A wet week ahead for Northern California

Winter, it seems, has very quickly arrived on our doorstep. A quick look at recent satellite imagery indicates a very active jet stream and an associated tropical moisture plume stretching all the way back to Asia. This early-season breakthrough of the westerly jet  over the far Eastern Pacific is at least partially due to the ingestion of energy and moisture from former Super Typhoon Megi, which made two land landfalls over the past several days–one on Luzon Island in the Philippines as a category five storm, and the other on central China as a weakening category one storm. It’s interesting to note that last year’s powerful mid-October storm in Northern California also originated from the remnants of a formerly-powerful Western Pacific typhoon, and the dry season-ending rains in 2008 arrived with similar origins.

Rain has already begun in earnest across much of the northern half of the state, and will continue to increase in coverage and intensity overnight and through the day on Sunday. A precipitable water plume of nearly two inches is currently moving ashore in Northern California in conjunction with a strong 150 kt jet. Upper-level divergence and vigorous upward motion will lead to ample precipitation ahead of and along a cold front slated to move ashore tomorrow afternoon. In orographically-favored areas, 36-hour rainfall totals could approach or exceed 7 inches, and even the Central Valley will probably see at least 1-2 inches of early-season rainfall. Fairly strong southerly winds will develop over NorCal as a steep N-S pressure gradient sets up ahead of the cold front, and since these will coincide with the first significant widespread precipitation of the season, some trees and power lines may come down.  Any flood threat will be fairly minor due to the very dry antecedent conditions.  All in all, this will be a very respectable winter-like storm for Northern California and will provide a good start to the rainy season (though snow levels will be pretty high for the high-precipitation part of the event).

Numerical models are now indicating that this period of storminess won’t end early next week as previously indicated. Instead, it appears as if a long parade of Pacific storms will ride a persistently strong and extended East Pacific jet into the Pacific Northwest and affect at least Northern California. At least two additional significant storms are depicted by the GFS and ECMWF through next weekend. Each will have the potential to bring heavy rainfall and possibly strong winds to a fairly large portion of the Pacific coast. Given that we are currently in a fall transition period, a time when numerical models have an increased degree of difficulty with long-term projections, there remains some uncertainty regarding the strength of these late-week systems. Given the significant precipitation accumulations expected over the next 36 hours, though, subsequent heavy rains later in the week will have the potential to present some hydrological issues.  Stay tuned…

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