624
FXUS66 KSTO 190501
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
1000 PM PDT Fri Oct 18 2019

.SYNOPSIS...
Light showers are forecast for the northern mountains and far
Northern Sacramento Valley overnight tonight and into the day
Saturday. Otherwise, dry throughout the remainder of the forecast
period. Below normal daytime temperatures this weekend will warm
to above normal next week. Mid week could see slightly breezy
north winds in the Valley.

&&

.DISCUSSION...
The Evening Update: Pacific trof moving onshore with vort max
located off the WA/OR coast at 04z. This feature will be moving into
ID by 00z Sun keeping the threat of precip over the Nrn/NErn Sierra
mtns. Clouds are preceding the vort max covering all of Shasta Co at
press time and as far S as RBL. Light radar echoes are just N of our
CWA and only spotty/isolated showers are forecast by the NAM WAVE
over the Nrn mtns overnite. Scattered light showers expected Sat
morning over the Nrn zones as the vort max (energy within the trof)
moves into ORE around 18z. As this feature moves into Srn ID during
the afternoon the showers should generally diminish in Norcal. Looks
as if there will be more cloud cover over the Srn portion of the CWA
in the afternoon due to the timing of the energy and with satellite
trends showing enhanced cloud cover SW of the vort max which
should spread over our CWA on Sat.  JHM

.Previous Discussion...

A weak trough will slide over southern Oregon and northern
California tonight which should bring light rain to our northern
most areas. Most of the precipitation threat will be over the
mountains north of interstate 80 and over the far northern
Sacramento valley with most of the CWA will see at least some cloud
cover. Saturday evening, the chance for precip will come to an end
as high pressure builds over the West Coast. This will allow for
some breezier north winds on Sunday; however, RH values are expected
to have moderate recovery but not especially strong will have
moderate recoveries. Come Monday, northerly winds will increase
slightly, with RH values starting to falter slightly.
Current thinking is that there will be some elevated fire weather
concerns Monday into Tuesday, but not enough to warrant any sort
of fire weather product.

&&

.EXTENDED DISCUSSION (Tuesday THROUGH Friday)...A stout upper
ridge sitting just offshore of the state on Tuesday morning will
gradually retrograde toward 140-130W longitude. As this occurs, a
potent trough drops down from western Canada with a track likely
into the Rockies/High Plains. The squeeze play of sorts between
these two systems will enhance height/pressure gradients across
the region. All of the guidance depict a strengthening
thermal/pressure trough up the Valley while higher pressures are
off toward the north and east. The 12Z GFS is currently stronger
than the 12Z ECMWF with the former depicting KSAC-KMFR and KSAC-
KRNO gradients in the 12 to 14 mb range. Such numbers are quite
impressive for the typical north to easterly flow events. Given
dry fuels and relative humidities significantly dropping on
Wednesday and Thursday, increasing fire weather concerns exist.
Given the forecast is still 5 to 6 days away, subtle changes in
the configuration of this weather pattern will alter impacts
across the region. One thing for certain is it will be a dry
period with Valley high temperatures in the low to mid 80s which
is 5 to 10 degrees above climatology. ~BRO

&&

.AVIATION...

VFR conditions the next 24 hours at TAF sites. Sustained winds
generally below 12 knots. A weather system will bring light
rain to our northern most TAF sites, mainly after 06z Saturday.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$