HMT discussion for 11-March-07
The plume of moisture in the eastern Pacific continues from near Hawaii to Washington and Vancouver this morning associated with a trough that extends north to the upper low in the Gulf of Alaska. The southern piece of this system is what breaks off to form a weak closed low that drifts to the California Coast later this week in the GFS but without any precipitation in the model forecasts. Other models are not so distinct with this feature. The main portion of this trough moves across the Pacific Northwest through Monday and then dives into a developing mean eastern CONUS trough while ridge building commences along the West Coast and the HMT area.
To the west in the Pacific another strong system is found near the Dateline at about 35 N, and then farther to the west a strong wave is exiting Asia into the western Pacific. The one near the Dateline phases with the overall Alaskan upper low and helps to build the ridge downstream this week. This is all in decent agreement amongst the models, but then things diverge by next weekend (17-18 March). The 00z GFS splits the trough well offshore and slowly sends a closed low, this time with moisture, into California mid to late week (~22-23 March), similar to what we saw in yesterday's 12z run. The 00z ECMWF run has similar timing but more of a phased trough without a distinct southern closed low. The deterministic Canadian model run through 240 h from last night is similar to the ECMWF run. The GFS ensemble forecasts from 00z are in good agreement with a potential system (varying somewhat in timing from early to midweek of the 18-24 March week to later in the week) and generally split between the closed low solution in the GFS deterministic run and the more phased trough found in the ECMWF and Canadian runs. As usual more spread in the Canadian ensemble (the one from the Penn State e-wall) but still a number of members with a strong trough into the West Coast in this timeframe.
The 12z GFS run just in is similar to the 00z GFS with a southern closed low solution but is even slower in moving it onto the coast, waiting until 25 Mar to do so and then without a lot of precipitation. Probably an attempt to frustrate HMT planners...the 12z ensemble has a majority of members with a closed low solution but most have better timing (mid to late in the 18-24 Mar week) than the deterministic run and there are a few solutions with a faster open wave solution similar to the 00z runs of the Canadian and ECMWF models. No 12z ECMWF run in yet with the time change.
ed szoke NOAA/GSD