Forecast Updated Tue 10:25 PM — Tonight’s models have begun to come in. The trend is for both Wednesday night’s impulse and the second one to take an increasingly southern track. Areas from Philadephia and north will have 1-2 “, but areas south (southern Delaware, Chester counties and southern NJ) may have 2-4 inches.  The second disturbance later Thursday into Friday may miss us entirely.   Stay tuned .

Forecast Updated Tue 5:53 PM
— The setup described yesterday continues to evolve into a complex and uncertain forecast as several impulses develop from a plunge of cold arctic air. The first impulse will move through late Wednesday evening into Thursday morning.   Snow totals of about 3 inches are possible by noontime Thursday morning, but the snow may continue into Friday as the second impulse moves in.

The forecast beyond noontime Thursday becomes unclear.

Some models have moved the track of this second impulse on Friday to our south;  the second impulse may miss us.

SREF forecast Thursday morning showing the two impulses. (Click on image for a larger view.)


It’s unclear if these impulses will arrive as discrete, separate systems or whether they will be diffuse less potent and more disorganized.

The model blend (NBM) has snowfall from Thursday into Friday with periods of heavier snow Thursday and again on Friday.    There is much uncertainty with these multiple systems.

The GEFS continues to forecast a nor’easter for us late Saturday into Sunday

GEFS showing plunge of cold air (blue arrow) and track of coastal low (white arrow) (Click on image for a larger view.)

Stay tuned…

Forecast Updated Tue 9:03 AM — The very light snow/freezing rain that had been expected early today is likely to stay to our north this morning.  The areas of positive vorticity as depicted on yesterday’s map  will be the only areas getting any precip from this weak disturbance.

The latest ICON model shows a more significant snowfall for late Wednesday through Thursday with a generalized 4-5 inches of snow total by Friday morning

ICON Snow Forecast for late Wednesday through Thursday (Friday morning totals) (Click on image for a larger view.)

We all know that these numbers are not worth that much at this point in time, but it’s useful to know that significantly more snow is possible.

Stay tuned


From Monday at 5:54 PM…

Cold air that had been sequestered in Canada for much of January has begun to plunge into the continental US.  The plunge of cold air is such that it will push the jet stream south, providing a boundary and track to bring stormy weather to the Philadelphia area over the next week or so.

GEFS Temperature forecast for Sunday showing region of likely storm development (Click on image for a larger view.)

As this week progresses, several weaker disturbances will affect our area as this boundary develops into a highly amplified jet flow.  Two relatively weak disturbances will affect us prior to Sunday’s storm.


A mid-level disturbance (areas of “vorticity” at 14,000  feet and above) will move through in the early morning Tuesday.  There’s hardly a surface low pressure development with this disturbance, but it shows as vorticity maxima—  

NAM 700 mb and 500 mb absolute vorticity Tuesday 6 AM.  We’ll be in the area of negative vorticity (blue and violet), limiting the precipitation, but areas to our north will have significantly more precip.(Click on image for a larger view.)

Though moisture is limited, there will be enough lift to cause precipitation.   The precipitation will move in prior to temperatures warming up.  There’s a very good chance of very light snow and very light freezing rain, maybe sleet, during the period 6 AM -10 AM Tuesday morning.

NAM-NEST PTYPE 9 AM Tuesday (red =freezing rain)  (Click on image for a larger view.)


Most models are on-board with this forecast, but be advised that in recent storms, the models have been too cold at the surface and freezing rain hasn’t materialized. This time, both the GFS and NAM model groups have the freezing line just south of Philadelphia at the start.   Accumulations won’t be an issue here in Philadelphia,  but slippery conditions are likely.  (Total QPF is about 0.10 inches water)

Wednesday Night into Thursday

A similar setup is expected to move in late Wednesday into Thursday. This disturbance will have more moisture and is expected to be stronger than Tuesday’s with maximum lift area over the PHL area. The Canadian models have been consistent moving this in Wednesday afternoon, but most other models keep the onset timing early Wednesday evening through Thursday morning.  Current snow forecasts range from 1-3 inches.

GFS forecast 8 AM Thursday morning (Click on image for a larger view.)



A more significant coastal storm is expect to affect us on Sunday, the result of the plunge of cold air and the resultant amplified jet configuration, as mentioned at the beginning of this post.  No details right now, but significant snow is possible.  Some models have this storm approaching as early Saturday evening.

GEFS Surface Forecast Sunday 1 PM (White line is near ground level freezing isotherm)  (Click on image for a larger view.)


Stay tuned.