FXUS66 KSTO 211015
AFDSTO
Area Forecast Discussion
National Weather Service Sacramento CA
315 AM PDT Wed Oct 21 2020
SYNOPSIS...
Dry weather and breezy northerly flow will continue this week with
periods of stronger winds resulting in elevated fire weather
conditions. Cooler temperatures by the weekend with potentially
another strong north to east wind event Sunday into Monday.
.DISCUSSION
...
Nearly cloud-free skies cover the region early this morning.
Moderately strong northerly surface pressure gradient remains
across NorCal early this morning while the easterly gradient has
decreased a bit compared to 24 hours ago. Local northerly gusts in
the teens to mid 20s mph are present across the northern
Sacramento Valley and surrounding terrain, and the increase in
wind is keeping temperatures in the mid 70s across the Redding and
Red Bluff areas.
The persistent ridge over NorCal is forecast to flatten somewhat
today as the upper trough drops down the BC coast into the PacNW.
A dry cold front will push into far northern California this
afternoon, and then sweep southward tonight and Thursday. The
northerly surface pressure gradient is forecast to increase again
this afternoon and tonight with more widespread breezy conditions
expected to return to the Sacramento Valley.
Easterly gradient will increase tonight as the front moves south,
so we`ll see an increase in winds again across the northern
Sierra foothills. The tight surface gradient will linger Thursday
before subsiding on Friday. Gusts of 40-50 mph will be possible
across the windier locales, and combined with low RH and very dry
fuels, critical fire weather conditions are nearly certain.
A brief lull is expected on Saturday, then the next batch of
short-wave energy is forecast to drop down from the north on
Saturday night into early Sunday as the upper pattern amplifies.
Another dry cold front is forecast to move south Saturday night
bringing a renewed increase in surface pressure gradients.
.EXTENDED DISCUSSION
(Sunday THROUGH Wednesday)...
A cold upper level trough will be digging into the Great Basin
for the start of the extended period. This will bring active
weather to NorCal mainly in the form of strong winds across the
Sacramento Valley Sunday and then over the Sierra for Sunday night
into Monday. Currently looking like a 10+ mb MFR to SAC gradient
by mid Sunday afternoon and a SAC to RNO gradient of 11+ mb Sunday
night. This should result in strong winds. We will also see a
push of cold air with this trough and overnight lows Sunday night
will be quite chilly falling into the teens over the Sierra and
into the 40s in the Valley. The cold air in place over the Sierra
and over NV will likely enhance some of the gusty downslope winds
Sunday night. Wind will be the main impact of the trough but a few
light rain/snow showers will be possible in the high Sierra I-80
south Sunday. Forcing looks weak and there will be a lack of
moisture and therefore little to no rain or snow accumulation is
expected.
As we head into the Monday the trough will be digging into the
Desert SW. This will leave an increased gradient over NorCal with
breezy winds continuing. They do not look to be as strong as
Sunday though. Very dry daytime humidity and poor to moderate
overnight recoveries are expected along with the winds and this
will lead to elevated fire weather concerns during this period.
Quieter weather settles in for the rest of the period as upper
level ridging begins to build in. High temperatures will remain
near to just below normal along with chilly overnight lows.
Daytime humidity will remain very dry with moderate overnight
recoveries. -CJM
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