Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1028 am PST Friday Dec 19 2014
Synopsis...light to moderate rain will spread south and east over
the area today. Some showers will linger into Saturday especially
north of San Francisco but this will be followed by a ridge of
high pressure building over the West Coast resulting in a drying
trend through at least most of next week including Christmas.
Discussion...as of 9:00 am PST Friday...based off radar returns this
morning plus gauges and chp reports decided to issue an Urban and
Small Stream Flood Advisory for most of the North Bay. Rain rates
have been impressive and quite a bit more than forecast. In
fact...some stations have received more than 1 inch an hour which
almost exceeds our forecast for the entire day. Just did a
forecast update this morning to greatly increase quantitative precipitation forecast numbers plus
the pop values into the afternoon.
Models are generally in good agreement that the rain will slowly
progress through to the south into the early afternoon hours and
stretch across the sf Bay region...the far East Bay..and the South
Bay. However there is some disagreement for the second half of the
day with some of the models showing the progression slowing and
almost becoming nearly stationary between sf and San Jose into the
early evening hours. Other models are a bit more progressive and
have it advancing to the Monterey Bay region. Either way the main
focus will be north of Monterey County with the highest daily
totals forecast over the North Bay.
Main focus of the shift will be the rain and any associated Hydro
issues. Will be keeping a very close eye at rainfall rates as the
main band moves into sf and the East Bay over the next hour. That
should give US a good indication of how widespread problems may
turn out to be.
Dry weather still on tap for the weekend for points south of sf.
Latest models in bring in a good chance for rain to the North Bay
so will have to look at those pop grids and adjust accordingly.
Previous discussion...light rain showers have been affecting the
North Bay since last evening with weak echoes also moving across
the greater San Francisco Bay area. These light showers are
associated with the warm front ahead of the main system still
located to the north and west. Models bring the main cold frontal
band over the North Bay during the morning... and across the San
Francisco area by midday. The front is prognosticated to weaken as it
pushes east and south...so the bulk of the rainfall is expected to
focus on the North Bay. Up to an inch of rain total is possible in
the North Bay mountains with a third to three quarters of an inch
elsewhere...higher values north and in the coastal hills.
Showers will taper off overnight with a few residual showers
lingering into Saturday. High pressure aloft is then forecast to
build over the eastern Pacific and spread over the coast. This
will push the storm track to the north and bring in a warming and
drying trend to the district that should persist through at least
midweek. Medium range models still indicate the possibility of
bringing an inside slider type trough across northern California
and Nevada by late Wednesday night and into Thursday. However the
latest model solutions generally are keeping the precipitation chances
more east and north...so for now...keeping the extended portion of
the forecast dry.
Aviation...as of 10:15 am PST Friday...for 18z tafs. A low
amplitude West Coast trough centered over the Gulf of Alaska will
result in weak to moderate frontal precipitation through the region
today. Expect S-southeast winds 5-15kts ahead of the frontal passage...
turning light west-northwest in the wake of the front. Model soundings show winds
veering west just above the surface (500-3000 feet) and increasing
to 20-25kts. Models hint that the frontal band could stall late
this afternoon/early this evening... so confidence on timing is
moderate. Improving flight conditions this weekend as high
pressure builds aloft.
Vicinity of ksfo...MVFR ceilings will prevail through the daylight
hours today. Light southeast winds expected to continue ahead of
the approaching front. Winds will gradually veer westerly after
frontal passage this afternoon. Moderate confidence for VFR conds
through the night and tomorrow morning.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...mainly VFR ceilings today. Gradually
lowering ceilings to around 3000-4000 feet by later this afternoon. Light
southeast winds expected to continue ahead of the approaching
front. Winds will gradually veer to the west after frontal passage
this afternoon. A few light showers this afternoon... with the
best chance of precipitation after 02z Sat.
Marine...as of 9:00 am PST Friday...a weak front will move across
the region through the day today before high pressure builds into
the region through the weekend. Expect south to southeast winds up
to 20kts ahead of the frontal passage and light west to northwest
winds in the wake of the passage. Northwest swell will prevail
through the forecast period with swell in excess of 15 feet
possible over the open ocean 10nm or more offshore. Hazardous seas
over the ocean and near the coast for much of the weekend.
... Surf advisory...North Bay from 3 PM
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm from 3 PM
Small Craft Advisory...pt Arena to pt Reyes 10-60 nm from 3 PM
Small Craft Advisory...pt Reyes to Pigeon pt 10-60 nm from 3 PM
Small Craft Advisory...Pigeon pt to pt pinos 10-60 nm from 3 PM
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm from 3 PM
Public forecast: Bell/Sims
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