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HMT Forecast Discussion: 10 March 2008 at 1945 UTC

The current satellite loop shows the next progressive shortwave trough approaching the WA/BC coast as of midmorning. A well-defined comma-cloud head is currently centered near 48N/136W and moving steadily ENEward toward Vancouver Island. A long, skinny cold-frontal comma-cloud tail extends Sward from the circulation center to ~30N; it is approaching the CA coast. The models take this shortwave across the PacNW and into the N Rockies late today and tonight. The trailing cold front limps across the ARB tonight, yielding very weak dynamics and orographics with limited moisture. Hence, precip should be less than 0.1-0.2” of liquid equivalent tonight. Snow levels will descend from ~6kft to <5kft in those areas where light precip. might fall. Clearly, this is not IOP material.

Midtrop heights rebound tomorrow (Tue) afternoon into Wednesday in response to transient ridge building along the West Coast. Hence, Tuesday and Wednesday will be dry.

The ridge promptly flattens Wednesday night and Thursday in response to the approach and landfall of a shortwave trough ejecting from a developing cyclonic circulation over the Gulf of Alaska. Zonal (i.e., onshore) flow will increase to 20-25 kt at 850 mb and 35-45 kt at 700 mb late Wednesday night & Thursday with moistening at low levels, although the flow is *not* progged to back around to SWerly. Hence, orographic forcing will be modest but not strong. Modest large-scale dynamics with a cold-frontal passage will contribute to atmospheric lift on Thursday. The Univ. of Hawaii website of the GFS shows a coherent, thin plume of PW (i.e., an atmospheric river) extending from from N of the HI Islands to the SF Bay area early Thu., although the PW values within the plume barely scratch 3 cm and it quickly dies out while moving Sward on Thu. Hence, we should expect to see measureable (but not copious) precip across the ARB on Thursday, perhaps in the 0.5-1.0” range (with possibly larger amounts in extreme N CA).

Cold air filters into the region behind the cold front on Thursday and beyond. In addition, midtrop heights continue to fall toward the end of the week and into the weekend in response to the approaching Gulf-of-Alaska storm system. The lower-trop flow may become more SWerly by the weekend. This will result in a more favorable orographic flow direction, during which time unstable cold-core conditions will encompass the Sierras. Hence, convective snow showers will likely impact the ARB during the weekend, although the storm system will probably not be a major precip producer given it’s cold-core character and lack of a warm-source moisture feed. Snow levels will likely descend to 3kft or lower during the weekend.

West Coast ridging may return by early next week, as the deepening trough aloft moves eastward into the central U.S.

Paul Neiman