Philadelphia Area Weather Forecasts

Philly Snowstorm Outlook

The models have been remarkably consistent predicting the nor’easter that’s expected to develop over the Delmarva coastline this coming Monday night into Tuesday.

As mentioned, an upper air vorticity moving in from the northwest in the jet stream flow will interact with an inverted coastal front along the southeastern states late Monday and spawn an intense nor’easter over the Delmarva area  Monday night into Tuesday.

All global models, including the ECMWF (European), the CMC (Canadian), the NAVGEM (Navy) and the GFS (US Global Forecast System) have been onboard with this development.

What is uncertain is the exact position of the storm early Tuesday, with a 25-50 mile discrepancy between the NJ coastline and the Delaware Bay resulting in either a major snowstorm  or a snowfall that mixes with rain for Philadelphia.  (Areas to the north and west are expected to have all snow at this time.)

As of the current time, it appears that snow will start Monday evening and become heavy after midnight, giving us a heavy snowfall by  daybreak Tuesday.  The snow will either continue or mix with rain in Philadelphia.   Last night’s GFS model had 18 inches of snow for PHL.

If this verifies, it would be the first time this season that an extended range forecast didn’t self-destruct a day or two before the event.   So despite the overwhelming forecast evidence, it’s too soon to be definite about this.

 

Philadelphia Area Weather Forecasts

Philly Weekend Weather Forecast & Snow Outlook

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The arctic front has moved through the area and very cold air will filter in for the weekend and much of next week.

Saturday will be sunny but will also  have some instability cloudiness, especially during the late morning and mid day hours.  It will be windy and cold.  High temperatures near 31.

Sunday will be similar to Saturday with windy conditions and some periods of instability stratocumulus, especially mid-day. High temperatures near 35.

Following a cold weekend, attention will be turning to a strong coastal low pressure system that is expected to develop late Monday night from an upper air disturbance dropping down from the northwest and interacting with an inverted coastal front off the southeastern coastline.   This potential system was remarkably predicted by the GFS model as much as two weeks ago.

If things continue to play out, this system will be the most intense to affect our area this entire winter.   The current question is how close to the coast will it develop; too close and we will have mostly a mix of snow and  heavy rain, with high winds.   Slightly off-shore and we might have a major snowstorm.  The latest GFS model favors a large snowstorm, with up to 15 inches of snow around PHL.

That said, the devil is in the details and the long range experts at the NWS feel this one is going to be tough one to predict accurately, even in the short range.

This storm will differ from today’s snow maker.   The ground temperatures will be cold and the QPF values will be much higher.  The March sun angle is an issue, but a large portion of the snow may occur before daybreak on Tuesday.     Stay tuned.

Winter Weather

Philly Friday Snow Update

Friday 6 AM update:  Last night’s models bumped the QPF to about 0.35 inches water.  Snow accumulations more likely to be 2 inches around PHL, mostly on grassy surfaces, little remaining on roadways, mostly slush.

The latest NAM data has become available.  It’s QPF value has risen to 0.26 inches water for PHL.    Tonight’s GFS model data won’t be available until about 10:40 PM, so I’ll be basing this discussion on this afternoon’s GFS data.

There continues to be differences in the models-  The GFS starts the snow about 3 AM, while the NAM model doesn’t have the snow starting until about 7AM for the immediate PHL area.

The GFS has double the QPF of the NAM, or about 0.50 inches water.   That would make a big difference in the snowfall totals.

What’s interesting is that typically the NAM QPF is almost always higher than the GFS QPF.  But with this situation, it is reversed.    In past situations where the NAM QPF has been significantly lower than the GFS, the NAM has been correct.

Temperatures will be near or above freezing for most of the time. Ground temperatures are very warm.   Solar insolation through clouds in March has a significant negative effect on accumulations. As mentioned, predicting accumulations for daytime snows with low QPF values can be just an academic exercise in March.

So what does all this mean?  I’m going with the NAM model, although the differences between the models makes the confidence in this forecast lower than usual.

Here’s my best shot:  Snow starts just before 7 AM in PHL and the immediate suburbs, (earlier start further north from PHL)  and accumulates about 1-2 inches on grassy surfaces.  It will be a  wet snow.  Little lingering accumulation on roadways, especially as the morning progresses, although there will be periods of moderate snow between 7 and 10 AM which may be slushy at times.   Snow tapers late morning with lighter snow showers into early afternoon.

Areas north of PHL will have more accumulation, possibly 3-4 inches, mostly on grassy surfaces.

It will become windy and cold during the afternoon and wet areas will freeze during the evening.

 

Weather… and Other Things 'Up in the Air'