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588
FXUS66 KSTO 290935
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
235 AM PDT Sat Jun 29 2024

.SYNOPSIS...
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Near to slightly above normal temperatures continue through the
weekend, before a stretch of hot weather with attendant widespread
Major HeatRisk and areas of extreme HeatRisk arrive from Tuesday
into next weekend. Additionally, periodically breezy northerly winds
and dry conditions will introduce fire weather concerns in the
Valley and adjacent foothills through midweek.
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&&

Key Points
- Long-duration, major heat event expected this week impacting
  holiday outdoor events.
- An Excessive Heat Watch remains in effect for the Valley, Delta
  and foothills from Tuesday morning through Saturday evening.
- A Fire Weather Watch remains in effect for the western side of
  the northern and central Sacramento Valley and adjacent
  elevations below 3000 feet Tuesday morning into the evening.
- Elevated to critical fire weather conditions Monday through
  Wednesday.

.Discussion...
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Latest GOES-West satellite imagery shows nothing but clear skies
across interior NorCal early this morning. Despite this, relatively
stagnant weather conditions at the surface are keeping overnight
temperatures a few degrees higher than last night. Resultant low
early morning low temperatures in the mid to upper 60s are
anticipated throughout the Valley and foothills, with mid 40s to low
60s at higher elevations.

Broad troughing aloft is expected to remain in place across interior
NorCal through the weekend while an anomalous ridge begins to build
in across the eastern Pacific. An embedded shortwave within the flow
pattern looks to traverse the Pacific Northwest this weekend as
well, and while appreciable impacts will be limited across interior
NorCal, an isolated pop up shower or thunderstorm cannot be
completely ruled out this afternoon and Sunday afternoon along the
crest of the southern Cascades and far northern Sierra. Current
probabilities of thunderstorm development remain around 5 to 10% at
this time, with the expectation that any developing cells will
rapidly move eastward off of the crest. Otherwise, onshore flow
will continue to dominate the southern Sacramento and northern San
Joaquin Valleys, with a shift toward more northerly surface winds
for central and northern Sacramento Valley locations today.

This overall pattern will keep temperatures near to slightly above
normal this weekend, with high temperatures in the 90s to low triple
digits in the Valley and adjacent foothills, with 70s to 80s at
higher elevations. The majority of ensemble guidance indicates that
the eastern Pacific ridge will continue building into the week
ahead, pushing the broad troughing eastward through the early week
period. This will usher in a warming and drying trend for the week
ahead, with breezy to locally gusty northerly winds anticipated
through midweek as a result of the exiting trough and arriving ridge
interaction.

Winds gusting 20 to 30 mph are expected to occur late Monday night
through Tuesday afternoon, with strongest winds through western
portions of the northern and central Sacramento Valley. Additionally
breezy northerly winds, with gusts 15 to 25 mph, will be possible on
Monday and Wednesday as well. Given the northerly component to the
wind, a rapid reduction in afternoon humidities is anticipated with
primarily teens to 20s on Monday, falling to single digits to mid
teens by Tuesday. As a result, a Fire Weather Watch is in effect for
the western side of the northern Sacramento Valley and adjacent
foothills on Tuesday.

Furthermore, a more robust heating and limited overnight recovery
trend is expected to begin by Tuesday. While the ridge axis
itself looks to remain offshore through much of the week at this
time, the anomalous strength off the ridge in tandem with
typically warming northerly winds, will result in a likely
prolonged period of well above normal temperatures this week. High
temperatures well into the triple digits are expected throughout
the Valley by Tuesday afternoon, with 80s to 90s expected at
higher elevations. This corresponds to a notable jump from
widespread moderate HeatRisk on Monday to widespread major
HeatRisk by Tuesday.
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&&

.EXTENDED DISCUSSION (Wednesday THROUGH Saturday)...

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Cluster analysis remains more or less in lockstep with respect to
the upper level pattern through the end of the week. Anomalous
offshore ridging and attendant high heights aloft look to
dominate. This will result in a continuation of seasonably hot
weather for interior NorCal. High temperatures well into the
triple digits look to persist throughout the Valley and adjacent
foothills, with mid 80s to upper 90s still possible at higher
elevations. Current probabilities of exceeding 100F are 95+% in
the Valley and foothills on Wednesday and 75% to 95% probabilities
through the remainder of the week.

With broad ridging being the primary catalyst aloft, very little
onshore flow and attendant overnight relief is anticipated.
Resultant overnight low temperatures in the 60s to low 70s are
expected for Valley and foothills locations. As a result,
widespread major HeatRisk continues to be highlighted with areas
of extreme HeatRisk also showing up from Wednesday into next
weekend. Moving into the weekend, there are some ensemble
discrepancies regarding the upper level pattern, namely the
interaction of the eastern Pacific ridge with the retrograding
southern CONUS ridge. However, current consensus indicates a
continuation of at least above normal temperatures and widespread
major HeatRisk through next weekend.
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&&

.AVIATION...

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VFR conditions next 24 hours. Surface wind mainly below 12 knots
except vicinity west Delta southwesterly local gusts 15-25 kts.
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.STO WATCHES/WARNINGS/ADVISORIES...
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Fire Weather Watch from late Monday night through Tuesday
evening for Central Sacramento Valley in Glenn, Colusa, Yuba,
Northern Sutter, and Butte County Below 1000 Ft-Eastern
Mendocino NF-Northern Sacramento Valley to Southern Tehama
County Line Below 1000 Ft-Southeast Edge Shasta-Trinity NF and
Western Portions of Tehama-Glenn Unit.

Excessive Heat Watch from Tuesday morning through Saturday
evening for Carquinez Strait and Delta-Central Sacramento Valley-
Motherlode-Mountains Southwestern Shasta County to Western
Colusa County-Northeast Foothills/Sacramento Valley-Northern
Sacramento Valley-Northern San Joaquin Valley-Southern
Sacramento Valley.
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$$