2007-01-26 Wx Forecast and Discussion
Current conditions
Much the same weather pattern today. The 500 mb cutoff low remains in position just off the N CA coast. It has moved southward, perhaps, 50-100 miles. IPW at Bodega Bay has increased between 0.6 and 0.7 inches.
Like yesterday, the Valley is foggy with temperatures in the mid 30s. Temperature at Blue Canyon and Forrest Hill is in the low 40s. At higher elevation, where there is snowpack, the temperature is well below freezing.
00-72 Hour forecast
There is an outside chance of precipitation exceeding a few tenths of an inch. IPW values have increased a bit, as expected in yesterday's NWP guidance, but upslope flow is nearly nill at the moment. The perfect storm scenario for this setup would include an abrupt eastward movement of a cut-off low that is now positioned over old Mexico, which could permit the cut-off low off the CA coast to tap into a bit of tropical moisture. The cutoff-low off the CA shore would then need to move onshore at just the right location to produce upslope flow. With a slight increase in IPW, perhaps to 0.9, there would be a chance for precipitation as the cut-off low moves onshore, but, again, far from the amount that is of interest to HMT, not to mention this scenario is a long-shot anyway.
3-10 days
As discussed yesterday, the polar vortex is forecasted to undergo a major change in the 5-10 day period. First, over the next 3 days, the broad north Pacific trough is forecasted to sharpen significantly west of the Aleutians. The effect is that the ridge over the West Coast is forecasted to move westward, as the vorticity dynamics intensify west of the Aleutians, and to become increasingly pinched off between day 5 and day 10. The polar vortex stretches southward during days 5 and 10 to create a series of intense mid-latitude storms that will move across the Pacific while retaining fairly high amplitude. As discussed in the HMT phone call, the vortex is being stretched in a number of directions: Siberia/China, eastern US, and Europe. This is part of the reason the response over the northern Pacific is less coherent than in early December. With the ongoing flare-up of Tropical convection, a stream of Tropical moisture can be tapped by these high amplitude storms. The pattern that is forecasted to set up would support a series, perhaps 3 to 5, of high amplitude and moist waves moving across the Pacifc. This differs from last season's iop4, the poster child, in that the flow this year would be less zonal so that a multi-day period of heavy rainfall would be unlikely, but, instead, a day of very heavy precipitation, much heavier than HMT has seen to date, is the more likely scenario to occur within this pattern.
In terms of the likelihood and timing of specific events. The GFS and CMC ensembles indicate the earliest day of interest is Feb 4, as a couple of members have the first wave making a direct hit on ARB. While this is a low-probability event given the forecast lead time, it provides a sense of how different these storms could be compared to what has been observed thus far. In the most intense precipitation event, the 12-hour precipitation total exceeds 100 mm for two consecutive 12-hour periods, before shutting off.
10-days and beyond
Ed and Klaus think Tropical convection will intensify in the Indian Ocean and west of the Dateline (~150-160E), reaching a level that could impact the flow over the central Pacific about week 2 and, especially, later. This could lead to a retrogression of the trough, perhaps permitting more vigorous cold events or westerly flow undercutting the ridge.
Chris Anderson, GSD/FAB