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November 30, 2007

HMT forecast discussion for Friday 30 November 2007

It doesn't seem so long ago that we were doing this....we begin HMT 2008 with the excitement in Southern California and the Southwest as a very moist upper level system off Baja gets pulled up ahead of the trough in the Northwest. This will become a big storm for much of the nation this weekend but not the HMT area where things are quiet albeit chilly at this time. A very high amplitude ridge is found farther west off the West Coast extending across central Alaska to the North Pole. West of this is a deep trough with a strong Pacific jet trailing all the way back to Asia. The current pattern will undergo quite a bit of change over the next couple of days as the current shortwave trough over far Northern California combines with the system hitting the Southwest and lifts into the mid-CONUS. As this happens, some of the northern energy in the current trough over the Pacific Northwest will retrograde, while a strong wave in the southern stream in the trough now west of the high amplitude ridge will push east, undercutting the ridge. So by Sunday the ridge will have been totally pinched off with the northern part over Alaska, and a very nice plume of moisture stretching into the West Coast. Unfortunately, this plume will be positioned just to the north of the HMT area Sunday and again on Monday, and then the question is whether it ever sinks farther south during the week before it weekens too much to be of interest. Some model forecasts earlier in the week had put quite a bullseye of precipitation into the HMT area around Tue/Wed, (4-5 Dec), but have subsequently backed off on this forecast and kept things farther to the north.

This should be quite an event farther north with several inches of rain in Oregon/Washington into early next week but may end up just being a tease for the HMT area, with a threat of precip (though questionable if enough to be an event for the HMT) maybe holding off for the HMT area until about Wed/5 Dec with the passage of a shortwave trough. Enough uncertainty that it will need to be monitored, with a chance the plume could sink far enough to the south to affect the area even by Tue while it is still moisture laden, although at this point the models would favor a close call only.

In the longer range general consensus has a ridge building back up along the West Coast by later next week which keeps the area dry and systems tending to dig farther to the east. This forecast prevails through most of the 0000 UTC GFS ensemble members. The Canadian ensemble system has, as usual, more diversity, with a tendency to have more members showing the ridge positioned farther west, thereby keeping the HMT area under some potential for systems approaching from the northwest. The latest HPC longrange discussion seems to indicate that the ECMWF ensembles also have more spread than the GFS, and so suggest more uncertainty in the forecast for later next week than the GFS system would indicate. The latest 1200 UTC GFS ensembles follow this trend in showing a little more uncertainty than the 0000 UTC group.

Ed Szoke NOAA/CIRA/GSD