WX Discussion and Forecast for 5-March-06
Precipitation is moving into the area about as everyone felt yesterday with the heavier precipitation at the moment hanging up a bit north of the ARW area. Latest totals indicate over 2" so far along the coastal regions north of San Francisco and over an inch in the higher areas north of the ARW already. Satellite imagery shows an extensive band of deep cloudiness ahead of the cold front extending from northern California southwest to what appears to be a wave near 135W/30N. There was at least (and may still be) some thin connection with the extensive tropical plume that is found from Hawaii west and northwards but it is tough to say how much has made it into this moisture band. SSMI imagery supports a definite connection in a thin band that still extends all the way back to a piece of the Hawaiin system with PW values of about 1.5 inches. Another separate band of tropical connection is found from Southern California well back to the southwest well to the east of Hawaii but this is aimed more for areas south of the ARW, although there is a chance this could merge with the main plume of moisture later today. Judging by the way things are moving along it seems like the 12z models may be a little slow bringing the onset of heavier precipitation into the ARW, but otherwise paint a picture that is in good agreement with the overall scenario outlined over the last couple of days: increasing precipitation and wind this afternoon with at least a couple of inches of precipitation with the frontal band through about 18z Mon when the front passes, then less organized precipitation with the initial high (5000 ft +) snow levels dropping to 3000-4000 ft, then another onset of heavier precipitation Monday night ahead of the trough passage around 18z Tue (another inch at least in the Eta and GFS latest 12z runs), with the snow levels lowering to near where they were with this last event as the precipitation then becomes spotty by 00z/Wed and ends at higher areas during Tue night. The latest Eta and GFS both agree on about 4" of maximum precipitation but it seems the amounts will depend on how much moisture from the tropical plume made it in, any stalling of the frontal band, and of course the amount of convection after frontal passage. Would expect some areas to come in with 4-6" total by late Tuesday I would think with 3-4 feet or more of total mountain snows. The 3 km LAPS 15z WRF run has local max precip amounts for the 24-h ending 15z/Monday at 3 to 5 inches. South winds are already gusting to 30 mph at Blue Canyon with a temp of 36 and the Sacremento WFO forecast has S winds up to 60 mph in gusts later today and 65 mph tonight.
Beyond the current system things appear somewhat more uncertain today then they did yesterday in some regards. The next system is still on track to quickly move in probably Wed night into midday Thu/9 March with a quick shot of potentially heavy precipitation as it does seem to have a connection to the Hawaiin tropical plume. Snow levels start out around 4000 or so feet then plunge quite low by late Thursday. The main issue after Thursday is how quickly the overall trough position shifts into the Rockies, with the 12z GFS slowing this process down as it deepens an intense closed low off the Pacific Northwest next weekend that then slowly retrogrades slightly and finally moves through Northern California midweek (around 16 March). This is much slower than what looked like an end to precipitation on yesterday's runs (and last night's GFS) more like very early next week. The 12z GFS solution is supported by the 00z Canadian and 12z NOGAPS, but the 00z ECMWF is more progressive. The ensembles from the 00z GFS have some spread though are not as dramatic at holding the trough back as the latest GFS. Either way what all this means is that following the initial next system late Wed-Thu we enter an extensive period, at least through the weekend and possibly well into next week, of waves of precipitation moving in with timing and amount very uncertain. One relative certainty is that at least for later this week snow levels would drop to extremely low values not yet seen, I believe. If the latest GFS is correct they could then rise again and there would be the potential for a more moist episode or two of precipitation from say Sun/12 March through about Wed/15 March. All this may be moot if resources have run out, but wanted to point out that 1) uncertainty in the late Wed-Thu system, 2) potential for VERY cold weather with anything later in the week or weekend.
Ed Szoke - ESRL/GSD