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HMT Forecast Discussion for Sunday 9 December 2007

All models point to the next 6 days being dry over HMT domain, but a more favorable large-sacle should set up starting approximately 2007121600, and it should last until the Holidays shutdown around 2007122200 (1600 PST Friday DEC 21)

The most current 2007120912 NAM, 2007120912 GFS and ECMWF runs remain adamant about keeping the ARB dry through 2007121612; the CMC calibrated probabilities and ESRL refcst analogs support the dry interpretation. Although the HMT domain will remains under dry N-NW flow aloft during the next 6 days, the flow becomes more W-SW in week two as a ridge develops near 170W, and a downstream trof around 140W depending upon model and ens member. The 170W ridge, early in week 2, has been a consistent signal in all models runs the past 3-4 days; it is the evolution of the downstream trof and associated sfc cyclones that varies considerably.

The 1200 GFS has precip reaching BBY-CZC btw 2007121700-2007121712, and ARB 12 hours after that; its primary sfc cyclone tracks NE into Van. Island. MAX 12 h precip over ARB is btw 1.0-1.5 in. starting 2007121712. The 2007120912 ECMWF is faster (a significant change from the 2007120900 run that was later) and starts precip over BBY-CZC 12 h earlier. The EC run deepens a secondary 995 mb low at 39N 128W at 200712172 that tracks ESE, and 12 h precip max approaches 2 in. over ARB.

The NAESF plumes continue to show the period 2007121712-2007122012 as wetter than normal over KSMF. The calibrated CMC ens precipitation fcsts suggest the 5-7 day period starting 2007121600 as the most likely to be the wettest over HMT domain, with the 24 period 2007121712-2007121812 looking the most promising; thus the CMC runs support the GFS and ECMWF deterministic solutions early next week.

In summary, the 3-4 days starting 2007121612 appear very promising. A candidate IOP could begin as early as 2007121618-2007121700 (Sun 1000-1600 PST). The situation will become clearer in the next 2-3 days as the models and ensemble should begin to converge somewhat in their synoptic details.

Steve Mullen
200712091930