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HMT Discussion for Monday 17 December 2007

HMT 12/17/2007 Forecast Discussion

Executive Summary
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Model guidance is nearly in complete agreement that a heavy precipitation period will occur between 12 UTC 12/18 to 00 UTC 12/19, though the NAM is a bit slower at ending the event. The heaviest precipitation period is 18 UTC 12/18 to 00 UTC 12/19 with all guidance producing at least 1" of precipitation. The total precipitation between 12 UTC 12/18 and 06 UTC 12/19 ranges 1.5" to 3.5", with most guidance in the 2.0" to 2.5" range.

NWS forecast offices pointed out that precipitation rate would increase prior to 12 UTC 12/18. I concur. QPF from CNRFC and GSD Ensemble in the 6-hour periods ending 12 UTC, 18 UTC, and 00 UTC are 0.25", 0.75", and 1.15" and 0.25", 0.5" and 1.25", respectively.

Model guidance is coming to a consensus on a second heavy precipitation period beginning 00 UTC 12/20 and ending 00 UTC 12/21. The ingredients are in place for larger precipitation amounts than in the current IOP.

Current Conditions
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Over the past 24 hours ending 12 UTC, Huysink reports 0.36", Sac reports 0.16".
At 16 UTC, 30F at BLU with light snow and 46F at SAC with fog.

Bodega Bay PW is 0.6"; freezing level is 5000 ft at 6 UTC
Sloughhouse freezing level is 5000 ft at 6-12 UTC
Chico freezing level is 4500 ft at 2-5 UTC

A short-wave trough is evident in NRL water vapor imagery along the latitude of N CA at 130W. The wave has slightly higher amplitude in the GFS compared to Canadian analyses. SSM/I imagery suggests the PW is topping out at 2 cm. The GFS initialization is a bit higher, pushing 23 mm.

Another more potent short-wave trough is apparent along the latitude of OR at 145W. This wave is also apparent in both the Canadian and GFS analysis as a sharp trough. PW values associated with this wave are quite a bit higher into the 3-3.5 cm range.

0-72 hour & 3-5 day forecast
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Model guidance is nearly unanimous on the timing of the next short-wave and taking the short-wave right over the ARB. PW values are a bit higher than in previous forecasts, pushing to just over 1", as well. Thus, precipitation amounts from nearly all model forecasts are higher than in previous cycles.

The NAM continues to have the highest precipitation totals, exceeding 3" for the 12-hour period. Also, it also allows the precipitation to linger. My first thought is to discount the NAM precipitation. However, the PSD seminar that described the mesoscale effects of the mountain chain make me hesitant to throw out the high-end NAM results. And, it appears from the PSD Analogue ensemble that >3" has occurred in the past as the probability of >2" has 84% probability.
As a result, I'm bumping up the precipitation forecast from yesterday.

Period of heaviest precipitation: 12 UTC 12/18 - 00 UTC 12/19
Most likely precipitation amount: 2.0"-2.5" (can't rule out 3+")


A second short-wave is forecasted to occur later in the week. The model guidance on this wave is coming into focus. All GFS ensemble members, 9 out of 12 CMC ensemble members and the NAM have pegged the onset of precipitation to be around 00 UTC 12/20 and have heavy precipitation through 00 UTC 12/21. This system is forecasted to have higher PW values, and, so, has a chance to produce more precipitation than the current system. It also has a very distinct frontal passage marking the end of the precipitation period.

5-10 day forecast
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GFS continues to have one more event in the long-range, though the current cycle pushes it back another day to Monday and takes the short-wave trough into the Pacific NW and northward. The trailing front that pushes through the ARB has a far less interesting precipitation scenario than in previous cycles.

10 day and beyond forecast
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Ridging expected by middle of next week in the PSD reforecast ensemble.