HMT-West 2007: Forecast

Forecast prepared by Chris Anderson, ESRL/GSD/FAB

2006-11-30 12 UTC Synopsis
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As expected, a fast moving, small amplitude, upper-level trough has moved over WA. IPW values exceeding 1" are crashing into the OR coast.

Current conditions in the ARB region
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Dry. Clear skies reported across ARB. Temperatures in the low to mid 40s from the valley upto Blue Canyon.

0-72 hour forecast
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Dry.

A fast moving front will pass along the CA coast midday today through midday Friday, giving the ARB zero chance of heavy rain but an excellent chance of high clouds.

>72 hour forecast
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For the third consecutive the long-range model guidance has indicated a fundamental shift in the circulation over the Pacific will occur late next week. A broad trough is progged to develop in the Northern Pacific, where ridging had been the general pattern. The consequence is zonal flow extending across the Pacific and nearly impenging on CA. Today's GFS has a slightly different storm track compared to yesterday's, not surprisingly, taking the first major storm south of ARB and into New Mexico. This is no reason to fret, however, as a long train of storms follow, with IPW values 1"-1.5" being entrained into the systems.

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2006-11-29 12 UTC Synopsis
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The large upper-level trough positioned over British Columbia two days ago has shifted eastward and is creating havoc in the central US. An upper-level ridge is in place in the trough's wake.

Current conditions in the ARB region
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Dry.

0-72 hour forecast
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Dry.

Although a fast moving shortwave is expected to push a front along the CA coastline 18 UTC Thursday through 18 UTC Friday, on-shore, and more importantly, upslope flow will be nearly absent from this system. The likelihood of a heavy precipitation event is zero, although there is a small probability of very light precipitation.

>>72 hour forecast

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Another wave is forecast to move through the NW early next week. The weather conditions
are expected to be mostly dry, very similar to the system moving through Thursday-Friday this week.

Seven to ten day model guidance continues to indicate much more favorable conditions for heavy precipitaiton in the ARB. The Pacific jet stream is progged to become more zonal with IPW values ranging 1.0"-1.5" embedded within the generally westerly flow. The GFS seems to be particularly excited about a system on Friday, Dec 9, having a peak 6-hr precipitation amount exceeding 1".

Anderson

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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

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2006-11-27 12 UTC Synopsis
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Massive, cold upper-level low is positioned along the NW US and British Columbia coastline. Tropopause pressure in the core is about 375mb. Core of highest speed upper-level winds appear to be west of Quillayute, WA. One aircraft report exceeded 140 knots at 300 mb. There are multiple reports of 100-120 knots extending from northern CA into OR. Precipitable H2O values in the 12Z GFS analysis are in the 0.6-0.7" 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 15:30 UTC is 0.5". The juiciest air appears to be south of the target region with 0.8-0.9" pcp h2o near Santa Maria in the GFS analysis.

The system may be characterized as one with vigorous upper-level dynamics with modest moisture supply. It is also a cold system with below 0C temperatures near the core at Quillayute, WA. Closer to the ARB region, Bodega Bay reports snow-rain line at 2000 meters.

A front passed through the ARB region Sunday afternoon around 19 UTC. Bodega Bay reported 0.6" of rain during the frontal passage and a wind shift from 110 to 300 degrees. Precipitation reports for the period Sunday 4a - Monday 4a PST are: Blue Canyon 1.03", Auburn 0.2", Georgetown, 0.2", Huysink 0.32", Farad 0.04". Much of this precipitation fell as snow in the Blue Canyon region. The surface met from the ETL Blue Canyon site reports and increase of the snow depth fom 0.5" to about 7" since 00 UTC, during that time abou 0.9" fo precipitation was recorded.

Current conditions in the ARB region
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Snotel measurements at 7a PST are mostly 1-6" and some reports of 6-12". 17 UTC surface obs show upslope wind direction with wind speeds about 10 knots. Overcast skies reported in the valley and freezing fog and snow reported at Blue Canyon and Truckee, respectively. Temperatures are in the low to mid 40s in the valley and upper 20s in the Blue Canyon to Truckee region.

Bodega Bay reports 50F and 0.55" of precipitable h2o. Wind direction is southerly at the surface and veering to westerly by 2000 meters. Wind speeds in the 2000 to 4000 meter layer are 30-35 knots. Precipitaiton is currently not being reported, and the freezing level is near 2000 meters.

0-72 hour forecast
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The core of the jet stream will continue to dive southward, pulling the upper-level low with it. The forecast questions are: what will the be southern extent of the trajectory of the core, how quickly will it move eastward, and how much of that 0.5" of pcp h2o can it squeeze out. Specific model information is given below. In summary, the forecasts are in very close agreement on the trajectory of the low, pushing it into the four corners region by Thursday midnight. Expect precipitation to be light but steady between now and 00 UTC Tuesday. Three hourly accumulation is not expected to exceed 0.2", and the storm total precipitation should be not much more than 1.0" range at higher elevation, all of which should fall as snow.

The NAM solution moves the core southeastward over the four corners by midnight Thursday. An upslope orientation to the 700 mb height field persists between now and late Tuesday afternoon. Three-hour precipitation rates peak at 0.2" early Tuesday morning, when, apparently, the combination of on-shore flow and upper-level dynamics provides the most vigorous flow.

The GFS has similar timing and trajectory of the upper-level low. PCP h2o remains steady near 0.5" through today and drops rapidly overnight as the 700 mb flow begins to have a northerly component. Precipitation amounts remain steady at 0.1" per 6-hours through Tuesday afternoon.

>>72 hour forecast

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A shortwave is progged to move through the NW Wednesday into Thursday. As a trailing front pushes into central CA Thursday night, pcp h2o values in the 0.7-0.9" move into the area, producing a chance at a brief (less than 12 hours) preciptiation event. More details about this system will be provided in tomorrow's forecast discussion.

Generally, a ridge will be positioned in the Gulf of Alaska. This will create unfavorable conditions for heavy precipitation in ARB through next week.

Anderson

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