Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
928 am PST Tuesday Dec 1 2015
Synopsis...it's a quiet start to December and temperatures are
milder this morning with only some high level cloudiness moving
into the area. High pressure will build over the area through
tonight. A trough of low pressure over the eastern Pacific will
then arrive early Thursday and produce a period of rain and wind
over the Bay area and central coast.
Discussion...as of 9:30am PST Tuesday... a very weak upper
disturbance offshore of the California/Nevada Stateline is introducing a few
high level clouds this morning meanwhile radar shows a few light
returns over the coastal waters. Precipitation from this system will stay
north of our forecast area as a ridge of high pressure persists
over our area through tomorrow. Temperatures are forecast to warm
today and into tomorrow as the high pressure strengthens aloft.
This mornings temperatures already reflect this warming trend as
most official observational locations are 3 to 6 degrees warmer
than 24 hours ago. Temperatures are expected to climb to their
seasonal normals this afternoon and then a few degrees above
normals by tomorrow afternoon.
The upcoming synoptic scale pattern appears much more progressive
over the previous week now that the Rex block that brought US
several days of subfreezing temperatures has been ejected from the
intermountain west. Models continue to converge on Thursday
morning for the arrival of the next storm system. This storm
system is expected to be the strongest of system of the last few
that have brushed through the area. The latest model runs all show
that the bulk of this systems energy and moisture will sweep
through the northern half of our forecast area and northward into
northwest California and the pacnw. Previous model runs showed a further
southward progression of the base of the trough... so there is
still a bit of uncertainty there. That said... the GFS/Euro both
show the base of the trough roughly positioned near the Bay area.
The surface frontal band will push further south into the Monterey
Bay region but weaken as it propagates southeastward from the
upper support. Amounts in the North Bay will range from 1/3" to
1" where as the Monterey Bay region could see as little as a
several hundredths to a few tenths. Onshore winds will become
breezy inland by Thursday afternoon and taper off overnight.
Showers linger into Friday.
Weak ridging develops for Friday and Saturday in the wake of the
aforementioned storm system which will keep temperatures near
normal these days. For Sunday... long term models continue to show
a weak trough approaching the coast. Models have trended this
system weaker and weaker each run and with the latest runs keep
all precipitation out of our forecast area as the system fizzles
on the Lee of saturday's ridge. On the longer time scale... the
GFS is hinting at a potent moisture plume aimed at the pacnw and
northern California late next week. This is definitely something
to watch closely.
Previous discussion...as of 4:40 am PST Tuesday...broad upper
level ridging over California/Nevada will persist into Wednesday. Milder and
dry weather is in store for the area today and Wednesday. 850 mb /
925 mb levels temperatures will climb upwards by 6-7c / 2.5-5c
respectively per the 06z NAM. It should be a nice recovery after
the recent cold stretch. Temperatures today and Wednesday will
climb back to or slightly above seasonal normals. Readings will be
mostly in the 50s to lower 60s today warmest north central coast.
On Wednesday temperatures range from the upper 50s to middle 60s area-wide.
Longer nights and drier air will still bring cool/chilly night-
time lows...but not as cold as recent. Light winds both days.
500 mb height ridging over California will weaken later Wednesday becoming
replaced by a fairly vigorous and quickly moving epac upper level
trough. The polar jet flowing through the trough will amplify the
system as it approaches from the west late Wednesday then as it
crosses the area Thursday. There's been run to run consistency
showing the polar jet splitting prior to reaching norcal which
often translates to systems losing some strength. Additionally in
recent days the model forecasts have shown some variability with
the behavior of this system as it approaches from the west mostly
in the forward speed of the cold front. At this point model output
agree the cold front will move quickly east over the area Thursday
pushed along by deep layered strongly aligned westerly Post frontal
winds. The front could become a bit "inactive" with this type of
wind pattern. Strongest forcing may fall in to a narrower time
frame thus quantitative precipitation forecast totals may not be as high compared to a relatively
slower less steeply sloped frontal passage. Southerly pre-frontal
winds may induce rain shadow effects for areas downstream of
higher terrain such as the South Bay...San Benito and interior
Monterey counties as well as along the southern Monterey Bay area.
Blending GFS/European model (ecmwf) quantitative precipitation forecast forecasts total 1" North Bay to 1/4"-1/3"
east and South Bay areas as well as the Santa Cruz Mountains quantitative precipitation forecast
amounts are 1/4" or less over Monterey and San Benito counties.
South facing slopes could pick up higher rain amounts due to
orographic lift / possibly by a factor of 2x than mentioned.
500 mb height ridging returns late Friday and continues at least
into Saturday. It's at this point when there's increasing model
differences mostly surrounding the duration of the middle level ridging
and whether or not it'll dissipate the next incoming trough. Models
have been increasingly consistent however with 500 mb heights pushing
to 572-576 decameters...above normal for Dec. Quantitative precipitation forecast forecast totals
with the next trough late weekend and early next week are up to
1/2" North Bay to 1/10" or less southern counties.
Next week's weather appears to be mostly dominated by above normal
ridging. Broad west-east sub-tropical ridge over central Pacific
gradually edges east possibly poking NE up to California forcing the storm
track northward into the Pacific northwest. GFS/European model (ecmwf) indicating 575-585
decameters next week...dry and warmer.
Aviation...as of 9:27 am PST Tuesday...VFR through the period
with mainly high clouds with light to moderate winds.
Vicinity of ksfo...VFR. Light to moderate winds.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR.
Marine...as of 09:26 am PST Tuesday...high pressure located off
Southern California will keep light to moderate winds over the
coastal waters through Wednesday. Winds will increase Wednesday
night and Thursday as a storm system approaches the region. A long
period swell will gradually decrease through the week...but a
larger swell will arrive late in the week.
Public forecast: drp
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