Flooding and Strong Wind in California, April 6 to 7, 2018

Area Forecast Discussion
National Weather Service Sacramento CA
344 AM PDT Fri Apr 6 2018

.SYNOPSIS...
A warm and wet system will move through NorCal today and tonight
with a transition to showers by Saturday afternoon. Moderate to
occasionally heavy precipitation is expected with breezy
winds. Colder air will move in late Saturday with a few inches of
snow at pass level. Sunday and Monday look dry with the next
chance of precipitation over Norcal next Tuesday.

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.DISCUSSION...
Light overrunning type precipitation has been falling over most of
the CWA through the overnight hours. So far precipitation amounts
have ranged from a few tenths of an inch over Shasta county to
only a few hundredths of an inch over Stanislaus county. Snow
levels remain elevated under upper level ridging at above 8000
feet so travel impacts over the mountains are minimal. A Pacific
frontal band has begun moving in to northwest California this
morning and this front will bring with it significant
precipitation thanks to a subtropical moisture tap spanning back
beyond Hawaii. The period of heaviest precipitation appears will
be from later this morning through Saturday morning. Storm totals
during this 30 hour period still on track to range from 1 1/2 to 2
1/2 inches in the valley to several inches over the mountains.
Snow levels will remain high during this period at above pass
levels so travel impacts over the mountains will stay minor. High
snow levels will bring some snow melt however which combined with
moderate to heavy rainfall could bring some localized flood issues
and so the flood watch will stay in place. Winds will be breezy
this afternoon and overnight and may even reach wind advisory
criteria briefly but likely only with the passage of the cold
front.

The rain will drop off fairly quickly on Saturday afternoon as the
front passes east of the state. At this time the main upper low
associated with the front will be pushing into the Pacific
Northwest leaving Norcal under weak troughing. Therefore,showers
will remain a threat Saturday afternoon with a slight threat of
thunderstorms as well although stability progs are not showing a
great deal of instability at this time. Snow levels will drop
below pass levels Saturday in the cold air associated with the low
but precipitation amounts will have dropped off enough that only
an inch or two of snow is expected at Sierra pass elevations.

Building upper ridge over the eastern Pacific and west coast will
lift the shower threat northward on Sunday with the entire CWA
under dry conditions by Sunday afternoon. Clearing skies will
allow temperatures to warm to above normal by Sunday afternoon.
The upper ridge is forecast by amplify over the west coast on
Monday bringing fair skies and warming temperatures with highs 10
degrees or more above normal and pushing back up to around 80
throughout the central valley.

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.EXTENDED DISCUSSION (Tuesday THROUGH Friday)

Weak system forecast to move through the West Coast ridge Tuesday
bringing a quick shot of showers to areas mainly north of I-80. A
colder system may bring showers further south Wednesday night and
Thursday before moving east by Friday.
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.AVIATION...

Moist Pacific weather system will bring moderate to occasionally
heavy precipitation over the next 24 hours. Widespread MVFR with
areas of IFR in the Central Valley with IFR/LIFR over the northern
Sierra Nevada. Snow levels above 100. Southerly winds gusts 20-30
kts develop by 19Z.
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.STO WATCHES/WARNINGS/ADVISORIES...
Flood Watch from this afternoon through Sunday morning for
Burney Basin / Eastern Shasta County-Carquinez Strait and Delta-
Central Sacramento Valley-Clear Lake/Southern Lake County-
Motherlode-Mountains Southwestern Shasta County to Northern Lake
County-Northeast Foothills/Sacramento Valley-Northern Sacramento
Valley-Northern San Joaquin Valley-Shasta Lake Area / Northern
Shasta County-Southern Sacramento Valley-West Slope Northern
Sierra Nevada-Western Plumas County/Lassen Park.

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