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Icelandic Indigestion, Incorporated

A. Camden Walker @ 2:02 AM

Shopping Forecast

Pre-dawn readers planning on camping out for those Black Friday deals... BUNDLE UP with temperatures in the upper TEENS pervasive in our area. Special Discount! One Day Only! How about temperatures over 20 degrees cooler than yesterday? It won't encourage you to get out and walk off some Turkey.

Windchills will be in the single digits! Slow improvement begins with our 7:01am rising Sun dominating a clear sky, maximum temps cracking the freezing mark after noon, and winds relaxing closer to sunset. Even with highs today in the upper-30s, gusty breezes will lop 10 degrees off of temperature perceived. Shop early, shop warmly!

Tuckahoe Wind: Downsloping Effect off of the Blue Ridge Mountains

The Rockies might have the Chinook, sure, but we have the Tuckahoe!
With the Appalachian Mountain chain just to our West, yet one more topographic feature affects our geographic region's weather. They aren't nearly as tall as the Rockies out west, and so their influence is often written-off as insignificant. A Chinook event compressionally warms the High Plains to the east of the continental divide out in the central United States--sometimes over 30 degrees. Westerly winds off of our nearby mountains result in an analogous effect here in the Mid-Atlantic region because of the same process--just to a slightly smaller degree and scale.
Kevin Ambrose looks West from Leesburg towards Blue Ridge foothillsA gorgeous November sky perfect for Thanksgiving Day flurries...ahh, great holiday weather!
The term Tuckahoe is a word with regional etymology. Prior to WWII, in the Piedmont of Virginia for instance, the term "Tuckahoe" was used as a noun part-of-speech when referring to a person who grew up EAST of the Blue Ridge Mountains. Similarly, the influential power this wind wields is over locales east of I-81 (built very near the crest of the Blue Ridge). Chinook and Tuckahoe winds both belong to the "Katabatic wind" family-- those winds around the world which move DOWN an incline to a lower elevation.

When air is forced over and then down a mountain it sinks. This sinking to a lower level of the atmosphere warms the air because of compression by the atmosphere's mass above it! Besides temperature, cloud/precipitation patters are also influenced. Before sinking, the opposite process occurs on the western-facing slopes of the Blue Ridge. On that side of the mountain chain's crest, cooling & condensing of rising air produces clouds & precipitation. Similar in nature to rain ahead of a cold front which lifts warm air ahead of it producing precip...except the mountain chain stays STATIONARY while the air LIFTS ITSELF OVER. This forcing that the topography "performs" on the air by simply sitting there, causes moisture to be "rained-out" of the air over the western slopes of the Blue Ridge.

We have been subject to the Blue Ridge's wall-like influence twice in the last week.
1)Tuesday's Nor'easter exited the area & quickly revealed the sun as its cloud shield suddenly broke behind it. Temperatures did not plunge as quickly as they would after sunset--partially offset by Tuckahoe warming.
2)Wednesday night's Alberta Clipper gave a much more generous amount of snow to points well N&W of the District. Frostburg, Winchester, Martinsburg, & Garrett Co. all received double our fleeting accumulation amounts.

These two satellite images below center on DC during Tuesday's Nor'easter departure. Notice especially in the Infrared satellite copy (right) the pronounced dry slot between Delaware & the I-81 corridor--you can see there are no shades of blue over D.C. 11/23 IR DCregional via Wunderground.com11/23 VIS DCregional via
A Visible shot(right) from the same satellite, also taken at 1pm Tuesday, confirms skies are clear over DC as clouds break up over the Piedmont area. Why is air flowing off of the mountains via a westerly wind? The large Nor'easter moving to DC's north is forcing large-scale wind directions to spin around it, in a counter-clockwise fashion. A very simplified way to view clouds interacting with mountains is that they are being "caught" like hair is caught in a comb. However, our Blue Ridge range is not only drying the westerly-moving air but this same process is also warming the air thereby reducing relative humidity to levels below which clouds can be maintained.

Just like our big brother Chinook wind in the Rockies, wind moves over and through the mountains, the moisture in the air condenses, warming the air by releasing latent heat. Then, as it moves downhill, the colder air above presses down upon it, resulting in further warming through adiabatic compression. The resultant air (that heads toward DC) is warmer and much drier than it was when it approached the crest of the Blue Ridge.

Our local landmass proves an effective barrier and weather creator!

Tropics: Can I help ya, help ya, help ya?

Why yes, Delta, you may certainly help me.. by becoming the new record-setting 25th named storm of the Hurricane Season. NHC thinks it is still possible for Tropical Storm Delta to become a hurricane in the next 36 hrs. After that, chances become slim due to an increase in wind shear piecemealing convection near its all-important center of circulation. So far only a storm for the fishes, marine interests will watch this storm... unless you are like us here at blog central: addicted to any form of intellectually stimulating weather! The requisite foreboding statement: It isn't unheard of for a tropical system to form in December.
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Let's revel in this Washington Post article on our season's first snowfall. It doesn't appear our second snowfall is within the foreseeable 10-day future.

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