Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Sacramento California
930 PM PDT Sat Apr 19 2014

Synopsis...
building high pressure on Sunday will result in warmer temperatures.
A stronger Pacific trough will approach the coast Monday bringing
increasing clouds and a chance of rain and higher elevation snow on
Monday night and Tuesday. Some areas may also see isolated
thunderstorms. Weak high pressure will dry things out some Wednesday
and Thursday but a threat of precipitation will continue across the
far northern sections of the state. Another trough will bring
another chance of rain to all of interior northern California Friday
into Saturday.

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Short term discussion...
onshore flow and the Delta breeze will be decreasing overnight and
into sun as 500 mb heights rise over norcal in advance of the deepening
ern pac trof that will affect norcal wx early next week. After the
good Delta push into the valley today (sat) and only a minor 20m
height rises on sun...the nly pressure gradients will not be as
impactful for a good katabatic wind effect. The namgrad shows the
rdd-SAC gradient only up to 1 mb by 12z...decreasing after that to
near zero. In fact the sfo-SAC gradient increases to 2+ mbs Sun
afternoon and a modest Delta breeze should return by evening. We
updated the sun Max T grids/forecasts to reflect this...with the
maxes correlating more closely to the 850-1000mb dry adiabatic lapse
rate...generally low to mid 80s in the valley. Jhm

Previous discussion...

Cooler and possibly wet weather Monday into Tuesday with a
stronger trough affecting northern California. The timing of
trough has been varying some between the models and model runs.
The 18z NAM is a little slower than the 12z NAM but the 18z GFS
is a little faster than 12z. The quicker timing may bring
precipitation into the coastal range and Shasta County during the
day on Monday. The best chance of precipitation will be Monday
night into early Tuesday. The trough moving through Tuesday
morning makes convection less favorable than moving through
in afternoon. Slight chance of thunderstorms Tuesday morning then
the best instability spreads east Tuesday afternoon for a threat of
thunderstorms over the east side of Sacramento Valley and into
western Plumas County as trough shifts east. Snow levels will
start out around 7000 ft Monday night and lowering to around 5000
ft Tuesday morning. Snow accumulations generally expected to be
less than 4 inches near pass levels.



Extended discussion (wednesday through saturday)

The upper level trough will be exiting to the east on Wednesday,
with drying conditions and high temperatures warming to near
normal levels. Could see a few lingering showers over the northern
mountains, with dry weather elsewhere. Ridging on Thursday will
bring a return to above normal highs, with valley highs in the mid
to upper 70s to near 80. A shortwave with a surface cold front is
expected to move into the area on Friday, spreading chances of
precipitation over much of the area by afternoon. Although there
are timing differences, models continue to show a trough pattern
with unsettled and cool conditions for the end of the week.
Additional shortwaves will continue through the weekend, with more
widespread precipitation and below normal temperatures. Models are
hinting that late Saturday and Sunday could be potentially quite
wet, with a significant moisture plume. A caveat to this is the
12z GFS, which, contrary to earlier runs, is trying to build in
some weak ridging in by late Sunday. Will be watching this system
closely to see how it evolves over the coming days. Ek

&&

Aviation...

VFR conditions will continue the next 24 hours across interior
northern California. South to west winds 5-15 kt with local gusts to
30 kt near the Delta will gradually weaken overnight. Winds will
shift to the north 5-15 kt across the valley on Sunday.

Dang

&&

Sto watches/warnings/advisories...
none.
&&

$$