Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 270540
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
1040 PM PDT Mon Jul 26 2021

.SYNOPSIS...
Widespread sprinkles, or light showers, possible through Tuesday.
Mainly late day thunderstorm chances expected over the Sierra
Nevada through the week. Cooler temperatures early this week
before warming to above average again by mid-week.

&&

.DISCUSSION...
The Evening Update: Appears this evening`s CAMs have picked up on
the decaying MCS over/near the Sierra Crest in our Tuolumne and
Alpine Counties at press time that will spread mainly light showers
or sprinkles over our CWA tonight and then working up to the north
during the day on Tue. Folks in the Valley should not be surprised
if they "smell rain" or experience some sprinkles/light showers.
Looks as if the shower threat will end by around 1800 Tue (6 pm).
Still does not appear there will be enough elevated instability for
thunder. In fact the strongest forecast REF from the NAM 3km is
forecast N of SAC early Tue morning with a dBZ in the low 30s which
usually means a noticeable light shower.

The cloud tops of the MCS are warming quite rapidly so the thunder
threat is most likely to remain over the higher elevations of the
Sierra mainly S of Tahoe/Hwy 50. Chances are low that thunder will
occur at the lower elevations of our CWA. In fact the HREF has about
an 11% chance of Prob of REFL > 40 dBZ in the aforementioned area
just N of SAC. The 20 pct probabilities are all outside of our CWA
thru Tue.

As mentioned last nite, the cloud cover did a number on the max
temps today, and the cloud cover will linger through most of the day
up N, but will gradually erode/thin by the middle of the day over
the Srn portion of the CWA where more solar insolation will warm it
up into the mid 90s or so. Marine layer has already lowered slightly
from yesterday and KSUU seems to have lost some onshore momentum
already, so the warming in the Srn half of the Valley looks
definite.

Next chance of thunder in our CWA should be over the Sierra again
Wed afternoon. JHM

.Previous Discussion...
Monsoon moisture continues to build north into NorCal around a
strong upper level ridge set up over the central part of the
country. PWATs have pushed into the 0.60 to 1.10" range across the
area and we will continue to see them increase overnight with them
maxing out over an inch tomorrow morning into the afternoon. The
building moisture has brought quite a bit of cloud cover today and
even a few sprinkles across the region. Overall instability is
limited over our area this afternoon but there is some along the and
east of the Sierra Crest and that is where some more intense showers
have developed along with some isolated thunderstorms. This trend
will continue mainly along the Sierra crest from around I-80 south
into the early evening.

The main feature we are keeping an eye on is a short wave trough
currently in SoCal that is spinning around the ridge. This short
wave trough will continue to track north into the overnight and
tomorrow and will be accompanied by the higher moisture values.
This will bring light shower chance to the entire area later
tonight and will continue into tomorrow. The best chances will be
over the higher elevations. Upper level instability will be
limited and should keep activity mainly as showers. Some weak
instability will begin to build in during the afternoon but even
that looks to be limited given the cloud cover we will see with
this. Overall the showers are not going to produce much rain given
the amount of dry air under the mid-level moisture but a brief
downpour can`t be ruled out mainly over the higher elevations.
The cloud cover will hold daytime highs down tomorrow with them
running near average in southern areas to as much as 15 degrees
below average in northern areas. The forcing will push north by
tomorrow night with dry conditions setting back in and clearing
skies.

The upper level ridge will slowly push east Wednesday into the end
of the week which will bring a warming trend to the region and we
will see highs push back into the triple digits across the Valley.
A trough off the coast of CA will cut off the Delta flow also
which will limit cooling in the Delta and Sacramento area.
Widespread moderate heat risk is expected by Thursday. Monsoon
moisture will remain in place but we will see a lack of forcing.
This will keep thunderstorm chances to the Sierra during the
afternoon and early evening mainly from about I-80 south. Gusty
erratic winds and brief downpours will be possible with any
thunderstorms we see this week.

-CJM

&&

.EXTENDED DISCUSSION (Friday THROUGH Monday)...
Interior NorCal remains situated between ridging over the Rockies
and troughing over the Eastern Pacific through the extended. This
should bring above normal temperatures that gradually cool
towards the end of the period. Monsoonal moisture surge could
cause a few thunderstorms to develop along the Sierra crest,
mainly south of I-80.


&&

.AVIATION...
VFR conditions at TAF sites next 24 hours. Local MVFR to IFR
conditions in wildfire smoke, mainly over Plumas County. Isolated
to scattered thunderstorms possible over the mountains. Scattered
-SHRA developing over the foothills this evening then ISOLD to
 Scattered -SHRA over the Vly after 07Z. Local southerly wind
 gusts up to 20 kts from KSAC northward. Local southwesterly
 surface wind gusts 25-35 kts vicinity west Delta.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$


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