Special Weather Statement, NWS Sacramento, CA

Area Forecast Discussion
National Weather Service Sacramento CA
331 PM PDT Wed Mar 27 2019

.SYNOPSIS...
Pacific storm moves through today into Thursday with rain,
mountain snow, and possible thunderstorms. Drier weather Friday
into the weekend with unsettled weather early next week.

&&

.DISCUSSION...
The Pacific cold front attendant to the offshore upper low has
just moved across the Greater Sacramento area. This has been
accompanied by a broad band of slow moving southwest-northeast
oriented showers. Somewhat unusual for this region, pulse-like
thunderstorms are firing ahead of this line within a pool of
instability. Normally this would be expected in the wake within
the dome of cool, unstable air aloft. Vertical shear profiles are
favorable for weak rotation so would not be surprised to see a
random funnel cloud report. Otherwise, heavy downpours and the
potential for gusty winds and small hail would be more likely in
the stronger cells. The entire swath of locally heavy rainfall
will continue advancing eastward toward the foothills/mountains
this afternoon. To the north, recent HRRR model runs have shown
indications of the Shasta County Convergence Zone setting up.
This would support an enhanced period of rainfall generally in the
vicinity of Redding. Overall, some lingering showers are possible
across Valley locations into the evening with additional chances
on Thursday given trailing cyclonic flow overhead.

Across mountain locations, the late-season winter storm is well
underway with heavy snow picking up this afternoon as the
baroclinic zone shifts focus to inland locations. This could lead
to a significant reduction in visibilities as well as hazardous
travel. Thus, as mentioned in previous messages, mountain travel
is not advised until the system clears on Thursday morning. The
S-band radar is showing initial snow levels around 5,500 to 6,000
feet. Given the arrival of colder air aloft overnight, expecting
the snow levels to drop to the 3,500 to 4,000 foot range early
Thursday. Like the Valley, numerous showers will continue into
Thursday with brief travel impacts where such precipitation falls.
When it is all said and done, elevations 7,000 feet and above can
expect 1 to 3 feet with locally higher amounts possible.

While some showers could linger into Friday morning over northern
counties (Shasta, Lassen, Tehama, and Plumas), much of the region
will begin to dry out as an upstream ridge approaches. The mean
ridge axis should settle over the state for the weekend ensuring a
dry, mild period. High temperatures are forecast to reach the low
to mid 70s over the Valley with upper 40s to lower 50s in the
mountains.  ~BRO
&&
.EXTENDED DISCUSSION (Sunday THROUGH Wednesday)

The extended period will start out dry with the transient ridge
maintaining its presence over the southwestern U.S. There is
decent agreement operational and ensemble model agreement that a
shortwave will reach the northwestern California coast by Monday
afternoon. An increase in cloud cover accompanied by some warm
advection precipitation is expected for Monday with snow levels
likely to be quite high, generally 7,500 to 8,000 feet. Once this
storm clears the region, model uncertainty grows as the main
operational models (GFS/ECMWF) are at odds with one another. At
the very least the models show run-to-run consistency, albeit with
different forecasts. The GFS favors a building of the eastern
Pacific ridge while the ECMWF depicts influences from a northern
stream trough. Thus, too much uncertainty exists to declare a dry
forecast into the middle of next week.

Looking ahead, the Atmospheric River Detection Tool and other
corresponding ensemble solutions are advertising a wet, active
pattern toward the end of next week and into the weekend. This has
also been highlighted in the Climate Prediction Center Day 8-14
experimental Heavy Precipitation graphics. The blocking upper high
over Alaska bounding an active Pacific jet are the key players in
the pattern change. While there is confidence in this set up,
the southward extent of its influences are a big question mark.
Ensemble probabilities remain highest between 40-45 degree north
latitudes. Will definitely be something to keep an eye on heading
to the period of April 4-8. ~BRO
&&

.AVIATION...

Segments/lines and clusters of thunderstorms over much of
interior Norcal persisting into the evening with MVFR/IFR/LIFR
conditions obscuring higher terrain. Scattered showers and mainly
VFR/MVFR conditions for the remainder of the night after 06z.
Mainly south winds 10-20 gusts to 30 knots through 00z mainly
northern Sac Vly, then lighter south winds the remainder of the
night. Unstable air mass continues into Thu with scattered
thunderstorms redeveloping late morning/afternoon and into early
evening.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 5 AM PDT Thursday for West Slope
Northern Sierra Nevada-Western Plumas County/Lassen Park.

&&