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HMT Discussion for Wednesday 19 December 2007

HMT 12/19/2007 Forecast Discussion

Executive Summary
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Model guidance begins precipitation between 00 and 06 UTC 12/20 and ends it between 18 UTC 12/20 and 00 UTC 12/21. The PW plume passes the ARB shortly after 18 UTC 12/20. Forecast amounts for the 24-hour period 00 UTC 12/20 to 00 UTC 12/21 range 1.5"-2.0" with 2.5+" possible at Blue Canyon, though much of the precipitation falls within the 12-hour period between 06 and 18 UTC 12/20. Freezing levels should be higher than in the previous storm during the early stages; the NWS is thinking freezing levels will drop around 12 UTC 12/21 to perhaps below 4000 ft.

Current Conditions
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Water vapor imagery shows a short-wave trough coming on shore at WA. A slug of moisture is evident in the warm sector coming on shore in OR. SSM/I estimates of PW are just under 2 cm with the initial moisture push but exceed 3 cm in a large pool extending from 130W to 160W.

A second short-wave trough is positioned on the latitude parallel to OR at 145W.

0-72 hour & 3-5 day forecast
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Model guidance is nearly unanimous in the speed of propogation of the short-wave trough positioned at 145W and PW values associated with it. The warm advection sector is over N CA by 06 UTC 12/20. The trough makes landfall in central CA around 12 UTC 12/20 and moves into Nevada by 00 UTC 12/21. Onset of precipitation occurs between 00 and 06 UTC 12/20, though ~0.01" has accumulated by 00 UTC. The NAM produces the highest QPF, as it did with the previous system. Twenty-four hour totals in the 00 UTC 12/20 to 00 UTC 12/21 period are 2.5-3.0" (NAM), 1.4-1.9" (GSD), 1.75-2.25" (GFS), 2.25"-2.50" (CNRFC), and probability of >1" and >2" of 84% and 56% in the PSD Analogue ensemble. The period of heaviest precipitation is 06-18 UTC 12/20 when the PW plume is approaching and impenging on the ARB with 700 mb flow as high as 50 kts.

My biggest concern is that the trough is fairly broad in the water vapor imagery, and it might be a little too potent in the analysis for the model initialization. Everyone has seen troughs fracture as they approach the Coast. Most likely this is just a problem with sparse measurements in the central Pacific. If this trough does fracture, it seems more likely that the warm advection period could be longer than forecasted. I don't see anyway that significant precipitation rates occur prior to 00 UTC 12/20, even if the trough is not accurately depicted in the model initial state.

5-10 day forecast
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Very little hope in the model guidance for precipitation south of extreme N CA.

10 day and beyond forecast
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The flow becomes more zonal by the New Year.

Followup to IOP2: Forecast and observed precipitation
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Precipitation forecasts and amounts for the 24-hour period 00 UTC 12/18 to 00 UTC 12/19.
GSD NAM GFS PSD CNRFC
36-hour forecast 1.5-2.0" 2.0-2.5" 1.0-1.5" 80%>1", 58%>2"
12-hour forecast 1.35-2.25" 2.4-3.25" 1.4-1.75" 92%>1", 84%>2" 2-2.25"
OBS
Sloughhouse: 0.6"
Alta: 1.8-1.9"
Colfax: 2.0"
Blue Canyon: 2.75-2.9"

Chris Anderson