Category Archives: Winter Storm Outlook

Winter Storm Outlook

MORE ACTIVE WEATHER

Wed 09:41 PM Update — As mentioned earlier, this possible snow event will be in two parts, a warm front type snow as early as Sunday followed by a secondary coastal low. The time frame is beyond the forecast horizon of the shorter range high resolution models.

FYI: The newer model runs of the major global models, the GEFS, CMC, and the ECMWF don’t become available until well after midnight. (The GFS model data for this forecast time frame becomes available about 11:10. ) Your favorite TV weather entertainer won’t have any meaningful updated forecast information this evening.

I’ll update tomorrow morning.

Wed 05:58 PM Update —There’s still much uncertainty regarding the amount of development and track of the secondary low late Sunday into Monday. Just about every model has some snow.  The latest European model is much slower with the development but brings a major snowfall to our area, especially areas just west of Philadelphia as late as Tuesday.   It’s way too early to post specific numbers.  Recent storm forecasts have over-stated the snowfall totals this many days in advance.  (The actual storm is well past the 84-89 hour range of the higher resolution models.)   But stay tuned.

BTW,  I saw this interesting article in the Washington Post.  

from earlier…

Cold air has built up in Canada and it will make its plunge into the eastern US over the next few days resulting in an amplified jet stream.

After several frigid days through Saturday, low pressure in the Midwest will try to bring warm air and moisture over us aloft (basically a warm front) on Sunday.  This should result in snow for us some time on Sunday afternoon or evening.   These scenarios (warm advection snows) can produce an inch or two of snow, sometimes more.

Of interest, several models have this Midwest low pressure system spawning a secondary coastal low late Sunday night—

GEFS model showing coastal secondary low formation Sunday night. (Click on image for a larger view.)

Depending upon the exact location of this secondary low, we could get additional snow.  Right now, the dip in the jet flow does NOT show an ideal track for development needed for heavy snow in our immediate area—

GFS model 300 mb jet stream flow. The dip isn’t sharp enough to bring the developing coastal storm close enough for heavy snow in Philadelphia.  (Click on image for a larger view.)


As we saw with today’s system, model storm forecasts several days in advance can easily change.  (The southward impulse (white arrow)  could phase more, causing a sharper bend in the flow.)   This one looks like it has potential, but the trend over the past day has been for a more eastward track.  

However, of interest is the German ICON model whose current forecast extends to 120 hours (Monday 1 AM).  It has very heavy snow tracking just south of our area late Sunday—

ICON model Snow Precipitation Rate Monday 1AM  (Click on image for a larger view.)

 

Using some new software, here’s the GFS projected snow totals for this potential storm—

GFS projected snow totals  (Click on image for a larger view.)

 

Stay tuned.

Winter Storm Outlook

ACTIVE WEATHER WEEK

Mon 05:46 PM Update — This is complex forecast.  Snow is NOT the issue. Precipitation type (sleet, freezing rain, rain)  is a question.

There are  differences between various models, especially regarding surface temperatures and temperatures below 6000 feet between 6 AM and 11 AM.  There are different forecasts for precipitation type. 

The NAM/NAM-NEST group have temperatures below 32º while the GFS/RAP group have it a above freezing.   So the NAM group has more freezing type precip (sleet/freezing rain) after daybreak.

In the old days (before I  had direct model data), I would use the NAM FOUS data for my forecast.  The FOUS shows little snow for our area, but maintains a longer period of sleet/freezing rain/rain into the mid morning hours.  So, I’m leaning towards the NAM group for this forecast.  The graphic below captures the parameters of the NAM FOUS data—

NAM NEST PTYPE forecast with 32ºF isotherms 9 AM Tues  (Similar NAM FOUS)  (Click on image for a larger view.)

 

Both model groups have the majority of precip in the immediate PHL starting later,  after 8 AM, although it starts earlier far southwest of the city.

Areas southwest of the city will have precip start before daybreak; these  areas are more likely to get a coating of snow before the changeover.    

If things change, I’ll update between 10 and 11 this evening

 

Mon 01:35 PM Update — A quick update.  This morning’s GFS has virtually NO snow accumulation for the immediate PHL area.   Surface temperatures rise above freezing early, so most areas won’t see any freezing rain either.   Just light rain.
GFS model 8 AM Tuesday PTYPE (note that these areas become all rain quickly)  (Click on image for a larger view.)

 

Thursday’s expected storm will miss us entirely, according to the latest GFS.

Sun 10:46 PM Update — Tonight’s models suggest several hours of light sleet and freezing rain after daybreak Tuesday morning following a light coating of snow that falls before daybreak. It’s not clear if the models are over-predicting this freezing precipitation potential.  We’ll need to see tomorrow’s models to get a better idea of the thermal profile on Tuesday.


Sun 04:53 PM Update — Today’s models continue to downplay the snow for Tuesday.
NAM NEST snow totals Tuesday. Philadelphia has a coating

 Warm air moves in at 2000-3000  feet, resulting in a period of sleet and freezing rain instead of snow after daybreak Tuesday.  Little accumulation in and near the city. 

 

The storm for Thursday, while more developed, appears only to brush us with some snow.  Again, an inch is the most likely amount.

From Saturday evening…

The upcoming week looks to be a relatively active weather week compared to the past few weeks.

Tuesday—

The storm expected to pass to our south on on Tuesday is showing less development and less moisture. Additionally, it now appears to move in later Monday evening and gradually exits during the day Tuesday.

Warm air appears to be brought in about 2000-3000 feet above ground level.  The predominant precipitation type will be sleet/freezing rain, then light rain on Tuesday instead of snow, after a small accumulation Monday night.  Total accumulations have reduced to under 1 inch for most of the immediate Philadelphia area—

GFS snow totals 10 AM Tuesday  (Click on image for a larger view.)

It appears that light rain showers, may linger on and off into the day Tuesday, tamping down or melting some of the small accumulation we receiving Monday night.  The main issue may be early Tuesday morning with some icy areas and perhaps some freezing rain in some areas—

GFS Precipitation Type and Surface Temp Tuesday 7 AM  (Click on image for a larger view.)  Please note that the GFS has been over-predicting freezing rain in recent storms.

Thursday—

Yet another storm is now showing moving to our south on Thursday. This storm is expected to brush us with light snow Thursday morning, however there is a possibility of greater intensification with this system—

GFS Precip forecast Thursday 10 AM (violet = snow)  (Click on image for a larger view.)

Currently, only about 1 inch of snow is expected with this Thursday storm.

Stay tuned.