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WX Discussion and Forecast for 6-March-06

The heavy band of precipitation with the main frontal band unfortunately
ended up hanging to the north of the ARB longer than forecast with the
result that the period of heavier precipitation, which is wrapping up
now, was shorter than anticipated. It didn't miss the ARB by much;
looking at the 24-h precip reports there was a band of 3-4" accumulation
about 60 or so miles to the north, from Brush Creek north of Blue Canyon
wsw back to north of San Francisco. It looks like this band will be
passing east of the area the next couple of hours (what is left of the
cold front) and with gradually decreasing temperatures aloft there
should be widespread convection this afternoon, which would decrease
somewhat after dark. In the higher areas (like Blue Canyon) would
expect more or less continuous precipitation with snow levels now around
4000 feet or so gradually dropping. The trough axis still looks to pass
near 18z Tuesday and would expect more steady precipitation to develop
ahead of this beginning around 09z and continuing through 18z Tue, with
the 12z Eta and GFS both having about an inch new of precipitation in
the 24-h ending 00z/Wed. Our latest (12z) 3 km run nicely shows the
more convective nature of the precipitation this afternoon then the
above noted timing for more widespread precipitaiton later tonight.
Following trough passage snow levels would lower to about 2500 ft or so
but precip will be winding down tomorrow afternoon and should mostly end
even in the high country Tue night.

The models seem to be coming into somewhat better agreement on the
overall broad pattern for the next week plus, with less spread than was
shown in yesterday's runs. The general trend is to shift the longwave
trough inland over the next week so that by early next week an upper
level ridge will be near the West Coast which could set up an extended
period of dry weather. There is still some spread in the GFS 00z
ensembles but not as much as yesterday, and the 12z GFS run is also in
agreement in showing dry weather for the most part in the ARB area from
about Sun/12 March through Tue/21 March. The mean 8-10 day ECMWF
pattern is similar to the GFS in showing this trend.

In the interim between the current system and next week there is active
weather as waves continue to affect the region in the NW flow. The
first such wave now appears to be more in the Thursday timeframe with
the bulk of the precipitation from about 12z/Thu through 06z/Fri 10 Mar.
The GFS shows only about an inch or so accumulation but, as noted
yesterday, there is a connection to the tropical plume extending north
from the system west of Hawaii as the next wave rides eastward near the
Aleutians, so there could be more of a heavier burst of precipitation
with this first wave, and, additionally, it is not all that cold to
start. After this relatively fast moving wave timing of any further
identifiable systems gets a little trickier, with the best guess Friday
night into Saturday (11 Mar), as the overall trough tries to close off
and shift south and east, with mostly scattered precipitation around by
about Sunday. Whatever happens from Friday and thereafter it will be
very cold, with thicknesses down into the mid 520s (dam), so very low
snow levels but moisture amounts could be an issue.