Winter Weather Update

The latest NAM data has become available.  Here are the trends:

The precipitation moves in earlier,  now beginning between 3 and 5 PM.  The NAM is showing a warming trend in all levels of the atmosphere that will extend further north than previously predicted.

Precipitation possibly starts as snow but quickly mixes with sleet and rain and changes to all rain by 7 PM in Philadelphia and by 9 PM in the northern and western suburbs.  A coating to less than an inch of accumulation in and around Philadelphia followed by heavy rain Saturday night.  Total QPF values about 1.90 inches water.

The arctic front moves through about 8 AM Sunday morning. Some changeover to snow showers possible, but little or no accumulation in and around Philadelphia.  High winds and gusts.  Temperatures rapidly plunge to below freezing by 1 PM and into the teens by evening.

NAM total snowfall prediction

With the high winds, open roadways may dry but areas of pooled water or drainage will quickly freeze by afternoon.

So this is really looking like just a heavy rain storm and a sharp cold front for Philadelphia and the immediate areas.

WINTER WEATHER UPDATE – FRIDAY

Winter Weather Update Fri 5 PM
Another quick update. This afternoon’s NAM and GFS show a faster storm onset. Precipitation starts about between 3-5pm Saturday afternoon. It’s likely to start as snow, with 1, possibly 3 inches before a rapid changeover to sleet and then rain.

Heavy rain by early evening Saturday. (Freezing rain likely far northwest in Allentown up until midnight.) Rain tapers in the morning Sunday as temperatures plummet and winds pick up. Minimal accumulation expected in the immediate PHL area when all is said and done.

Temperatures are forecast to drop from 49 (GFS) or 42 (NAM NBM) to 30 by 1 PM, then falling further during the afternoon to 20 by 7 PM and single digits at night. Windy conditions may dry some open roadways, reducing icing, but standing areas of water or draining areas from the side of roadways will freeze.

The next model runs come out between 9 PM (NAM) and 10:45 (GFS). I’ll update before 11 PM.

Winter Weather Update Fri 10 AM
A quick update. This morning’s NAM shows a faster storm onset. Precipitation starts about 4pm Saturday. A very fast changeover to sleet and then rain. Freezing rain possible far northwest. Heavy rain by early evening Saturday. Rain tapers in the morning Sunday as temperatures plummet and winds pick up. Minimal accumulation expected in the immediate PHL area when all is said and done.

NAM accumulations
NAM forecast total accumulations by Sunday noon.

Last night’s models did very well with the light snow that should be ending within the next hour.

Here’s the latest on the Saturday to Sunday storm. For Philadelphia and its immediate suburbs, a total storm QPF is about 1.95 inches water. Most of this precipitation will be rain, but the beginning and end of the storm will have snow or mixed precipitation.

Based on the latest 06Z run of the NAM and GFS models (1AM last night), the NAM is holding off much of the precipitation until temperatures rise enough when the precipitation begins towards evening. Temperatures in all levels of the atmosphere will minimally support snow, so between 5 and 7 PM Saturday, expect some light snow rapidly mixing with sleet and rain and then all rain by 9 PM. In the far northwestern suburbs, an extended period of sleet and freezing rain may be occur and areas near Allentown may remain a frozen mix until midnight.

The GFS also has the atmosphere warming, but it has more precipitation starting somewhat earlier in the late afternoon, with greater mixed precipitation accumulating before the changeover to rain about 9 PM.

The Arctic front moves through about daybreak Sunday. Temperatures plummet from about 40 at 7 AM to 28 at 1 PM and 17 by 7 PM. Little moisture is left with this drop in temperatures. A change back to snow is expected Sunday morning, but accumulations in the immediate PHL area and surround suburbs will be minimal Sunday, according to latest models.

Stay tuned for more updates. We’ll be in better range for specifics tonight and tomorrow morning.