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HMT Forecast Discussion: 5 March 2008 at 1945 UTC

The latest satellite imagery shows clear skies with ridge conditions firmly entrenched along the West Coast for today, including a sunny bluejay day in store for the ARB. Meanwhile, the first shortwave trough is quite evident in the IR imagery as a N-S band of cold cloud tops moving Eward beyond the 140th meridian. This wave will cut through the ridge Thursday into Friday, splitting in the process. By the time the remnants of this wave reach the ARB, it will be devoid of substantive large-scale dynamics and moisture, and there will be no lower-trop orographic forcing. Hence, expect little more than scattered middle/high clouds and slightly cooler temperatures.

The next progressive shortwave trough rolls into the PacNW and N CA on Friday night into Saturday and should remain somewhat more coherent at landfall than its predecessor. Nevertheless, because it will propagate through the (flattening) ridge along the West Coast, the large-scale dynamics and orographic forcing will be modest at best and should remain well N of the ARB. The best chance for precip will be for extreme northern coastal CA into the PacNW. The ARB should remain mostly dry.

Shortwave #3 originally slated for Sunday into Monday is looking less impressive with each successive U.S. model run, and in line with persistent EC solutions showing ridging over the ARB during this period. Most or all significant precip is progged to remain N of the ARB.

The potentially significant storm for next Mon/Tue is looking less impressive in the latest 00Z and 12Z GFS model solutions, and in line with the EC prognosis for this event. The current solutions show a quick-moving, shortwave trough and well-defined cold front making landfall across the ARB on Mon/Tue. The core of the dynamics stay N of the forecast area. Additionally, the lower-trop onshore flow only briefly goes around to WSW before veering to WNW during fropa, hence the orographic forcing does not appear to be ideal. Given that a prominent 200 mb jet stream (>150kts) presently extends across western Pacific Basin, we should not yet write off this storm for the ARB early next week. However, it must be acknowledged that d(model)/dt is going in the wrong direction.

Paul Neiman