Area Forecast Discussion
Issued by NWS Sacramento, CA
FXUS66 KSTO 272212
Area Forecast Discussion
National Weather Service Sacramento CA
312 PM PDT SAT AUG 27 2016
Mostly a dry weather pattern with average temperatures into next
week. A fast moving trough could bring scattered showers and even an
isolated thunderstorm to areas north of Interstate 80 Monday night
and Tuesday morning.
Inland coastal stratus retreating to the coast in typical diurnal
fashion this afternoon. Temps running near to slightly cooler than
yesterday...the coolest in the far Nrn Sac Vly which should end up
just a few degrees cooler than yesterday. Winds and pressure
gradients lighter/weaker than yesterday in the Srn portion of the
CWA where temps should near persistence for maxes this afternoon.
Marine layer holding around 2600-3000 ft this afternoon and should
lower Sun/Mon as subsidence from building high pressure increases
over the region. The synoptic pattern is rather benign over CA as
the main synoptic features are distant from our area which is
generally in between 3 main wx features. The first is the large low
over Nrn Canada which retrogrades and acts as the kicker for the Rex
Block (#2) currently in the vcnty 35N/140W to move into our area
late Mon nite and Tue. In the interim...the retrogression of the Nrn
Canadian low allows subtropical high pressure (#3) to build Nwd
allowing for slight warming Sun/Mon and a slight "squashing" of
the marine layer.
Model consensus suggests the Rex Block will move over our area early
Tue as an open wave ahead of a deepening upper trof next week. The
NAM elevated instability progs suggest at least a small chance of
precip will be possible in our CWA mainly N of the I-80 corridor.
For now...the best chance of precip in our CWA would be the 12z-18z
Tue time frame...but the timing is likely to vary due to
retrogression and deepening wx systems. The Rex Block does have some
moisture with it on the water vapor imagery with a small area of
visible cloud...and some subtropical moisture from the reservoir of
higher PW air vcnty of 20N/140W is forecast to work NEwd towards and
into Norcal as the moisture becomes entrained into the SWly flow
ahead of the Rex low/trof. This makes elevated convection and
terrain forcing the most likely triggers for convection in our CWA
than surface based instability...with typical inverted-V soundings.
Although our PoP area is rather large...the PoP at a point will be
low with little QPF.
The deeper trough developing over the eastern Pacific next week will
lead to cooler and breezier weather. JHM
.EXTENDED DISCUSSION (Wednesday THROUGH Saturday)
Northern California will remain under a longwave trough pattern
through the extended period, bringing cooler than normal
temperatures for next week. Medium-range models have shown poor
consistency handling a deeper, more robust trough late next week.
However, there continues to be enough confidence to mention a
slight chance of showers mainly along the mountains for Labor Day
VFR conditions across TAF sites next 24 hours. Generally light
onshore winds around 5 to 10 kts except for gusts up to 25 knots
near the Delta.