Monthly Archives: March 2019

Winter Storm Outlook

WINTER WEATHER UPDATE SAT 5PM

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This afternoon’s NAM model just became available. This is the “off hour” model run that does not include new weather balloon upper air (radiosonde)  data.

Here are the changes—Temperatures are warmer and precipitation ends earlier, about 4 AM Monday. Precipitation starts about 4 PM Sunday, probably as snow or a mix.Temperatures at the surface are above freezing, so the first amounts won’t accumulate very much.

Temperatures at critical levels of the atmosphere become too warm for snow about 8:00 PM in Philadelphia and move north.

8:00 PM Critical Temperatures

The warm air aloft moves north to a position shown below by midnight. This is the northern extent of the sleet/rain line.

Areas  north and west of the white line below are all-snow for the storm: 

Midnight Sunday critical temp- Maximum northward movement- white line is transition line

By 2 AM, cold air line moves back through Philadelphia:

Sunday 2 AM critical temp

[su_note note_color=”#d9f2da”]So this means that between 8:30 PM and 2 AM, areas below the white line in the middle graphic are a mix of sleet and rain instead of snow.

This will seriously limit the snow accumulations in the immediate Philadelphia area. Combine this with marginally freezing surface temperatures in the 32-33 degree range means limited snow.[/su_note]

This is an off-hour run.  We really will need to wait until Sunday morning’s model runs for the final call.

 

 

Winter Storm Outlook

WINTER WEATHER UPDATE -SAT 11 AM

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I’ve been reviewing the latest NAM and GFS data.  Here’s the latest on the expected storm for Sunday afternoon into Sunday night.  Total QPF values 0.50 GFS and 0.74 NAM

Basic scenario: snow to sleet and rain in the city, then back to snow. Details below:

Precipitation starts as wet snow in the afternoon, about 2-4 PM.  Initially, surface temperatures are above freezing in areas near the city, preventing accumulation.  Temperatures  then drop due to dynamic cooling and there’s some snow accumulation in the city.  However…

During the early evening, the snow in the city will mix with and change to sleet and rain.  By 9 PM, it will be mostly rain and some sleet in the city.  The northern extent of this changeover is depicted in the following map at 10 PM:

Northernmost extent of changeover to sleet at 10 PM Sunday night is depicted as white line.

North of this white line, snow continues throughout the storm.

South of this white line (10PM), the sleet and rain will gradually change back to snow about 12-1 AM  and will continue as snow until morning.

North, the QPF values are less, but the temperatures are colder; calculations for the snow totals become a bit complicated.

North of this white line, final accumulations will be 4-8 inches.

South of this white line, the final accumulations will be 2-5 from the city northward.  These are preliminary estimates.

This is a complex storm. We won’t have a final handle on the thermal profiles  and final QPF until tomorrow morning’s model runs.

Winter Weather

WINTER WEATHER FORECAST- UPDATE

[su_box title=”Forecast for Saturday and Sunday ” box_color=”#defcdc” title_color=”#000000″]After the precipitation ends about 6-7 AM Saturday morning, skies will be mostly cloudy with brightening skies and possibly some sun during the mid to late afternoon. High 40-42.

Latest models are showing snow for late Sunday.

For Sunday, increasing clouds. Snow begins in the mid afternoon and ends Monday morning. Current trends is for snow for Philadelphia and north and west. Accumulations 3-5 inches, based on current QPF. [/su_box]

Prior Forecast for Friday night…

The early NAM data has become available.  QPF values are 0.39 inches water.  Precipitation is just moving into the Philadelphia area now.

There are some changes– temperatures at critical levels in the atmosphere appear to remain too warm for snow in Philadelphia and the immediate suburban areas for much of this precipitation.

Areas far north and west (Doylestown and north and Pottstown and west) will have rain/sleet changing to snow about 2-3 AM.  A few hours of snow are possible before ending between 6 and 7 AM.  2  or so inches possible.

For the immediate suburbs of Philadelphia-  mostly rain or rain with some sleet….cold air does move in aloft towards the end the storm and there may be a transition to snow with some light accumulation before ending between 6 and 7 AM.   Nothing significant, as it’s March and anything that falls will melt soon enough tomorrow.

As we can see, the forecast can change for these borderline thermal profile events.

I’ll be checking the GFS later this evening when it becomes available.

There’s still uncertainty with the Sunday storm.  Stay tuned.