Complex Storm Consolidating Over West Today

Much of the West is already experiencing rain or snow this morning, as the large storm system consolidates. Here in Arizona snow is falling at higher elevations across the northwest third of the state.

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The 1200 UTC 500 mb analysis from NCAR RAP (top) indicates that the short wave is strung out north-to-south from western Canada (where 500 mb temps are -40C and colder) to off the southern California coast (where the previously mentioned subtropical moisture plume has come ashore, extending far into the interior West).

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The nearly concurrent satellite water vapor image (middle graphic - the water vapor channel is centered above 500 mb but it is very useful for diagnosing the structures within this system) for the eastern Pacific indicates a complex structure. The moisture plume from north of Hawaii is apparent, and it appears that a short wave has pulled eastward with this plume, comprising the southern part of the storm system, and located west of southern California. The very cold, northern short wave/cutoff is also apparent along the coast of Washington and over Vancouver Island. The comma-shaped feature near Yuma, Arizona, is likely an artifact due to orographic cloud streaming off the coastal mountains.

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The 12 UTC NAM 500 mb analysis, with vorticity, is shown in the bottom panel. The model analysis appears to confirm the complexity of structures present. There are several disorganized, orographic-related?, vorticity maxima inland over the central and northern Rockies. The short wave associated with the Hawaiian moisture plume is indicated by a distinct vorticity maximum from the east Pacific into central California. The primary vorticity maxima off the Washington coast is being fed by an intense vorticity ribbon, or streamer or tail, that is originating somewhere off to the far west of Alaska.

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As the 500 mb cutoff consolidates and digs southward there will be abundant, small-scale, short term forecasting challenges for western US forecasters. I'll discuss a few of these in the next post.