Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 210439

935 PM PDT Sun Apr 20 2014

A Pacific trough will approach the coast Monday bringing
increasing clouds and a chance of rain and higher elevation snow
Monday night and Tuesday. Another trough will bring unsettled
conditions Friday and Saturday.



Darkening in the water vapor imagery this evening indicative of a
well defined vort max rotating Ewd with the Ern Pac trof. This trof
is tapping into a modest PW plume of about six to seven tenths of an
inch north of 35N...with the higher PWs to the south. Thus...the
potential for a modest precip event will be challenged by the lower
PW air off the coast which the frontal band must navigate on MON
likely eroding the plume to some degree...and the forecast splitting
of the vort max as the trof approaches/nears Norcal. The apparent
timing of the precip seems to indicate that interior Norcal will
remain dry until the late afternoon and evening hours when some
scattered showers should develop...per the WRF.

Most of the rain is likely to hold off until the Mon nite (after
midnite)/Tue morning period...with widespread showers over the
Siernev Tue afternoon...and decreasing activity in the Valley. Looks
as if the WRF indicates (forecasts) a decent band of convective
precip along the I-80/Hwy 50 corridor early Tue afternoon
corresponding to the passage of the upper trof axis.

This weekend`s Bufkit forecast hodographs for the valley on Tue
mainly showed straight line hodographs indicative of more
multicellular convection than supercellular convection. Generally...
some of the storms may contain small hail rather than rotating
updrafts this time around.

A much cooler day on Tue with 850 mbs temps and low to mid level
thicknesses some 13 deg/20 dam lower than Sun. Max temps in the
valley should only reach into the 60s.    JHM

.Previous Discussion...

Upper trough will move east towards the coast late tonight with
increasing clouds after midnight...but no precip. Trough will
deepen as it moves along the coast tomorrow. Scattered showers will
move over the coastal range and northern Shasta county during the
afternoon. High temperatures will cool into the mid to upper
70`s at most valley spots...due to the increased cloud cover.
These readings are still about 5 degrees above normal.

Band of showers with frontal system will move across Norcal after
midnight. Sufficient instability is indicated by modified
totals/totals over the northern and central Sacramento valley for
thunderstorms and decided to add slight thunderstorm chances for
these areas. Lift will also be aided by initial vort lobe swinging
through the area.

Skies will turn partly cloudy Tuesday morning behind leading
moisture band. Models prog best instability over the valley 11am-
4pm Tuesday and showers and isolated thunderstorm should develop
with surface heating. Snow levels will drop to 4500 feet during
the afternoon. Up to 2 inches of snow will be possible at pass
levels during the afternoon and evening. Qpf`s will generally be
light outside of thunderstorms with this system as most valley
locations receive 0.10 inches or less. High temperatures will
cool into the mid and upper 60`s for valleys.

Trough will move into the great basin region on Wednesday...but an
unsettled northwest flow will keep shower chances over the
northern mountains and Shasta county. Temperatures will warm into
low 70`s at most valley locations. Another trough will approach
the area Thursday increasing the shower chances over northern and
western areas.

.Extended Discussion (Thursday through Sunday)

Ridging Thursday will bring a brief return to above normal highs,
before a cool shortwave from the Gulf of Alaska begins edging into
the area late afternoon/evening. Showers will begin to spread into
the northern and Coastal Mountains, then into most of area by
Friday. Peak precipitation over the north is expected Friday
afternoon, over the Sierra Friday evening (snow levels around 6000
ft). Lingering showers continue int Saturday. Precipitation totals
of an inch or possibly 2 possible for the higher mountains.

Another wave may arrive Saturday night/Sunday across most of the
area (ECMWF) or weak ridging with some moisture over the northern
mountains. Will lean towards the wetter ECMWF for now, with the
weekend remaining relatively wet and with below normal
temperatures. EK



VFR conditions will continue through Monday afternoon across
interior Northern California. Generally light Valley winds will
become southerly 5-15 kt across the Valley ahead of a low pressure
system Monday. Near KRDD-KRBL, south winds 15-20 kt with gusts to 35
kt possible Monday afternoon. Look for precipitation to spread into
Northern Interior CA after 00z Tuesday, with IFR/LIFR conditions
possible over the mountains.



.STO Watches/Warnings/Advisories...

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