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2006-03-03 WX Discussion and Forecast

A solid band of rain with a few embedded thunderstorms is moving across
the HMT area this morning just ahead of the trough axis moving
onshore. Trough passage across the HMT is expected around 21z today
with the winds becoming more westerly and the precipitation decreasing
in coverage, though convective showers will remain over higher areas
through most of the night for some additional snows. the snow level is
already quite low, down to around 1500 to 2000 feet, and may lower a
bit more as the trough moves by. The LAPS run has the well-defined main
band of precipitation persisting until about 22z before breaking up into
less organized convection. The 3 km run max precip amounts 15z today
thru 15z tomorrow are up to 1.5 to 2" in the HMT area which could be a
bit high. The ski report this morning from Squaw Valley says they have
had 24" of new snow in the last 2 days.

Saturday should be quiet between systems but as noted yesterday the
next trough now crossing the eastern Aleutians is quickly diving
southeastward and intensifying for the next event. This system will be
stronger than the current one and, as noted yesterday, still looks to
be tapping into some of the moisture associated with the broad trough
near Hawaii. This moisture plume appears to be contained mainly in the
initial strong rainband that will arrive in the HMT area later Sunday
and persist until around 18z on Monday. It looks like todays model
runs are a little faster with the start of this event and significant
precipitation in the HMT area could well be underway by later (~21z)
Sunday, perhaps even 18z, with initial precipitation beginning in the
morning. In addition, strong SW to SSW winds are predicted to accompany
the first main rainband, with speeds forecast by the Eta and GFS to be
about 50 knots sustained near 925 mb and 55 to 60 knots at 850 mb.
This strong southerly flow will push warmer air in with the freezing
level rising to 850 mb so initially snow level will be fairly high
(4500-5500 feet) but eventually lowering to near what is happening now
as the trough passes on Tuesday, which probably happens around 12z or so,
with precipitation likely ending by later in the day on Tuesday. With
the system tapping into more tropical moisture than the current event and
a longer duration the 00z GFS is forecasting about 5" maximum precip
amounts for this event which look reasonable. The 12z GFS is a little
lower but still plenty wet.

Yet another wave closely follows the Sunday-Tuesday storm and this one
at least temporarily shifts the mean trough position over the Rockies
with upper level ridging shifting eastward off the West Coast extending
north to the Aleutians. This system is now forecast to arrive on
Wednesday/8 March and will be the coldest so far with thicknesses
forecast to drop into the lower 520's (they are near 530 dam with the
current event). After the initial Wed-early Thu burst of heavy
precipitation (perhaps aided with at least some contribution from the
same tropical source) there will be an extended showery period likely
perhaps into Friday. Beyond this somewhat uncertain with a number of
the GFS 00z ensembles indicating a deep trough setting up farther off
the West Coast mid-March while others regain something akin to the
current pattern, but there does seem to be a chance of a gap in the
events after the Wed-Friday storm of this coming week.

Ed Szoke - ESRL/GSD