2006-12-03 Wx Discussion and Forecast
Forecast prepared by Chris Anderson, ESRL/GSD/FAB
2006-12-03 12 UTC Synopsis
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An upper-level ridge has moved over head. Winds are light northwesterly from near the surface upwards. A shortwave trough is evident in the Gulf of Alaska.
Current conditions in the ARB region
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Severe Clear. Snow depth measured by the ETL instruments at Blue Canyon is steady at 5.2". The 1"-6" is the most common report of snow depth in the ARB by SNOTEL measurements. fog is reported in the Sacramento Valley. Temperatures are in the low 40s from the Valley up the slope of the slopes of the Sierra.
0-72 hour forecast
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Expect a dry frontal passage Monday night into Tuesday with the most noticeable impact being an increase of IPW values reported by Bedoga Bay.
>72 hour forecast
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The northern Pacific regime shift is underway. A broad trough now extends across much of the north Pacific from Russia to the Bering Sea in the latest GFS analysis. A potent shortwave trough is progged to travel south of the Bering Sea on Tuesday, establishing a persistent flow of cold air and troughing in that region for a week or so.
Though the first system affecting the US West Coast after the regime shift is expected on Thursday, the odds are that it won't appreciably affect ARB. Thursday is now the Day 5 period in the Hamill-Whitaker anologue forecast technique. Their technique provides calibrated probability of 24-hour precipitation exceeding selectable thresholds. Probability of measurable rain in the 24-hour period 00 UTC Thursday through 00 UTC Friday is about 10%. On Day 6, that is 00 UTC Friday through 00 UTC Saturday, pops for measurable, >0.5", and >1.0" are 30%, 20%, and 10% with the actual frequency of occurrence given these pops being 30%, 15%, and 5%. On Day 7 (00 UTC Saturday through 00 UTC Sunday), pops of measurable, >0.5", and >1.0" are nearly 40%, 25%, and nearly 20% with actual frequency of occurrence being 37%, 22%, and 10%.
The climatological robability of >1.0" is about 15%. Since the probabilities of >1.0" on Day 6 and Day 7
are slightly less than climo, these storms are not expected to produce unusually heavy precipitation amounts, although they are the most potent thusfar of the HMT campaign.