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HMT forecast Discussion 12/12/2007

HMT 12/12/2007 Forecast Discussion
Current Conditions
At 17 UTC, Low clouds are hanging around the ARB and eastward, but are not production precipitation. They are an indicator of a short-wave trough that at 12 UTC was positioned off the CA coast directly west of ARB. Associated with the trough are high mid-level relative humidity values. The cyclonic vorticity advection and most of the forcing for ascent have passed by the ARB by now. The main impact of this trough will be to continue cold air advection into the ARB region.Chances of precipitation events of interest to HMT over the next 48-hours are nil.

0-72 hour forecast
ARB will be in a pattern of anticyclonic advection throughout this period. Additionally, cold air advection will occur during most of the period, with a bit of warming at the very end of the period ahead of the next short-wave trough. Chance of HMT precipitation events is zero.

3-5 day forecast
A potent short-wave trough is forecasted to hit the Pacific NW shoreline in the 3-4 day period. GFS, CMC, and UK model guidance, including ensembles, take the short-wave track Idaho, draping behind a diffuse frontal zone that makes it to the ARB in some projections but not others. Adding more gloom to the forecast are PW values not exceeding 1" along the so called front. The chance that this period will have an precipitation event of interest to HMT is no more than 5%.

5-10 day forecast
Good news for HMT during this period perhaps. A series of potent short-wave troughs are evident in GFS and CMC ensembles. The first of these waves may affect the ARB by late Monday. The good news is that a pool of PW vales greater than 1" develops offshore and remains entact as multiple waves come through the area. It appears there could be two strong waves separated by 2 days, though my experience looking at GFS/CMC over the past couple of years would suggest the waves will be more fragmented. The chance for an HMT precipitation event is very high. I would almost call it a certainty, so give it a 95% chance.

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.