Winter Storm in California, April 24 to 25, 2014

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

.Synopsis...
Mild temperatures are forecast today. Then another 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.

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.Short Term 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. SREF plume for BLU shows a
mean of about 7 inches. Would have to consider a couple of inches of
melting/settling could lower the actual accumulations.

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

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.Aviation...

Showers impacting the Northern Sacramento valley this evening as a
cold front system moves across NorCal tonight through Friday.
For the next 24 hours, valley conditions will generally be VFR
except for local MVFR/IFR as showers track eastward. 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

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.STO Watches/Warnings/Advisories...
winter weather advisory from 11 pm this evening to 5 am pdt
saturday above 5000 feet in the west slope northern sierra
nevada...western plumas county/lassen park.

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