...A PAIR OF UPPER LOWS WILL BRING WIDESPREAD PRECIP TO THE REGION
LATER TODAY THROUGH THE WEEKEND...
...T-STORMS POSSIBLE OVER NRN/CNTRL CA/NV IN THE AFT/EVE THE NEXT
FEW DAYS...

.METEOROLOGICAL CONDITIONS (WED PM - TUE AM)...

Overall, forecast remains on track for a pair of upper lows to bring
widespread precip and cooler temperatures to the region later today
through the weekend. Det models have come a little closer together
on the 500 mb height pattern differences on Sunday, but disagreement
remains. Ensemble spreads have not changed much from this morning
either. 24 hr QPF at Blue Canyon in the northern Sierra shows
spreads roughly between 0.50" to 2.50" for Saturday and Sunday with
spreads at Arcata 0.50" to 1.50". Variations on the exact
positioning of the system, even slight ones, make a difference as
precip is expected to be terrain driven. Focus is still across the
Sierra and the higher terrain areas of nrn CA. Where the higher
amounts will focus remains a source of uncertainty in spite of the
12z det models getting a little closer together than this morning.
Where this particularly helps is with regard to freezing levels. The
positioning of the low on Sunday were different enough to result in
nearly 1000 ft of differences in freezing level forecasts over parts
of the Sierra between the GFS and the ECMWF. That difference is now
more on the order of 500 ft or so. How much precip will spill over
into the valleys is also uncertain, often with these offshore
systems models may initially over predict that spillover and then
dial in and reduce QPF over the valleys as we get closer to arrival
time. Thunderstorms remain possible as well, over nrn CA today and
nrn/cntrl CA/NV tomorrow/Friday.

The afternoon update was mainly a blend of WPC guidance and the
latest NBM that just came in. This continues to shift precip a bit
northward with increase of 0.25-1" north of the Monterey Bay and
decreases of 0.10-0.50" to the south. Total QPF for the next 6 days:
2-5.75" Sierra, 2-5.50" Shasta, 1.50-3.75" nrn CA coast, 0.50-1.50"
greater Bay Area (over 2" possible Santa Cruz mountains), 0.25-0.75"
coastal srn CA (up to or just over 1" Transverse), and 0.30-1.75"
down the valleys.