Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sacramento California
418 am PDT Thursday Sep 3 2015
below average temperatures through Saturday as a weather system
impacts the region. Showers and thunderstorms possible across the
northern portion of the state Friday into Saturday morning. Warmer
weather returns Sunday into next week.
the axis of an approaching low pressure trough is centered near
the 127w longitude line and will gradually track across northern
California now through Saturday. This will bring cooler than normal
weather along with the possibility of showers and thunderstorms.
Temperatures at 3 am were running 1-4 degrees cooler than 24 hours
ago thanks to a moderate Delta breeze of 15-20 mph overnight.
Onshore flow will continue as the trough approaches so highs today
will be in the low 80s for much of the valley, 70s in the Delta,
60s to mid 70s in higher terrain.
Tomorrow, the trough will deepen into norcal bringing a colder
airmass causing Friday to be the coolest day in the forecast.
Valley highs will be in the upper 70s to low 80s...a good 8 to 15
degrees below normal. The air will become more unstable on Friday
while also bringing possible precipitation to portions of norcal
north of Interstate 80. Precipitation should start in Shasta
County late morning then spread southward towards Interstate 80
through the day with best chances in higher terrain. Cape &
modified total totals indices indicate that the most unstable
period will be Friday between about noon into late evening which
is when thunderstorm development will be favored. Snow levels
will also lower between 7000 to 8000 ft so the Lassen park area
could see a dusting of snow. West to southwest winds will be
breezy as the trough moves across our region on
Friday...especially through the Delta and over the mountains.
The trough axis shifts into Nevada by Saturday afternoon which
means showers will diminsh quickly in our County Warning Area. Daytime highs will
gently warm on Saturday but still be about 5-10 degrees below
normal. By Sunday, 500 mb heights will rise as the cooler air
exits and temperatures finally become more seasonal. Jbb
Extended discussion (monday through thursday)
Another short wave trough moves through longwave Monday across
far northern portions of California. Models differ with strength
of the wave with oper GFS strongest and ec weakest. Overall
dynamics and moisture however look limited and expect main impact
for the area will be some locally breezy wind over higher
terrain. Upper ridging from the epac progged to gradually build
over norcal Tuesday into Thursday resulting in dry weather with
high temperatures rising to slightly above normal.
Upper low over b.C. Digs S along west CST with westerly flow. VFR conds over
interior norcal next 24 hours exc local MVFR ceilings poss in showers over Shasta
mountains after 12z Friday. Local southwesterly surface wind gusts to 25 kts through Delta and
over hyr terrain this afternoon into evening.