HMT forecast discussion for Sunday 2 December 2007
The main changes for this forecast cycle include increased optimism for a potential event late Thursday through Friday and perhaps into Saturday (6-8 Dec), and then a pattern that could keep the area vulnerable to systems out of the northwest in the longer range. Right now the changes that we've been talking about have occurred, with the upper level ridge now pinched off over northern Alaska, while a strong wave sits below it near 150W/40N. To the east of this strong system is a shortwave trough that will hit the Oregon coast shortly with increasing wind and precipitation. The heavy precip extends from Washington south to near Eureka at this time, and only a slight southward shift is expected through most of the event through Monday, with many inches of rainfall up the coast through Washington. There could be a little snow shower activity through the higher HMT elevations this afternoon through Monday. It looks like a better chance of some rain/snow would prevail on Tuesday as the remains of the plume sag south. At this time it still looks like the plume will be much diminished by this point but would not be surprised to see something in the order of 0.25" or so in some areas on Tuesday, or at least some more substantial echoes in the area. Until then high winds will be the main threat for the higher HMT areas. The 0600 UTC high-res NCEP WRF windows both have very little precip for the HMT area, but from 7 to 9 inches total through 1200 UTC Tue from Eureka northwards.
The upper level flow is forecast to shift to west-northwest by Wed/5 Dec with drier conditions moving into the area. However not far away is another wave traveling across the Pacific that showed signs of deepening while still approaching the California coast yesterday, and this trend continues with todays 0000 and 1200 UTC model runs. For instance, today the GFS 1200 UTC run now has a pretty strong upper low about to hit north-central California on 1200 UTC Fri/7 Dec. A new twist also is that this system is forecast by both the GFS and the 0000 and now 1200 UTC ECMWF to slow down and head more to the south towards Southern California with time, keeping the HMT area potentially in precipitation with southwest flow for a longer period, perhaps from late Thursday into Saturday. There is some risk now that the heaviest precip could actually be too far south, but other models (Canadian, for example, and to a lesser extent the UKMET) are quite a bit farther north and do not close the system off over Southern California. GFS 0000 UTC ensembles favor something eventually cutting off but only slightly over a more progressive but deep trough. The Canadian ensemble set from 0000 UTC favors a more progressive trough with the main precip threat for HMT on a Friday time frame. One interesting note is that there could be a tap into some moisture associated with an upper low that is hanging out nnw of Hawaii, shown best in the GFS PW loops at the U of Hawaii site.
Beyond this period both ensembles keep the ridge far enough west off the coast in enough members to have some potential for the following week (9-14 Dec). A similar message from the latest GFS 1200 UTC ensembles.
ed szoke NOAA/CIRA/GSD