The weak upper-level wave that the models have been bringing across the coast the past few days is just now crossing the coast a little N of the Bay Area. Earlier this mrng there were a few radar echoes in the Trinity Alps/Shasta area, and at present there are a few small showers moving through the American River Basin. I expect no accumulations of consequence.
This upper level trough is now embedded in the broader
scale difluence aloft along the West Coast, and had
120kt NW flow on its back side at 12Z this mrng, based on aircraft reports. The NAM digs this
all the way down to over Sonora state, Mexico, by 00Z
3 Jan, in the process preserving the broader-scale
difluence along the coast.
The IOP prospect for 4 Jan discussed yda is still on
track. Attm, the weather feature in question is a broad, low-amplitude trof in the
central Pacific with a 150kt, basically westerly
jet on its S side. Consistent with consensus over the
past several days, this trof is fcst to become confluent as
it apchs the Gulf of Alaska, and the subtropical high
underneath the anticyclonic shear on the south side
of the upper trough strengthens to 1040+ mb
and moves eastward out of the central Pacific to between 135 and 145W and to near
the latitude of SFO by 4 Jan.
The deep surface lows associated with the trough aloft
are moving into the Gulf of Alaska and bring a trailing front with
copious pcpn into the Pacific NW and BC by tomorrow. This
front slowly slips southward as the trough aloft comes
eastward. Rain begins from Cape Mendocino northward
by 00Z 4 Jan. However, as the front comes southward,
the surface geostrophic onshore flow ahead of it
weakens, again in agreement with earlier forecasts,
so that despite good onshore flow aloft,
by the time the front is into Nrn Ca on the
4th, onshore geostrophic flow at the sfc is weak,
and northerly flow at the surface is strengthening
behind the front. In other words, deep warm advection
south of the front is weak or nonexistent by 00Z
All this plus the GFS-predicted pcpn suggests to me
that chances of Blue Canyon getting at least 1" of pcpn
out of this are ~60%. However, I think > 2"
is very unlikely, no more than 10%. Indications from the progs are
that the event in the ARB will start no earlier than about 10pm Pacific time on
the 3rd, and end by late afternoon on the 4th (say near 00Z 5 Jan). Heaviest pcpn
is anticipated to be roughly 4am to noon on the 4th.
Snow levels will drop during the event. Taking an average of my forecast of 5,500 feet initially and the CNRFC's of 7,000 feet initially puts the starting snow level at above 6,000 feet. Snow levels are xpctd to drop to near 3,500 to 4,000 feet by the storm's end.