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243
FXUS66 KSTO 250501
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
1001 PM PDT Thu Apr 24 2014

.Synopsis...
A weather system will bring a chance of widespread precipitation
along with cooler temperatures and lowering snow levels tonight
into early Saturday. Potential for isolated thunderstorms on
Friday.

&&

.Discussion...
Showers continue to increase this evening as upper trough drops
south along the coast. A band of strong thunderstorms developed
earlier this evening near Chico with plenty of lightning and a
few small hail reports. Best shower chances overnight will shift
south over the southern Sacramento valley and central Sierra
as shown by latest Hrrr run. Have updated forecast to raise pops
in these areas overnight. Showers will increase over northern
areas towards morning with upper low dropping south.

.Previous Discussion (Today through Sunday)... WSW to SWly flow
aloft over Norcal moistening as .80+" TPW plume moves onto the Nrn
CA coast. Mostly Light precip forecast through most of tonite as
the frontal band moves towards the I-80 corridor by around
midnight...and well SE of our CWA by early Fri morning. Synoptic
scale or larger scale forcing is forecast to increase over Norcal
on Fri (per convergence of Q vectors) as short wave and jet energy
on the backside of Ern Pac trof digs the trof along the coast
during the day. Then...during Friday night into early Sat this
energy continues to dig rapidly into Socal. Thus...for our CWA the
bulk of the precip is expected on Fri in widespread and bands of
heavier showers/storms. Liquid amounts over an inch in the Siernev
are possible and up to a third of an inch in the Valley. QPF
should then be decreasing from N to S during Fri afternoon and
evening.

Cloud top enhancement west of the CA coast beginning to occur this
afternoon as short wave energy near 40N/135W begins to interact with
the moisture plume. This feature moves Ewd tonite and rain or
showers should become more widespread over Norcal during the
night. Convergence and surface heating along the E side of the
coastal range may result in an isolated storm this evening...
otherwise decreasing intensity until dynamics increase Fri
morning.

Heavier precip is expected on Fri as the digging trof will bring
colder/unstable air into Norcal...not only lowering snow
levels...but resulting in deep convection (showers/thunderstorms).
One...snow levels should lower to around around 4000 Nrn mtns to
4500 to 5500 feet over the Siernev on Fri. Two...the best
instability and greatest thunderstorm potential looks to be over the
central and southern Sacramento valley in deference to the location
of the NE quad of the vort max during the 18z-00z Fri time frame.
Increasing onshore gradients on Fri will result in Delta Breezes and
topographic channeling of the winds on the E side of the Sac vly.
Convergence there could be the focus for stronger storms with
possibility of hail.

Up to nearly a foot of snow is expected over the higher elevations
of the Siernev and 5-9 inches or so near pass levels...although
melting/settling at this time of year may account for a couple of
inches less accumulation. Greatest snowfall amounts expected to be
from the I-80/Hwy-50 corridor Swd in zone 69 given the track of
the short wave energy within the upper trof.

This rapidly digging trof maintains a neutral tilt over CA. Forecast
hodographs for the valley on Fri are mostly cyclonically curved...
not broadly looping and favoring left moving storms with negative
helicity. This is not the typical TOR set-up for Norcal...and favors
more the hail producing storms given the steep lapse rates forecast
with the advection of colder air.

As this system moves across the Four-Corners area Sat nite...
another short-wave is forecast to bring a chance of light precip
mainly over the mtns into Sun morning. By Sun afternoon...ridging
should prevail over Norcal with warmer/drier wx.   JHM

.Extended Discussion (Monday through Thursday)

Medium range models amplify EPAC upper ridge through first half of
the extended forecast period for dry weather and warming.
Amplifying EPAC upper ridge Monday is forecast to shift inland
Tue/Wed by the GFS/GEM. The ECMWF-HiRes keeps the ridge axis off
the West Coast as it digs an upper low southward through the
Great Basin. The GFS/GEM solutions suggest more warming than the
Euro which points to breezier conditions Tue/Wed. All point to
temperatures well above normal early next week with highs from the
mid 80s to near 90 in the Central Valley and upper 60s to lower
80s for the mountains and foothills.

Models diverge beyond midweek but all suggest minor cooling as
the ridge weakens Thursday, either from an approaching Pacific
short wave system or from a backdoor trough dropping in from the
east. Thus have lowered temps a few degrees day seven with
continued dry.

PCH

&&

.Aviation...
A cold front system continues to impact NorCal tonight into
Friday. For the next 24 hours, valley conditions will generally be
VFR except for local MVFR/IFR when showers develop. For the
mountains, areas of MVFR/IFR expected with local IFR conditions in
heavier showers. Snow levels will lower overnight down to
4500-5500 ft by Friday afternoon. Thunderstorms are possible
across the valley and mountains between 12z Fri and 03z Sat.

Generally W to S winds in the valley now through Friday with
enhanced southerly winds in portions of the valley Fri morning and
afternoon with local gusts up to 30 kts between KCIC and KSMF.
Mountain winds will be SW with gusts up to 35 kts...locally higher
over Sierra ridgetops. JBB

&&

.STO Watches/Warnings/Advisories...
winter weather advisory until 5 am pdt saturday above 5000 feet
in the west slope northern sierra nevada...western plumas
county/lassen park.

&&

$$








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