« HMT Discussion for Wednesday 19 December 2007 | Main | HMT forecast discussion for 26 Dec 2007 »

HMT Discussion for Thursday 20 December 2007

HMT 12/20/2007 Forecast Discussion

Current Conditions
PW plume passage occurred this morning at 06 UTC at Bodega Bay and at 08 UTC at Truckee. As of 14 UTC, the ARB is on the northern edge of the PW Plume. Light radar echoes cover the region with a band of moderate echo in the valley. The moderate echoes are pushing southward signaling an slow end to the precipitation in the ARB. Expect another 0.25" by 18 UTC and complete shut down of precipitation around 18 UTC.

The short-wave trough did not fracture as it approached the California Coast, and it is positioned just offshore and just south of the ARB. This puts the ARB on the cold side of the thickness gradient, just along the northern edge of the warm advection region.

Long-range guidance continues to emphasize the Pacific NW as a landing point for strong waves. Occasionally a few ensemble members push a front into the ARB. One such situation occurs on Christmas Day. With the forecast of a high-amplitude ridge in place, the PW plume does originate near Hawaii in this forecast rather than the central Pacific; however, precipitation amounts are modest as the front becomes disconnected from the high-speed upper-air flow. This pattern is forecast to continue through about Dec 28.

Followup to IOP3: Forecast and observed precipitation
Precipitation forecasts and amounts for the 18-hour period 00 UTC 12/20 to 18 UTC 12/21.
36-hour forecast 1.6-2.1" 1.80" 1.95" 56%>1", 41%>2" 1.8"
12-hour forecast 1.5-2.0" 1.75" 1.55-1.80" 84%>1", 56%>2" 1.65-2.25"
OBS, Sloughhouse: 0.4-0.45"
Colfax: 1.65-1.7"
Alta: 0.8-0.85"
Blue Canyon: Missing obs
Big Bend: 2.35-2.50"
Norden: 2.35-2.50"
Truckee: 0.25-0.30"