Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA
FXUS66 KMTR 240403
Area Forecast Discussion
National Weather Service San Francisco Bay Area
903 PM PDT Thu Mar 23 2017
...Rain and gusty southerly winds return on Friday...
.SYNOPSIS...Rain and moderate southerly winds return to the North
Bay overnight as a frontal system approaches. Rain and wind will
spread southward Friday as the 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 the region.
.DISCUSSION...as of 9:03 PM PDT Thursday...Radar starting to
slowly fill in with a few bucket tips just starting to show up in
northern Sonoma county. Rain event is on track with 00z models
keeping similar timing with rain gradually increasing over the
North Bay tonight reaching the greater Bay Area for Friday morning
commute. Looking at satellite ir imagery there are cooling cloud
tops well offshore and would expect to see some baroclinic leaf
structure development as trough digs and jet streak passes over
the region. AMSU/SSMI blended satellite scans show that the
moisture plume has tpw values from 2.0-2.3 inches, well in excess
of 200% of normal for late March. This depth of moisture with a
potentially slow moving front does raise concerns.
As the front moves southward through midday the models have been
consistent in developing a wave with an associated surface low
just west of Half of Moon Bay. As this occurs the band of rain
will likely slow with latest projections showing the Santa Cruz
mountains once again taking the brunt of the rainfall with the
Santa Lucia range above Big Sur taking a close second. Heavy rain
rates will once again inundate small streams and any compromised
hill sides will once again be tested due to short term heavy rain
rates. The one bit of good news is of course we`ve had a little
bit of dry weather since the rain earlier this week and the main
stem rivers are running low. The Friday afternoon commute will be
a mess as well from San Francisco southward to the Central Coast
with winter storm warnings up in the Sierra as well so make your
weekend travel plans accordingly. In addition to the rainfall,
winds will once again be problematic ahead of the front with gusty
southerly winds from 35 to 45 mph in many coastal and hillside
locations. It appears that many trees are still stressed from the
long term drought and now saturated soils so more 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.
Rain chances return, and look likely by Sunday afternoon for the
Bay Area and North Bay as another cold upper trough passes across
the state. This system wont have near the moisture tap but any
showers will bring moderate to heavy downpours with numerous hail
reports likely and snow levels down to around 4000 feet by the
tail end. Rain totals for the Sunday afternoon/night system look
to be around 0.25-0.50 with local amounts 0.75 inches. All rain
looks to be east of the area by Monday morning.
Beyond that much of next week now looking dry.
.PREV DISCUSSION...as of 1:40 PM PDT Thursday...After an active
weather day yesterday, today -- as expected -- has been tranquil
with just scattered clouds. Temperatures have been running close
to values from Tuesday with most spots in the upper 50s to the
lower 60s. Hard to believe in less than 24 hours we will be
dealing with another round of active weather.
Synoptically a cold front associated with a 988 MB low heading
toward British Columbia will move through our region on Friday.
Models have shown very little variation from previous runs, so the
forecast was changed very little during the shift. Rain will move
north-to-south starting overnight in the North Bay then down to
the SF Bay Area early in the morning and to Monterey Bay Region
close to noon. Rain rates will pick up with the approaching front
and could be moderate to heavy at times. Current timing brings the
main band through SF Bay late in the morning or early in the
afternoon. In fact, if the current NAM verifies the bulk of the
rain will fall after morning rush and before the afternoon
commute. Along with the rain southerly winds will increase with
many spots forecast to see gusts of 25 to 35 mph. Winds for higher
elevation locations will be stronger -- gusts over 45 mph are
Rainfall totals still look like 1.5" to 3" for the North Bay with
2/3" to 1.5" around SF Bay with generally less than 2/3" for
points to the south. All of the coastal ranges are expected to be
in the 2-4" range with a few local spots possibly seeing slightly
Rain will switch to showers behind the front with light
precipitation possible through Friday night. However, by mid-day
Saturday all of the rain should be well to our south and east.
Similar to today`s time period, that break will be short as
another system moves across on Sunday. This one should be quite a
bit weaker and will have less moisture to utilize. In general
amounts will be around half of what we get out of the first round.
Rain will switch over to showers going into Monday.
A ridge of high pressure will build back to our region starting
next Tuesday, although a system will dive down from the north late
Wednesday/early Thursday possibly bringing a few light showers.
However, after it departs a more pronounced pattern change will
take place as a stronger ridge of high pressure returns to our
region. This should give us dry conditions with warmer
temperatures toward the end of next week possibly through the
.AVIATION...As of 4:55 PM PDT Thursday...For 00z TAFs. VFR
conditions expected to prevail through the evening hours with
westerly winds at 10-15 kt. Showers expected to move in from North
to South beginning with KSTS around 04z Friday. Steady rain
expected across all terminals by tomorrow morning. MVFR ceilings
possible early Friday morning for Bay Area terminals. Low
confidence on timing of the rain.
Winds are expected become southerly on Friday with possible gusts
exceeding 20 to 25 kt by 18Z. High confidence on wind direction,
low confidence on the timing and gust magnitude.
Vicinity of KSFO...VFR this evening and overnight transitioning to
MVFR ceilings and rain by Friday morning. West winds 10-15 kt this
evening becoming southerly by 6 to 8Z with gusts exceeding 20 kt
possible by 15 to 18Z.
SFO Bridge Approach...Same as KSFO.
Monterey Bay Terminals...VFR this evening and overnight with
generally light winds and occasional gusts at KSNS before sunset.
Rain showers are expected to begin impacting after 12 to 16Z.
.MARINE...as of 08:37 PM PDT Thursday...A storm system is
currently approaching the waters from the west. this will cause
winds to increase and turn southerly late this afternoon.
occasional gale force gusts are possible late tonight and into
tomorrow...especially for the northern coastal waters. rough seas
possible just ahead of and behind the frontal passage, but should
begin to decrease by saturday. expect northwest winds to return by
saturday as well before briefly becoming southerly on sunday as
another weak system moves over the waters.
.Tngt...SCA...Pigeon Pt to Pt Pinos 0-10 nm from 5 AM
SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 5 AM
SCA...Pt Arena to Pigeon Pt 10-60 nm
SCA...Pt Reyes to Pigeon Pt 0-10 nm from 9 PM
SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 9 PM
SCA...Pt Arena to Pt Reyes 0-10 nm
SCA...SF Bay from 5 AM
PUBLIC FORECAST: RWW
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