Area forecast discussion
National Weather Service Sacramento California
315 PM PDT Sat may 18 2013
Short term discussion...
Upper ridge over the epac extending into norcal is being dented by
short-wave trough moving into the pacnw. This system is bringing
some clouds to far northern California this afternoon and may lead
to a few showers into this evening across the northern and eastern
portion of Shasta County. Temperatures early this afternoon are
running around 3 to 6 degrees warmer than 24 hours ago even with
the cloudiness across the far north.
Short-wave skirts to the east through the Great Basin and into The
Rockies on Sunday allowing high pressure to quickly rebuild across
norcal. Northerly surface gradients tighten considerably later
tonight into Sunday and will lead to breezy north winds through
the Central Valley and foothills along with warmer overnight
temperatures at the north end of the SAC valley. Gradient becomes
more easterly Sunday night into early Monday leading to down
canyon winds in the northern Sierra and continuing north winds
along the west side of the Sacramento Valley. Gradual warming of
temperatures is expected with milder overnight lows in the thermal
belts along the foothills.
Upper ridge builds over norcal Monday with continued warming
expected with lighter winds and subsidence over the area. Monday
is expected to be the warmest day with high temperatures in the lower 90s
in the Central Valley. Upstream trough dropping down the West
Coast of British Columbia begins to influence norcal weather Tuesday as stronger
onshore flow develops ahead of it and lowering heights aloft and
thickness cooling begin to lead to general airmass cooling.
&&
Extended discussion (wednesday through saturday)
A closed upper-low rotates into the Pacific northwest by
Wednesday...bringing cooler weather to much of the West Coast and
chances of precipitation as far south as norcal. Models continue
to show high certainty in the formation of the low and slowing of
its progression as it moves inland. The forecast becomes less
certain later into the week as secondary waves or even secondary
lows rotate around the parent low and brush norcal. In
general...chances of precipitation will increase further north. Shasta
County has the best chance currently of our forecast area to see
some rain showers...particularly Friday morning according to the
European model (ecmwf). Temperatures will drop by around 10 degrees Wednesday as
cooler moves in...and look to remain on the cool side of average
through the period.
Looking on towards next weekend...the European model (ecmwf) flattens the low into
a broad trough along the West Coast...which would dry our area
but keep US cool. The GFS meanwhile has been trending deeper with
a secondary wave and as of the 12z run today is creating another
closed low just offshore...supporting continued unsettled weather
over our northern areas. As a result of models indicating some
activity but a lack of consensus...have gone with near to
slightly above climatological chance of precipitation for the mountains
from around Red Bluff northward...with cool temperatures for all
areas. -Dvc
&&
Aviation...
VFR conditions continue over interior norcal next 24 hours. A
weak low pressure system will brush the area with a few middle-level
clouds from around krbl north and some isolated showers in Shasta
County through the early evening. North winds will increase in the
Central Valley tomorrow morning...with local gusts over 30kts in
the afternoon. -Dvc
&&
Sto watches/warnings/advisories...
none.
&&
$$
