HMT-West 2007: Forecast

2006-11-28 12 UTC Synopsis
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Massive, cold upper-level low is centered on the Idaho-Canada border. Tropopause pressure in the core is about 450mb and lowest pressure levels are in the southwest quadrant of the low, where the ore of highest 300 mb wind speed is located. Oakland reports 125 knots at 300 mb, while, in Oregon, Medford and Salem reported incomplete wind profiles. RUC analyzed the core 300 mb wind speed just off the OR coast with values exceeding 130 knots. The core might extend farther south than the RUC thinks, given the Oakland report. Wind direction at all levels is nearly parallel to the coastline and along the spine of the Sierra Mountains. IWV values in the 12Z GFS analysis are in the 0.4-0.5" range in the San Francisco Bay area and westward over ocean points. SSM/I passes between 05 and 09 UTC similarly show values less than 1", and Bodega Bay IPW at 16:00 UTC is just over 0.4".

Precipitation reports posted on the CNRFC webpage for the 24-hour period 4a Mon through 4a Tue are: 0.08", Auburn; 0.52", Georgetown; 0.55", Blue Canyon; 0.80", Greek Store; 1.00", Huysink; Missing report, Truckee and Farad.

Snow depth at the ETL surface met station at Blue Canyon is nearly 9". Nearly all SNOTEL sites in and around the ARB report between either 6-12" or 12-24".

Current conditions in the ARB region
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There are no radar echoes in the ARB region, though 16 UTC surface reports indicate overcast conditions and low clouds or fog is present in the IR. All precipitation reports were 0" or missing in the 6-hour period 10p-4a. Temperatures are in the mid-30s in the valley and low- to mid-20s in the Sierra Mtns.

Bodega Bay reports 51F and 0.45" of IPW. Wind direction is northwesterly from the surface through 3000 meters. Wind speeds in the 2000 to 4000 meter layer are 25-30 knots.

0-72 hour forecast
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With the core of the upper-level jet still west and south of the upper-level low center, it is expected that the upper-level low will continue to move southward. It appears to have taken a track a bit east of what was forecasted in yesterday's NAM and GFS output. The consequence for ARB is that precipitation ended earlier than forecasted. This may have occurred for a couple of reasons. One being the orientation of the low-level flow never became westerly as expected ahead of an upper-level low with a more southward trajectory. Instead, the ARB region was along the periphery of the upper-level low where low-level heights were oriented north-northwest. The other impact was the upper-level dynamics shifted eastward rather than moving over the ARB region. Thus precipitation ended late last night rather than late this afternoon as forecasted. As the upper-level low continues to have a more eastward trajectory than progged yesterday, expect little or no precipitation over the next 24 hours.

A fast moving shortwave is expected to make landfall along the British Columbia, Washington, Oregon coast line between 36 and 42 hours. The 15 UTC SSM/I shows a swath of IPW between 1-1.5". NAM and GFS forecasts peg the timing of contact between this high IPW and the CA coastline to a 24-hour period beginning 18 UTC Thursday and ending 18 UTC Friday. Maximum IPW values at Bedoga Bay appear to be 0.9"-1.0" and inland values in the 0.4-0.6" range can be expected. However, the northwesterly orientation of 700 mb height field will foul up any chance of heavy precipitation. Even the possibility the ARB will get non-zero pcp during this period is remote - the Hamill-Whitaker analogue technique has pops of pcp>0.01" of only 10% - and contingent upon precipitating clouds advecting over the ARB from northern CA. Should precipitation occur, expect all amounts to be less than 0.10". A report approaching 0.25" would be a shocker.

>>72 hour forecast

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The next period of interest for ARB appears to be Monday, Dec 4 into Tuesday, Dec 5. Model
guidance indicates a very large spread of potential trajectories. A few trajectories bring the wave into the ARB region, others carry it into Canada, while most have an evolution similar to the wave expected later this week. The Hamill-Whitaker anlogue technique predicts pops for pcp>0.01" of 15%. While this is not an encouraging outlook for this particular storm, the model guidance suggests the Pacific jet stream will flatten out, becoming more zonal, and IPW values will be generally higher toward the end of next week. In the 8-14 day period, Hamill-Whitaker analogues predict pops for pcp>0.01" of 70-75%, which is about equal to the climatological probability.

Anderson