HMT discussion for 10-March-07
A nice plume of moisture in the Eastern Pacific stretching back to Hawaii but unfortunately directed even farther north than yesterday towards Vancouver and Washington. The plume will sink to the south by late Sunday into early Monday as the wave moves onshore in the Pacific Northwest but precipitation will remain well north of the HMT area. Slight ridge building in California behind this wave but then the ridge is flattened a bit as a strong jet and associated fast-moving shortwave moves around the Alaska upper level low in zonal flow across the Pacific Northwest on Tuesday. Precipitation with this feature remains well to the north of the HMT. This is the wave that then dives into the eastern CONUS to create the trough there by later in the week with the substantial upstream ridging that then takes place along the West Coast. Excellent model agreement through the week with this scenario. So, overall, not much different from what has been talked about the last couple of days, dry for the HMT area.
A slight wild card on the 12z deterministic run is that it takes a piece of the southern energy from a strong wave moving across the Pacific and separates a closed low that then drifts to the West Coast by next weekend, but confidence in this would be fairly low. Additionally, the 12z GFS has absolutely no precipitation with this particular feature. If that feature doesn't amount to anything than the hope remains on an overall change to a more western trough for the following week (period of about March 20 and beyond). Last night's deterministic run of the GFS was not very favorable for precipitation along the West Coast as it set up the trough too far inland, but this morning's 12z run is substantially different, with a full-latitude trough approaching the West Coast on Tue/20 March with the southern portion cutting off and plowing right into the West Coast by Thu/22 March with lots of precipitation. The eventual result is a far weaker western trough than earlier runs of the GFS and the GFS ensembles. It appears the differences in the runs are related to whether the associated system coming across the Pacific later this week closes off and stays well to the west in a position north of Hawaii (as in last night's GFS and ECMWF run) or phases at least for awhile with the Alaskan upper level low that opens up. Just looked at the latest 12z ECMWF run out through 240 h and it supports the 12z GFS solution.
The above model differences are reflected in a spread in the GFS and Canadian model ensemble forecasts from 00z, with spread increasing considerably after good agreement through next weekend (17-18 Mar). A number of the members support the 12z GFS solution for a potential event midweek (20-22 March). The 12z GFS ensembles are generally similar to the 00z set from the GFS, with greater spread by the last week of the program but several members supporting a potential IOP midweek (20-23 March or so).
ed szoke NOAA/GSD