HMT discussion for Saturday 17-March-07
As we get closer the approaching system is still on track but remains marginal for an IOP and a tough decision to probably make today either way. The trend in the 00z models and the 12z runs is for a slightly slower evolution (moreso now in the 12z runs) with precipitation holding off until about 00z/Tue/20 Mar and then lasting until around 21z or so (maybe a little later) on Tue before becoming more showery and ending. Consensus on the timing of trough axis passage at the HMT area would be 00z/Wed. Amounts are down a bit, max in the 00z GFS (using web graphics today) in the 0.75-1.0 inch range, less on the NAM. Other models with precipitation also generally down a bit, below an inch total. ECMWF and NOGAPS are even a little slower than the the GFS but the RH field from the ECMWF (no precip available) looks less than yesterday. The 12z GFS has a broad area in the 0.5-.75 inch range with a tiny spot above .75 just north of the HMT, so down a little more from 00z. The NAM is a little less, in the 0.5-0.7 inch range. Just saw the 12z Canadian run and it agrees with a slightly slower solution, so the bulk of the precip probably 09z Tue through 00z/Wed with a trough axis passage around 00z Wed, and it is a little more moist than the 00z run.
We are getting into the range of the short range ensembles, and a look at the 09z set shows a mean total precipitation in the 0.5 to 0.75 inch range, with most runs in this vicinity, and a couple in the inch or more category. The 12z GFS ensembles are trickling in. For the Tue storm they agree on the slightly slower trend seen in the 12z deterministic run, with a few runs having more southern energy in the system.
A look at our wildcard Pacific plumes noted yesterday shows they are still there, the closest near 140W aligned more north to south but in the satellite loop gradually weakening with time. The U of Hawaii site had a 6-h forecast total PW from the 06z GFS and it suggests up to 1.5 inches max in this plume which looks to be in agreement with the SSMI imagery, but by 24 h into the forecast really diminishes the moisture. It has not weakened to that extent in the last 24 h according to the SSMI imagery, so this is something we could recheck on Sunday to see if the model is weakening all this too much. The wave for Tue is also carrying a moisture plume but the PW analysis and forecast shows it as VERY narrow (though the water vapor imagery is more impressive) and also weakening with time. Still, they are forecast to combine offshore and this is the moisture hitting the area on Tuesday. Based on all this and the latest 12z ensembles showing some slowing potential, would think this is still worth waiting another day to decide on, especially as it does not seem soundings would be needed before 00z/Tue.
Looking beyond this system the Pacific does remain active and waves moving off Asia tap into the tropical moisture in the western Pacific. A very strong jet (greater than 180 kts in the forecast from the 12z GFS) is predicted to move out of Asia early next week and progress across the Pacific. One potential wave of interest for next weekend (24-25 March), though it may stay to the north. However, last night's ECMWF run was most favorable with a closed low sitting off the central California coast by 00z Tue/27 March. And the 12z set of GFS ensembles coming in now are trending to having more members with potential for something on Sunday 25 March. Other systems are then in both the 00z and 12z ensemble forecasts for the last week of March with a lot of spread and large timing differences, but certainly potential as a number of the members show closed lows forming at least within the area of interest. Given this and that massive jet, appears we will be tempted to hang on the way it looks now, but something for the long range crew to sort out on Monday's telcon.
ed szoke NOAA/GSD