NATIONAL WEATHER SERVICE FORECAST DISCUSSION
Area Forecast Discussion
Issued by The National Weather Service
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000
FXUS66 KSTO 242125
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
225 PM PDT Thu May 24 2018

.SYNOPSIS...

Afternoon and evening thunderstorms possible mainly over higher
terrain this week. These are expected to extend across the Valley
Friday. Slightly cooler than normal temperatures continue across
most of the region through Saturday, warming to well above normal
early next week.

&&

.DISCUSSION...

Main forecast concerns for the short term period are continued
chances for mountain thunderstorms, rain and isolated thunder for
the valley Friday, snow accumulations for high elevations of the
Sierra south of highway 50 Friday through Saturday.

Latest water vapor and model analysis shows the cut off upper
level low has now moved well outside the region of weather
influence as another disturbance approaches northern California
from the west. Upper flow has returned to the northwest, which
will continue to allow for thunderstorm development, but storms
should move to the east of the crest this afternoon and tonight
instead of to the west. Brief heavy downpours and some small hail
can be expected in the stronger storms.

Friday will be the coolest day of the week as the upper low passes
directly overhead. Temperatures across the area will range between
10 and 20 degrees below normal with chances for valley rain and
isolated thunder, mainly in the morning. Latest model runs have
the majority of the instability over the mountains Friday, so
valley thunder is expected to be minimal. However, scattered rain
showers in the morning for the valley commute is possible.

With the cold system moving overhead, chances for snow showers are
possible for the high Sierra (8000 feet and above) for locations
south of Highway 50. Model forecasts have swayed a bit on the
actual accumulation amounts, but WPC 50th percentiles have
somewhere between 1 and 3 inches of snow expected by Saturday. Due
to Memorial Day weekend outdoor recreation, went with a Winter
Advisory for locations near the area.

Heading into the weekend, and upper level ridge builds and
temperatures will begin to warm back to and slightly above normal.
Temperatures across northern California are expected to increase
by 15 to 30 degrees from Friday to Sunday.

&&

.EXTENDED DISCUSSION (Monday THROUGH Thursday)

We will see a ridge to start the extended period and that will
bring a big warm up for NorCal. We will see some of the warmest
highs so far this year and they will run 30-40 degrees warmer than
Friday`s expected highs. If you have outdoor plans for the
holiday weekend you will want to be prepared for the heat. With
the ridge overhead we will see dry conditions. As we head into the
mid part of the week we will see a trough push the ridge
eastward. This will cool are daytime highs back to near normal.
This trough will also bring back the chance for mountain showers
and thunderstorms mainly during the afternoon and evenings.
Looking at soundings and CAPE values it does look rather weak and
that should keep the activity isolated.

-CJM

&&

.AVIATION...

VFR conditions are expected at the TAF sites through 9z. We will
see increasing clouds this evening with overcast skies between
3500 and 5000 feet developing in the late overnight. Light
scattered showers are expected at the TAF sites after 9z and will
taper off around the southern terminals by 18z. Showers will
linger up north into the evening. MVFR/IFR will be possible in the
vicinity of any showers. Overall we will see 5-11 knots winds but
10-18 knots during the evening.

-CJM

&&

.STO WATCHES/WARNINGS/ADVISORIES...

Winter Weather Advisory from 5 AM Friday to 5 AM PDT Saturday
for West Slope Northern Sierra Nevada.

&&

$$
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