River Flood Warning 19, NWS Sacramento, CA

Area Forecast Discussion
National Weather Service Sacramento CA
830 AM PST Wed Mar 6 2019

.SYNOPSIS...
Occasional showers and thunderstorms with mountain snow today.
Mountain travel will be impacted and additional rain may increase
flood potential. Showers continue Thursday into the weekend.

&&

.DISCUSSION...
Morning Update: Front has progressed into the Ern portion of our CWA
this morning, over the W Slope Sierra, and we should begin to see
the surface winds turn from SE to S/SW over the Srn portion of the
CWA. BUFKIT indicates this will result in more straight-line
hodographs over the Srn portion of the CWA, but with
onshore/upvalley flow and topographic forcing of the wind in the
Central/Nrn Sac Valley, the wind profiles are still favorable for
anticyclonic curving hodographs up to 1 km and favorable CAPE values
for thunderstorms, and the potential for rotating storms, say N of
KMYV this afternoon.

Another concern and active monitoring is the potential for a
convective precip band that could set-up over Shasta Co this
afternoon and evening. The higher resolution QPF/REF progs indicate
a heavy rain band developing this afternoon and lasting well into
the evening over mainly Shasta Co. This heavy precip band sets up
along a nearly stationary/quasi-stationary front developing mainly
due to topographic forcing which often persists until the synoptic
forcing relaxes. The remnants of the closed upper low is just off
the Nrn CA coast and is forecast to move inland this afternoon over
that portion of the CWA, while other subtle and weaker waves move
inland over the rest of the CWA which could trigger other convective
activity over the rest of the CWA.

Snow profilers this morning indicate the snow level around 6000 ft.
   JHM

.Previous Discussion...
Front moving inland early this morning accompanied by a line of
showers with mainly light to moderate rain. Gusty south to southeast
winds have increased ahead of the front and are currently gusting
around 25-35 mph across much of the region. Snow levels remain
fairly high (around 6000-7000 ft per profiler data) and KBLU precip
remains rain.

Front and widespread precipitation moves through early this
morning followed by scattered showers and thunderstorms from late
morning through the afternoon as weaker upstream short-wave
approaches. Forecast soundings in the post-front environment this
afternoon indicate the potential for a few longer-lived storms
with their attendant risks, heavy rain, hail and gusty winds.

Higher res models also continue to indicate the potential for a
persistent line of showers and thunderstorms over Shasta County
this afternoon into this evening with the setup of the north
valley convergence zone.

12 to 18 inches of additional snow can be expected across the
passes today, with 2 feet or more over the peaks, before snow
tapers a bit tonight. Showers may bring another foot or more of
accumulation through Thursday morning. Additional valley rainfall
of 1/4 to 1/2 inch is expected today with much lighter amounts
expected tonight and Thursday.

With saturated soils, and streams, creeks and main stem rivers
running high, additional rain may increase the potential for
flooding, especially near thunderstorms which are capable of
bringing higher amounts in a short period of time.

Showers will persist Thursday and Friday as a longer wave trough
moves through the West. Lowering snow levels during this time
will result in some snow below the BLU level which could lead to a
downgrade of the winter storm warning to an advisory.

&&

.EXTENDED DISCUSSION (Sunday THROUGH Wednesday)
A broad positively-tilted upper trough will settle offshore of
the state on Sunday. Forecast guidance has been fairly consistent
in carrying the parent upper low toward northern Baja California
by early next week. Some snow showers are possible over the
mountains (primarily from Highway 50 southward) to conclude the
weekend. Given the general lack of moisture, perhaps an inch of
snow could be squeezed out over regions which see such a shower.

As the axis of lower heights exits toward northern Mexico, dry
northerly winds are anticipated to commence next week. The next
system to keep an eye on would be Tuesday as a longwave trough
advances through the Pacific Northwest. It remains to be seen
where the southern extension of this storm will reside. Currently
the 00Z ECMWF is flatter while the 06Z/00Z GFS shows a more
amplified solution. While the latter does have ensemble support,
would like to see the ECMWF join this wetter scenario. Thus, have
kept the forecast mainly dry for the early/middle part of next
week for now.

The temperature forecast supports slightly below average
temperatures for early to mid March. Valley high temperatures
should sit anywhere from the mid 50s to low 60s while mountain
locales remain in the 30s and 40s. ~BRO

&&

.AVIATION...
TAF sites should generally remain in MVFR conditions throughout
the forecast given passing showers reducing visibilities. KRDD and
KRBL may see conditions deteriorate to IFR due to low ceilings.
Between 21Z-00Z, scattered thunderstorms could affect Sacramento
airports down to KSCK and KMOD. Forecast winds will remain out of
the south to southeasterly direction with gusts possible up to 25
knots. After 06Z, wind speeds are expected to decrease a bit, but
will stay in the 10 to 15 knot range. ~BRO

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 10 AM PST Thursday for Western Plumas
County/Lassen Park.

Winter Storm Warning until 10 AM PST Thursday for West Slope
Northern Sierra Nevada.

Flood Watch through Thursday morning for Carquinez Strait and
Delta-Central Sacramento Valley-Clear Lake/Southern Lake County-
Motherlode-Mountains Southwestern Shasta County to Northern Lake
County-Northeast Foothills/Sacramento Valley-Northern Sacramento
Valley-Northern San Joaquin Valley-Shasta Lake Area / Northern
Shasta County-Southern Sacramento Valley.

&&