Area Forecast Discussion
Issued by NWS Sacramento, CA
FXUS66 KSTO 091054
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
354 AM PDT Sun Mar 9 2014
A stream of Pacific moisture will bring precipitation to the
region today and early Monday, especially for areas north of I-80.
Breezy winds are possible Tuesday. Dry and mild conditions return
for the remainder of the week.
A 1.25-1.5" precipitable water plume (per latest satellite
estimates) is on track to bring precipitation to NorCal next 36
hours. Areas north of I-80 will see the most precip with periods
of rain today into early tomorrow, while areas south may not see
anything until later tonight. At the surface, recent KSMF
aircraft soundings indicate a very dry layer persisting around
800mb. One would expect precip aloft to have a tough time
reaching the surface given this dry layer, yet light rain has
been reported tonight from around Yuba City northward, indicating
more saturated profiles north of KSMF. High-res models have had a
tough time handling this situation as none of them are indicating
any Valley precip until later today (although the HRRR has
trended wetter in recent runs). Meanwhile, the latest radar
mosaic shows this early round of precip waning offshore, with the
more focused precip associated with a frontal band much further
north. All this makes for a complicated precip forecast today
from Redding south. Have opted for a slight chance of rain from
Sac vicinity northward, with better chances in the Redding/Red
Bluff area and a good bet over the mountains.
The action picks up late today into tonight as the trough axis
moves inland and an associated upper-level jetstreak brushes the
region. Closer to the surface, a cold front (most evident in
frontogenesis plots from 850-700mb) pushes southeastward across
NorCal with the moisture plume at its leading edge. The jet
position puts our area in the favorable right entrance region,
while the frontogenesis provides extra dynamics. Meanwhile, the
NAM is showing some marginal instability close to the surface
associated with the front. Have thus included a slight chance of
thunderstorms tonight, with the timing matching expecting frontal
passage. The front is currently expected to reach the North Sac
Valley around 3z-9z and the I-80 corridor by 9-15z. While this
timing is not the best for thunderstorms, our recent convective
event a few days ago demonstrated that frontal convection can
still go on overnight. Several difference worth noting between
this event and the last is the preceding surface dewpoints will
not be as high and surface convergence will not be as strong,
making the current event less favorable for convection compared
to the last.
Precipitation across the area should come to an end quickly by
mid-day Monday as drier air moves in and developing subsidence
aloft works against any lingering showers. Only lingering chance
could be some orographically-enhanced showers over the mountains.
Due to the subtropical origin of the moisture plume, snow levels
will be quite high and above Sierra passes today. Snow levels
will quickly lower behind the cold front to around 5500 to 6000
ft by mid-day tomorrow, although precip will be winding down by
that point. All told, we`re looking at 2-6 inches of snow near
Sierra pass levels with up to 10 inches over the higher peaks.
Lower elevations in the mountains can expect 1-2+ inches of rain,
with the Feather River area and parts of Shasta County likely
seeing the most. In the Valley, 0.5-1" is expected in the North
Sac Valley, tapering to a quarter inch or less from Sacramento
Tight surface pressure gradients will develop on the backside of
the trough by Tuesday along with strong winds around 700mb. This
would bring gusty north winds to the north and west sides of the
Sacramento Valley, strong east winds for the west slope of the
Northern Sierra, and very strong winds to mountain ridgetops.
.Extended Discussion (Thursday through Sunday)
Increasing confidence that high pressure will strengthen across
the region later this week and next weekend. This will result in
dry weather with very mild temperatures for the interior of
NorCal. Look for Central Valley highs to warm through the 70s into
the lower 80s by the weekend with mountain readings warming from
the 50s and 60s into the 60s and 70s.
Little change in the pattern expected through 06Z Monday as
moisture continues to stream up into NorCal from the southwest.
VFR conditions will continue with CIGS generally 070-100 with sctd
-SHRA mainly N of I-80. Front moves S aft 06Z with widespread MVFR
conditions developing with IFR conditions over higher terrain.