Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 171135

Area Forecast Discussion
National Weather Service Sacramento CA
435 AM PDT Tue Sep 17 2019

Dry conditions with warmer temperatures expected today. Another
weather system is forecast to bring showers and a few
thunderstorms Tuesday night and Wednesday, particularly over the
mountains. Conditions will dry out with warm temperatures
returning late this week and continuing into the weekend.


A progressive Pacific pattern has ushered yesterday`s system into
the Central Great Basin. Brief height rises via a shortwave ridge
will afford a temporary period of dry weather. 24-hour mid-level
height comparisons show rises in upwards of 12 dm which offers
warmer temperatures today. Valley highs should range from the
upper 70s in the northern Sacramento Valley to low 80s for points
southward. While only a few degrees warmer than yesterday, much of
the day should be mostly sunny with high clouds arriving late.

The second Gulf of Alaska system in the chain will arrive late
Tuesday and continue to impact the region Wednesday, possibly
extending into Thursday morning over the high Sierra. Broad
ascent ahead of the trough passage will spread light to moderate
showers from north to south across interior northern California.
The 06Z NAM continues to show ample moisture advection per PWATs
in the 1 to 1.25 inch range along the cold front. Relative to
climatology, this is roughly in the 90th to 95th percentile. Cold
frontal showers will spread through the region, while instability
driven activity falls in the wake underneath the cool, unstable
upper low. Model CAPE forecasts and simulated reflectivities show
a bulk of the latter activity would be over the northern
Sacramento Valley. Overall, the highest rainfall amounts are
forecast over the southern Cascades and Sierra with 0.25 to 0.50
inches possible.

Unsettled weather should exit the region on Thursday with skies
clearing out from west to east. With the mean trough centered over
Nevada, meridional height gradients set up supporting a period of
dry, northerly flow over the Valley on Thursday and continuing
into Friday. Wind fields do not appear particularly strong
although speeds could reach 15 mph or so on Thursday
afternoon/evening. A continued moderation in the temperatures will
accompany this shift in the winds with highs in the mid 80s on
Friday. This still sits around 2 to 5 degrees below average in
most locations.  ~BRO

A warming trend will continue into the weekend as an offshore
ridge tracks across the West Coast. Consequently, Valley highs are
forecast to reach the upper 80s to low 90s, while upper 50s to
mid 70s will be more commonplace across mountain locales. While
conditions remain warm over the weekend, a shift in the pattern is
noted in the ensemble guidance. By late Sunday, a digging trough
sets its sights on the western U.S. with a position rather up in
the air. Ensemble spaghetti plots shown by the 00Z GEFS indicate a
great deal of west-east spread. The path the resultant upper
low/trough takes will dictate whether the region sees another
northerly flow event or perhaps more precipitation. The past few
runs of the GFS favor a track east of the state into Nevada while
the 00Z ECMWF takes the system overhead. It remains difficult to
trust any individual model given the continued fluctuating tracks.
Definitely worth keeping an eye on given the degree of troughing
suggested by the guidance. ~BRO



VFR conditions next 24 hours. Winds less than 12 knots, except local
southwesterly wind gusts 15-20 kts vicinity Delta after 00z
Wednesday. A weather system will bring showers across interior
NorCal after 06z Wednesday.


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