Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
440 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 late Wednesday into Thursday and produce a period of
rain and wind over the Bay area and central coast. A few showers
will linger into Friday then another chance for rain will arrive
on Sunday. The storm track may lift northward next week as high
pressure builds in from the southwest.
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 3:50 am PST Tuesday...middle and upper level
clouds expected to pass over area terminals today. Winds will be
light offshore or locally variable through forecast period.
Vicinity of ksfo...VFR conditions through the period. Mainly
high/middle level ceilings and light offshore flow.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR conditions through the period.
Mainly high/middle level ceilings and light offshore flow.
Marine...as of 2:05 am PST Tuesday...light to moderate winds
will continue through midweek over the coastal waters. 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: canepa
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