Area Forecast Discussion
Issued by NWS Sacramento, CA
FXUS66 KSTO 191035
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
335 AM PDT Sun Oct 19 2014
High pressure will provide dry weather and seasonable temperatures
today. A frontal system will provide cooler temperatures and
precipitation chances Monday and Monday night.
One more mild day over Norcal to end the weekend...and then a
cool...showery start to the work week. An upper level trof axis
along 140W will edge Ewd today amplifying the downstream ridge over
the Intermountain West. Synoptic cooling will account for a few
degrees of cooling to the Nrn zones...while near persistence is
expected elsewhere. Meanwhile...the SE portion of the CWA (i.e.
zones 67/69) may see slight warming from yesterday.
The upper trof will move Ewd inside 130W early Mon and begin to
effect the wx over interior Norcal. The last couple of runs show a
slightly more negative tilt with the vort max moving inland Mon
nite. This may slow the progression of the trof a few hrs from
earlier runs...but precip should move across most of the CWA by late
Mon afternoon...except for the far SErn portion of the CWA...and
then over that area Mon evening. Estimated QPFs range from up to .70
inch in the Shasta Co mtns tapering to a couple hundredths in the
Nrn San Joaquin Vly to about .25 inch in the far Nrn Sac Vly. These
values could be augmented by any convection that may develop in the
NErn quad of the approaching vort max behind the surface front.
We blended the NAM/GFS instability progs and the synoptic forcing to
arrive at an area of potential thunderstorms...mainly from Butte/Wrn
Plums counties Southward...and then east of I-5...or roughly Hwy
99/70 Ewd to near the Siernev crest. Bufkit forecast soundings
indicate a strong cap on some of the profiles making the convective
forecast problematic at this time. In addition...the thunderstorm
potential appears to be limited to a short time window...roughly 3
PM til about dark...or up to 4 hrs or so. A shift in the vort max
farther N as suggested by the ECMWF may result in convection over
the Shasta Mtns and in line with the WRF. Have leaned to the NAM for
now due to the digging jet on the backside of the trof.
The trof will progress more rapidly Ewd on Mon...and by Tue morning
will be E of our CWA. Breezy winds with the front and trof will
decrease with drier and milder wx Tue/Wed. A large/deep upper low
over the GOA will maintain a flat ridge over Norcal for the
midweek period which would likely be susceptible to be overrun by
a lot of mid/high cloudiness...aka a "dirty ridge". A frontal
boundary will be strung out across the Pac NW but precip is
expected to remain NW of our CWA. JHM
.Extended Discussion (Thursday through Sunday)
For most of our CWA, the latter half of the week should be fairly
mild, dry weather with valley highs in the 70s to low 80s.
However, extended models continue to hint that the upper low in
the Gulf of Alaska could keep a slight chance of precipitation
along the Coastal range. The models diverge Friday into the
weekend as the GFS wants to track the low into the west coast and
spread the chance of precipitation across all of NorCal.
Meanwhile, the ECMWF keeps a weak ridge over NorCal. So have
blended the two solutions and kept mention of precipitation over
the Coastal range until models come into better agreement. JBB
VFR conditions and mainly light winds continue the next 24 hours.