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HMT Discussion for Tuesday 18 December 2007

HMT 12/18/2007 Forecast Discussion

Executive Summary
Forecasted precipitation accumulation in the 18 UTC 12/18 to 00 UTC 12/19 period is about 0.25" less than yesterday. Expect about 1.0" to accumuluate during this period. Light precipitation should persist through 06 UTC 12/19.

Consistent with yesterday's forecast, model guidance pegs the next heavy precipitation event to begin around 00 UTC 12/20 and to shut off abruptly after 00 UTC 12/21. Forecast amounts for the 24-hour period range 1.5"-2.0", with 2.0+" in isolated areas along ridge lines. The NAM continues to be wetter than all other models with amounts in the 2"-3" range.

Current Conditions
Precipitation began about 08 UTC.
Blue Canyon 00-12 UTC accumulation is ~0.5" and 12-16 UTC accumulation is ~0.25".
Sloughhouse 00-12 UTC accumulation is ~0.4" and 12-16 UTC accumulation is ~0.2"; freezing level is 5500 ft 12-14 UTC.
Bodega Bay freezing level is 7000 ft; PW is 1" or a tick higher.

The short-wave trough took a slightly more southern route than expected, and the base of the trough at 12 UTC was located parallel to SF Bay. At 12 UTC, it had a bit of a fractured appearance in the Canadian analysis, with stronger winds moving inland in N CA and S OR.

Water vapor imagery shows a moisture plume moved through the area between 08 and 17 UTC; another skinnier plume is moving into the area. Moderate radar echo is evident just north of ARB. SSM/I PW estimates are 2+ cm offshore.

A short-wave is positioned near 150W parallel Washington. A slug of moist air with SSM/I PW 3+ cm is associated with this trough.

0-72 hour & 3-5 day forecast
Model guidance continues to pinpoint the 18 UTC 12/18 to 00 UTC 12/19 period as the heaviest precipitation period. NAM has the highest totals during that period with values approaching 2". All other guidance puts the 6-hr total in the .5"-1.0" range, which is about 0.25" below yesterday's projection. Also, NAM has the precipitation lingering to 12 UTC 12/19, whereas other guidance suggests a light precipitation through 06 UTC 12/19 and nearly complete shutoff after that. The region has seen ~0.75", and expect another ~0.75" this afternoon.

Period of heaviest precipitation: 12 UTC 12/18 - 00 UTC 12/19
Most likely precipitation amount: 1.25"-1.75" (slightly down from yesterday's 2.0"-2.5" forecast)

Another potent wave is forecasted to hit the CA coast around 00 UTC 12/20. Model guidance has not reached a consensus on the timing of this wave. The CMC ensemble brings it onshore prior to 00 UTC; whereas, the GFS ensemble has precipitation onset after 00 UTC. Models are in agreement about an abrupt shutoff shortly after 00 UTC 12/21. Precipitation totals during this 24-hour period will likely to range 1.5-2.0" with the possibility of 2.0+" along the ridge lines.

5-10 day forecast
Most of ensemble members are focusing the short-wave track on the Pacific NW. Some ensemble members continue to have a trough pass over ARB on Monday with a PW plume extending across most of the central Pacific containing PW values >3 cm.

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

Chris Anderson