Area Forecast Discussion
Issued by NWS Sacramento, CA
FXUS66 KSTO 122257
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
257 PM PST Thu Dec 12 2013
A weak system brushes far Northern California bringing a slight
chance of showers over the coastal range and Shasta Co. mountains
overnight. Temperatures will continue to warm through early next
week. The potential for another weak system exists for northern
California next Wednesday.
.Short Term Discussion (Today through Sunday)...
Temperatures are trending a few degrees warmer today than
yesterday from high pressure, though daytime temperatures are
around normal for this time of year. The pattern shifts inland as
a weak trough approaches the west coast tonight. This will
increase cloud cover and bring a slight chance of showers for the
coastal range and Shasta County mountains overnight. The
morning low temperatures will be slightly warmer than previous
mornings due to the cloud cover and slightly modified air mass.
Patchy frost may still be possible for some of the locally colder
valley locations, but a hard freeze is not likely.
A dry and warm ridge pattern quickly builds back in Friday and
through the weekend which will continue the warming temperature
trend. Near to slightly below overnight temperatures are still
forecast the next several nights, but with no significant threat
of freezes. Some gusty easterly mountain winds are possible Friday
night/early Saturday as surface high pressure strengthens over the
Great Basin. Shen
.Extended Discussion (Monday through Thursday)
First part of the EFP shows the evolution of the Omega-Rex pattern
into a weak trof along the W coast. The ECMWF/GEM are more
progressive than the GFS in rotating the trof inland which may bring
a chance of some light precip to our nrn mtns by midweek. For
Wed/Thu we did expand the PoPs swd over the Siernev due to
uncertainty regarding the apparent dry NWly flow of the two foreign
soil models...while the native GFS model slides the old "Rex"
portion of the current pattern along the CA coast. Diffluent flow
from this scenario may allow some light precip over the Siernev.
NTL...the QPF is very low. The main impact regardless of model
choice is cooling temps...after what is forecast to be a warm/mild
Mon afternoon. Nly flow should produce some adiabatic warming
effects nudging max temps towards 70 in the nrn Sac Vly.
Toward the latter half of next week...another highly amplitude
pattern is forecast to evolve with a trof over wrn NOAM and a ridge
over the ern Pac. The GFS model trend tool has been trending
deeper/stronger with the trof/ridge pattern. Air flow trajectory is
forecast to be from AK and the Yukon instead of from the Arctic and
thus not quite as cold for the latter half of next week as our
recent past. While there is usually more spread in the modeling with
deep trofs...there is generally more reliability in the ridges...and
by next Fri (20th) this pattern has a reasonable chance of evolving
for the 6-8 day forecast period. Although the models show moisture
spilling over the ridge and sliding into the Pac NW...the bulk of
the moisture is forecast to remain north of our CWA as no short-waves
or low centers are forecast to slide down the NW flow at this time.
Thus...a very strong ern Pac block pattern is setting up which
spoils the chances of significant precip in Norcal. If the pattern
doesn`t change...and quickly...for the last 10 to 11 days of the
calendar year...we would be looking at the driest calendar year on
record over a (good) portion of our CWA. JHM
Mainly VFR conditions the next 24 hours, except local MVFR/IFR
HZ/BR tonight thru early Fri in the southern Sacramento and
northern San Joaquin Valleys. Weakening frontal system will bring
clouds mainly AOA 10 kft roughly during the 06Z-15Z Fri time
frame north of Sacramento...and possibly scattered-areas cigs AOA
5 kft over Siernev. Precip mostly limited to sprinkles/flurries
over coastal and Shasta Co mtns. Light winds today becoming Nly
5-10 kts Fri.