Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 061657
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
955 AM PDT Wed May 6 2015

.Synopsis...
Sierra thunderstorms possible this afternoon as transition to more
unsettled pattern begins.  Low pressure dropping southward from
the Pacific Northwest will bring a chance for showers and
thunderstorms, mainly to the mountains through the end of the
week. Near normal temperatures through the end of the week then
warming over the weekend. Breezy north winds Thursday. Chance of
showers or thunderstorms again early next week mainly northern
mountains.

&&

.Discussion (Today through Friday)...
Initial short wave dropping SEwd over Norcal today is digging the
Wrn coastal states trof. A band of mid/high clouds is associated
with this short wave from Lake/Napa Cos...NEwd into Plumas/Lassen
Cos. The convective chances in the Siernev zones for today/this
afternoon look good as models continue to forecast instability in
the area. In addition...the morning REV RAOB shows an LI of nearly
-2. Behind the short wave...subsidence is resulting in clear skies
and increasing NWly winds with gusts to 25 kts...which are generally
expected to decrease this afternoon as the lower level winds
decrease.   JHM

.Previous Discussion...

Upper low over Pacific Northwest dropping slowly southward today.
Main impact will be cooling most areas and a threat of showers or
thunderstorms over the Sierra mainly north of Tahoe. Most other
areas are likely to see some increased cloud cover as well. Models
now in fairly good agreement in dropping low center down the
northern Sacramento valley overnight. Main instability with this
low position would be from the northern Sacramento valley
eastward. This is where main shower and or thunderstorm threat
will be Thursday and Thursday night although the coastal range
could still see a few light showers. Snow levels will drop low
enough to impact higher Sierra passes. Current forecasts has best
chance of heaviest showers south of about U.S 50 Thursday
afternoon and evening. Heavier showers could produce several
inches of snowfall in this area so have issued a winter weather
advisory for this possibility as several passes are now open for
the summer season. As the low drops southward...northerly surface
gradient increases with a nearly 10 mb gradient from MFR to SAC
developing by Thursday morning. Could see winds over the northern
Sacramento valley reaching close to wind advisory criteria but
believe this will be a fairly short lived event with models
dropping surface gradient quickly in the afternoon. Upper low
shifts into the Great Basin on Friday but is forecast to move
fairly slowly so Sierra could remain impacted by shower activity
through through the day. Low finally moves into the eastern Great
Basin on Saturday which should bring an end to the Sierra shower
threat and warming temperatures.


.Extended Discussion (Sunday through Wednesday)

A weak wave pushes through the transitory ridge on Sunday with a
few showers possible over the Coastal Range into Shasta County.
The GFS has a southward-moving closed low inside 130W and over
Central CA Monday into Tuesday while the EC keeps the low near the
Oregon/California border. Even if the track split the difference,
showers and thunderstorms would likely development over the much
of the mountains and adjacent northern Sac Valley. The low drifts
east of area on Wednesday, but we still have a chance for showers
over northern mountains. Temperatures will remain near normal.
JClapp

&&

.Aviation...

Winds becoming NW over the Sacramento Valley today...gusting to
25 kt after 18z. Northerly winds over north and central Sac
Valley could be 30kt around 1000-2000 ft AGL. Some showers
developing over the Sierra and foothills later this morning, with
some afternoon thunderstorms.      JClapp


&&

.STO Watches/Warnings/Advisories...
winter weather advisory from 5 am thursday to 5 am pdt friday
west slope northern sierra nevada above 6500 feet from U.S 50
southward.

&&

$$





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