Weather Service Report


270 
FXUS66 KMTR 241521
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
821 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 CWA. 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
locations.

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 PW 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 04:40 AM PDT Friday...For 12Z TAFs. KMUX radar 
shows light to moderate rain across the district at this hour. So 
far, only KSTS showing MVFR, but expect CIGs to lower around 
2000-2500 ft later this morning at all terminals, as more 
moderate, possibly heavy at times, showers filter in from the 
west/southwest. 

Southeast winds will turn more southerly and southwesterly and 
pick up in speed through the morning hours. Wind gusts could reach
25-30 kt at times through about 00Z, as a cold front passes. Rain
will taper off north to south, transitioning to showers after the
cold front passes. Rain chances diminish by the evening hours. 
CIGs should dominantly be MVFR today, with IFR possible in heavier
rain bands. 

Moderate forecast confidence. 

Vicinity of KSFO...MVFR ceilings and with rain by 12Z-13Z. 
Southerly winds will increase later this morning with gusts 
exceeding 20 kt around 15Z and continuing into the afternoon and
early evening.

SFO Bridge Approach...Same as KSFO.

Monterey Bay Terminals...Generally VFR through at least 18Z. CIGs
will lower to MVFR, 2000-2500 ft, with rain. Southeast winds 
turning south/southwest by 18Z-20Z, with gusts up to 20-25 kt 
possible. KSNS should remain southeast through the day, with gusts
up to 30 kt possible through about 00Z. 

&&

.MARINE...as of 2:21 AM PDT Friday...Occasional gale force gusts 
are possible through the day 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.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Wind Advisory...North Bay Mountains from 6 AM this
              morning to 3 PM PDT this afternoon
             Wind Advisory...San Francisco Peninsula Coast-East Bay
              Hills and the Diablo Range-Santa Cruz Mountains from
              10 AM this morning to 7 PM PDT this evening
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 9 AM
             SCA...SF Bay
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm until 3 PM
             SCA...Mry Bay from 9 AM

&&

$$

PUBLIC FORECAST: Bell/Sims
AVIATION: BAM
MARINE: BAM


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