Weather Service Report


693 
FXUS66 KSTO 310451
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
951 PM PDT Thu Oct 30 2014

.Synopsis...
A cold front will impact interior NorCal Friday into Saturday.
The front will bring rain, mountain snow, cooler temperatures, and
a slight chance of thunderstorms. Snow levels will lower to around
5000-6000 ft by early Saturday morning. Showers are expected
behind the front on Saturday then end Sunday. Drier and milder
weather early next week.

&&

.Discussion...

Some pre-frontal rain has spread/developed over the far NWrn portion 
of Norcal in the WAA zone. The frontal precip is nearing the coast 
and should move into the Wrn portion of our CWA (coastal range and W 
side of the Valley) Fri morning. Energy moving into the base of the 
Ern Pac trof will shift the trof axis Ewd tonight...causing the 
front to move inland. The front should slow (or in layman's 
terms...get "hung-up") in the Siernev Fri night when the bulk of the 
precip should occur for our Siernev and warning areas. BUFKIT for 
BLU indicates the precip rates will remain below flash flood/debris 
flow guidance...with only 5 of the 24 SREF QPF plume members 
suggesting that the guidance will be exceeded. The convective 
potential from the hi-res column max REF progs basically shows a 
line of convective cells developing after 21z mainly along and E of 
I-5...and moving Ewd into the Siernev by 03z. So the best estimate 
for the highest hourly precip rates should be in the early evening 
for the 50/80 corridor. This line does include SAC and especially 
the Nrn SJV where the bulk of the energy or dynamics will mainly 
affect the Srn portion of our CWA. There may be more widespread 
convection from SCK-MOD Ewd into the Srn Motherlode and over the Rim 
Fire Burn Scar than farther N. A secondary area of convection is 
also forecast over the RDD/Shasta area to a lesser degree. 

Confidence is low in snow levels because of the deepening trof and 
energy passing S of our area Fri nite and Sat. There will be plenty 
of mid level cold air with the minus 24 deg C isotherm over our CWA. 
The mid level temps also suggest the potential for snow levels to 
lower to 5500 feet...with thickness values suggesting a thousand 
feet or so lower possibly due to diabatic processes. With strong 
dynamics and synoptic scale forcing...there will be the potential 
for convective bursts of snow and larger snowflakes can fall farther 
before melting than smaller flakes. In an environment less than 
saturation...cooling by sublimation/vaporization is on an order of 
magnitude larger than the melting effect which can lead to lower 
snow levels. When the heavier showers arrive over the Siernev Fri 
evening this may occur. BUFKIT suggests snow down to around BLU by 
around 09z Sat...for example.   JHM


.Previous Discussion...

Have made some changes to this afternoon's forecast. Latest models
are showing higher snow levels, slower timing, and that the
greater snow impacts will be farther south than anticipated. We have
cancelled the Winter Storm Watch for Western Plumas County and
Lassen Park. Parts of Lassen Park (especially the peak of Mt.
Lassen) will still see some snow, but the coverage is not very
widespread. The Western Sierra slopes south of Plumas county will
have greater snow impacts so we upgraded the Winter Storm Watch
for that region to a Warning. The other changes to this Warning are
that we raised the elevation to above 6000 ft (the Watch was
at 5000 ft) and adjusted the timing from 5 pm Friday through 2 pm
Saturday. Motorists in the Sierra need to be prepared for winter driving.

The big focus is how will the weather impact Halloween. First off,
the forecast continues to call for a wet day across NorCal with a
slight chance of thunderstorms. The brunt of precipitation in the valley is
still on track to arrive before 5 pm, but light rain will continue
during typical Trick-or-Treating hours. If you are at higher
elevations, expect rain to turn to snow above 6000 ft sometime
after 5 pm. Bottom line: have an umbrella, be careful driving as
roads will be slick, and stay sheltered if you hear thunder or see
lightning nearby.

Daytime highs on Halloween will be notably cooler than the past
couple of days. Valley highs will be in the low to upper 60s while
mountain regions peak in the 50s. After 5 pm, temperatures during
Trick-or-Treat hours will be: Valley - upper 50s to low 60s,
Mountains - 40s to 50s.

Showers are likely to continue on Saturday but are expected to
diminish by late morning/early afternoon. Saturday highs under
the cooler airmass are forecast to be about 10 degrees below
normal. Lingering light showers along the Sierra could continue
into Sunday morning, but rebuilding high pressure will end precipitation
by Sunday afternoon. Sunday highs will start to move upward again
as a warmer and drier trend settles in for the first half of next week.
JBB

&&

.Extended Discussion (Monday through Thursday)

High pressure ridging along the west coast will be the dominant
weather feature through the extended period. Daytime highs will
slowly be climbing Monday through Thursday reaching to around
normal. Extended models move a weak system over the north end of
the ridge and into the Pacific Northwest around Tuesday but keep
any precipitation north of the forecast area. Therefore...extended
period remains dry for now. Extended models hinting at a return to
a wetter pattern towards the end of next week but have been rather
inconsistent both run to run and model to model so confidence in
this change not especially high at this time.

&&

.Aviation...

VFR conditions with generally light southerly winds will continue
at terminals through Friday morning ahead of cold front that is 
currently approaching NorCal coast. Hires models have trended a
bit slower since earlier runs and have pushed back the arrival of
precip a few hours. Light rain is expected to overspread the
Coastal Range by early morning and into the Valley during the
morning to early afternoon hours. Showers expected to continue
thru the end of the taf period. Periods of MVFR conditions
expected in heavier showers through 18z at KRDD and KRBL and
through 21-22z from KSMF south as front slowly moves through. A
few thunderstorms can't be ruled out in the afternoon and evening
but confidence too low for inclusion at this time. Southerly winds
will remain in the 5 to 12kt range on Friday before shifting more
southwesterly by the end of the taf period.

CEO


&&

.STO Watches/Warnings/Advisories...
winter storm warning from 5 pm friday to 2 pm pdt saturday above 
6000 feet in the west slope northern sierra nevada.

&&

$$





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