000
FXUS66 KSTO 292102
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
202 PM PDT Thu Jul 29 2021

.SYNOPSIS...
Late day mountain thunderstorm chances expected through the
weekend. Above normal temperatures are expected through Friday
with a slight cooling trend for the weekend and early next week.

&&

.DISCUSSION...
Ridge of high pressure remains centered over much of the Central
US with the western edge situated over California. This will
continue to bring southwest flow into interior Northern
California, drawing in monsoon moisture into the area.
Afternoon and evening shower and thunderstorm chances continue
over the next several days. Thunderstorms already began forming
over the Sierra late this morning, and best chances will remain
along and east of the Sierra crest south of Plumas County and
along the Coastal Range. Better chances of more widespread
mountain showers and thunderstorms are possible tomorrow as the
potential spreads north into the Shasta County mountains. Any
thunderstorm may be accompanied by gusty and erratic winds, brief
downpours, and lightning.

Temperatures are forecast to remain above normal through tomorrow
as high pressure remains over the area. Current temperatures
around 1pm are overall similar to yesterday, and highs are
forecast to top out around 100 to 108 degrees in the Valley and
upper 70s to upper 90s in the mountains and foothills. Overnight
temperatures are forecast to be fairly toasty once again tonight,
only falling to the low to mid 70s in the Valley and foothills
with some thermal belt locations in the foothills only falling to
the low 80s. Wildfire smoke continues to impact temperatures
around the Dixie Fire. Smoke is forecast to continue the diurnal
flow that`s generally been observed by draining into the northern
Valley in the early morning hours before the southwest flow
increases and pushes it back to the north/northeast.

An upper level trough forming off of the eastern Pacific Coast
will lower heights over Northern California this weekend,
moderating temperatures. Temperatures are forecast to fall back to
near normal by Saturday, which is 90s across the Valley and
foothills and 70s to upper 80s in the mountains. -HEC

&&

.EXTENDED DISCUSSION (Monday THROUGH Thursday)...
Northern California will likely see southwesterly onshore flow
over the region throughout the extended forecast period. Cluster
analysis is in agreement high pressure over the Southwestern US
and troughing off the Pacific Northwest Coast will keep Northern
California near seasonal temperatures with little day to day
change in the forecast. Thermal belts will also continue to see
mild overnight lows. This pattern will also support a Delta
Breeze. Some discrepancies emerge by the later part of the week in
terms of the depth and location of the trough off the Pacific
Northwest Coast. This could lead to some changes in the forecast
from persistence by the weekend.

&&

.AVIATION...
VFR conditions at TAF sites with passing mid to high level clouds
AOA 15K ft AGL. Local MVFR/IFR conditions in wildfire smoke over
Plumas and Butte counties. Surface winds under 12 kts except local
southwest gusts 25-30 kts vicinity west Delta. Isolated-scattered
showers and thunderstorms over high elevations of the Sierra
south of Tahoe with isolated activity north of Tahoe through 03z,
weakening to isolated showers and dissipating after 07z. Chance of
MVFR weather in heavier storms with isolated downdrafts G40 kts
lee-side of Sierra. Temps AOA 105 deg F will result in density
altitude 3000-3500 ft Thu and Fri afternoons.

&&

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

&&

$$