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HMT Forecast Wed 2/20/2008 2000UTC

We've had precip over the ARB since yesterday afternoon and 24 hour totals
ending 16Z vary with Blue Canyon being representative for the mountains
having 0.28". Higher amounts were observed at lower elevations such as 0.52"
at Folsom. This is consistent with the LAPS 24 hr precip accumulation as of
12Z. Over the past few hours the precip has been becoming restricted to the
NERN ARB. Easterly flow aloft as the cutoff slides by to the S is currently
favoring the Reno area. The precip should be spotty over the ARB this afternoon
and precip chances should end by about 01Z Thu according to today's 12Z ESRL
WRF-NMM run.

The PacJet is poised to reach CA on Thu. Waves within the jet will be
amplifying as we get into the weekend. A prominent atmospheric river shows
up from 132W across the Pacific at 00Z Thu. It starts to break up after
that, though we see about 24mm IWV near SFO at 18Z Thu.

The Thu/Fri event is on track as a negatively tilted open wave followed by
a second even more open wave. This double structure has been shown by the GFS
in some of the previous runs. The start time looks a bit earlier at 16Z Thu
continuing until 12Z Sat. There would be a double maximum of precip at about
22Z Thu and 21Z Fri, with about 1.0" total in the GFS. The first burst of
precip should be a bit warmer with 850mb temps of +1C, and the second shot
a bit colder at -0.5C (probably all snow at Blue Canyon). The ECMWF Hi-res has
about the same amount of total precip, while the ESRL WRF-NMM run suggests
favored areas could receive 2" (starting 19Z Thu).

The weekend event has the 500mb low surviving landfall farther south than
we've seen, along the northern CA coast at 18Z Sun. The precip in the GFS
occurs between 21Z Sat and 09Z Mon with a total of 3.0" (similar to yesterday's
12Z run). 850mb temps start out fairly warm at +3C gradually dropping to +0.5C
near the end, thus after much rain some snow is possible at Blue Canyon
towards the end. The ECMWF Hi-res has a deeper low a bit more to the north
and it moves a bit slower suggesting a slightly later end time. However this
model gives less precip with 1.7" over the ARB with the highest amounts a bit
to the south.

The GFS IWV field has about 22mm nosing in over SAC on 00Z Sun with higher
amounts around 30mm upstream to the SW just offshore between Santa Maria
and Monterey. At this point the overall atmospheric river has a scalloped
shape.

Longer term it looks like a wave on Wed Feb 27 will ride up over a west coast
ridge and hit mostly in BC. The GFS hints that the PacJet may redevelop in
the WRN Pac around Mar 1 with an associated atmospheric river forming.
This is analogous to what we're seeing this week so we might see some
WX in the ARB around Mar 4.

Steve Albers