Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 171540
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
840 AM PDT Wed Sep 17 2014

.Synopsis...
Temperatures cooling to below normal Thursday with a chance of
showers and thunderstorms. Warmer and drier weather returns for
the weekend.

&&
.Discussion...
Forecast still on track. System is just off shore and will push
into the forecast area starting tonight. System then starts to
close off Thursday afternoon over NorCal. All the while the system
is weakening so any resultant activity should be pretty minimal.
Looks like biggest impact will be cooler temperatures and light
moisture chances. All this is covered very well in current
forecast. Next concern will be with upper low as it closes off
over southern California. This will turn winds around to the east
Friday into Saturday. NAM showing a fair amount of instability
wrapping around on back side of the upper low but GFS much weaker.
Will take a closer look at this for afternoon package but maybe
biggest impact with this would be possibility of winds blowing
some smoke down into valley areas during this time. Will also look
closer at this for afternoon run as well.  Rasch

.Previous Discussion...
Upper trough axis between 130-140W early this morning. Most of the
region is seeing some mid and high clouds ahead of the trough,
though the far southern portion of the forecast area remains
clear. The marine layer remains quite shallow and onshore
gradients are weak.For the most part, temperatures are a little
warmer than 24 hours ago with the increase in clouds and range
from the 50s in the cooler mountain valleys to the 60s in the
Central Valley.

The trough will continue to approach the region today, however it
looks like precip chances inland will likely hold off until
tonight. We`ll see a mix of clouds and sun today. Temperatures
will likely cool a bit further and winds will increase.

The bigger changes will hold off until later tonight and early
Thursday as the trough nears the coast and a weakening frontal
system moves onshore. Scattered showers can be expected across the
region Thursday along with a chance of thunderstorms in the
afternoon as the cooler air aloft moves overhead. Despite pretty
impressive TPW approaching 1.5 inches, overall QPF is forecast to
be light given the rather weak nature of the system. Some areas
may see a 1/4 inch or so, but most of interior NorCal will likely
see less than a tenth of an inch. The cooler airmass along with
the clouds and showers will result in considerably cooler
temperatures on Thursday.

System forecast to close off and drops south to off the SoCal
coast Friday and Saturday before lifting out to our east across
the Great Basin on Sunday. The eastern fringes of the forecast
area may see some clouds and a slight chance of showers and
thunderstorms, but the remainder of the region will warm up and
dry out.

Some considerable model differences develop next week with over
200m height differences forecast at 500 mb in the latest
operational runs over NorCal depending on which model you look at.
With ensemble means still favoring a ridge, will continue the dry
and warm/hot forecast for now.

&&

.Extended Discussion (Sunday through Wednesday)

Some considerable model differences develop next week with over
200m height differences forecast at 500 mb in the latest
operational runs over NorCal depending on which model you look at.
With ensemble means still favoring a ridge, will continue the dry
and warm/hot forecast for now.


&&

.Aviation...

Mainly VFR conditions next 24 hours with isolated
showers/thunderstorms possible over western Shasta County and
Coastal range beginning this afternoon and spreading SE over
valley and into Sierra by tonight. Local SWerly surface wind gusts
up to 35 to 45 kts possible over higher mountain peaks this
afn/eve and overnight. SWerly gusts up to 20-25 kts through delta
and Sac Valley this afternoon.

&&

.STO Watches/Warnings/Advisories...
None.
&&

$$







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