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HMT forecast discussion for Thursday 6 December 2007

Operations associated with IOP-1 are well underway. As of 19Z, mostly light-moderate amounts (0.30 in. at BLU) are indicated by the radar estimates and gage reports to the north and west of ARB. Stronger echoes (>30 dBz) moving from the west over the ARB at this time, which suggests the ~18 h period of most intense precipitation has begun.

The UA flow has moderate westerly jet with 250 mb winds of ~120kts over the HMT region at 15Z (+3 h NAM). The westerly flow extends down to ~700 mb, turns to WSW at 850 mb suggesting WAA, and is SSW over the Central Valley as the flow impinges against the Sierras. The westerly flow is ahead of a sharp tough that is intensifying as it digs down the West Coast to a NAM position over SAN by 12Z Sun Dec 9th. 500 mb trough axis crosses over ARB 12-15Z 7 DEC, with FROPA 09-12Z 7 DEC. GFS is the fastest, and I tend to favor GFS on timing for West Coast systems (better numerics, better handling downstream energy propagation from Pacific). With FROPA and commencement of CAA, precip will become more showery and snow levels should drop to 4.5K.

Look at the 12Z 6 DEC NAM and GFS and 09Z 6DEC SREF (www.spc.noaa.gov/exper/sref) runs suggests a shutdown of ARB precip around 15Z 7 DEC 2007. NAM has MAX ARB storm total of just under 2.00 inches, and the coarser GFS has only a tad less. MAX SREF uncalibrated 24h Prob>1.00 in. = ~0.70 (Prob>2.00in. =0) ending 15Z 7 DEC, suggesting a Prob>1.50-2.00 in. ~0.70 in favored orographic regions. The HMT 3km, lagged ensemble for the 24 h of 18Z 6 DEC to 18Z 7 DEC shows Prob>1.00in. > 0.50 on favored ridges. I leave SREF downscaling to the local hydromet experts; ARB is not an area where I can claim experience.

On the longer range, the NAEFS plumes for SAC suggest zero precip for HMT thru Tues. DEC 11 as CA is dominated by N-NW flow aloft. A few members show a weak S/W trough embedded in WNW steering flow on Wed. DEC 12, but precip chances look nil as the system propagates into difluent steering over CA. The outlook for week two suggests that after the weekend of DEC 15-16, especially starting Mon-Tues DEC 17-18, COULD bode well as a few outlier NAEFS members show ppt >25 mm and a strong 500 mb low off the CA coast. The time frame is several days beyond the predictability limit for synoptic systems. That caveat in mind, the NCEP MREF members for 12Z DEC 16 (+day 10) (www.meteo.psu.edu/~gadomski/ENSHGTNH_12z/ensloopmref.html) are consistent with building a 500 mb LW ridge just east of the dateline. The pattern deepens a significant trough near 160W in a few members (say ~20 percent), which progresses toward CA by day 12 (12Z DEC 18) and has strong SW flow ahead of it. The scenario has the potential for a more classic Atmo. River set up. The CMC ensemble (http://www.meteo.gc.ca/ensemble/) from 00Z 6 DEC is in a general agreement (~15 percent of members look favorable). The situation should come into better focus by the end of my shift (DEC 12th).

Steve Mullen
20:00 UTC 6 December 2007