Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 182204

Area Forecast Discussion
National Weather Service Sacramento CA
304 PM PDT Tue Oct 18 2016

Drier weather returns to NorCal this week as the storm track
shifts north of the region.


Short-wave moving east across the Great Basin this afternoon has
allowed drier NW flow behind it to bring clearing to Norcal. Cumulus
clouds over the mtns (mainly N of Tahoe) will dissipate after dark
to be followed by the high level cloudiness spreading over Norcal
ahead of the upstream trof nearing 140W. This cloudiness will lift
Nwd late Wed as high pressure builds during the next couple of days.

Warmer daytime temps and cool overnight lows are expected the next
couple of days as the airmass dries. Locally breezy north winds will
be possible at times and enhance the drying process. Generally these
katabatic winds are fog inhibitors for the Central Valley...while the
wind-sheltered mtn valleys/basins in the Sierra that received
substantial rainfall will still be conducive for fog/stratus under
strong subsidence and radiative inversions. 925 mbs winds indicate
moderate Nly winds developing tonite (after 06z Wed) mainly on the W
side of the Valley/lee side of the Coastal Range as offshore
gradients increase. These winds should preclude fog in the Lake Co
area and have removed this phenomenon from the grids for tonight.
Temp/dewpoint spreads are expected to be wide enough in the Valley
to preclude fog even though winds should decouple under strong
radiative inversions the next couple of nights.

Max temps were adjusted upward slightly most locations the next
couple of days due to adiabatic warming effects from the Nly winds.
Although max temps may not warm to 850 mbs this time of year...they
should fall within a few degrees of dry adiabatic descent from 850
mbs. Less warming effects are expected on Thu as the Nly 925 mbs
winds weaken.

Cooling should begin on Fri and into the weekend as the ridge shifts
inland and the GOA/NErn Pac trof begins to deepen and the flow turns
onshore. A band of clouds is expected to move inland Fri/Fri nite but
no rain is expected until late in the weekend/early next week.   JHM


On Saturday, Northern California will be in-between a ridge to our
east and an incoming trough to our west. Temperatures will be a
few degree cooler than Friday and right near seasonal normals. The
trough is expected to move further inland on Sunday providing a
few more degrees of cooling and high cloud cover for our area.
It is not until late Sunday night into early Monday morning does
the ECMWF brings in some precip for our area. In contrast, the
GFS brings in a much weaker wave and delays it by about 12 hours.

Beyond this, both the GFS and ECMWF depict a broad trough that
will remain just offshore and rotate energy through our area.
While there are still disagreements on the timing, strength, and
placement of each wave, our confidence is growing that early next
week will be cool and cloudy with periods of rain. Wilson


VFR conditions expected over the next 24 hours as the dry northerly
flow continues to erode any cloud deck. This should also prevent
any fog formation across the Sacramento and Northern San Joaquin
valleys tonight. A very similar day on tap for tomorrow with
north winds around 10 kts for most locations. Wilson


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