Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
351 am PST Monday Nov 30 2015
Synopsis...a weak frontal system may produce light rain
today...mainly near the coast and in the North Bay. Otherwise dry
weather is expected through Wednesday along with warmer
temperatures. A stronger Pacific storm will likely bring rain and
gusty winds to the region on Thursday.
Discussion...as of 3:50 am PST Monday...by 2 am temperatures had
already dropped below freezing at several inland locations and
into the upper 20s in some spots. High clouds ahead of the
incoming weak frontal system are beginning to spread across the
northern portion of our area this time...which already appears to
be bringing the cooling to a halt in the North Bay. But farther
south...skies should remain mostly clear for the next few hours
which will allow for further cooling prior to sunrise. Will keep
the freeze warning going for now...although it's possible the
warning could be cancelled prior to 9 am as the front draws closer
and warmer air begins to mix into the boundary layer. This will be
our last cold night.
The incoming front does not yet show signs of weakening and a
narrow band of precipitable water values near 1 inch continue
to be detected along the front. It's still likely that the front
will weaken as it continues to push eastward and into the dry
airmass currently over California. But recent model output
indicates there is a decent chance that coastal areas from Santa
Cruz County northward will see some light rain today. Based on
the latest hrrr...light rain will reach the North Bay coast by middle
morning...and San Francisco and San Mateo County early in the
afternoon. Most inland areas are expected to remain dry. Probability of precipitation
have been increased in coastal areas today. Rainfall amounts today
are forecast to be less than a tenth of an inch. A few sprinkles
or light showers may linger through the early evening...but dry
weather is expected to develop by tonight. Dry weather is then
forecast to continue for Tuesday and Wednesday...along with a
warming trend as a shortwave ridge develops over California.
The models agree in bringing a relatively robust Pacific weather
system into northern California on Thursday...although there are
some differences on timing. A moist southwesterly flow is forecast
to develop across northwest California as early as Tuesday
night...and the 00z European model (ecmwf) develops some warm advection precipitation as
far south as Sonoma County by Wednesday morning. However...the NAM
and GFS keep this moisture to the north of our area. Rain chances
won't increase significantly for most of our area until Thursday
when the upper trough approaches northern California. The European model (ecmwf) is
faster and spreads the bulk of the precipitation across our area
on Thursday morning...while the GFS and NAM hold off until
Thursday afternoon. Aside from these timing differences...the
models agree that this system will be capable of producing at
least a short period of moderate to locally heavy rain rates as
it pushes through on Thursday. But because the system is forecast
to move through relatively quickly...overall rainfall amounts are
not expected to exceed an inch in most areas. Model quantitative precipitation forecast for the
Thursday event is generally between a half inch and an inch in
the sf Bay area and Santa Cruz County...and about a third of an
inch of less for Monterey and San Benito counties.
At least a few hours of strong winds are expected with this system
as the front moves through on Thursday. The models forecast a
surface low of about 990 mb west of Cape Mendocino on
Thursday...which would result in wind gusts of up to 50 miles per hour or
more along the coast and in the hills. Wind advisories may be
needed late Wednesday night into Thursday.
Showers are forecast to continue into Thursday night...and mostly
end by Friday. Dry and mild weather is forecast for
Saturday...with the next system possibly bringing another round of
rain into our region by late in the weekend.
Aviation...as of 10:30 PM PST Sunday...VFR conditions tonight
under light offshore winds. A dissipating front will bring middle
level clouds to the area Monday afternoon. Local models show very
light quantitative precipitation forecast reaching the northern sfo Bay area and North Bay. Lower
level moisture could bring clouds down to MVFR category Monday
Vicinity of ksfo...VFR through Monday.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR through Monday.
Marine...as of 3:15 am PST Monday...a weak frontal system will
dissipate over the coastal waters today. A stronger frontal system
approaches the area mid-week. Larger swell heights will arrive
late in the week.
... Warning...inland valleys and mountains
until 9 am.
Public forecast: dykema
aviation: west pi
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