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HMT Forecast Discussion for Saturday, 8 December 2007

The HMT/ARB region is currently between two 500 mb S/W trofs, underneath N steering flow aloft. IOP-1 was associated with first trof, located over KLAS at 15Z. The upwind trof is over the CA-OR border at 15Z, and it is fcst to be over ARB domain later this afternoon. The system will be associated with scattered instability showers, mostly over the foothills and higher terrain of ARB. The 2007120812 NAM shows only isolated showers, while the GFS is once again more aggressive in coverage and amounts. I think the NAM offers the best solution for ARB, as there is little available moisture. MAX QPF for the ARB, if any precip occurs, should be less than 0.05-0.10 in. range at the wettest locations.

Both the NAM and GFS are adamant about keeping ARB dry from 2007120912 to 200712120 (end of NAM run). No arguments here, as the HMT domain remains under N-NNW flow aloft and low-level offshore geostrophic flow, effectively shutting off any Pacific moisture into ARB. Confidence is very high for dry early Sunday (tomorrow) through early Wednesday LST.

On the longer range, the 2007120812 GFS shows the 500 mb steering flow becoming NW starting Wed (2007121212) as a modest S/W trof comes onshore over WA-OR. A more vigorous systems crashes into the PNW on 2007121412, as the offshore UA ridge continues to flatten. By the end of the run (2007121712), the GFS shows a vigorous 500 mb closed circulation off the CA coast with a good moisture tap from low latitudes. The 200712012 ECMWF run (The preliminary discussion erroneously referred to the 00Z EC run as the 12 Z run.) has the same broad message as the GFS in terms of dry through the end of next week, but it erodes the offshore ridge 1-2 days faster. The EC has precip starting over HMT-BBY/CZC by 2007121700 (1600 PST Sunday) and ARB by 2007121800; most importantly, it shows signs of a moisture tap down to 30N, generally consistent with the GFS idea.

The 2007120812 GFS ensemble (http://www.meteo.psu.edu/~gadomski/ewallmref.html) for 500 mb height suggests dry through 2007121312 (0400 PST Thursday). The 2007120812 CMC ensembles
(http://www.meteo.psu.edu/~gadomski/ewallmref.html) now have no members that suggest wet, but so it too screams dry through 2007121312 (0400 PST Thursday). The NAEFS plumes from 2007120812 IC's (http://www.meteo.gc.ca/ensemble/EPSgrams_e.html) suggest the period 20071217-20071221 has the highest probability of being wet for KSMF. Calibrated probabilities from the CMC system (http://www.meteo.gc.ca/ensemble/charts_e.html?Hour=192&Day=0&RunTime=12&Type=prob)
begin to raise the likelihood of 12 h precip to near 20 percent by 2007121612. The ESRL ens refcst-analogs (http://www.cdc.noaa.gov/reforecast/narr/) from 2007120800 IC's indicate an approx 40 percent chance of >50.0mm over ARB during the 5-day period 2007121300-2007121800, and approx 60 percent chance of >50.0mm over ARB during the week two period 2007121500-200712200; the upswing in probability seems consistent with the latest GFS-EC runs and conservative CMC calibration.

The overall picture that I piece together from the GFS-EC runs, MREF-CMC-NAEFS members and ESRL refcst-analogs is that next 6-7 days will be dry, period, with a minimal (2-5 percent) chance of precip btw 200712091200-200712130000, but the week of 15-22 DEC looks far more promising for a candidate IOP system.

Steve Mullen