Monthly Archives: January 2021

Winter Weather

TUESDAY’S COMPLEX WEATHER FORECAST

Tue 07:21 AM Forecast Review —The ‘very light possible snow’ last night didn’t happen.  The NBM over-predicted the snow.  Current temperatures are warmer than predicted by last night’s NAM and are closer in line with yesterday’s GFS. 
Current (7 AM ) radar with temperatures.  (Click on image for a larger view.)

While the model’s PTYPE is still for sleet mid morning,  I think we can rule out freezing rain in Philadelphia and immediate suburbs.  A changeover to rain still expected.  The NAM did poorly with PTYPE, despite my preference for the model from past experience.   

from Monday night…..

Tuesday’s weather forecast continues to be complex regarding precipitation type (“PTYPE”)— snow, sleet, freezing rain and rain.

First, this will NOT be a snowstorm.

Tonight’s 00z models are just becoming available.  The model blend (NBM) has some very light snow possible tonight.  Some minor snow accumulation is possible before a changeover to sleet and freezing rain, according to the NBM—

NBM snow totals for Tuesday. (Click on image for a larger view.)

 

The heavier precipitation arrives during the morning hours, sometime between 9 and noon as sleet, freezing rain, and rain. 

The complexity of the forecast revolves around the transition and the PTYPE.  The newest HRRR and NAM show heavier precipitation around noontime with some dynamic cooling. 

Surface temperatures for most of the area will be above freezing according to the HRRR, but below freezing according to the most recent NAM—

NAM PTYPE and 32º isotherm (white line) at noon.  (pink =sleet, red=freezing rain) Click on image for a larger view.)

 

Here’s the somewhat warmer HRRR—

HRRR PTYPE with 32º isotherm.  Notice that the the 32º isotherm (freezing line) is far north of the city.

 

Just available, the HIRESW-MEM2 is very similar to the NAM

In these situations, I have historically gone with the NAM.  So a prolonged period of sleet is expected, with possible prolonged period of freezing rain in areas north of the city.

The HRRR changes everything to rain about 2- 3 PM, but the NAM maintains freezing rain north of the city until 3-5 PM.

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.