HMT Forecast Discussion for Friday 14 Mar 08 (entered 1930z 14 Mar)
Showery precipitation is now over to snow at KBLU and should remain that way as the next trough moves across the region with falling temperatures through Saturday and limited precipitation. This big trough will move slowly across the West over the weekend with a ridge building along the West Coast, downstream of a deep upper low centered near the western Aleutians. There is a strong jet on the south side of the Aleutian low, and a nice plume of moisture south of this jet extending into the tropical Pacific. Over the coming week an upper low will remain positioned over western Alaska, while the current system centered near the western Aleutians slowly works eastward across the Gulf of Alaska. The associated jet stream then is forecast to stretch to the northeast and hit the British Columbia coast by Sunday, with the moisture plume to its south extending into the Pacific Northwest. Then early next week the jet and moisture plume gradually shift southward, reaching northern CA by late Tue/18 Mar, but with the moisture weakening with time. There is decent agreement then that what is left of this plume reaches the ARB by Wed-Thu, but the deterministic forecasts never produce much precip in the ARB. There are some ensemble members from the 00z/NAEFS that have considerably more precip extending in a nice plume fashion to the wsw, but the trend in the latest 12z GFS supports an overall weakening of the plume and not much precip. The most favorable aspect of this whole thing would be mild enough temperatures for rain at BLU, but taking everything into account at this point suggests that it would be a long shot to get an IOP-worthy event midweek out of this. A look at the latest 12z ECMWF run shows a plume working down to the OR/CA border by late Tue, but then any precip waits until an a shortwave passes later on Wed into Thu and by then things cool down so the precip might actually be some snow, so not very favorable. Similar picture from the 12z Global Canadian model; a nice but very narrow plume doesn't quite make it far enough south so just some light precip late Wed with the shortwave. During the telcon it was noted that we will want to watch the trend of this feature and if it still does not look that favorable Monday would likely be the last telcon.
Beyond this feature, the main upper low gradually sinks south from Alaska into the Gulf of Alaska and then possibly farther south into Easter weekend and beyond. This brings heavy precip into the Pacific NW for most of next week, and then perhaps into the ARB by later next weekend and beyond, all after the time frame of the end of the experiment on Friday/21 Mar. There is certainly spread in how far south this system will sink and when the precip would finally get into the ARB, and of course how much. It could be a big event, as early as Sun/23 Mar, or maybe just another rather cold one with any rain going over to snow at BLU with the strong dynamics.
ed szoke NOAA/GSD/CIRA