HMT forecast discussion for Saturday 1 December 2007
The very narrow and high-amplitude ridge remains in place from the North Pole south across central Alaska and then further south near 140W this morning, but is about to be undercut by an eastward moving shortwave near 40N/150W, as well as a retrograding trough now in British Columbia. This will set up the strong west-southwest flow and moisture plume aligned at the Pacific Northwest that was discussed yesterday. The aforementioned shortwave in the Pacific will hit the Oregon coast near 1800 UTC on Sunday and bring strong winds and rains south into Northern California as well. This may be the closest tease for the onslaught into the Pacific Northwest, and one would think there could be at least a little precip perhaps into the higher elevations of the HMT area, but it looks like a sharp cutoff will be in place with the heavier rains/snow staying too far north. The hi-resolution runs of the WRF (ARW and NMM NCEP windows) run at ~5 km resolution both have accumulated precip up to about 0.30" in the higher elevations of the HMT area, mainly falling between 0600-1500 UTC 2 Dec, then another close call later on Sunday. The strong onshore flow will continue aimed at the Pacific Northwest through Monday and gradually weaken by Tuesday, but not before some areas from extreme Northern California to Washington receive 5 to maybe 10 inches of rain, so quite an event. Cold enough for snow in some areas and all kinds of warnings are in place, but more wind warnings than precip by the time you head south to near the latitude of the HMT area. The 1200 UTC NAM is a little farther south with the main precip than the latest GFS run. Overall though pretty good agreement with a close call but the heavy rains staying too far north. Same with the short range ensembles, though again some members do bring a little precip into the higher terrain of the HMT area on Sunday.
A couple of possibilities are present after the big event weakens on Tuesday. One remains whether there may be something left to the plume as it sinks a bit south resulting in precipitation possible in the HMT area in the late Tue through Wed time frame. Right now this looks marginal but certainly we'll keep an eye on it. A better chance might be with a shortwave later this week that amplifies as the upper level ridge gets reestablished off the West Coast. This was a minor feature yesterday that amplified farther inland but today there is more spread with this feature, with some models developing it more before it reaches the California coast. This would be a threat of a relatively quick moving and cold event in the Friday/7 December time frame. We have time to watch this one.
Looking further ahead the ensembles generally continue to predict the upper level ridge to build off the West Coast with most systems dropping farther inland, so a dry pattern for the HMT. However, a little more uncertainty now in the GFS ensembles, (and some remaining in the Canadian ensembles) with the chance the systems could amplify more as they approach from the northwest and before passing the coast. This would not likely lead to any sustained atmospheric river into the area, but a potential event out of the northwest with reasonable precip possible. The uncertainty is seen by comparing just the ECMWF 240h from last night's run with the latest GFS, with the EC having a decent wave by early next week in the northwest flow.
ed szoke NOAA/CIRA/GSD