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HMT forecast discussion for Saturday 5 January 2008

The event continues across the ARB with a moist westerly flow now in place behind yesterday's strong wave. A brief review of yesterday's storm: the power to the ETL gage at BLU went down just as the precip began to pick up, totals at the KBLU METAR indicate 2.56" of melted, but not sure how trustworthy that would be. WFO/SAC in the telcon mentioned the reports of more than 4 feet of snow at nearby ski areas and that I80 was shut down for a time late yesterday and overnight. They noted that some spotters at 6000 ft reported nearly 4 ft of snow, with an automated sensor near Lassen Peak reporting 9" of liquid precip. Automated gages include 1.51 in 24-h at SAC, and a max near the ARB of 4.92 at Huysink in the 24-h ending at 7am local time. The front passed KBLU a couple of hours faster than yesterday's forecast, according to the METAR obs FROPA was near 22z, with temperature dropping to 34 quickly and S+ by 2235z. Overnight KBLU reported some freezing rain and sleet at times, if this is accurate not sure. Peak wind gust yesterday at KBLU was 180/63 kts at 2113z. This morning KBLU has been down since ~1330z, the last ob just before then had 1 mi in S-, 210/14 g30kt, and a temp of -3C (27F).

Current analyses show a very strong jet is still in place extending from 100 kts+ in sw UT west across central CA, then wnw to an elongated jet near 40-45N stretching back across the Pacific with a 170-180 kt jet max from ~140W to the Dateline, the the jet bends to the sw into the south-central Pacific, where there is a moisture plume. This area of moisture to the nw and w of Hawaii does get tapped by the systems next week. At 500 mb an upper low remains over Alaska with the trough extending south off the West Coast. Strong w-wnw flow impinging on the coast with embedded shortwave troughs. One of these is a potent one now approaching the Northern California coast with an organized area of echoes that will hit the ARB this afternoon and tonight. Snow level will be well below KBLU through this with the NWS predicting about 2 feet more by Sunday morning. One last wave later Sunday into Monday with precip finally decreasing after 12z/Mon and some clearing possible.

The next wave does move rapidly across the Pacific, arriving across the ARB on Tue/8 Jan. Precip totals from this event vary some but it looks like max amounts in the 1.5-2.5 inch range would be likely. Startup of precip varies some, the GFS and NAM have some precip beginning a little before 12z/Tue, but other models are a tad later. Latest ECMWF model looks like it has roughly similar timing for this and the next wave to what the GFS is forecasting, and the Canadian Global is also in general agreement. At this time too tough to call, the plowing out of the radar site may determine when folks could get there. This system winds down by ~06z/Wed, then the next wave comes in right behind it with precip beginning around 00-06z/Thu/10 Jan, lasting again about 15-18h. More uncertainty with this wave, but an intriguing aspect of it is that it too could tap the elongated moisture plume. Snow levels with both waves should rise considerably from where they are now, but at least for the first one KBLU could be close to the transistion zone, and this discussion in the telcon indicated that the GFS probably brings the snow level above Blue Canyon, but right now thinking is it could be quite close to KBLU.

After the potential Thursday event ridging builds up near the West Coast with a mean trough over the CONUS. One last potential wave is possible before this occurs though on Sat/12 Jan, though ensembles indicate this has only about a 30% chance of bringing anything of interest to the ARB. The ensembles generally favor an extended dry period then from 13 Jan through 20 Jan. There is a chance, shown in some members (especially in the Canadian ensemble system) that the CONUS trough could retrograde enough to begin to extend back across the West Coast during the week of 14-20 Jan, which could make things interesting again, but this is quite uncertain at this time.

ed szoke NOAA/CIRA/GSD