Daily Archives: Wednesday, December 16, 2020 4:06 pm

Updates

LOWER SNOW TOTALS FORECAST ON-TRACK

Despite the impressive snow falling at 3 PM, the revised forecast of only 1-2 (3) inches in the Philadelphia area and immediate northern and western suburbs remains on-track from this morning’s update.

Based on radar, the changeover has just moved north of the Delaware-PA state border.

The high winds (50-60 mph) are still expected this evening and night-time hours.

The latest NAM and NAM-NEST continue to show warm air entering the region at 6000 feet (800 mb) with a change to sleet, freezing rain and rain in the 6  – 8 PM timeframe.  The higher resolution NAM-NEST shows a possible alternation of precipitation types during the evening hours, especially far north of the city.  A changeover to back to snow will occur 2- 5 AM depending upon location.  The snow will gradually taper during the morning hours. 

The latest NAM NEST snow totals—

Latest NAM-NEST snow totals by 7 AM Thursday (Click on image for a larger view.)

Weather Updates

MAJOR CHANGE IN SNOW TOTAL FORECAST

Wed 09:51 AM Update — The most significant issue may be freezing rain, starting late afternoon.

I’ve been trying to ignore the NAM model forecasts in favor of the newer fancier models.  But when I go back to my forecasts that have really shined in past years, the temperature profiles of the NAM seem to be the most important factor, secondary to total QPF.

With the newest NAM model data just available, I can no longer ignore its forecast and its temperature profiles.

The NAM shows a large area of above freezing temperatures at 6000 feet resulting in much more sleet, freezing rain and rain than previously forecast—

Here’s the location of the 32º temperature at 6000 feet above ground at  11PM—

NAM 800 mb temperatures at 11 PM  (Click on image for a larger view.)

As a result, the NAM is predicting final snow totals on Thursday morning to be much lower than previously forecast—

NAM Snow Accumulation 7 AM Thursday  (Click on image for a larger view.)

The snow totals will only be 1-2 inches in the Philadelphia area and adjacent western suburbs by Thursday morning.

The start time for the snow is still about the same 12-2 PM, maybe 3PM. After snow accumulates by late afternoon a changeover to sleet and rain will seriously reduce additional snow totals. A change back to snow will occur before ending early Thursday morning around daybreak.

I know I’m going out on a limb with this forecast, but that’s the reason you’re reading this blog instead of watching the TV forecasts!