923
FXUS66 KSTO 232233
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
333 PM PDT Thu May 23 2019

.SYNOPSIS...
Scattered rain showers and thunderstorms with locally heavy rain
and flooding over the Sierra today and all interior Norcal
mountains Friday as one low pressure system moves east and another
drops in to take its place. Scattered showers and thunderstorms
most areas over the weekend with below normal daytime
temperatures. Afternoon mountain showers continuing through mid
week.

&&

.DISCUSSION...
Afternoon thunderstorms are producing heavy rain in the Sierra
and foothills south of I80. A Flash Flood Warning has been issued
through 6 pm for western Placer and southern Nevada County, in the
area around Colfax. CHP reports 1 1/2 feet of water in the west
bound lane of I80. Roadway flooding has also been reported on I80
from Drum Forebay to Crystal Springs. Motorists are strongly
urged to avoid that area. If that is not possible, slow down and
avoid any flooded portions of the road! An Urban and Small Stream
Flood Advisory has been issued through 6 pm for the Motherlode and
adjacent Sierra as additional storms bring very heavy rain and
local flooding.

Divergent flow aloft and orographics will again bring afternoon
showers and thunderstorms to the mountains Friday afternoon with
some threat of showers to the northern Sacramento valley as well.
The low over the Pacific Northwest now looks to be an inside
slider type dropping into the Great Basin on Saturday. Therefore,
the greatest risk of any shower activity will be over the eastern
CWA with enough instability for thunderstorms over the Sierra
Cascade range. Another cold upper low is now forecast to drop out
of the Pacific Northwest on Sunday but this system should take a
more westerly track. As a result, scattered showers will be a
possibility over the entire CWA and north state with a threat of
thunderstorms over the Sierra Cascades.

Daytime highs will be taking a drop Saturday and Sunday as these
two cold systems drop through the north state especially on Sunday
when daytime highs are expected to come in from 10 to 15 degrees
below normal. For the most part, snow levels should be above pass
levels so mountain travel impacts should be minimal this weekend
although snow levels may drop to pass levels at times causing
periods of slick roads. Sunday`s cold upper low is forecast to
move east of the state Sunday night but will likely continue to
have impacts on the CWA weather going in to the early part of next
week.

&&

.EXTENDED DISCUSSION (Monday THROUGH Thursday)
An upper low will make its way from the southwest US towards the
Central Rockies on Monday limiting precipitation chances to just
the northern and eastern portion of the foothills and mountains in
NorCal. Models indicate a small chance for some thunderstorms
mainly over the most northern portion of California, and have
added thunder to the forecast for Monday afternoon. A drier
pattern will be seen next week although there are some scattered
chances for showers over the Sierra crest throughout the week.
Temperatures will be on the rise through the week with Valley
highs in the mid 70s Monday, reaching into the mid and upper 80s
by Thursday.

&&

.AVIATION...
VFR conditions are expected at sites over the next 24 hours.
North winds may gust up to 25 knots through 00Z in the Sacramento
Valley, mainly north of KOVE. Localized IFR/MVFR conditions may be
seen over the mountain and foothill regions after 18Z through
late tonight associated with isolated showers and thunderstorms.

&&

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

$$