Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
454 am PST Wed Dec 7 2016
Synopsis...cool overnight temperatures are expected region-wide
with patchy frost through the morning hours. A weather pattern
change will then bring unsettled conditions to the region from
later today through the upcoming weekend. Periods of rain can be
expected at times across the region, especially tonight into
Thursday morning and again Thursday night into Friday.
Discussion...as of 3:00 am PST Wednesday...chilly temperatures
this morning thanks to an offshore flow, light winds, clear skies,
and fairly low dew point values. Most locations are running
anywhere 4 to 10 degrees under yesterday's values leading to
widespread 30s to lower 40s. San Francisco is one of the "warm"
spots with a temperature currently at 47. Worth noting that based
on current readings and dew point values, looks like we will only
see minor pockets of 20s by sunrise. As forecast, majority of
Metro locations should remain in the 35 to 40 range.
Conditions will quickly change as the overall pattern switches
from cool and dry to quite wet as the overall flow transitions to
zonal by tonight. Synoptically a plume of fairly high precipitable water values
(traced west of hawaii) will become entrained in the flow and move
to our coast by late in the evening. As dew point values increase,
rainfall will return to our region late today as a surface warm
front pushes to the coast. Precipitation will start over the North
Bay as early as this afternoon and then slowly work its way to the
south tonight into Friday. Rain rates will pick up with the
highest amounts forecast for sf Bay Metro locations tomorrow
afternoon. At the same time south to southeast winds will increase as
the gradient tightens. Gusts in excess of 35 miles per hour can be expected
along the coast with many inland locations getting at least into
Rain will continue off and on into Friday before a second piece of
energy moves through and brings another shot of moderate to
briefly heavy rainfall. That energy will likely be focused south
of San Francisco with the Santa Cruz Mountains looking at their
best shot of getting substantial numbers. Precipitable water values will remain
high and could exceed 1.10" for that region.
Rainfall totals by Saturday evening will range from more than 4"
for some coastal hills, to generally 1 to 2.5" for the bulk of our
urban locations. Lesser amours can be expected for southern
sections especially inland spots.
Rain will greatly diminish for the weekend into at least Monday
before a third slug of moisture GOES through around
Tuesday/Wednesday. Longer range guidance favors wetter than normal
conditions continuing beyond that period.
Aviation...as of 04:45 am PST Wednesday... VFR conditions this
morning ahead of an approaching series of unsettled weather
systems. The first of these will bring deteriorating ceilings down
throughout the early morning before -ra/rain arrives later this
morning. Intermittent rain will continue through the remainder of
the day and into the weekend, leading to wet runways and reduction
to visibility. MVFR cigs will accompany this storm system with
cloud bases generally 2000 feet or above through today. East to
southeast winds will prevail through the day while some taf
locations will gradually veer towards the south by the end of the
taf period. Winds will become briefly gusty at times as the storm
system pushes through the area.
Vicinity of ksfo...VFR thru early afternoon. High clouds early
this morning will gradually lower to 2500-4000 ft range by this
afternoon. Offshore wind will prevail. Rain possible as early as
21-22z. Rain will persist through the end of taf period. MVFR
later this afternoon and overnight possible.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR thru early afternoon. Offshore wind
will prevail with strong winds possible in Salinas valley late in
the taf period. MVFR cigs posbl later this afternoon and evening.
Intermittent rain through the end of the taf period.
Marine...as of 04:34 am PST Wednesday...an approaching storm
system will bring increasing south to southeast winds across the
waters today with the strongest winds over the northern outer
waters. Gale force winds will be possible over the northern outer
waters. Winds weaken and veer west overnight and into tomorrow.
Rain possible later today through at least Saturday.
... Arena to Pigeon pt 10-60 nm from 9 am
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm from 3 PM
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm from 3 PM
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm from 9 am
Public forecast: Bell
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