HMT discussion for Mon/12 Mar
A distinct plume of moisture extends from a shortwave moving through the Pacific Northwest back to a separate southern wave near 30N/140W. This southern wave will remain separate from the overall upper level low over Alaska. West of the plume in the Central Pacific is a strong wave that has just crossed the Dateline near 45N, and farther to the west a strong wave has moved off of Asia just north of Japan, so pretty active in the Pacific. Quiet though this week for the HMT with the building of an upper level ridge commencing later Wednesday after the passage of today's system and another shortwave tomorrow. Very toasty temperatures with dry conditions will prevail this week. The southern system currently near 30N/140W that was mentioned yesterday as drifting to the east in the GFS forecast to near the California coast does not seem to be in the more recent forecasts although some other even weaker wave is that apparently interacts with the southern wave, but all of no consequence to the HMT this week as there is no moisture with any of this.
So attention remains on what might come next week. The 00z GFS went to an open trough solution, reaching the West Coast on late Tue/20 Mar to early Wed/21 March, followed by another wave somewhat farther north and then an intermountain trough by the weekend into the following week. The timing for midweek and the open trough is similar to the forecast from the 00z ECMWF run and previous runs, and also similar to the open trough solution in the 00z Canadian run that has a moist system hitting the HMT area on Wednesday. The latest (12z) GFS is similar to last night's run, though a tad faster, with a potential IOP beginning on Tue/20 Mar. All of these runs, while supporting a potential IOP next week, do forecast a fairly fast-moving system. A look at the 12z GFS solution on the U of Hawaii site for precipitable water shows a nice plume as the system approaches towards the West Coast this weekend but it weakens with time considerably and is less well-defined by the time the trough makes it onshore. The 00z/GFS ensemble now has most members supporting the open trough solution and not the closed upper level low we saw on a couple of the GFS runs. The Canadian 00z ensemble solutions show more spread but generally at least through midweek next week agree with a potential system. The solutions beyond the first wave for midweek next week generally do not support any additional IOP possibilities at this time, with some spread and mostly a trough farther inland in the West, although certainly more variability leaving open some possibilities. The latest 12z/GFS ensembles just in have all but one member now with an open wave solution that is even faster, so more like a potential Tue/20 Mar IOP, and quite diffferent from the deterministic GFS runs we saw yesterday and the day before. And, hot off the presses, the 12z ECMWF run is in very close agreement with timing and the structure of the system through day 10 (12z/Thu/22 March).
The bottom line then at this point is a quiet and warm week this week through the weekend, then good agreement on a potential IOP next week, now favoring the Tue-Wed/20-21 March timeframe rather than later in the week.
ed szoke NOAA/GSD