NATIONAL WEATHER SERVICE FORECAST DISCUSSION
Area Forecast Discussion
Issued by The National Weather Service
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000
FXUS66 KSTO 212212
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
312 PM PDT Mon May 21 2018

.SYNOPSIS...
Afternoon and evening thunderstorms possible mainly over higher
terrain this week. These may extend through the Valley late in the
week. Seasonal temperatures continue across the region.

&&

.DISCUSSION...
An upper level low is centered over the Mojave Desert, with
easterly return flow into Northern California. Showers and
thunderstorms are beginning to develop once again this afternoon
and continue into this evening. They are focused over the
northern Sierra and southern Cascades. At 3 pm, the strongest
storm was in eastern Shasta County near Shingletown, with other
thunderstorms near Butte Meadows in northern Butte County,
extending to the south across Tuolumne County. Some showers could
extend into the eastern side of the Valley, but will weaken as
the move downslope. Storms overall are not expected to be severe,
but could have a brief periods of heavy rain, small hail, and
lightning. These are expected to continue through the evening,
with some isolated showers along the Sierra crest overnight. The northern
Coastal Range should get a bit of a break, now that the low is
further south and east.

Thunderstorms are expected to become more widespread Tuesday and
Wednesday, across the mountains and potentially extending into
the northern Sacramento Valley. Late Thursday mountain and
foothill thunderstorms and widespread Valley showers are forecast
as an upper low moves onshore.

Temperature are warmer today than over the weekend, currently
close to 90 degrees over the northern Sacramento Valley. A weaker
Delta breeze is bringing relatively cooler temperatures to nearby
influenced areas. This will strengthen tonight, bringing more
significant cooling for Tuesday. Temperatures across the area
trend downward through Thursday, as another cool upper low
approaches. Highs on Thursday are expected to be below normal,
generally in the upper 70s to around 80 in the Valley. EK

&&

.EXTENDED DISCUSSION (Friday THROUGH Monday)
An upper level closed low is expected to approach NorCal late
this week. Current model runs show this system impacting NorCal
more than originally expected. Both the GFS and the ECMWF are in
fairly good agreement that the center of the low will move right
over NorCal, bringing widespread showers and thunderstorms to the
area Friday, lingering into early Saturday. As such, I have
significantly increased the chance of precipitation for these
days. After Friday, there are slight differences in timing and
location of the low, though generally it will continue moving east
and out of the forecast area by Saturday night. Showers and
thunderstorms may linger over the mountains later Saturday into
early Sunday. The weather system will cool daytime temperatures
into the 70s across the valley and 40s to 60s for the foothills
and mountains, a whopping 10 to 15 degrees below normal.
Temperatures will rise back to near normal by Saturday. Ridging
begins to build Sunday and is expected to continue into early next
week. Temperatures will warm under the ridge with valley
locations nearing 90 degrees by Monday. HEC &&

.AVIATION...


Showers and thunderstorms have formed over the northern Sierra
Nevada and will continue through the evening. Local MVFR/IFR
conditions possible in these. Otherwise, VFR conditions over
interior NorCal next 24 hours. Gusts up to 25 kts in Delta until
00z Tuesday, increasing to 30 kts overnight. Winds less than 15
knots at TAF sites. HEC
&&

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

$$
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