Winter Storm Warning 5, NWS Sacramento, CA

Area Forecast Discussion
National Weather Service Sacramento CA
358 AM PST Thu Jan 31 2019

.SYNOPSIS...
Light precipitation possible south of Sacramento this morning.
Light precipitation spreading across the forecast area Friday.
Heavy precipitation Friday night through the weekend with
significant mountain snow and strong winds at times. Lower snow
levels early next week. Central valley thunderstorms possible
Saturday. Dry late Tuesday through the rest of the week.

&&

.DISCUSSION...
Upper level low pressure system now located off the central
California coast bringing some scattered light showers to the
southwest most forecast area this morning. Delta and northern San
Joaquin valley may see a few sprinkles but this is about it for
today. Some light fog has formed in the central valley again this
morning but nothing too dense. This afternoon and evening look
mainly dry before precipitation starts spreading over the
forecast area late tonight and Friday morning. This initial
precipitation Friday will be light as weak disturbances pivot out
ahead of an approaching front. Even with light rain, cloud cover
and cooling, daytime highs Friday will still come in around normal
for this time of year.

Models fairly consistent in bringing cold frontal band on to the
coast Friday evening. GFS shows a PW plume feeding this front of
over an inch and a quarter so this looks to be a fairly wet
system and significant precipitation is expected. Current timing
puts the front over the central valley by mid day Saturday.
Strongest winds with this system will precede the front early
Saturday morning. Nam shows 925 mb winds over the central valley
topping 50 knots. Surface gradients are not ideal but it will
still likely be quite gusty Friday night and early Saturday for
at least a short period of time ahead of the frontal passage.

By 00z Sunday, models move main frontal band into the Sierra.
Heavy snow is indicated Friday night through Saturday so winter
storm warning still on track especially when combined with
expected strong winds. Upper trough still forecast to push into
Norcal Saturday afternoon increasing instability. With main
frontal ban over the Sierra 00z Sunday, may see a little clearing
over the valley for surface instability and a chance of
thunderstorms. With expected cloud cover, thunderstorms should
stay fairly isolated.

A secondary shortwave trough is forecast to swing across Norcal on
Sunday bringing another wave of increased precipitation. Snow
levels drop from 5000-6000 feet Saturday to as low as 4000 feet on
Sunday as cold air filters in behind initial cold front. Winter
storm warning stays in place through Sunday but lowering snow
levels Sunday night into early next week may require new winter
products for lower elevations.

&&

.EXTENDED DISCUSSION (Monday THROUGH Thursday)

As the initial onslaught of shortwaves exit the region over the
weekend, an anomalously cold upper trough swings through Monday
afternoon. With an origin over far western British Columbia, it
will transport frigid 850-500 mb temperatures into Northern
California. More specifically, 700-mb temperatures are expected to
drop down into the -15 to -18C range. Snow levels ultimately
crash down toward the valley floor in many of the recent models.
The biggest thing working against this system is a dearth of
moisture given a frontal passage will have taken place earlier
that morning. However, the upper forcing with the trough passage
should spawn a broad axis of precipitation which exits the region
Monday night/Tuesday morning. With snow levels across the more
northern counties dropping down considerably, a dusting may be
possible down to elevations around 1,500 feet. Once the system
clears the region, continued cold advection will maintain below
normal temperatures through midweek. On Monday and Tuesday, highs
will generally sit in the upper 40s/low 50s across Valley
locations, while the higher elevations remains in the 20s and 30s.
Overnight temperatures will easily plummet to the freezing mark
over the Valleys with single digits and teens likely in the
mountains. A gradual warming trend is anticipated bringing
temperatures closer to normal as mid/upper level ridging builds
over the western U.S. Some break down of this ridge is possible by
next Friday (Feb 8), but model uncertainty is rather high at this
point. BRO

&&

.AVIATION...

Areas of mist have lowered visibilities to MVFR conditions around
Sacramento. Showers in the vicinity of KSCK/KMOD have put the
northern San Joaquin Valley down to IFR. Some improvement expected
over all these sites after 21Z-00Z. Across northern TAF sites
(KRDD/KRBL), scattered showers near the region by 06Z. All winds
remain light and below 10 knots.  BRO
&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Friday afternoon through late Sunday
night for West Slope Northern Sierra Nevada-Western Plumas
County/Lassen Park.

&&