Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Sacramento California
234 PM PST Tue Jan 16 2018
active weather pattern continues, with a stronger and colder
storm system ariving Thursday-Friday. A similarly cold and wet
storm system is expected Sunday-Monday.
our weak frontal system from last night has moved east of the
region, with a couple of showers lingering across the region.
Precipitation totals were light for the most part, with readings
generally below a third of an inch. Fog and low stratus has slowly
eroded across most of the valley today, save for a few patches
near the Delta and San Joaquin Valley. A shortwave ridge of high
pressure will build over the area tonight into Wednesday. With
increasing subsidence, fog and low clouds are likely to return to
much of the Central Valley late tonight and Wednesday morning. A
few showers will also be possible along the northern mountains
tonight into Wednesday morning.
The next Pacific storm system will begin to move inland Wednesday
evening, with the front passing through the interior during the
day Thursday. Showery precipitation should continue into Friday.
This system will be stronger than the current one, but rain
shadowing may occur across portions of the valley, limiting lower
elevation precipitation. Orographics should be quite strong for
the Shasta drainage and Sierra however. This system will be a fair
bit colder than recent storms, with snow levels starting at or
below major Sierra pass levels, and lowering to 2500-3500 feet on
Friday. Our current forecast calls for 12-18 inches of snow above
6000 feet along the Sierra, and a dusting possible down to around
3000 feet. Light snow accumulations will also be possible above
2000 ft along Interstate 5 in Shasta County.
As the trough axis passes overhead, a bit of instability is
projected across portions of the Central Valley Friday afternoon.
A slight chance of thunderstorms will be possible, with the
primary impact being the potential for accumulating small hail /
Extended discussion (saturday through tuesday)
Overall we should see dry conditions on Saturday as a weak ridge
pushes off to the east. A trough will drop down into the pacnw on
Sunday and that will push a cold front into norcal. Widespread
rain can be expected across the valley and mountains by Sunday
afternoon continuing into Sunday night. Things look to dry out by
Monday afternoon with lingering mountain showers. The system does
look to be a wetter system and snow levels are expected to be on
the low side between 3500 to 4500 feet. Mountain travel will
likely be difficult Sunday and Monday.
Models diverge quite a bit as we head into Tuesday. The ec builds
in a ridge pushing the could front to our south and bringing in
dry weather. The GFS is weaker with the ridge and stalls the front
over norcal as a short wave tracks east, this would keep rain in
the forecast for the valley and mountains. Have rain chances in
the forecast for Tuesday but confidence in rather low right now.
We will see cool temperatures for the start of the period with
them running as much as 5-10 degrees below average in the higher
elevations. Temperatures will return to near average by the end
the period though.
Mainly VFR conditions this afternoon but local MVFR/IFR possible
at the southern terminals due to low cigs. Areas of valley fog
develop after 4z Wednesday and IFR/LIFR possible through 20z
Wednesday. Winds remain under 10 knots.
Winter Storm Watch from Thursday afternoon through Friday
afternoon for West Slope northern Sierra Nevada-western Plumas