Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 211100
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
400 AM PDT Tue Apr 21 2015

.Synopsis...
The threat of afternoon and evening showers and thunderstorms,
mainly over the mountains, will continue through the week with a
gradual cooling trend.

&&

.Discussion...
Partly cloudy skies cover portions of interior NorCal early this
morning. Some of the cloudiness is remnants from yesterday
afternoon and evening`s convection, while the clouds moving up
from central California are associated with the deformation zone
to the north of the weak closed mid/upper level low moving toward
SoCal.

Surface pressure gradients continue to trend toward stronger
onshore flow, and this is reflected in a continuing robust Delta
Breeze at Travis AFB (gusts over 30 kts) and temperatures that are
a little cooler through much of the valley compared to early
Monday morning. The marine layer is now around 2500 ft deep on the
Fort Ord profiler, and temp/dew point spreads are narrower in the
Sacramento area compared to 24 hours ago, so wouldn`t be surprised
to see some morning stratus extending inland into the Central
Valley. Delta Breeze season is back in force!

Satellite imagery shows the upstream trough now approaching 130W
stretching out as it bumps into the Rex Block along the west
coast. The trough will have enough strength to nudge the block
east, but it will split with a portion of the energy forecast to
drop southward off the coast toward SoCal while most of the energy
tracks well to our north through the PacNW.

Enough moisture and instability lingers across the region for more
afternoon and early evening thunderstorms over the mountains
through the end of the work week. The best chance for seeing
convection migrate off the mountains into the valley will be this
evening across the northern Sacramento Valley as the stronger
portion of the trough moves into the PacNW, and Wednesday evening
across the southern Sacramento Valley and northern San Joaquin
Valley as the southern split moves south off the coast setting up
an area of deformation along and south of I-80.

Cooling will spread further inland today as onshore flow
continues. Pressure gradients will likely reach some sort of
equilibrium by Wednesday as the cooler airmass spreads across the
region, so some areas may warm a bit depending on cloud cover.
Another short-wave dropping down from the north on Friday may
bring further cooling to the region.

&&

.Extended Discussion (Saturday through Tuesday)...
A flat ridge continues overhead for the weekend, allowing plenty
of higher level cloudiness to regulate temperatures. However,
enough instability will remain to generate some showers mainly
over the Shasta/Tehama county mountains. The GFS (US) keeps a
rather weak, broad wave extending into northern California early
next week, while the ECMWF (European) holds onto a ridge. Due to
the uncertainty, we`ve opted to keep a mention of precipitation
across the northern mountains (mainly, Shasta/Tehama/Plumas co)
into Monday/Tuesday. Since previous runs have been trending drier,
it will be tough to obtain some, if any, significant precipitation
late in the period. JClapp

&&

.Aviation...
Marine layer 2 to 2.5K ft thick with stratus into wrn Delta south
of KSUU. A 70 percent chance MVFR stratus reaches Sacramento TAF
sites between 12-16Z. KSMF may see stratus 1-2 hrs after KMHR/KSAC
as stratus expands westward from ern Sac county. Near the Delta,
southwest winds 20-32 kt will continue today. Isold aftn/evening
thunderstorms return over Sierra and Coastal Range aft 20Z. JClapp

&&

.STO Watches/Warnings/Advisories...None.

&&

$$






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