Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 221009

Area Forecast Discussion
National Weather Service Sacramento CA
309 AM PDT Sat Oct 22 2016

Cooling trend this weekend with a chance of precipitation
spreading over the forecast area Monday through next week as an
upper level disturbance approaches northern California.



A few high level clouds were moving across the I-80 corridor
overnight as moisture streams ahead of the incoming upper level
disturbance currently located over the eastern Pacific. Model
height analysis and water vapor imagery continued to show the
eastward progression of the system, which will impact northern
California for the start of this coming workweek.

Overnight clear skies and temperatures dropping into the upper 40s
and lower 50s have brought increased relative humidities,
therefore some locations could see some patchy fog this morning in
low lying areas especially near rivers and creek basins. Any
developing patchy fog this morning should dissipate by mid
morning. Cloud cover will gradually increase this weekend as the
upper low approaches from the west with high temperatures
remaining near normal in the 60s/70s across the forecast area.

The main forecast concern is the approaching storm system in the
eastern Pacific. Latest model guidance has slowed the upper
disturbance arrival to early Monday morning with precipitation
spreading in coverage from west to east over northern California
throughout the day. Little instability in the model atmospheric
profiles will allow for rain showers, but thunderstorms are not
expected at this time. Precipitable water values are forecasted to
be well above the 100th percentile, which increases confidence
that moderate to heavy rain is likely at times for many locations
and could impact travel from Monday through Tuesday night. Breezy
winds are possible, especially for higher elevations as a stronger
jet stream rounds the upper trough base on Monday into northern
California. Widespread rain will continue through Tuesday night.

Snow levels with this storm should remain mostly above 8000 feet
for locations along and south of Red Bluff, but northern mountains
could see snow levels drop to 7000 feet. Mountain passes will
receive little snow accumulations, but the main area of concern
for significant snowfall are the mountains near Lassen Park for
Monday night into Tuesday. Winds gusts in combination with heavy
snowfall at higher elevations near the park could create
dangerous whiteout conditions at times.



Models differ on timing of waves lifting out of the persistent
trough over the eastern Pacific into NorCal later next week, and
how much entrainment of deeper moisture from the south will occur,
as the ECMWF is generally wetter. Will leave POPS generally in the
chance category until better timing resolution develops.


VFR conds nxt 24 hrs with SCT-BKN250, exc local MVFR vsbys nrn SJ
Vly 13Z-16Z.


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