Friday, February 17, 2006
The Week-end Welcomes Waning Warmth
A. Camden Walker @ 2:02 AM
1pm update: The cold front has passed through the area. Look for slowly dropping temperatures and gradually clearing skies. Winds will continue to be gusty out of the northwest!
Experience two days within one day, today. Dress deliberately so that you aren't caught off-guard by cold wind ushering in Winter by sunset.
Our DC temperature will reach 60 by noon but under generally cloudy morning skies. We may see more sunshine this afternoon once a potent cold front blows through. Winds will shift from the southwest to northwest--this dryer, blustery air will make afternoon 50s feel cooler than that. After sunset, expect gusty winds to diminish a little but temperatures only to be in the 40s.
Kevin Ambrose captures bright sun, warm temperatures, and melting snow in Reston.
Snow Lover's Crystal Ball 
Next Chance of Accumulating Snow: Monday, Feb. 20
Probability: 20%
Potential Impact:
Commentary: I will keep the hopes alive for Monday, despite this snow event's potential being on life-support.
Two distinct, relative-to-average "settings" dictate whether said pressure fields formulate a "positive" oscillation setting for the North Atlantic, or instead a "negative" setting. Does each translate into distinct, DC weather? You can focus your attention on two, staple features that essentially run this show.
[1]"L" = Semi-permanent low pressure center between Greenland and Iceland ("Icelandic vortex")
[2]"H" = Semi-permanent high pressure off of the coast of Africa named the "Azores" high (for the island chain it often sits over)
When the NAO is in a "Positive" phase, both the Azores high and Icelandic (vortex) low are amplified. SEE IMAGE AT LEFT. Pressures over Iceland are very low, as storm energy swirls around a fast-moving Jet Stream. Warmth can be experienced in the Mediterranean as, simultaneously, the Azores high pressure system builds in strength.
Mild, moist conditions are usually experienced in DC and the Eastern Seaboard as the center of the Azores high moves a little further westward, building into and often connecting with our familiar "Bermuda high". However, despite giving you this DC weather correlation for Positive NAO, I should note that much more distinct "behavior modification" comes during Negative NAO. Almost always during wintertime Negative NAO phases, Washington DC's temperatures remain below-average--but I get ahead of myself.
When the NAO is in a "Negative" phase: weak, weak, weak, is the key word. SEE BOTTOM, RIGHT IMAGE. The Azores high moves eastward, hugging the African coast near its namesake islands. Ridging is suppressed--and thus warm air--to more southern, tropical regions.
The pressures over Iceland are relatively higher-than-average, since the Icelandic low vortex isn't asserting control. Slight Jet Stream ridging is possible, but more plausible are slower Jet Stream speeds through the area--since we all know the Jet Stream feeds on gradients in temperature and pressure--undermining the strength and number of storm systems.
More often than not, DC experiences below-average precipitation under this regime. However, most of our Arctic Air outbreaks also correlate to a "Negative" NAO phase. If the Icelandic low breaks down somewhat, there is a greater opportunity for a trough to develop in the Jet Stream just to the west of Greenland--over the eastern portions of the North American continent.
If this times properly (NAO playing an indirect role) cold air plunging southward during a Negative NAO can produce moderate snowstorms for DC when said Arctic Air comes in contact with warm Gulf, Atlantic waters.
Forecast: "Beltway Rollercoaster"
Experience two days within one day, today. Dress deliberately so that you aren't caught off-guard by cold wind ushering in Winter by sunset. Our DC temperature will reach 60 by noon but under generally cloudy morning skies. We may see more sunshine this afternoon once a potent cold front blows through. Winds will shift from the southwest to northwest--this dryer, blustery air will make afternoon 50s feel cooler than that. After sunset, expect gusty winds to diminish a little but temperatures only to be in the 40s.
Next Chance of Accumulating Snow: Monday, Feb. 20 Probability: 20%
Potential Impact:
Commentary: I will keep the hopes alive for Monday, despite this snow event's potential being on life-support.
FRIDAY FEATURE
The North Atlantic Oscillation (NAO): Whacky Warmth for DC when "positive"
Two distinct, relative-to-average "settings" dictate whether said pressure fields formulate a "positive" oscillation setting for the North Atlantic, or instead a "negative" setting. Does each translate into distinct, DC weather? You can focus your attention on two, staple features that essentially run this show.
[1]"L" = Semi-permanent low pressure center between Greenland and Iceland ("Icelandic vortex")
[2]"H" = Semi-permanent high pressure off of the coast of Africa named the "Azores" high (for the island chain it often sits over)
When the NAO is in a "Positive" phase, both the Azores high and Icelandic (vortex) low are amplified. SEE IMAGE AT LEFT. Pressures over Iceland are very low, as storm energy swirls around a fast-moving Jet Stream. Warmth can be experienced in the Mediterranean as, simultaneously, the Azores high pressure system builds in strength.
Mild, moist conditions are usually experienced in DC and the Eastern Seaboard as the center of the Azores high moves a little further westward, building into and often connecting with our familiar "Bermuda high". However, despite giving you this DC weather correlation for Positive NAO, I should note that much more distinct "behavior modification" comes during Negative NAO. Almost always during wintertime Negative NAO phases, Washington DC's temperatures remain below-average--but I get ahead of myself.
When the NAO is in a "Negative" phase: weak, weak, weak, is the key word. SEE BOTTOM, RIGHT IMAGE. The Azores high moves eastward, hugging the African coast near its namesake islands. Ridging is suppressed--and thus warm air--to more southern, tropical regions.
The pressures over Iceland are relatively higher-than-average, since the Icelandic low vortex isn't asserting control. Slight Jet Stream ridging is possible, but more plausible are slower Jet Stream speeds through the area--since we all know the Jet Stream feeds on gradients in temperature and pressure--undermining the strength and number of storm systems.
More often than not, DC experiences below-average precipitation under this regime. However, most of our Arctic Air outbreaks also correlate to a "Negative" NAO phase. If the Icelandic low breaks down somewhat, there is a greater opportunity for a trough to develop in the Jet Stream just to the west of Greenland--over the eastern portions of the North American continent.
If this times properly (NAO playing an indirect role) cold air plunging southward during a Negative NAO can produce moderate snowstorms for DC when said Arctic Air comes in contact with warm Gulf, Atlantic waters.
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