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HMT Forecast Discussion: 9 March 2008 at 1945 UTC

The current satellite loops show mostly sunny skies across CA today, as a ridge amplifies along the West Coast. Ridging and dry wx will persist through tomorrow afternoon, although high clouds may start streaming over the ridge Monday afternoon in advance of an approaching shortwave trough.

The new 12Z NAM has come around to mirror the global GFS and EC solutions for the landfall of the shortwave on Monday night and Tuesday morning. Namely, the large-scale dynamics and orographics primarily impact the PacNW, although the trailing cold front may generate some light precip along the N half of the Sierras Monday night. In addition, 850 mb wind speeds across the ARB briefly ramp up to the 20 kt range in prefrontal SWerly flow, so modest short-lived orographic forcing may briefly aid in precip generation early Monday night. However, subsident NWerly flow with stabilizing lapse rates will quickly take hold in the postfrontal environment on Tuesday, so don’t expect much in the way of postfrontal orographics. Total precip across the ARB should be less than 0.3-0.5” liquid equivalent. Snow levels will likely start out in the ~6.5kft range before dropping to ~5kft toward the end of the brief period of precip.

The ridge amplifies yet again along the West Coast Tuesday night and Wednesday, yielding dry and mild conditions. The ridge flattens in response to the approach and landfall of a shortwave trough from the Gulf of Alaska on Thursday into Friday. However, given that the storm will originate from a cold source region rather than from the tropics, expect relatively low snow levels and only modest amounts of precip. Low heights and unstable conditions may persist into next weekend, with the possibility of a little cold-core precip.

Paul Neiman