Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1012 PM PST Thursday Dec 18 2014
Synopsis...light to occasionally moderate rain will spread south
and east over the area tonight and Friday. Some showers will linger
Saturday but this will be followed by a ridge of high pressure over
the West Coast resulting in a drying trend through at least most of
next week including the Christmas Holiday.
Discussion...as of 9:07 PM PST Thursday...for December 18th it
was a mild day over the area...normal lows for Dec 18th run in the
middle 30s to middle 40s mildest closest to the ocean and Bay
waters...but the cloud cover that lingered over the area since
late last night helped sustain milder temperatures area-wide into
the daytime hours. Metar observation...mesowest observation and kmux radar showed
a few areas of periodically very light rain mostly over the Santa
Cruz Mountains and over the North Bay counties. Venado in northern
Sonoma County measured 0.04" this evening. Ksts Airport had a trace
A SW-NE oriented ridge axis is presently located from the central
coast to the Great Basin and will continue to flatten and move east
away from the Bay area overnight allowing the next in the series of
eastern Pacific shortwave troughs to arrive late tonight and Friday.
A low level warm air advection pattern will generate a steady light
rain from north to south beginning tonight through early Friday with
the rain trending to moderate at times by early Friday as a cold front
slides southeast over the Bay area. The 00z NAM model forecasts briefly
drier middle level air arriving in the North Bay by early Friday evening
but chilly and unstable northwest flow patterns like this often produce
additional light pockets rain or brief showers a bit longer and once
in a great while during periods when the models indicate dry weather/no
quantitative precipitation forecast. 24 hour quantitative precipitation forecast totals from the 00z NAM model ending 10 PM Friday
evening are running upwards of 8/10" rain North Bay...1/3" sf
peninsula...east and south bays...and about 2/10" north central coast.
This is a little wetter north and a little drier south compared to
the 18z NAM run. Isolated 1" rain amounts are possible in the
hills and mountains due to weak orographic enhancements on SW
winds. The rainfall on top of saturated soils area-wide still
presents the possibility of at least some minor flooding issues
Friday. Rainfall rates per the 00z NAM model appear to reach a
maximum by late morning and through the afternoon from north to
Light rain or showers may linger into Saturday and again Sat night
over the North Bay as a persistently moist epac flow brings yet
another warm frontal precipitation event to northern California. Most of
the measurable rainfall with this system is projected to reach
Mendocino County and the coastal ranges northward to the Pacific northwest.
This system should result in lesser rainfall amounts for the Bay area.
The 00z NAM and GFS models indicate upwards of 1/4"-1/2" rain highest
over northern Sonoma County and quickly lowering to trace amounts
southern Napa and Marin counties. By about this time in the
weekend the eastern Pacific Ridge amplifies some and shifts
eastward overlapping northern California bringing a longer period of dry
weather to all areas.
A moist zonal flow continues into the Pacific northwest next week. In
the extended and just beyond a longwave ridge will be developing
over the epac/Gulf of Alaska per recent National Weather Service cpc's outlooks and
discussions for the 6-10 day and 8-14 day periods suggesting
higher probability of a drier weather pattern for California. Comparing earlier
today's 12z European model (ecmwf)/GFS/Gem models to their respective previous model
runs they are indicating approx 50% reduction in 10 day total rainfall
forecasts for the Bay area and central coast.
Previous discussion...as of 3:00 PM PST Thursday...radar has
continued to pick up a few echos into the afternoon hours with a
handful of gauges picking up light amounts of precipitation today.
Showers will likely continue into the overnight hours in advance
of more substantial rain as our final system for the week takes
aim on our County Warning Area.
Rain will be on the increase for the evening and overnight
hours and start as warm frontal rain before switching to
widespread moderate rainfall Friday morning into the afternoon as
a weak surface low skirts down the coast. Appears that the morning
commute for Friday will be wet for almost all spots with the
potential for moderate rainfall hitting the North Bay close to sunrise...down
to sf Bay late in the morning...and to Monterey Bay and south by
noon to early afternoon. Winds will also pick up a bit especially
over the hills. Spots above 1000 feet cloud see local gusts over
30 miles per hour with speeds generally 15 to 25 miles per hour.
Behind the system the rain will switch over to scattered showers
at least into Friday evening. Rainfall totals by that point will be
mostly a third to two thirds for urban spots will locally more
than 1 inch for the North Bay. Possible we could see some Urban
and Small Stream issues and minor debris flows if the heavier rain
hits any of our recent problem areas. Still a chance for some
residual showers into Saturday over the North Bay while most other locations
should be dry before Saturday morning.
A ridge of high pressure will build back into our region starting
over the weekend allowing for drier and eventually warmer weather
to return. The storm track should stay to the north at least
through Christmas day.
Aviation...as of 10:00 PM PST Thursday...for 06z tafs. Doppler
radar is beginning to pick up a few showers over the North Bay.
Ceilings are gradually lowering with mainly calm winds at the surface.
Light southeast winds will continue ahead of the frontal system.
Light warm frontal rain is expected overnight with steady rain
expected by morning.
Vicinity of ksfo...VFR ceilings are anticipated to lower to MVFR
around 2000-2500 feet in the next few hours. Light southeast winds
expected to continue ahead of the approaching front. Winds will
gradually veer to the west after frontal passage Friday afternoon.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...mainly VFR ceilings through this evening with
ceilings lowering to MVFR around 3000 feet after midnight. Light
southeast winds expected to continue ahead of the approaching
front. Winds will gradually veer to the west after frontal passage
Marine...as of 3:00 PM PST Thursday...southerly winds this
evening ahead of an approaching front that will spread rain over
the northern waters tonight. Rain will spread southward Friday
morning as a cold front passes through the waters with winds
turning westerly. Behind the front large northwest swells will build into
the weekend. Building high pressure will bring lighter winds over
the weekend. The combination of large seas and unusually high
tides will create hazardous seas over the ocean and near the coast
for much of the weekend.
Public forecast: canepa/Bell
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