

A very strong coastal storm is expected to develop and move northeastward on Saturday, as much colder air moves down causing a dip in the jet flow.

The general forecast path of this storm has trended somewhat closer to the coast over the past several days, with a mostly rain event as warmer air will move in with the system. As the system moves away, there is a possibility that there will be change to snow as very cold air moves in behind the low. Very WINDY conditions developing.

The ECMWF and GEFS models are in pretty good agreement with the speed and path, but the ECMWF and GEFS have a more eastern track than the NAM —

Zooming in, this time with the latest GEFS which like the ECMWF also has a colder eastern track, we can see that it’s predicting a drop to freezing about 1 PM with a changeover to snow shortly thereafter.

(I should mention that the latest ICON model is very different. Not sure if it’s significant.)

Warmer temperatures preceding the storm along with high sun angle (it’s March) suggests low ability for snow to accumulate, especially on roadways.

But there will be a rapid drop in temperatures to the 20s by evening and some areas will ice up.

Cold weather for Sunday.

Stay tuned.

Update Tue @ 11:44 AM — This morning’s Canadian RGEM has consistently shown Wednesday’s storm to be mostly rain in most of our area, but with wet snow mixed in at start. No accumulation expected here.

Of greater interest is the large coastal storm expected Saturday. Stay tuned.

Update Tue @ 7:55 AM — Last night’s GFS and Canadian RGEM show virtually no snow accumulation on Wednesday in the Philadelphia area, except for the far northern and western suburbs. While temperatures just above 2000 feet will be cold enough for snow (at times) during the day, overall thermal profiles don’t support snow accumulation. The RGEM is even warmer with mostly rain falling.

High precipitation rates may cause dynamic cooling, enough to allow some of the precip to fall as snow at times, but it will melt on the surface and certainly any roadways.

I’ll keep an eye on it.

Update Mon @ 8:12 PM — So, I’ve been asleep at the wheel regarding Wednesday’s storm… I took a quick look at the forecast, saw the critical thickness line (red line) was north of our area, forecast rain, and I never looked further. But there’s more to this.

I haven’t watched the TV weather in a long time, but I could tell something must be brewing because I noticed a large uptick in my web traffic stats. So, I took a deeper look and I see that snow is possible for Wednesday. But is it really gonna happen?

Here’s the latest NBM forecast—

So, will it snow on Wednesday? Maybe wet snow or snow mixed with rain will fall. In favor of the snow are cold temperatures above 2000 feet and relatively high precipitation rate.

Will it accumulate?

I don’t think we’ll get accumulation in the immediate PHL area. We’re up against a March high sun angle and surface temps above freezing.

The NBM has this snow forecast—

I’ll keep an eye on it.

Update Mon @ 6:17 PM — After this evening’s showers, the next storm to affect us will be on Wednesday. Easterly winds and rain expected—

Of greater interest will be a major storm expected to develop on Friday night into Saturday. This has the makings of a significant coastal storm with snow in western Pennsylvania into parts of New England.

While I’m leaning towards, the warmer GFS, the ECMWF shows possible changeover to snow for us. Not so sure about that.

Update Mon @ 9:29 AM — Today’s HRRR shows elevated CAPE northwest of Philadelphia between 4-6 PM as the rain moves in. This suggests the possibility of thunderstorms embedded in the first area of rain just northwest of the city.

Previously Posted Sun 9:45 PM —

On Monday, high temperatures are expected to get to the upper 70s with a some areas possibly touching 80º. A cold front approaches with rain moving in from 4-5 PM. (Considerable cloudiness will limit what could have been even warmer temperatures.)

The unseasonably warm weather expected Monday will eventually give way to cold weather again over the coming weekend.

During this coming week, we’ll be transitioning from a ridge with a southwesterly flow to an eventual colder trough over the weekend.

Several low pressure systems are expected to form and move up from the southwest. After rain Monday night, we’ll likely see some rain again on Wednesday and yet again Friday into Saturday.

The strong contrast in temperatures is likely to set off some wild weather.

Saturday may have some “interesting weather” for us as a strong cold front moves through, according to the latest GEFS—

Stay tuned.