Area Forecast Discussion
Issued by NWS Sacramento, CA

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553
FXUS66 KSTO 202148
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
248 PM PDT Fri Apr 20 2018

.SYNOPSIS...
Dry and warm through early next week with just a chance of showers
or thunderstorms over the Shasta County mountains on Sunday. Chance
of showers or thunderstorms over the mountains mid to late next
week.

&&

.DISCUSSION...
Afternoon temps trending some 4 to 8 deg warmer over our CWA than
yesterday, and as much as 14-15 degrees at BLU and TVL. As the ridge
axis shifted inland today, there was a northerly zephyr (gentle
breeze) today which made it just a delightful spring day in Norcal.

This will be just the start of several days of quite warm, well
above normal max temps over most locations (as much as 8-15 degrees
above normal), and very spring-like temps, (60s mtns to 80s Valley)
into next week. Slower progression of the closed upper low over the
Ern Pac next week keeps the ridge and warm air mass over Norcal,
with Mon-Wed now looking to be the warmest days over most locales.
Forecast high temps for the RDD Airport are in the mid to upper 80s
on 4/23 & 4/24 near their record maxes for those dates, otherwise
the records for our main CLIMO sites are in the 90s which we are not
forecasting. A strong temperature inversion will form overnight
causing some good thermal belts to develop over the Foothills
through the weekend.

Short wave (currently off the Pac NW coast) will be moving across WA
on Sat and will increase Nly pressure gradients over the Nrn
mtns/Vly which could result in locally stronger winds/breezy
conditions Fri nite into Sat. Then on Sun, the models remain
consistent in their forecast of a progressive short wave trof moving
through Norcal during the day, and we will continue to forecast a
chance of showers over the Shasta Co/Lassen NP mtn zones mainly
during the afternoon/early evening, along with slight cooling into
Mon morning. There should be a modest Delta Breeze blowing during
the afternoon as the flow turns onshore. However, the cooling may
not reach the Nrn SJV where there usually is a day lag in cooling.
This will again be followed by some modest warming on Mon as high
pressure rebuilds over Norcal.

Other than the slight chance of afternoon storms over the Shasta
Co/Lassen NP mtns on Sun, dry wx is forecast for Norcal until the
chances for mountain thunderstorms increase during the middle to
latter part of next week as the closed upper low over the Ern Pac
moves closer to the CA coast. The Cloud mass currently near 155-160W
is the predecessor to this developing system.   JHM

&&


.EXTENDED DISCUSSION (Tuesday THROUGH Friday)

Warm temperatures continue Tuesday and Wednesday as upper level
ridge remains over NorCal. Daytime temperatures will remain in
the mid to upper 80s for the valley and mid 50s to upper 60s for
the mountains, around 10 to 15 degrees above seasonal normals. A
closed upper low lingers off the coast through Wednesday, creating
enough instability for a chance of a few afternoon thunderstorms
over the NorCal mountains. The closed low approaches the area
Thursday through Saturday, allowing temperatures to cool to near
normal by Friday. Precipitation could become more widespread over
the mountains Thursday and Friday as the low passes over the area,
though the details associated with this system remain uncertain
at this time. HEC
&&

.AVIATION...

VFR conditions at the TAF sites the next 24 hours. Winds
generally around 10 kts or less.

&&

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

&&

$$



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