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HMT forecast discussion for Tuesday 4 December 2007

There was some juice left in the plume as it finally drifted south over the HMT area this morning. The 1200 UTC sounding at SFO jumped to 1.5" of precipitable water with RNO up to 0.70". The Blue Canyon METAR had 0.50" as of 1800 UTC this morning. Lots more to the north and northwest over northern California. Radar imagery indicates the heaviest of the precip continues to drift to the south of the area so most of the rain should have occurred already. As for the next event, some conflicting model forecasts from 0000 UTC with the GFS the least favorable, driving the next wave too far south. But the 1200 UTC GFS is farther north, in agreement with other 0000 UTC runs that continued to maintain a stronger system more favorable to an event.

Right now there is an elongated upper level low in the Gulf of Alaska with the upper level ridge now elongated and positioned to the north of Alaska. The next system is a shortwave trough now located near 45N/165W and moving ese, ahead of a strong wave to its west that will lift to the northeast to a position west of Alaska, building the ridge behind our approaching shortwave for Thu-Fri. There is a southern wave near 30N/170W that is heading off to the south and will form a closed low north of Hawaii by Wed. Model forecasts show a brief tap into the moisture plume that will push to the north of this upper low by the progressive northern wave but this then gets pinched off by late Thu/6 Dec. Some details about the model forecasts...as noted above the 0000 GFS plunged the approaching wave to the south with not much sw flow at 700 mb, and at most about an inch of precip but more split around the HMT, falling mostly between 1800 UTC Thu and 1800 UTC Fri (6-7 Dec). But the ECMWF maintained a more northerly track, with 30 kts of wsw 700 mb flow at 0000 UTC Fri before the 700 mb low ends up pretty much right over the HMT area by 1200 UTC Fri, then some wrap-around as it heads to the east. Max precip through the event is ~1.5", with the heaviest falling from about 2200 UTC/Thu to 1500 UTC/Fri. The Canadian Global model from 0000 UTC is like the ECMWF, and quite generous with the precip, with up to 2.5". The UKMET from 0000 UTC has similar timing but is not as deep as the Canadian and ECMWF models. The 0000 UTC NOGAPS looks more like the ECMWF. The position of the approaching shortwave is very similar in the GFS and ECMWF all the way through 1200 UTC/Thu, then the GFS drops the wave much farther south. The 1200 UTC runs coming in are similar for the UKMET and Canadian, though a little faster in pushing the system east in the Canadian model so max precip is down to ~1-1.5". The big change is with the GFS, which is now farther north and deeper with the upper low, resulting in much better sw flow, reaching 25-30 kt at 700 mb at 0000 UTC/Fri, with the 700 mb low then passing just to the south of the HMT area by 1200 UTC/Fri, after which a period of wraparound moisture occurs in n to ne flow with snow levels droping. The max precip is more in the valley than into the higher terrain of the HMT, ranging from ~1.75" near SAC to ~1.5" in the higher terrain. The period of precip is rather long, beginning with light precip ~1200 UTC/Thu, heavier late Thu and overnight, then wraparound on Friday before ending ~0300 UTC/Sat/8 Dec. The 1200 UTC NOGAPS is similar to the 0000 UTC run, and close to the latest GFS, with an estimated 1-1.5" total precip. The 1200 UTC ECMWF run just in is still favorable, with a little farther northern position for the upper low that is not quite as strong as the GFS, but owing to its slightly more northern position gives about the same amount of precip as the GFS, with up to 2" just to the south of the HMT area. The 1200 UTC NAM is both weak and far south, with maybe 0.20" of precip in the area, but my faith in this would be quite a bit less than any of the global runs. There is a good deal of spread in the 0900 UTC SREF ensembles, but the ensemble mean does have a pretty decent event, beginning about 1500 UTC/Thu and ending late Fri, with ~0.5" of precip. The 0000 UTC GFS ensembles also have a lot of spread in the precip amounts for the storm, from very little to about what the operational run produced. On the other hand, the latest 1200 UTC GFS ensembles are more in agreement on a wetter system, with only one (of 12) member having very little for the event. Bottom line is this is not a huge system, it may be rather compact as it hits onshore, but if it takes the right path has potential for interesting amounts of precip, especially if it picks up something from the Hawain closed low. Another potential for more precip is that the system could linger in terms of wraparound longer than Friday as it deepens to the south and east. However, Its relatively size as it hits the coast implies that potential track changes could result in vastly different amounts of precip. Consensus from the conference call suggests that while precip could start as early as 1500 UTC/Thu the main precip would get going later, perhaps 2100-0000 UTC, with a trough passage 0600-1200 UTC/Fri, and then most of the precip ending. Expected snow levels will initially be quite high, around 8000 ft, but fall below ~4500 ft as the trough/upper low nears around 0600 UTC/Fri and continue some lowering beyond this time.

Once the system passes the HMT area cold northerly flow ensues as the upper level ridge builds west of the West Coast, with the trough digging into the Southwestern CONUS and slowly moving to the east. Timing differences then show up between when the first in a potential series of waves that will traverse the Pacific threaten the HMT area. The Canadian would have the first one as early as late Tue/11 Dec, the GFS more like Wed, and the ECMWF delayed until Thu/13 Dec. By 240h there is quite a large difference between the 0000 UTC model runs off the Pacific in both timing and intensity of troughs. The Canadian ensemble from 0000 UTC is trending in a number of members towards a bigger trough that lingers off the West Coast in the 15-19 Dec period. The GFS ensembles from both 0000 UTC and 1200 UTC look more progressive with weaker (though rather moist in some cases) shortwave troughs and considerable spread in timing in the members for this same period. It appears overall that there would be rather low confidence in the forecast in the longer range. This shows up quite nicely for the SAC meteogram as an enormous spread in potential daily precip for the period 12-19 Dec.

ed szoke NOAA/CIRA/GSD