Scientific Forecaster Discussion
fxus66 kmtr 241812
Area forecast discussion
National Weather Service San Francisco Bay area
1112 am PDT Fri Mar 24 2017
..rain and gusty southerly winds return today...
Synopsis...rain and wind will spread southward today across the
forecast area as a cold front moves across the region. A break in
the rain is expected Saturday with showers anticipated to return
Sunday afternoon and night as a low pressure system moves through.
Discussion...as of 8:10 am PDT Friday...everything is still on
track for widespread rainfall (potentially moderate to heavy at
times) later today along with gusty southerly winds. Kmux radar
shows the bulk of the rain is currently from sf Bay northward.
Over the past 30 minutes have noted an increase in moderate
cells moving across the waters and our County Warning Area. Model guidance
continues to bring the main band and associated cold front through
later this morning around sf Bay and down to Monterey Bay during
the afternoon. Latest cnrfc forecast has changed little from
yesterday, so still expecting 2/3" to 1.5" for many urban
locations around sf Bay. Coastal ranges will mostly be in the 2-4"
category although a few spots in the North Bay mountains could
see even higher values.
So far winds haven't been to high (typical windy spots still under
40 mph), although those values should increase shortly as the
front approaches. Gusts of 25 to 35 mph will be common in urban
Will be updating the forecast as the day unfolds, although do not
anticipate any major changes.
Previous discussion...the approaching system is taking its time
reaching US, but kmux radar is starting to fill in with stronger
reflectivities off the coast. So far, only a couple hundredths of
an inch have been picked up by rain gauges in the North Bay.
Latest model runs indicate a slightly slower solution with the
main rain band intensifying over the North Bay after sunrise, then
spreading south through the remainder of the Bay area during the
morning. The hrrr keeps the Monterey peninsula rain-free until
early afternoon. Satellite-estimated precipitable water values
still exceed 180-200% of normal offshore, with over 1.5" of precipitable water in
the moisture plume aimed currently aimed at the northern
California coast. Thus, once this system moves through, episodes
of locally heavy rain are possible, especially over coastal hills.
Rainfall totals through Saturday morning are forecast to range
from 1-3" in the North Bay and coastal hills and from 0.25-0.75"
in the lower elevations. Rain to turn to showers this afternoon
and end overnight.
In addition to rain, southerly winds are expected to ramp up in
advance of and with the front. Winds will be strongest over the
higher terrain from Santa Cruz County northward, as well as some
coastal areas. A Wind Advisory will be issued for these areas for
much of today. Downed limbs, trees and power lines seem likely.
Precip will turn showery by Friday night and end in the pre-dawn
hours of Saturday. Saturday looks dry but seasonably cool.
Another system will bring rain chances back to the area late
Sunday. This system does not look to be as wet, but an additional
0.25-0.50" with local amounts 0.75 inches possible. This rain is
expected to be east of the area by midday Monday.
Beyond that the remainder of next week now looks dry.
Aviation...as of 10:55 am PDT Friday...for 18z tafs. A heavy band
of showers is moving across the Bay area at this hour resulting in
decreased vsbys and IFR to MVFR cigs to area terminals. As the
band of rain slides south terminals currently ahead of the band
will begin to see lowered vsbys and cigs. Improving conditions
expected behind this band of showers later this afternoon and
Moderate southeasterly winds with locally stronger gusts will
continue across the forecast area. Wind gusts could reach 25-30 kt
at times through about 00z, as a cold front passes.
Moderately high confidence.
Vicinity of ksfo...MVFR cigs with periods of moderate rain and
gusty winds expected through 00z. Moderate southeasterly winds
with gusts around 25 kt or more expected through the afternoon
and early evening.
Sfo bridge approach...same as ksfo.
Monterey Bay terminals...VFR cigs prevail at this hour with cigs
expected to lower to MVFR around 20z with the arrival of the rain.
Southeast winds will turn increasingly to the south by the
afternoon, with gusts around 20 kt possible.
Marine...as of 10:49 am PDT Friday...occasional gale force gusts
are possible through the day today as a storm system impacts the
region with strong southerly winds and rain. Seas will be rough
with the frontal passage...but should begin to decrease by this
evening. Expect northwest winds to return by Saturday morning
before briefly becoming southerly on Sunday as another weak
system moves over the waters.
... Advisory...North Bay mountains from 6 am this
morning to 3 PM PDT this afternoon.
Small Craft Advisory...mry Bay
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...sf Bay
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm until 3 PM
Public forecast: Bell/Sims
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