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HMT Discussion for Saturday 15 December 2007

HMT 12/15/2007 Forecast Discussion
Current Conditions
At 16 UTC, clear skies over the area with some fog in the valley. The important feature are upper-level waves west of Oregon. A sharp-amplitude, large and fast moving trough was positioned at 12 UTC at approximately 150W, due west of Oregon. A smaller short-wave trough was positioned along the Oregon Coast as of 12 UTC.

The 11 UTC SSM/I estimates of IWV is less than 2 cm with the first short-wave. A slightly more moist pool is evident with the larger trough.

0-72 hour forecast
The weak short-wave trough hitting the Oregon Coast at this time is forecast to move quickly inland with a trailing front that briefly pushes into N CA before retreating as the Big Dog approaches.

The second trough is also forecasted to take make landfall on the OR coast and move eastward. However, the size of this system will create upslope flow into the ARB. Since the track of the trough is so far north, upper-level forcing of ascent will remain north and precipitation onset is unlikely to begin before upslope flow is established. 12 UTC model forecasts are becoming more similar with the propogation speed, but the NAM has lower PW values and the GFS has higher LI values; hence the GFS has an earlier onset to precipitation.
About half of 09 UTC SREF members has precipitation by 06 UTC 12/17 (Monday), similar to GFS. NAM has 12 UTC precipitation onset. Even in the 09 UTC SREF the NAM members have a later onset than the RSM members: after 12UTC compared to before 06 UTC. Monitoring of the accuaracy of the PW initialization is critical to determining the timing.

The high-resolution ensemble from GSD, which is initialized with NAM has a slightly earlier onset than the NAM with precipitation beginning between 06 UTC and 12 UTC 12/17 (Monday). So, only the NAM of all 12 UTC and 09 UTC forecasts has precipitation onset after 12 UTC.

In all cases, the early stages of the storm are forecasted to be the least intense periods of precipitation. Precipitation accumulation by 00 UTC 18 Dec (Tuesday) is forecasted to be at most 1", but much more frequently in the 0.3-0.5" range. Most of the model guidance has the heaviest precipitation occurring between 09 UTC 12/18 and 00 UTC 12/19 with as much as 3.5" but forecasts in the range of 1.5-2.5" are more common.

The PSG Analgue forecasts have 12% of measureable precipitation in the 24-hr period ending 00 UTC 12/17 (Monday), 78% ending 00 UTC 12/18, 88% ending 00 UTC 12/19, 76% ending 00 UTC 12/20, and 78% ending 00 UTC 12/21. The periods of highest likelihood of heavy precipitation are the 24-hour periods ending 00UTC 12/28 and 00 UTC 12/19 with approximately 58% chance of >1.0" and approximately 30% chance of >2.0" in both periods.

Taking into account all of this guidance, storm totals by 00 UTC 12/19 appear almost certain to exceed 1" and very likely to exceed 2". NWS forecasters are expecting snow levels to be lower with the first system than the second system. In the first system, the snow levels could begin as low as 3500 ft.

3-5 day forecast
Another very large trough is forecasted to move into the NW after 12 UTC 12/19 (Wednesday). There is less agreement about the timing, orientation of the height field, and PW values. The timing of this system is difficult to pin down. It is possible the main system could be preceeded by small short-waves that were shredded from the main short-wave, resulting in essentially no break in precipitation, especially at high elevation. Also, this system could have slightly lower snow levels if a bit of cold air advection sneaks in behind the first system.

5-10 day forecast
Another potent wave is forecasted to hit the ARB area Friday night into Saturday. The GFS forecast of this event has more of an appearance of a "Pineapple Express" configuration.

10 day and beyond forecast
PSD Reforecasts indicate a ridge building along the West Coast, but the flow remains very zonal and strong throughout the central Pacific. This appears to be linked to intensification of a tropical 150 mb streamfunction ridge in the Western Pacific above a region of heavy, widespread precipitation.

Chris Anderson