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242
FXUS66 KSTO 300809
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
109 AM PDT Sun Jun 30 2024

.SYNOPSIS...
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A general warming trend begins today, culminating in dangerous
heat and widespread major HeatRisk with areas of extreme HeatRisk
from Tuesday into next weekend. Critical fire weather conditions
are also anticipated in the Sacramento Valley from late Monday
through Wednesday.
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&&

Key Points
- Long-duration, major heat event expected this week impacting
  holiday outdoor events.
- Excessive Heat Warning remains in effect for the Valley, Delta
  and foothills from Tuesday morning through Saturday evening.
- Red Flag Warning is in effect for the western side of the
  northern and central Sacramento Valley and adjacent elevations
  below 3000 feet Tuesday morning into the evening.

.Discussion...
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A few straggling high clouds are lingering over interior NorCal
early this morning as a trough moves across the Pacific Northwest,
but otherwise clear skies are noted elsewhere. Some northerly
downslope wind gusts are being observed throughout the northern
Sacramento Valley as well, with the last gasps of onshore flow
holding on from I-80 southward across the Valley and Delta. As a
result, overnight low temperatures are only expected to fall into
the mid 60s to low 70s throughout the Valley and foothills, with mid
40s to low 60s at higher elevations.

The presence of broad troughing aloft today will allow the power
struggle between onshore flow and northerly flow at the surface to
continue today. High temperatures this afternoon are expected to
rise a few degrees compared to yesterday. Widespread 90s to low
triple digits are anticipated in the Valley and foothills, with 70s
to 80s at higher elevations by this afternoon. Occasional gusts of
15 to 25 mph will remain possible through the early evening while
the trough axis is overhead. A brief instance of an isolated shower
or thunderstorm cannot be completely ruled out this afternoon and
evening across portions of Plumas and Lassen counties, but
probabilities of thunderstorm development remain around 5 to 10% at
this time.

Moving into the week ahead, the trough aloft is expected to
gradually transition eastward as a robust ridge begins to build
across the eastern Pacific. This will spur a more notable warming
trend across interior NorCal through midweek, with little indication
of appreciable relief on the horizon. Monday looks to be the
"coolest" day of the week, with widespread high temperatures in the
low to mid triple digits in the Valley and adjacent foothills, while
80s to 90s are still expected at higher elevations. Temperatures
will continue to warm on Tuesday and Wednesday, with widespread mid
to upper triple digits (greater than 105, less than 110) anticipated
in the Valley and adjacent foothills by Tuesday afternoon and upper
triple digits to low 110s likely by Wednesday afternoon. With
little, if any, Delta breeze influence expected through midweek,
overnight low temperatures in the mid 60s to near 80 in some areas
throughout the Valley and foothills will offer little overnight
relief. As a result, widespread major HeatRisk and areas of
extreme HeatRisk look to arrive on Tuesday and remain in place
into at least next weekend.

In addition to the hot weather that looks to dominate the region
this week, a period of breezy to locally gusty northerly winds is
expected from late Monday night through Wednesday evening. Gusts of
20 to 30 mph will be possible throughout the Valley and adjacent
foothills, with strongest winds expected along western portions of
the northern and central Sacramento Valley. Given the typically dry,
northerly aspect of the winds, afternoon minimum humidities into the
single digits to low teens are anticipated on Tuesday and Wednesday.
Overnight humidity recovery looks to remain poor as well, especially
across the northern and central Sacramento Valley. As a result,
elevated fire weather conditions are expected on Monday, becoming
critical fire weather conditions for Tuesday and Wednesday.
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&&

.EXTENDED DISCUSSION (Thursday THROUGH Sunday)...

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Ensemble guidance then indicates a slight eastward transition of
the eastern Pacific ridge through the late week period as the
ridge axis looks to become centered over NorCal. There is some
subtle uncertainty as to the exact strength of the ridge aloft as
it moves inland, but the background trend of hot and dry weather
continuing will remain through the end of the week and into next
weekend. Resultant widespread high temperatures in the triple
digits are expected from Thursday through Sunday across the Valley
and adjacent foothills while 80s to upper 90s persist at higher
elevations. In turn, this will lead to a continuation of
widespread major HeatRisk and areas of extreme HeatRisk through
much of the weekend as well. Ensemble guidance does indicate the
potential for at least some semblance of Delta breeze initiation
across the weekend which may introduce some modest overnight
relief across the Delta and its immediate vicinity, but warm
overnight lows are still expected to prevail elsewhere.
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.AVIATION...

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VFR conditions next 24 hours. Local northwesterly surface wind
gusts 15-25 kts northern Sacramento Valley thru 06Z Monday, and
local southwest gusts 20-30 kts vicinity west Delta after 00Z
Monday, otherwise surface wind gusts generally below 12 kts.
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&&

.STO WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from 11 PM Monday to 8 PM PDT Wednesday for
Carquinez Strait and Delta-Central Sacramento Valley in Glenn,
Colusa, Yuba, Northern Sutter, and Butte County Below 1000 Ft-
Eastern Mendocino NF-Eastern Portion of Shasta/Trinity NF-
Northern Sacramento Valley to Southern Tehama County Line Below
1000 Ft-Northern Sierra Foothills from 1000 to 3000 Ft. Includes
portions of Shasta-Trinity and Butte Units-Southeast Edge Shasta-
Trinity NF and Western Portions of Tehama-Glenn Unit-Southern
Sacramento Valley in Yolo-Sacramento Far Western Placer,
southern Sutter and Solano County Below 1000 Ft.

Excessive Heat Warning from 11 AM Tuesday to 8 PM PDT Saturday
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.

&&

$$