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HMT Forecast Discussion: 12 March 2008 at 1945 UTC

Today will be the last dry day across the ARB domain until the end of the weekend. IR satellite loops show a minor cluster of enhanced cloud tops moving across CA as of midmorning in association with a transitory vorticity spoke aloft. No significant wx is tied to this feature, although it has flattened the midtrop ridge. Mid/high clouds will thin temporarily today as this feature moves eastward. Meanwhile, the cold-core storm system offshore continues to grind slowly Eward and is now centered at ~45N/150W. A prominent Eward-moving baroclinic cloud leaf is located SE of the cold-core circulation center and represents the ejecting S/W trough that will affect our area tonight and tomorrow. Clouds will thicken again this afternoon as the S/W approaches.

As the ejecting S/W takes aim at CA, zonal (i.e., onshore) flow will increase to 15-25 kt at 850 mb and 35-45 kt at 700 mb late tonight into Thursday with moistening at low levels, although this flow is *not* progged to back around to a more favorable SWerly direction. Hence, orographic forcing will be modest but not strong. Moderate large-scale dynamics with a cold-frontal passage will contribute to atmospheric lift late tonight and Thursday. The SSM/I satellite imagery shows a respectable PW plume with this S/W pointing toward CA, although core values over the eastern Pacific have decreased from ~3.5 cm yesterday to barely 3 cm today. This decreasing trend is consistent with the GFS solutions depicted on the Univ. of HI website, which show core values decreasing to <=2.5 cm upon landfall late tonight and tomorrow. This plume will also promptly sag Sward beyond the ARB during the day Thursday and Thursday night. Hence, we should expect to see measureable, but not copious, precip across the ARB starting late tonight and on Thursday, perhaps in the 0.5-1.0” range. 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 briefly become more SWerly on Saturday. This could result in a more favorable orographic flow direction, during which time unstable cold-core conditions will encompass the Sierras (500mb temps will be less than -30C). Hence, convective snow showers will likely impact the ARB on Saturday, 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. Locally moderate snow accumulations may occur in persistent regions of convection. Snow levels will likely descend to 3kft or lower during the weekend. Brrrr!

Dry wx should return to CA on Sunday as Nerly flow takes hold across the state. Low-amplitude ridging is progged to return for early next week, as the deepening trough aloft moves eastward into the central U.S. Moist zonal flow will likely impact the PacNW next week, although the long-range model solutions exhibit a high degree of uncertainty as to how much (if any) of the moist zonal flow will sag far enough S to impact the ARB. It will need to be watched.

Paul Neiman