Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
915 am PDT Friday Oct 31 2014
Synopsis...a significant storm system will drive a cold front
through central California today bringing widespread rainfall to
the region. There is also a possibility of thunderstorms
developing this afternoon and evening. Showers will linger into
Saturday with dry conditions returning Sunday through much of next
week as high pressure builds over the region.
Discussion...as of 9:15 am PDT Friday...all eyes are on the
approaching cold front with radar echoes intensifying as the rain
band approaches the coast. So far only light amounts have been
picked up from the North Bay through the greater San Francisco Bay
area...but that will change within the next hour when the frontal
band moves through. The heaviest rain in San Francisco is expected
to occur within the next hour and persist for less than an hour.
Rainfall amounts expected with this initial frontal band could
range from a quarter of an inch to 3/4 inch depending on where you
are with heavier amounts expected across higher terrain.
Frontal rains will change to showers by afternoon with locally heavy
showers and thunderstorms possible. The heaviest rainfall amounts
are forecast to occur from the Santa Cruz Mountains southward
along the central coast where isolated locations may pick up
2 inches or more. Regardless...widespread rainfall totals across the
North Bay will range from 0.25" to 0.50" with 0.50" to 1.00"
Showers will persist this evening and then taper off from north to
south overnight. Another short-wave trough will initiate additional
showers across the southern third of the County Warning Area...from Santa Cruz
County southward...Saturday morning. All showers will diminish by
Saturday evening when high pressure begins building along the
Temperatures will remain cool through the remainder of the
weekend with dry conditions returning by Sunday. A slow warming
trend is then forecast through at least midweek as a ridge of high
pressure builds over the coast.
Aviation...as of 5:00 am PDT Friday...for 12z tafs. A storm
system just offshore of the Pacific coastline will shift onshore
through the day and spread rain and MVFR ceilings across the area.
Southerly winds ahead of the front will shift to the west-
southwest following the frontal passage between 18z and 21z. As
the front moves closer to the coast showers will begin to spread
across the region from north to south and winds will turn south to
southeasterly. Mainly MVFR ceilings expected with the exception of
ksts that will see IFR ceilings through much of the period. Rain is
forecast to begin over the North Bay around 14z this morning with
the heaviest rain coming between 16 and 20z. Wet runways and south
plan expected for the morning rush Friday.
Vicinity of ksfo...MVFR ceilings expected this morning as the front
approaches. Southeast winds will develop overnight with light rain
likely by 14z Friday. Would expect to start the day with southeast
winds and wet runways. Periods of moderate to heavy downpours
possible between 16z and 20z. Wind shift to the SW also
anticipated between 16z and 20z.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...MVFR ceilings expected this morning as the
front approaches. Southeast winds will develop overnight with
light rain likely by 19z Friday. Periods of moderate to heavy
downpours possible between 18z and 22z. Wind shift to the SW
anticipated in the afternoon.
Marine...as of 08:44 am PDT Friday...a cold front will go across
our waters today leading to showers and possible thunderstorms.
Wind will briefly increase with the passage of the front as it
moves northwest to southeast. Behind the front lighter northwest winds
are forecast by the evening. On Saturday stronger northwest winds and
building seas to around 10 feet will develop creating some
hazardous seas for the start of crab season.
Public forecast: Sims
Visit US at weather.Gov/sanfrancisco
Follow US on facebook, twitter, and youtube at: