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HMT forecast Discussion for 12/14/2007

HMT 12/14/2007 Forecast Discussion
Current Conditions
At 16 UTC, clear skies and chilly. Northwest flow is present through the entire depth of the atmosphere as a large ridge is positioned off the Pacific NW coast. The ARB is embedded within a temperature gradient associated with the NW geostrophic flow.

0-72 hour forecast
A sharp amplitude short-wave trough is forecast to move onshore over the Pacific NW overnight tonight. Model guidance remains consistent among the many current forecasts and past forecasts with the wave staying north of the ARB, the surface front becoming diffuse as it approaches the ARB, and PW values less than 1" offshore of CA. Add it all up, and precipitation is expected to be light and confined to N CA.

3-5 day forecast
This is when the fun begins. 00 UTC prognosis from GFS ensemble, CMC ensemble, PSD retrospective ensemble, and 09 UTC NCEP SREF show a potent short wave crashing into the Pacific NW overnight Sunday into Monday. Despite a forecasted short-wave track similar to the current scenario, much more precipitation is forecasted for the ARB region. The differences between the forecast and current scenario are higher PW values (in excess of 1" in the forecast) and the possibility of multiple waves passing over for while the high PW values are in place off the CA shore.

Timing of the onset of precipitation is still murky. Most (about 2/3) forecasts suggest an event that begins after 12 UTC on Monday. Precipitation appears to be nearly continuous in most forecasts through 00 UTC Thursday, with the heaviest amount forecast for the 00 UTC Tuesday to 00 UTC Wednesday period. QPF in a few GFS members exceeds 4" in this 24-hour period. Similar amounts are evident in some members of the Canadian ensemble. In all, approximately 1/3 of the global ensemble forecasts (CMC + GFS) have 24-hour amounts exceeding 4" for the 00 UTC Tuesday to 00 UTC Wednesday period. The PSD Analogue ensemble, based upon the GFS ensemble mean precipitation, forecasts 15-20% chance of exceeding 2" in that period and 80% chance of exceeding 1".

In my experience over the past two HMT field projects, large waves like these often come through with a number of smaller waves, as if the large wave were torn into smaller pieces. It's probably premature to say the period of heaviest precipitation is highly likely to be 00 UTC Tuesday to 00 UTC Wednesday given the tendencies for waves to split apart. With the pool of high PW air offshore, a burst of heavy precipitation could occur in any 12-hour period after 12 UTC on Monday and before 00 UTC on Thurday.

Reno and Sacramento feel the snow levels will be 5-6K ft at the beginning and drop to 4500 ft during the event, though Reno thinks it is possible it could drop a little lower due to strong, synoptic scale ascent within the system. Their general impression of onset and heaviest period of pr
ecip is in line with the model discussion above, ie heaviest period Monday afternoon through Tues
day afernoon.

5-10 day forecast
Beyond Thursday, global model guidance indicates the potential for more quick moving waves coming through the ARB area through the weekend. The ensemble trend after the weekend is to build ridging in the Gulf of Alaska. In fact, the PSD reforecast ensemble mean generates intense convection in the Tropical western Pacific that creates zonal flow with high cyclone activity over the western Pacific that cause ridging as they approach the Gulf of Alaska.

10 day and beyond forecast
Something is bubbling in the Indian Ocean. Perhaps we'll hear about another MJO very soon, though the PSD reforecast ensemble mean indicates a ridging pattern and has active convection in the western rather thean central Pacific which is more in line with what is expected of La Nina conditions.

Chris Anderson, NOAA/GSD