Tag Archives: Cold Air Damming


A winter-time precipitation scenario is setting up for our area on Monday.

A front moving through this weekend will return as a warm front, but with over-running precipitation and cold air damming.

What was originally forecast to be a quick change from light snow to rain has evolved into a “cold air damming” scenario where cold air trapped at the surface likely will result in a prolonged period of sleet or freezing rain. (The models are showing mostly freezing rain.)  The GFS also shows some dynamic cooling- as the precip increases, the surface temperatures drop.

Some amount of wintry mix may be falling into the late afternoon and longer north and west of the city.

The QPF is expected to be light and the predicted snow accumulation is about 0.5 inches, but the prolonged freezing rain possibility may present a travel issue.

GFS Categorical Precipitation type for 2 PM on Monday. Click for larger image

The ‘event’ is more than 50 hours in the future time frame.  The higher resolution models, with the exception of the NAM NEST, only predict 36-48 hours in the future.

These things are tricky to forecast.  In our area, the models tend to under-predict the duration of the mix precip.

Stay tuned.



I’ve been looking at the latest GFS and NAM models from this morning. Sunday night into Tuesday promises some interesting (at least for this season) winter weather.

Two systems will affect us, one Sunday evening and the other starts Monday evening. The first approaches Sunday with light snow developing sometime during the evening and ends early morning Monday.

There are differences in the models with the QPF.  The NAM is showing 0.23 inches water, the GFS less. With the prior two winter weather events, the NAM out-performed the GFS.   So it appears that 2, possibly 3 inches of snow will have accumulated when it ends Monday morning.

Monday will be mostly cloudy as another more intense storm moves to our west.  Cold air in place at the surface with warm, moist air over-running the the cold air will result in snow initially.

The GFS has the snow starting early on Monday evening.  Critical temperatures in the upper atmosphere warm by 3-5 AM Tuesday morning, with the snow changing to sleet and freezing rain at that time.  It’s possible that 3-4 inches of snow will have accumulated before the changeover.

Surface temperatures may remain at or below freezing until 7 AM Tuesday, so things will be icy and messy Tuesday morning.

North and west of the city, the freezing rain and sleet may be prolonged into late morning.

By noon on Tuesday everything should have transitioned to all rain.   This looks messy and the temperature profiles and QPF predictions will likely change in the next few days.  Stay tuned.


[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.