The Wedge, Overrunning, Cold Air Damming

December 13, 2024

The Wedge, over running, (personally under cutting may just as good of a term to describe it) cold air damming. Regardless of the reference, this setup across the Alleghenies is a staple winter weather setup and occurs multiple times each winter. The boundaries are often in the same area. This year with the new Bear Rocks station in play, we can watch that area real time to see just how long it holds vs Cabin Mt.

In these setups, as long as you have the driving mechanism of the High Pressure sitting off to the Northeast driving cold dry air up the eastern Alleghenies, you get adiabatic cooling as the air rises and the process of evaporational cooling with the moisture incoming. That combo helps hold the high eastern Alleghenies cold, as long as you have the main player of that big cold dry high north.

For this post, we will use the ECMWF(Euro) products. It is slightly colder vs other modeling, but tends to handle the setup well.

• High Pressure New England

• The clockwise flow of winds at the surface around the High thats sitting in New England. Driving the colder dry air up the east side of the Alleghenies.

-The modeled dewpoints indicative of that dry air.

• While that’s one player, back to our west is our moisture that will be incoming. Thats accompanied with milder, moist counter clockwise SW winds

• Those SW winds are seen aloft. Lets look at the 850mb(roughly 5000′) winds. As that zooms in at 4pm look at the modeled temps at height. Above freezing aloft and that also would translate down to the high ground on the west side of the Alleghenies and also the lower elevations west. That driving wedge of the the colder dry air gets shallow and completely is gone as you move west of the Allegheny Front. So in your head, vision youre along the eastern Alleghenies at 3000′. You have a driving ESW wind, it’s 29° and that air is rising up the mountain undercutting what’s occuring above you. Up above you at 5000′ there is milder SW winds. The precip is melting in that layer and it’s either refreezing as sleet before reaching the ground or if the milder air sector is broader, it’s remaining liquid and freezing on the surface as freezing rain.

• The modeled 850(5000′) temps for the event. Look what happens in the process along the east side of the Alleghenies. Those 5000′ temps cool further as you get moisturing in that dry air mass lifting up towards the Allegheny Front.

So, the east side is impacted by the cold dry high. It’s not though as cold as what the high ground along the Central to eastern Alleghenies will be. However the entire column above cools enough that

1- The upper air is cooled just enough to see the entire column below freezing. Therefore you can see snow. This isn’t a setup where we have a departing frigid airmass and you see that cold air sitting there trapped in the deep valleys. Not the case. This setup can feature Cumberland getting snow at 32-34° while at near 3000′ at the Garrett County Airport it’s 29° and freezing rain.

• Lets look at that visual of what’s going on up above

-First the Garrett County Airport. I highlighted the freezing point with the blue line. Temperature/Dew Point/Humidity in blue, red, green. Notice as you increase vertically from the surface(that’s the bottom of the temp line) the temperature increases with height. That would indicate likely freezing rain at 4pm Sunday that may battle in some sleet at times as well. The cold is pretty shallow. The surface may be 28-30° at this time but jumps above freezing aloft fairly rapidly.

• Lets hop down to Cumberland at 700′ near the Potomac. The surface sits near 32°, but as you progress in height, the entire column remains just below freezing or very close. Indicating wet snow that may be mixed with a bit of sleet. That may be impacted by precip rates as well. Now if the model is off a degree aloft and at the surface. Straight liquid occurs. Climatology favors snow with the setup for the initial brunt of the precip. Not a major event, but a disruptive one

• Again vision this type of setup(example not for this specific event)

A look at how the wedge looks from a vertical profile. Depending on how strong the forcing for the undercutting of the dry cold east winds are, depends how far west the wintry makes it. Vision this wedge below advancing east or west. With that you can also see how some of the high 2800-3000′ ridges performs better than a 4000′ ridge when the east flow impact weakens.

• Now let’s translate this system over to a visual using the 6z Euro

-Surface Temps. Remember you can have slightly warmer temps at 700′ east of the Allegheny Front vs a central and eastern ridge at 2800-3000’+ and have snow down low while sleet or freezing rain up high.

• Again, using the 6z Euro which is a touch colder vs some other modeling. This post isn’t to say this scenario is locked in, it’s definitely a climatological favored look. However variances and changes can occur

– The euro snowfall zone

-The euro sleet zone. Keep in mind, this isn’t a mesoscale model and the area vs reality can be a bit broader

-The euro freezing rain setup

• In summary, a wintry event is on the table for Sunday. Timing, intensity, how strong the wedge is, is not exactly known. A trend slightly colder, or slightly less cold will impact overall results.
• Temps moderate Monday a.m, and whatever occurs begins to melt off. The next potential wintry system looks to be on the table later next week.

For now, enjoy and become fascinated by the unique setup of Sunday. The why of how these systems work holds lots of unique interest that our Alleghenies play a significant role in.