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HMT forecast discussion for Wednesday 9 January 2008/1945 UTC

A few snow showers over the higher terrain of the ARB this morning ending at KBLU at 1530z, so the end of IOP5. The KBLU METAR recorded a 24-h total of 0.97" at 12z (quite possibly an underestimate since it is a tipping bucket gage), with off and on S- accumulating another 0.01" in the 3h ending at 15z. Precip remained as snow through the event with temperatures at KBLU rising to -1C then peaking briefly at 0C at 23z. Precip timing during the event was about as forecast, lingering maybe a few hours longer with S- (0.18" was recorded by KBLU between 06-12z), but otherwise the heaviest period was as expected, with KBLU have 0.28" 18-21z, 0.23" 21-00z, then 0.13" over the next two 3-h periods. The automated mesonet at BLU was not reporting, but other ESRL mesonets were available. The one at Big Bend (elevation 1739 m = 5706 ft) showed precip (on the hot plate sensor) beginning to accumulate ~17z, tapering off at ~08z and pretty much over by 12z, with ~2 inches total. Temperatures at Big Bend rose to close to 0C just before 00z/Thu, then rising pressure thereafter. Not sure if the surface front passes around this time or not. The KBLU observations did not have any really notable wind shift, but rather a gradual turning from 170 to 190 from 23z through 04z. At Alta (elevation 1085 m = 3600 ft) the hot plate was not working, apparently, but the tipping bucket has precip beginning near 18z, with a steady climb to ~1.4" by 07z/Thu, then light precip till 13z with 1.6" total. Note that the 24-h totals from the ALERT gages were somewhat different, at BLU only 0.77", with a max at Huysink of 1.52", and 0.20 at SAC (0.32" from the KSAC METAR). Radar loop indicated the heaviest band, with reflectivities up to 50 dBZ (perhaps contaminated by bright band?), slipped just south of BLU around 23z. Trough passage at OAK, per the special soundings, looks to be ~01z at 700 mb, while at Sloughhouse it occurs at 04z/Thu. Max PW from the OAK RAOBs was 0.79" on the 18z/Tue sounding. So it appears the NAM forecast amounts were not too bad for this event, with the GFS/EC just a little low but close. Overall, a pretty good forecast for this event by the models.

As for the next wave, looping of water vapor satellite imagery with 500 mb analyses in the Pacific indicates a gradual weakening of the approaching wave, which in the analysis is a broad shortwave trough centered near 45N/155W (north of Hawaii) this morning, with the lead cloud shield approaching the OR/Northern CA coast. There is a moisture tap back to nw of Hawaii, but the plume is not as substantial as earlier in the week. Another, weaker wave is behind the trough centered near the Dateline. It is a wave behind this system that is forecast to split off from the main flow and become a cutoff low that lingers well off the West Coast (near 30-33N/150W) next week. Overall the models are consistent in forecasting a less organized and overall weaker system for Wed night through Thu, with precip amounts most likely remaining below IOP criteria. The 12z NAM begins the precip about 03/Thu/10 Jan, with 0.03" from 03-06z, 0.27" total by 12z/Thu, then 0.4" total by 00z/Fri, but lingering cold advection precip for 0.72" by 12z/Fri. More to the north with >1.3", and ~0.85 nearby to the west of ARB, but the highest amounts of 2.5" near OR/CA border on coast. 09z SREF: mean in 0.25-0.5" range for the entire event through 12z/Fri/11 Jan; individual members vary from 0.1-0.25 to 0.5-0.75" range. 00z Canadian Global, ~0.5" w/similar timing to NAM. The latest GFS is quite disorganized with the precip, with similar timing to the NAM but only 0.24" at KBLU through 12z/Fri. The 00z ECMWF run has ~0.3" at KBLU for the entire event. NOGAPS looks similar. Our local models have ~0.3 to near 0.5" by late Thu for this event. This event will be warmer, although certainly beginning as snow at KBLU elevation, then likely over to rain showers on Thursday with the freezing level possibly rising to near 8000 feet, at least on the NAM BUFR soundings. It is likely the real snow level will be lower, perhaps in the 6000-6500 ft range. General telcon agreement that this system should remain below IOP criteria, with only an outside chance the jet combined with the moisture plume could sneak out more precip than expected.

The trailing trough now near the Dateline mainly stays north of the ARB through the weekend, with the 12z Canadian Global model pretty much the only one bringing some light precip to the ARB. No ensemble members though have any precip over the weekend.

In the longer range, the 12z GFS continues the forecast of a high-amplitude ridge building just off the West Coast for all of next week, with the cutoff low staying well to the west, and a deep trough over the CONUS. The 00z ECMWF and Global Canadian model forecasts are pretty similar to the latest GFS. The 00z NOGAPS brings the upper low MUCH closer to the coast by midweek, but even with that the precip stays just off the coast. The NOGAPS solution is seen in some of the GFS and Canadian 00z ensemble members, and in several of the GFS 12z ensemble members, but for the most part only a few members bring precip onto the West Coast. This closed low will have to be watched since these things are generally difficult to forecast. Otherwise, the models keep it dry with the mean trough over the CONUS, retrograding by the following week (20-24 Jan) in at least some of the ensemble members. If this retrogression were to be even more prominent then that could be another way to eventually get the ARB back into precipitation. Alternatively, some members do the opposite, gradually shifting the CONUS trough eastward by the week of 20-24 Jan, with a shift eastward of the upper level ridge as well.

ed szoke NOAA/CIRA/GSD