NATIONAL WEATHER SERVICE FORECAST DISCUSSION
Area Forecast Discussion
Issued by The National Weather Service
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237
FXUS66 KSTO 211103
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
403 AM PDT Tue May 21 2019

.SYNOPSIS...
Another weather system will impact northern California today
bringing more showers and thunderstorms as well as heavy mountain
snow. Below average temperatures continue through the week with
additional mountain shower chances. There is the potential for
more widespread Memorial Day weekend precipitation.

&&

.DISCUSSION...
Early morning water vapor imagery showed a strong mid/upper-level
cyclone spinning off the Oregon coast. As was evident with the
previous couple of systems, heights and wind fields will again be
quite anomalous in nature. More specifically, 500-mb heights
should approach 4 standard deviations below average accompanied by
rather brisk tropospheric wind fields. One major difference with
this disturbance is its more north-northwest to south-southwest
track which is not able to tap into appreciable Pacific moisture.
Expected Valley rainfall totals will be in the 0.10 to 0.25 inch
range with isolated higher pockets in any thunderstorms which may
materialize. Moving from north to south across the region, the 03Z
SREF plume diagrams shows instability falls off considerably.
Consequently, among Valley locations, expect the best chances for
thunderstorms across the northern Sacramento Valley where MUCAPE
values could approach 500 J/kg.

Given mountain precipitation with rather low snow levels for
mid/late May, a Winter Weather Advisory remains in effect for the
Sierra-Cascade ranges for elevations 5,500 feet and above. Such
lower snow levels will lead to travel impacts at mountain pass
levels. Currently anticipating a widespread 6 to 12 inches of
snow with perhaps up to 18 inches around the ridgetops. The
Advisory is set to expire at 8 PM with scattered snow showers
possible in the wake through early Wednesday morning.

Today should prove to be the coolest day of the week given the
presence of the upper low and extensive cloudiness. Low to mid 60s
will be the norm for the Valley along with some breezy southerly
winds over the northern Sacramento Valley. Over the mountains,
some spots may struggle to escape the low to mid 30s. With the
upper low shifting over toward the central to northern Rockies,
the flow aloft will become northerly for Wednesday through Friday.
Temperatures should gradually warm back into the 70s which is
still below average. Mountain showers are forecast to be diurnally
forced each day with some activity potentially drifting into the
foothills. Looking to early in the weekend, yet another upper low
is expected to impact the region with more widespread shower
chances. ~BRO

&&

.EXTENDED DISCUSSION (Saturday THROUGH Tuesday)

555 DM upper low forecast to dig south off the CA coast through
the weekend, then push inland across SoCal late Sunday or Monday.
Models differ with progression of low, however oper GFS and GEFS
are similar and preferred. Unsettled weather expected through the
extended forecast period with the best chances for showers over
the foothills and mountains. Instability charts suggest the
potential for mountain thunderstorms on Saturday. Below normal
temperatures progged over the weekend into Monday with high temps
returning to near normal by Tuesday.

&&

.AVIATION...

NWly flow alf as upr low apchs tda, movg S thru Intr NorCal tngt
into Wed mrng. Areas of MVFR/IFR conds poss in pcpn nxt 24 hrs,
mnly ovr fthls and mtns. Isold tstm poss btwn 18z-03z. In Cntrl
Vly, areas Sly sfc wnd gsts to 25 kts, and ovr hyr trrn W-NW sfc
wnd gsts up to 35 kts poss til arnd 00z Wed.


&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 8 PM PDT this evening for West
Slope Northern Sierra Nevada-Western Plumas County/Lassen Park.

&&

$$
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