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HMT forecast discussion for Sunday 6 January 2008

Lull underway across the area this morning but the next and final wave in this system has echoes just now reaching the coast. Freezing level remains low, at ~3300' in this morning's OAK sounding. The snowfall in the forecast for BLU area out of WFO/SAC is in the order of 6" or so more with the snow ending late tonight. Snow should diminish according to the latest models by 12z/Mon with little further accumulation on Monday. Still not sure of total maximum precip and snow amounts; KBLU still not reporting. Huysink gage (west of KBLU) had 1.64" in 24-h ending at 7am local time this morning. SAC area around 0.5". Heavenly ski area at Lake Tahoe reported 108" storm total so far, with a breakdown of amounts of 12"/7am 4Jan, 36"/7am 5Jan, and 60"/7am 6 Jan, this on the RSN site, on their own homepage they list "only" 64-96" storm totals. WFO/SAC estimates from reports a little higher than BLU that perhaps in the order of 4-5 ft of snow could have fallen so far at the radar site.

Biggest change in the model forecasts is that all models generally predict less precip for both events this week. In fact the forecast amounts make them marginal for IOPs. There still is a nice pool of moisture west of Hawaii and both systems tap this, but the likely reason for less precip appears to be the systems are both fast-moving and weaker than in yesterday's runs. Perhaps playing into any decision on what to do would be how soon the radar site gets plowed out, but also the long range forecast that looks quite dry after these two storms, at this point through ~20 January.

The trend to lesser precip amounts was found in pretty much all the 00z model runs. For the first wave the focus for heavier precip was more in Northern California, with the ARB area in the GFS getting ~1-1.25" max. A little more is forecast by the 12z NAM, but the 12z GFS has a little less, just under an inch, with a slightly weaker shortwave than the NAM. SREF 09z forecasts have a mean precip amount of about 0.75" for event 1. Models generally agree on light precip beginning by 12z/Tue, but the main part of the system is from 18-21z/Tue thru 06z/Wed, then ending.

Clearing is then forecast between this and the next storm, which is forecast to be even a weaker wave in both 00z runs and the latest 12z forecasts. The 12z GFS has precip with storm 2 beginning near 00z/Thu with max in the 12-h period ending 18z/Thu/10 Jan, but only about 0.5" or so. This is similar to the 00z GFS, but the Canadian was juicier with a longer duration event, and also to a lesser extent the NOGAPS. The ECMWF (did not see precip amounts tho for this) does not seem too impressive with the event.

There could be a close call with another wave on Saturday into Sunday (12-13 Jan), but this looks unlikely overall. Then rather good agreement of a ridge building along the West Coast with mean trough over the CONUS. This CONUS trough then gradually retrogrades, with the ridge shifting west a bit, but not enough to get the ARB into any precip of note through the 15 day period (ending 22 Jan). Latest 12z GFS ensembles have a few members with more uncertainty for the week of 14-20 Jan but most members remain dry in the ARB region. This is also seen in the ECMWF 12z run (and also the latest GFS deterministic 12z run) that both have a detached closed low slowly moving towards central California by day 10 (12z/16 Jan).

ed szoke NOAA/CIRA/GSD