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HMT Forecast Discussion: 4 March 2008 at 1945 UTC

Satellite loops show the tail end of a transient shortwave trough having passed the ARB overnight. In its wake, midtrop ridge-building, dry N to NE flow, and subsidence is overtaking the region. As a result, expect mostly sunny skies and near seasonal temperatures along the Sierra foothills the next couple of days.

The first in a series of shortwave troughs embedded in strong Pacific zonal flow aloft will make landfall across the PacNW and extreme N CA Thursday into early Friday. This wave will rapidly minor out as it breaks through the W Coast ridge. In addition, this wave will not have significant dynamic/thermodynamic signatures at the surface. Low-level frontal forcing will be weak at best and orographics nil. Consequently, expect little more than light precip along the N CA coast region and perhaps some increase in high/middle clouds across the ARB on Thursday into Friday.

A couple more shortwave troughs are progged to come ashore along the PacNW and/or extreme N CA on Friday night into Saturday and again on Sunday into Monday, based on the GFS. Each successive wave will serve to further erode the midtrop ridge along the West Coast and perhaps yield a bit more precip than its predecessor. Most of the significant rain/snow should remain N of the ARB throughout the weekend, although we will need to keep an eye on the Sun/Mon wave given that we may have a period of favorable orographic forcing. It should be noted that the current EC solution shows transient ridging rather than a shortwave trough over CA for Sun/Mon, so confidence on this precip event is presently low. If/when precip falls with these waves, snow levels will likely be in the ~7kft range.

A stout 200 mb jet stream (>150kts) presently situated across the western Pacific should eventually extend eastward across the Pacific Basin and may yield a storm of IOP caliber for the ARB early next week. This potentially significant landfalling shortwave for the Monday night/Tuesday timeframe that John B. mentioned in previous forecast discussions is still represented in the various models and ensemble spreads, although the intensity of the wave varies from model run to run, across models, and across ensemble members. This storm also looks like it may be a repeat of storms earlier this winter, that is, a dynamically strong but cool event with relatively low snow levels (i.e., <6kft) rather than a warm atmospheric-river tap of subtropical moisture from the SW.

Thereafter, a period of cool, unsettled, onshore flow may be in the offing for the latter half of next week. Time will tell.

Paul Neiman
NOAA/PSD