Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
903 PM PST Sat Dec 20 2014
Synopsis...light rain and drizzle will possible across our area
tonight and slowly diminish overnight as high pressure builds over
the eastern Pacific. King tides will arrive to the coast on Sunday
resulting in coastal flooding along the immediate coast and San
Francisco Bay front...including coastal marshes and sloughs.
Discussion...as of 08:30 PM PST Saturday...light rain continues to
be reported across the North Bay this evening...with reports of
drizzle in portions of San Francisco. Most of this precipitation
is not being picked up on the kmux-88d radar and is likely
occurring in the lower levels due to warm air advection
interacting with precipitable water values greater than 1.00". Could see a few
spots pick up a few more hundredths of an inch through tonight and
into early Sunday morning. However...precipitation will tapper off
across much of the region Sunday morning as middle/upper level high
pressure builds along the West Coast. Given the abundant low level
moisture in place...overnight temperatures will once again be 10
to 15 degrees above seasonal averages for most locations.
Previous discussion...as of 2:39 PM Saturday...the current satellite
water vapor image continues to show moisture spilling over the
ridge of high pressure positioned over the eastern Pacific. Kmux
Doppler radar continues to show isolated showers associated with
this moisture passing over the forecast area.
The 1800z gfs40 and nam12 have initialized well with the current
synoptic pattern. The 1200z European model (ecmwf) has also initialized well and
all 3 models remain in good agreement through at least the next
84 hours. The models all suggest that isolated showers will stick
around through this evening with the greatest impact expected
over the North Bay area.
The models slowly build the east Pacific Ridge and slide it
closer to the coast through the weekend. This will bring an end
to the showers by Sunday afternoon...dry weather is then expected
through early Wednesday morning. The gfs40 and European model (ecmwf) continue to
advertise a weak trough of low pressure pushing into northern
California Wednesday. This weak trough will bring a possibility
of showers to the North Bay area Wednesday with showers possible
Christmas evening for the remainder of the forecast area. A slight
chance of showers will remain a possibility for the North Bay
area on Christmas day. Keep in mind that any impact of showers on
Christmas day will be minimal.
Aviation...as of 9:03 PM PST Saturday...a high content of water
vapor is present in the air mass. An upper level ridge over the
eastern Pacific will expand over norcal tonight through Sunday
while in the very near term low level warmer air transported by
the wind provides the necessary lift for areas of light rain to
spread southeast. Lowest confidence is in the fog potential
tonight due to moist air combined with long night-time cooling.
Low to middle level cloud cover as well as rainfall may help reduce
the areal coverage of fog during the period nonetheless would
expect at least localized IFR in ceilings/visibilities overnight.
Vicinity of ksfo...light southeast winds and VFR/MVFR ceilings forecast
during the period. Light rain/wet runways likely overnight.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...cloudy conditions leading to intermittent
light rain forecast for tonight. Light winds.
Marine...as of 2:11 PM PST Saturday...large long period swells
will begin to diminish through the afternoon and evening. Large
breaking waves will remain a possibility across the San Francisco
bar and offshore reefs and rock outcroppings through the remainder
of the day. High pressure will build off the California coast
tonight into early next week resulting in increasing northwest
winds over the southern waters beginning Sunday. Additionally...
the King tides will impact the coast and inland bays over the next
three days. King tide impacts for seafaring vessels will occur
primarily during low tides... with the possibilities of boat keels
touching Harbor bottom... strong currents in Harbor entrances...
and additional breaking waves over offshore reefs.
... Surf advisory...through 10 PM for coastal Sonoma County to
Small Craft Advisory...rough bar advisory for sf bar
Public forecast: rgass/Larry
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