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HMT Forecast Discussion for Friday, 7 December 2007

The HMT domain is beginning to experience strong CAA and drying as the UA trof for IOP-1 moves out of the HMT domain and the UA flow is transitions to N-NNW. Continuous precip for KBLU has stopped by 18Z; 24h total at KBLU for 18-18Z period was 2.96 in. Yesterday's forecast was too dry (up to 2.0 in.).

The 12Z DEC 7 NAM and GFS runs show a 500mb trof that is over BC this morning will move S, embedded in the northerly flow. It will be over HMT by late Saturday (00Z DEC 10). This system could be prodigious precip producer over the mtns of SoCal and the Baja the following day, but it looks harmless over HMT/ARB. The NAM shows widely scatter instability showers over the ARB, and while the GFS is a tad more aggressive in coverage and amounts (~0.05-0.10 in over ARB), it is not encouraging. Moist, low-level flow from the Pacific will have problems making it into the ARB domain. I am downplaying the system.

For the remainder of week one, the 12Z DEC 7 GFS shows zero precip for HMT thru Tues. DEC 11 as CA is dominated by N-NW flow aloft. By Wed. DEC 12, the system discussed yesterday shows minimal promise. Its track and strength bears a "some" resemblance to IOP-1, but the question is moisture; there is little evidence of significant moisture transport into CA as has the system's trajectory hugs the West Coast before it digs offshore to west of Baja by Friday 12Z DEC 14th. The 00Z DEC 7 ECMWF T799 deterministic run agrees with the GFS solution from fresher IC's. Paradoxically, the ESRL/CPC ens. refcsts from 00Z DEC 7 IC's (http://www.cpc.ncep.noaa.gov/products/predictions/short_range/tools/cdc.pub.html) show day 6-10 (DEC 13-17) with a slightly above normal signal (50-60 percent prob of being in the upper tercile). The ESRL/CPC wet outlook holds into week 2 (days 8-14, DEC 15-21). However, the NAEFS plumes from 00Z DEC 7 runs for KSMF now show DEC 18-20 as the dates with the most promise (http://www.meteo.gc.ca/ensemble/EPSgrams_e.html), or 1-2 days later than yesterday's 00Z runs. Survey of NAEFS ensemble members suggests little real hope over the next 9-10 days.

Of course, the situation will be monitored over the weekend just in case the progs begin to hint at an unforeseen (at this time) regime transition.

Steve Mullen
19:00 UTC 7 December 2007