Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 030452

952 PM PDT FRI OCT 2 2015



Upper-level low near the US/Canadian border progged to dig rapidly
Swd into Socal through Sun. The low is forecast to deepen as it
drops Swd with the digging polar jet. The continentality of the
developing/deepening system means a rather dry system...but dynamics
very strong and will squeeze out some precip mainly over the W Slope
of the Sierra. The valley may see some light showers/sprinkles.
Pressure falls ahead of the deepening system and then rapid pressure
rises behind it will result in increasing winds. Initially from the
S ahead of the system through the Carquinez Strait and into the
valley...but more so from the N as the strong upper low and
associated cold front drop Swd rapidly over Norcal.

About a 9 mbs SAC-MFR gradient is forecast by Sat afternoon...
tightening to about 13 mbs Sat evening mainly N of the I-80
corridor. Highly anomalous wind speeds are forecast in the Nrn half
of the Sac Vly and Coastal Range at 850 mbs...and to a slightly
lesser extent at 700 mbs over the Nrn mtn zones. The 850 mbs wind
anomaly and forecast 925 mbs winds suggest the relatively high
probability for wind advsry criteria to be met in the area of the
tightest pressure gradient from the Nrn Sac Vly into the Coastal
Range Sat afternoon/evening...while the 700 mbs anomaly suggests
gusty ridge or highest peak winds over the Nrn mtns Sat nite.
Reasonably confident in issuing wind advsry for the valley/coastal
range for Sat afternoon/evening in the evening update...but will
hold off elsewhere given the higher elevation and less anomalous
speeds forecast for the Nrn mtns. This will likely be a lot more
isolated and problematic...however Jarbo Gap and the higher mtn
peaks (e.g. Swain, Harkness, Humbug, Colby) could be susceptible to
40-50 mph gusts.   JHM

.Previous Discussion...

A warm afternoon with highs in the mid 80s to low 90s across the
valley. That`s a good 10-15 degree warmup compared to 24 hours ago!
However, today`s warm & dry weather will change notably by this time
tomorrow. Key take away as you read the discussion will be the
strong winds...

The models remain in good agreement about the tracking of an upper
level low (aka "inside slider") moving into NorCal by Saturday
afternoon/early evening. This storm will bring a cooler airmass to
the region so that daytime highs will be about 5-10 degrees
cooler than today for areas along and north of Interstate 80. In
addition, moisture with this storm will bring showers and possible
thunderstorms to portions of the valley, but the best chances will
be across the Sierra through the weekend and into Monday. Models
aren`t showing a ton of moisture with this system as PW values
only peak around 0.8". However, one concern is whether any
thunderstorms develop near burnscars as the brief, heavy downpours
may cause debris flows/mudslides.

The low will make it`s way southwestward towards the SF Bay area
overnight Saturday into Sunday morning. As this occurs, there
will be enhanced north to northeast winds across our region. The
strongest winds will occur over the Sacramento Valley, Coastal
Range (western Tehama through Lake counties), and down the mid to
lower slopes of the Sierra from Lassen Peak to I-80 after the low
passes. Wind gusts across the mountains could peak around 40-50
mph for both the Coastal & Sierra ranges...although some localized
spots on the Sierra Crest could near 60 mph. Strong wind gusts are
another main concern this weekend since so many trees are weak or
dead due to the drought and wildfires. Those windspeeds will
likely cause downed trees which may cause power outages in
mountain communities or could impact people still managing the
Valley or Butte wildfire areas.

On Sunday, chances for rain/thunderstorm activity will mainly be
along the Sierra south of Interstate 80 and into the eastern side
of the San Joaquin valley as the upper low continues to track
southward along the coast into SoCal. The center of the low will
linger in SoCal through Monday and wrap around moisture could
continue to impact our region along the Sierra.

Total precipitation amounts from Saturday through Monday will be
around a half inch for the higher elevations in the Sierra, about
0.05-0.15" in the foothills, and less than 0.10" in the valley.

The low pressure system expected to impact the area over the
weekend will be exiting to the southeast by early Tuesday. Dry
weather will continue through next week with upper level ridging
over the region. Winds will be light with temperatures remaining below
normal through Wednesday, then rising to above normal for the end
of the week. EK



VFR conditions expected TAF sites over the next 24 hours. Breezy
northerly winds through around 03z with gusts 20-25 knots down the
central valley and over higher Sierra terrain, then decreasing
overnight. Winds will increase Saturday afternoon over the
northern Sacramento Valley and the southern Cascades, where winds
may gust to 35 kt after 21z. There is also a slight chance of
rain showers with local MVFR conditions in the southern Cascades
after 18z. EK


WIND ADVISORY from 2 PM Saturday to Midnight PDT Saturday Night
for Central Sacramento Valley- Mountains Southwestern Shasta
County to Northern Lake County- Northern Sacramento Valley.

WIND ADVISORY from 5 PM Saturday to noon PDT Sunday for Clear
Lake/Southern Lake County.


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