HMT-West 2007: Forecast

For awhile there I thought I would not be doing any more forecasting for the project as it looked like all the resources would be used up by now, but the spigot shut off and now the quiet period continues for the HMT project area. It's not like there isn't a lot of action in the eastern Pacific; a look at the water vapor loop shows a nice plume of moisture connecting back to the tropics and all the way to the West Coast just to the south of an extensive jet. Unfortunately, all this is aimed more at the Pacific Northwest and this looks like it will remain the case through the weekend with the southern extent of the moisture perhaps getting tantalizingly close at times early in the weekend before more ridge building pushes things farther to the north. At this point there is no model solution to be found that would bring enough moisture far enough south this weekend to make for an IOP.

I am not very optimistic for the next 10 day period based on current deterministic runs and the GFS ensembles. As mentioned above the ridge actually starts to build along the West Coast by Sunday as a trough now passing the Dateline phases with the persistent deep upper low now in the Gulf of Alaska, pumping up the ridge downstream. By early to mid next week a decent shortwave trough emerges out of the Gulf of Alaska system and may change the weather around here mid to late week, but this system is well east of the HMT area which remains in a dry upper level ridge. This is the case through the next weekend (17-18 Mar) on the latest GFS, which by 240 h (12z/Sunday/18 Mar) has a longwave ridge centered over California and a broad and deep trough in the eastern CONUS. This is in excellent agreement with last night's GFS deterministic run as well as the 00z ECMWF 240 h forecast and the just-in 12z ECMWF run. The GFS ensembles from 00z as well as the Canadian ensemble forecasts have pretty decent agreement on this scenario.

Speculating beyond day 10, the 00z GFS forecasts a return to a western trough with a strong wave dropping southeastward in the nw flow off the Pacific and a subsequent retrogression of the upper level ridge. This may or may not lead to a potential wet system late in the week of 19-24 March, but it is a change and there is surprisingly good agreement at least in the GFS 00z ensembles with this change, as well as the Canadian ensembles. The 12z GFS ensembles just coming in are pretty much in agreement with the above scenario, especially through day 10, and then maybe just a bit faster for the potential system days 11-15. Bottom line for now is wet in the Pacific Northwest but very low chance of anything far enough south in the next few days, then most likely a prolonged dry period for some time.

ed szoke NOAA/GSD