Friday, December 16, 2005
a NICE FRIDAY, That's What! plus Black Ice & regional Cyclone Formation
A. Camden Walker @ 12:02 PM
Also: New guidance suggests increasing possibility of snow Sunday--but not a done deal. Additional discussion also available in comments.
We'll have mid-40s today under mainly sunny skies, thanks to some help of downsloping winds. Some quilted clouds will likely mix into the sky just after noon. Windbreakers will help fend off a perceptible breeze from the west, however wind chills will not be extremely bitter. Thank you, Friday!Storm Recap
Use the comments at the bottom of this post for "spotting" to us your locale's accumulation.Some spots way to the western edge of our reading area received 3 to 4 inches of snow before a slight mix/changeover later yesterday evening (e.g. Winchester). Ice accumulations were substantial for a few hours just outside the NW portions of the Beltway near White Flint & Gaithersburg. Most accumulations were reportedly washed away in Fredericksburg by mid-afternoon. Brick sidewalks & marble curbs or stairs were treacherous even in downtown DC until 2am. Point being, this most recent storm's impact varied widely within our region's distinct geo-climatologic zones. Smaller, local variables played a more pivotal role--so I personally would be interested in hearing the amount AND composition of your accumulations. I must say we dodged the "Ice Storm!" bullet to a large extent; Further, I want my teammates to take credit for a great forecast.
Photo by Kevin Ambrose: Snowman Survives Through Two Winter Storms (unofficial title)
Friday Feature: Our Region's Affinity for Cyclogenesis
Our atmosphere makes life possible through distribution of heat energy--life exists in the hottest & coldest places on this Earth. The planet maintains this Life-supporting temperature range in a threshold that astronomers & exobiologists agree is astonishingly small--and not yet found elsewhere. Want to guess what is the pervasive manifestation of our planet's highly evolved heat distribution engine with which our astonishing thermostat is maintained? Weather. So now, stepping off the lectern, I can focus our meteorological discussion on temperature gradients. The close proximity of air masses to air or water masses possessing highly different temperatures (or overall characteristics) spawn a very efficient, focused heat distribution mechanism: Cyclones.
The classic set up is cold arctic air charging out of Canada straight for the warm Gulf Stream waters (at right) just off the East Coast. Notice the cusp in the Stream where it ricochets--if you will--off of the Outer Banks of North Carolina. So not only are two characteristically different bodies of water mixing between Cape Hatteras & the Virginia border, but a cold air mass aloft suddenly finds itself over a non-uniform environment! Up until the advection (traveling) of this air mass into the area, it had traversed hundreds of miles of land area without being substantially modified. The properties of the air mass now begin to shift rapidly--adjusting to this boundary in the water. The Gulf Stream virtually sets up a permanent water "front" just off our coastline; cold water to the north, warm water to the south.
As we've discussed in previous posts, liquid water does not lose its heat content nearly as fast as the gaseous atmosphere. The unfathomable heat capacity of the oceans correlates to only small incremental changes (relatively) in temperature during hemispheric summers or winters--as solar energy changes. On land, we have average temperatures drop a degree every 4 days in DC's month of December! It might take a large body of water weeks to accomplish this benchmark.
Specifically off of our coast here in the Mid-Atlantic/SouthEast, an atmospheric trough or weak front may develop parallel to the Gulf Stream. This air that stagnates (sits in-place) over the water boundary will begin to acquire its characteristics AND boundary orientation. This sets the stage for air to rise and move along the coastline wherein the aforementioned "air mass" actually divides into two air masses. It loses its synoptic (large-scale) identity maintained all the way from the Northwest Territories. A cyclone rides along this weakness in the atmosphere. Along this trough--if aligned ideally with the water-induced atmospheric isotherm lines--a cyclone will ride northeastward... feeding & twisting the dynamic temperature environment. Not to forget, there is less friction when air moves off of a land mass and hits more "smooth" water. This diffluence (acceleration-induced atmospheric vacuum that encourages air to rise vertically) could perhaps be talked about next week? Along with what truly drives the entire North Atlantic current water/heat-exchange cycle. To conclude for this week:
DC is perfectly centered on a continent that has access to a core of arctic air in Canada unable to modify (warm) on its plunge toward our permanent warm Gulf Stream waters just offshore. Such tightly grouped atmospheric conditions are difficult to find collocated anywhere else in the world. Thus in a stormy pattern--if induced by cold air plunging southward--Nor'easters (our fondly named, local version of a cold-core cyclone) may be persistently generated up our coast.
Katrina Ramifications: Still Surfacing plus one last tropical tease?
The Washington Post yesterday ran one online and one print article, both dealing with the recovering New Orleans' (sizeable reading alone) woes after Hurricane Katrina. The double trouble stems in discovered elevation of Lead levels in soil samples around the city plus the political controversy of federally funded levee reconstruction. I'll cordon off my personal commentary in the comments section later today, if anyone would like to discuss the ongoing plight of beleaguered New Orleans.
For my fellow tropical junkies, the National Hurricane Center is watching one feature:
A 1012 MB LOW NEAR 25N30W IS ALSO PRODUCING MODERATE SCATTERED SHOWERS ESPECIALLY TO THE NE OF THE LOW. QUIKSCAT SHOWS SOME 30 KT WIND VECTORS...
Next Chance of Accumulating Snow: Starting midday Sunday
Probability: 15%
Potential Impact:
Commentary: A shield of snow currently forecast over central & eastern Virginia could jog northward as models get a handle on a potential kink in the sub-tropical Jet Stream flow over the warm Gulf Stream waters offshore. Maximum potential DC accumulation is 1.5 inches.
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