Tag Archives: Cold Air Damming


[su_note note_color=”#d9f2da”]Thurs Noon update: Wow, more snow than I had forecast! The low level atmosphere temperatures are at or below freezing. The NAM upper atmosphere critical thickness level seems to have fooled me this time around. It works in the winter, apparently not in November.    😱         I’ll make a note of it!

So, let’s try this again. Based on the latest NBM (which I’ve been ignoring, except for the precipitation start time), the changeover to sleet and then rain will occur about 1 -2 PM in PHL.    This is a tough business.


Based on the latest NAM and NBM model data, the forecast from last night is on track.

Precipitation is moving in, as shown on radar, at 10:30 AM.  On this radar image, it’s easy to see how the precipitation is hitting a wall of cold air, eroded at the northern edges—

Radar Thurs 10:20 AM, courtesy of Weathertap.com

This is a sleet to rain event, with some snowflakes possible at the start.   The latest NAM supports the possibility of a brief change to snow before daybreak Friday.  (Little or no accumulation.)




An interesting weather setup is being forecast by most models for Thursday into early Friday for our area.

There’s a blocking high pressure system and an upper cut off low in the midwest.

NAM Model
Upper Low- Blocking High Setup NAM model-  Thursday evening.

The blocking high pressure will give us the low level cold damming effect, trapping cold air at the surface.  This is what will give us the chance of sleet/freezing rain Thursday morning.

The upper low in the midwest will merge with the surface low on the coast.  This will cause intensification of the surface low Thursday night, bringing in enough cold air to turn precipitation to snow Thursday night into early Friday morning.

Let me start by saying that Thursday early morning rush hour should be OK, but moisture moving in from the south during the morning will fall mostly as sleet and freezing rain, especially north and west.  (There may be a few snowflakes at the very start.)

I do NOT think we will get snow in the immediate PHL area and surrounding suburbs on Thursday morning or afternoon.   This is a sleet/rain event for much of Thursday.  Despite the fact that the GFS is calling for snow, based on critical atmospheric thickness levels and the NAM, I think this will be sleet/freezing rain to rain event for early Thursday into Thursday afternoon.

Here’s where things get interesting for late Thursday into Friday,  something I alluded to yesterday—

Sometime late Thursday into Thursday evening, the upper cut off low is expected move and merge with the coastal system, causing rapid intensification.  The increase in the precipitation rate and the cold air associated with the upper low will cause precipitation to change to snow in the immediate PHL area Thursday night into daybreak Friday.  Too early to determine if there will be any accumulations.  (The latest GFS doesn’t bring in that much cold air aloft.)

As stated in recent posts, the models have been anything but consistent over the past week.  But this scenario has been solidifying over the past two days.    Stay tuned.