Wildfire in California, August 14 to 31, 2021

Area Forecast Discussion
National Weather Service Sacramento CA
124 PM PDT Sat Aug 14 2021

.SYNOPSIS...
High to locally very high heat risk is expected for much of the
region this weekend. Mostly dry weather forecast except for a few
isolated mountain thunderstorms through the weekend. Temperatures
will moderate early next week.

&&

.DISCUSSION...
GOES-West visible imagery shows wildfire smoke continuing to
impact parts of northern California again this afternoon. Compared
to yesterday, areal smoke coverage and thickness isn't quite as
high. This, combined with the strengthening upper level ridge over
the Great Basin, is boosting temperatures region-wide with most
Valley communities running 5 to 10 deg F hotter compared to 24
hours ago. Forecast highs this afternoon range from the upper 90s
to around 108 deg F across the Sacramento and northern San Joaquin
Valleys. This corresponds with widespread high heat risk, thus
the prompting Heat Advisory that is in place through the weekend.

The big caveat with temperatures this time of year: smoke from
wildfires. Smoke, like cloud cover, partially reflects incoming
solar radiation. If incoming solar is decreased, daytime heating
potential is decreased. At night, smoke can act as a "blanket" and
insulate, keeping low temperatures warmer than they ordinarily
would be. As smoke coverage varies from hour-to-hour and from
location-to-location, this introduces plenty of challenges with
both the high and low temperature forecast. No matter how you
look at it, the next several afternoons will generally fall into
one of two categories, either 1) clear and very hot, or 2) smoky
and slightly less-hot.

As a result of the building ridge, afternoon/evening shower and
thunderstorm potential will be fairly limited in the mountains. If
any convection does materialize, the best potential (10 to 15
percent chance) would be along/near the Sierra crest.

Upper level trough approaching the Pacific Northwest will begin
to cut into the ridge on Monday, and will allow for 500 mb
geopotential highs to begin scaling back over northern California.
This will provide gradual day-to-day synoptic cooling with the
biggest change from Monday to Tuesday. As this ridge breaks down,
expect some locally gusty onshore winds, particularly in the
Coastal Range and Sierra/Cascades through the favored ridgetops,
gaps and canyons, as well as in/around the Delta as early as
Sunday afternoon/evening, and repeating again on Monday and
possibly Tuesday. This will bring some elevated fire weather
concerns over portions of northern California. // Rowe

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.EXTENDED DISCUSSION (Wednesday THROUGH Saturday)...
A trough will be just east of California on Wednesday, which may
linger over the Central US through the mid to late week period,
while high pressure continues to build over the Pacific. Winds
look to shift offshore for a bit mid-week due to this trough
passage, which could bring area wildfire smoke back into the
Valley for a period. Otherwise, dry weather continues with high
temperatures near or slightly above average for mid August. As
stated in previous discussions, wildfire smoke will continue to
impact exact temperature forecasts.

&&

.AVIATION...
VFR conditions expected the next 24 hours, except
for areas of MVFR/IFR conditions vicinity wildfires. MVFR likely
to predominate at KRDD, possibly RBL at times. Elevated smoke
layers from 040-060. Local southwest wind gusts 25-30 kts vicinity
west Delta, otherwise gusts generally less than 12 kts.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 7 PM PDT Sunday for Central Sacramento
Valley-Motherlode-Mountains Southwestern Shasta County to
Western Colusa County-Northeast Foothills/Sacramento Valley-
Northern Sacramento Valley-Northern San Joaquin Valley-Shasta
Lake Area / Northern Shasta County-Southern Sacramento Valley.

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