Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 192257

Area Forecast Discussion
National Weather Service Sacramento CA
357 PM PDT Fri Apr 19 2019

Chance of showers and thunderstorms across Norcal through Saturday
as low pressure from the Pacific moves inland. Significantly cooler
Saturday, then warming well above normal through the middle of
next week.


High cloudiness from the upper low vcnty 31N/123W has spread over
the CWA as the low moves towards the Socal coast. The incoming
neutral to slightly positively tilted upper trof will "kick" the
upper low Ewd while dragging a frontal boundary across our CWA late
tonight and Sat when precip chances should increase in our CWA. NAM
19/00z REF prog suggested some higher REFs and possible
thunderstorms developing E of I-5 mainly over Shasta Co around
sunrise on Sat, with a few cells developing during the late
afternoon and early evening over the Srn portion of the CWA and the
W slope of the Sierra. Forecast BUFKIT soundings indicate the
potential for elevated instability above 4-5 kft, with large scale
ascent from the dynamics from the upper trof. CAPE is rather "thin"
so hail is not the main concern, but light wind fields (slower-
moving cells) and PW over an inch in the morning could yield some
locally heavy rain rates, especially with the cluster or short
segments of cells east of I-5 in Shasta Co, and in the central Sac
Valley in the afternoon. There is a small chance of seeing some half
inch per hour rates, but no heavier rates.

The trof is forecast to be narrower than yesterday, so the precip
chances should wind down in our CWA Sat nite, then dry and milder wx
on Sun on the backside of the trof. Energy dropping Swd on the
backside of the trof reforms the closed low over Srn NV on Sun with
Nly flow developing in its wake. This should allow skies should
clear in time for Sun morning services with a modifying air mass
during the day.

Still expecting 10 to 15 degrees cooler temps on Sat over today, and
then milder temps on Sun, up to 9 degrees above normal across the
Nrn portion of the CWA, to near or slightly below normal temps Srn
portion of the CWA.

Warming accelerates Mon and into next week as upper ridging builds
over the region. Daytime highs are forecast to warm to 10 to 15
degrees or more above normal.   JHM



The extended forecast period beginning Tuesday remains intact as a
sprawling mid/upper-level ridge moves through the southwestern
U.S. Relative to climatology, 500-mb heights should be around 1.5
to possibly 2 standard deviations above average. Ample subsidence
within the anticyclone will afford a few warm days across the
region. Valley high temperatures on Tuesday and Wednesday will
rise into the mid/upper 80s with perhaps a few 90 degree readings
mixed in over the northern Sacramento and northern San Joaquin
Valleys. Toward the conclusion of the work week, the eastern
extent of an offshore upper trough will increase cloud cover over
Northern California. Given the guidance diverge on the forecast
and exhibit poor run-to-run continuity, it is difficult to say
what the impacts will be. At this point, have slightly increased
the chances for isolated showers and thunderstorms across the
mountains on Friday. ~BRO



VFR conditions will prevail today with high clouds spread over the
region. Showers and isolated thunderstorms will arrive to the
northern most TAF sites late this afternoon and evening, before
spreading southward on Saturday. Any such thunderstorm could
diminish visibility and ceilings leading to a brief period of MVFR
to even IFR flight rules.


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