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HMT Forecast Discussion: 11 March 2008 at 1945 UTC

As last night’s shortwave trough quickly exits the region without much effect, transient ridging is quickly taking hold across CA for today into tomorrow. Consequently, the string of dry wx will persist for two final days before a cold-core storm system impacts the ARB later in the week and over the weekend.

The current IR satellite imagery shows this cold-core system taking shape over the eastern Pacific. As of midmorning, it’s circulation was centered at ~44N/160W, while a secondary comma-cloud head and baroclinic leaf resided ~1000 km E-SE of the primary circulation center. This latter feature represents a leading shortwave trough that is progged to eject Eward in advance of the trailing cold-core storm system.

The low amplitude ridge over CA promptly flattens Wednesday and Thursday in response to the approach and landfall of this initial ejecting shortwave trough. High clouds are already approaching the OR/CA coast with this S/W, and should overtake the ARB later today and tomorrow. The cloud deck will thicken tomorrow as the S/W nears. Zonal (i.e., onshore) flow will increase to 15-25 kt at 850 mb and 35-45 kt at 700 mb late Wednesday night into 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 late Wednesday night and Thursday. A transient PW plume will accompany the cold fropa, but peak values w/in the plume will likely remain under ~2.5 cm. Hence, we should expect to see measureable, but not copious, precip across the ARB starting late Wednesday night and on Thursday, perhaps in the 0.5-1.0” range (with possibly larger amounts in extreme N CA). Snow levels will be in the 6-7kft range at the onset of precip, lowering steadily on Thursday and Thursday night in the cold advection, eventually settling in on the 4.0-4.5kft range by early Friday.

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 (500mb temps will range between -30 to -35C). 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. Brrrr!

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

Paul Neiman