Friday, January 13, 2006
Fog to Storms: DC's "1000 Weather Combinations" Reported!
A. Camden Walker @ 2:02 AM
STRONG STORMS, WIND ...POSSIBLE THIS EVENING
DENSE FOG ADVISORY EXPIRED AT 1PM TODAY
9:29am: Wilson Bridge fire is OUT--some lanes becoming available--traffic logjam will take time to work through, especially with a separate Route 1 issue/closure being reported
8:31am: WILSON BRIDGE CLOSED both directions reportedly due to fog-related accident & fire--Find alternatives to I-95 North & all of I-495
8:31am: WILSON BRIDGE CLOSED both directions reportedly due to fog-related accident & fire--Find alternatives to I-95 North & all of I-495
Friday Forecast: Murky then Warm & Windy
Everyone deserves a seemingly pleasant end to their week. In the classic calm before the storm, around 62 degrees today (away from water) for a high makes yet another warm Friday!
Fog will burn off slowly this morning since we are under more clouds than expected above the fog. Clouds should break some by noon and show us some blue sky during the early afternoon. To accompany this, gusty 10-20mph winds out of the South. In a winter rarity, we'll only experience negligible windchills. I like my usual recommendation of wearing a windbreaker with your one-layer top-- in case of a stray late-afternoon shower, be sure the windbreaker is waterproof. Drive Safely this morning with patchy dense fog in low-lying areas. 
Time Line: Your morning, depicted below by Kevin Ambrose...and then?
Most importantly, the weather won't start going downhill until after Rush Hour this evening.
- Storm risk increases greatly after 8pm tonight.
- A heavy, potentially severe batch looks to focus its attention on DC around 2:30am-4:30am tonight.
- Rain tapers to showers by 6:00am Saturday.
- It will be cloudy, with some peeks of sun. Any accumulating rain will be miniscule throughout the daytime Saturday.
- Snow will become the visible yet LIGHT precipitation type by late-afternoon Saturday in western locations such as Winchester. In the immediate Metro Area this will not happen until after 8pm Saturday.
Nocturnal Storms: They "go" where (& when) the sun don't shine
In the summer, we half-expect afternoon thunderstorms here in DC. But what happens when storms march through the area at night, waking us all up? Why is the atmosphere so unstable and buoyant when the sun isn't making things hot and there isn't the oppressive summertime stickiness to the air?
You've often heard the weather teaser "upper-level energy is moving through the area" or the snippet "the atmosphere has a lot of lift tonight" perhaps on your 11pm TV news station. I want to focus on air-flow patterns in the middle & upper echelons of the troposphere. I've created a small glossary, which lists some important factors for all storm formation. Some of these items will also play a role in our perhaps after-dark turbulence tonight. But, hey, even if chances fizzle by 2am Saturday, you'll be armed and ready--with palatable information, hopefully--the next thunderous night!
Lifted Index: helps measure "stability" based on how easily or difficult an air parcel might rise in the atmosphere all based on relative temperature changes among air parcels in close proximity. Atmospheric temperature generally decreases with height. When surface air rises, it also cools. However, sometimes it cools more slowly than its surrounding environment. When rising air is warmer and less dense than surrounding air, storm development is facilitated. The more energy required to lift, cool, and condense a piece of air inside a cloud-- the more energy a storm expends just trying to maintain simple cloud structure! A storm will be less volatile if it intersects an area in which LI is decreasing (becoming less negative in value)-- an area that has more stubborn air parcels not readily ingestible into a Cumulonimbus (thunderstorm) cloud. DC doesn't have inordinately negative LI values for Friday.
Convective Available Potential Energy (CAPE): another "stability index" -- tonight DC will not have over 1000J/kg of atmospheric energy which is arguably a threshold needed for "Severe" level storms. I think I just got too technical...but we do have atmospheric "CAPE" moving over us tonight, nonetheless. Again, it takes just a portion of these ingredients collocating to raise our storm chances.
Precipitable Water (PWat): the amount of water available for a storm. This becomes very impressive overnight, late on Friday night. This offers a thunderstorm or larger, synoptic storm system to use evaporational cooling to energize itself. The more water that a storm can lift into higher levels of the atmosphere, air parcels cool (again, just like the chill experienced when stepping out of the shower into a ventilated bathroom), then their density forces their fall back into the lower part of this developing cyclical loop vertically building in the wet atmosphere. Of course this can also mean heavier rain from a storm, too. Water vapor is available for any purpose if there is high "PWat".
Helicity: tornadic conditions exist when Helicity is high thus indicating spin of parcels around parcels NOT spinning to the same, fast degree. Sometimes the Jet Stream can develop a sharp trough and at the edge of these troughs, the gradient of motion is great. Directly at the edge of a fast moving, narrow stream one can often observe spinning eddies of water that swirl about. The average "turning speed" for our DC atmosphere Friday is interesting in that it registers as EXISTENT (perceptible spinning) levels of Helicity; but, nothing alarming. This is a tricky concept-- I welcome input!
Vorticty: is localized rotation greater than spin naturally placed on air by the spin of the Earth. It will be a big player tonight (and enhancing snow Sat. night) as POSITIVE Vorticity sweeps through the mid-levels of the atmosphere. Direction is important. A parcel rotating clockwise is said to have negative vorticity, and a parcel rotating counterclockwise is said to have positive vorticity. There are two types of vorticity: SHEAR vorticity, which arises from changes in wind speed over a horizontal distance, and CURVATURE vorticity, which is due to turning of the wind flow. BOTH will be rotating and blowing through the DC area tonight & tomorrow...hanging just to our west for a while. Surface low pressure systems often develop just to the east of these maximum areas of "atmospheric vacuum." Ask any questions in comments (click link below post)!
Convective Inhibition (CIN): is simply the amount of negative (subsiding) energy the atmosphere must overcome to achieve a state of free convection, or thunderstorm development. Its units are also in Joules/kilogram just like CAPE. There is no inhibition today over DC. Above the boundary layer, parcels (cubes) of air can begin rising to aid Cumulonimbus development, unless there is high CIN.
Next Chance of Snow: Saturday late night (Beltway area)
Probability: 55%
Potential Impact:
Commentary: Tomorrow night, a second piece of energy behind tonight's front-spawned storms, may coat the greater Metro area's grassy surfaces. Rural roads may possess a couple slick spots around dawn Sunday. Timeframe when snow will BARK worse than its predicted 0.5" bite: 10:30pm-4am Sunday. See comments section of this post for more lengthy discussion throughout the day.
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