Strong Wind in California, April 13 to 14, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
326 PM PDT Mon Apr 13 2015

.Synopsis...
A fast-moving weather system moves through tonight into early
Tuesday bringing cooling temperatures...and a chance of showers,
mainly over the mountains north of Highway 50. Breezy to windy
conditions the next several days. Warming mid-week through Friday
under building high pressure.

&&

.Discussion...
Upper trof along 130W preceded by surface cold front is approaching
the coast and has initiated a cooling trend over interior Norcal.
Onshore flow and tightening/increasing pressure gradients
increased the Delta Breeze and up valley flow this afternoon
resulting in cooling especially in the immediate Delta influenced
areas. Although max temps today will be a handful of degrees above
normal...they were several degrees cooler than Sun.

Impressive cold pool/strong vort max associated with the upper trof
will be moving across our CWA between 06z-18z Tue. Surface front
will precede the upper trof by a few hrs with timing estimated to be
N of I-80 at 06z...roughly a KSVE-KCIC-KSTS line...and then S of our
CWA by 12z. The TPW along offshore frontal band is about .70" to
.80"...but is forecast to erode to about .50" to .60" as the front
moves inland. Moisture is forecast to be very shallow in our CWA
which will limit QPFs to a tenth or two mostly over the Nrn Siernev
and Nrn Mtns.

WBZs and 850 mbs/1000-500 mbs thickness charts suggest the snow
level will drop rapidly to 2-3 kft but only for a brief period of
time (around 12z Tue) and then rise rapidly. So in the 06z-12z Tue
time frame we may see some brief periods of lower snow levels as the
front passes. The progressive movement of the trof will account for
a rapid changes in wx...both deteriorating/improving overnight and
early Tue morning.

Winds will shift from Sly to Nly behind the front tonight with
breezy/windy conditions on Tue into Wed. Strongest 925 mbs winds are
forecast to be at nite/early morning which makes wind advsry
criteria a little more problematic for Valley locations. Would
prefer to see the stronger 925 mbs winds during the day when mixing
is more pronounced. Strong ridge winds expected over the Siernev/Nrn
mtns as the wx system moves over the area tonight and in the wake of
the system Tue/Wed.  The Nly winds will mainly be on the W side of
the Valley into Solano/Napa Cos late tonight and Tue and again Tue
nite and Wed. A typical Nly Barrier jet pattern is setting up for
Tue nite/Wed on the W side of the Sac Vly as high pressure builds
across ORE and into NV. This will also result in strong downslope
flow in some of the SW-NE oriented canyons in the Siernev. This
would be more of an example of a katabatic wind as colder air from
the NE plateau/Wrn Great Basin areas is forced downslope/downhill by
the pressure gradient and gravity.

Max temps forecast to cool to near normal on Tue...then warm above
normal mid-week. Some of the CAA on Tue will be offset by adiabatic
warming effects of the Nly winds...otherwise a rapid warmup as high
pressure builds in behind the departing trof/upper low Wed/Thu.
Lingering Nly flow will increase or have a positive effect on
temps.   JHM

.Extended Discussion (Friday through Monday)

High pressure over the area on Friday continuing above normal
daytime temperatures for this time of year with highs in the mid
80s in the Valley and upper 50s to mid 70s in the mountains.
Temperatures are expected to remain above normal into Monday. A weak
system moves into the Pacific Northwest late Friday into Saturday.
This system may brush through Northern California Saturday
bringing a slight chance of showers over the mountains. The ECMWF
is more progressive with this system than the GFS that forms a
closed low off the California coast on Sunday. Slight chance of
showers over the Sierra on Sunday with low off central Cal coast. Low
confidence in the precipitation forecast for the weekend with
uncertainty of the track of the low. Kept forecast dry on Monday
for now but may change with the track of the low.

&&

.Aviation...

Generally VFR conditions across Valley TAF sites the next 24 hours
local MVFR conditions possible around KRDD after 02Z. A weak
weather system will pass across far NorCal this evening and
tonight, and will bring scattered IFR/LIFR rain and snow showers
mainly to the mountains along and north of I-80.

Across the Valley...generally light south winds this morning will
increase to 10-20 kts this afternoon. Over the mountains,
southwest wind gusts up to 50 kt possible this afternoon.
Northerly winds 10-22 kts are expected behind the system aft 06z.
Gusts up to 30 kts possible, especially on the West side of the
Sacramento Valley.

&&

.STO Watches/Warnings/Advisories...
None.
&&