High Wind Watch 5, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1003 PM PST Thu Nov 28 2019

.SYNOPSIS....Cold temperatures tonight will bring the threat of
freezing temperatures to the North Bay and frost in the valleys of
the East and South Bay. Widely scattered showers will persist
across the southern portion of the forecast area tonight and
Friday. Otherwise generally dry and cool weather is forecast
through Friday. The next storm system will bring widespread rain
to the region starting Saturday afternoon. This will be a slow
moving system that will potentially maintain periods of rain
across the region through Tuesday, or even Wednesday. Heavy rain
is possible at times, along with strong and gusty winds,
especially from Saturday night through Sunday night. Heaviest
rain is projected to fall from the Santa Cruz Mountains southward.

&&

.DISCUSSION...As of 9:15 PM PST Thursday...Current KMUX radar
shows that widely scattered showers persist across the southern
portion of our region this evening, mainly over Monterey and San
Benito Counties. Skies have cleared across the North Bay and
temperatures there have already dropped into the mid to upper 30s.
Temperatures in the North Bay Valleys are forecast to drop as low
as the upper 20s by daybreak Friday and a Freeze Warning is in
effect until 9 am Friday for the North Bay Valleys and Mountains.
Widespread frost is also likely close to the coast in the North
Bay as well as in the East Bay Interior Valleys and Southern Santa
Clara Valley. A Frost Advisory is in effect for these areas.

A cold upper low is currently centered over the San Francisco Bay
Area and is forecast to move gradually east and over the Great
Basin by late Friday. Models agree that enough moisture and
instability will remain on the west and southwest flank of the
upper low to maintain widely scattered showers over primarily the
southern half of our forecast area into Friday afternoon.
Otherwise, expect dry and cool conditions on Friday.

The break from widespread precipitation won't last long. A system
currently developing well offshore beyond 140W is forecast to
quickly strengthen and track eastward over the next two days. This
system has tapped into a rich supply of moisture from just north
of Hawaii where precipitable water values are currently in excess
of 2 inches. This system and it's moisture plume (atmospheric
river) are forecast to approach the California coast over the
weekend and bring widespread rain to our region starting Saturday
afternoon. As it approaches the coast it is forecast to stall,
which will mean an extended period of rainfall through at least
Monday of next week, or possibly all the way into Wednesday.
Current models indicate the period of heaviest rain will be from
Saturday night through Sunday evening. This is also the period
when we can expect strongest winds. Models indicate the strongest
winds will be over the coastal waters, but locally strong and
gusty winds are likely over land as well, especially along the
coast and in the hills. Wind Advisories are possible.

Preliminary estimates of rainfall totals from Saturday through
Monday range from 1.5-3 inches at lower elevations and generally
from 2-5 inches in the hills, with upwards of 8-10 inches in the
Santa Lucias where the most sustained heavy rains are projected to
occur. One potential factor that might result in locally lower
rainfall totals than expected in the coastal ranges is wind
direction. Because this system will be approaching from the west,
winds in advance of it will be from the southeast, as opposed to
south or southwest. This will limit the amount of orographic
enhancement along the coastal ranges and thus possibly lower
rainfall amounts.

This incoming system will be much warmer than the system that has
impacted our area over the past few days. Thus, snow levels will
rise rapidly over the weekend, as will temperatures, especially
nighttime temperatures. Overnight lows will begin to moderate on
Friday night as clouds begin to increase ahead of the approaching
system. By Saturday night, nighttime temperatures will be as much
as 15-20 degrees warmer than tonight's temps. The National Blend
of Models (NBM) is much too cool with nighttime temperatures once
precipitation starts. A forecast update was completed earlier
this evening to adjust minimum temperatures up by as much as 10
degrees from Saturday night onward.

Longer range forecasts maintain an active weather pattern with a
potentially strong storm system moving in from the northwest by
late next week and into the following weekend.

&&

.AVIATION...as of 9:55 PM PST Thursday...for 06Z TAFs. VFR to
MVFR conditions throughout the Bay Area and Central Coast. Winds
are light. Clouds linger and any accumulation overnight can still
be expected to lower slightly, should still remain no lower than
MVFR. The one exception is KSTS, where cool temperatures and
higher moisture may present low vis/fog in the early morning.
Mostly VFR conditions Friday with locally breezy winds.

Vicinity of KSFO...Clouds linger near the terminal, but VFR
conditions at the time of the 06z TAFs. Any building of cloud
cover over the night could lower cigs, but not forecast to be
lower than MVFR conditions. Winds will be light through the night
and early Friday, where they will be locally breezy. VFR to MVFR
conditions for Friday.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Cloud cover will keep VFR to MVFR
conditions through the evening as clouds thicken and lower
slightly. There are still a few isolated cells close to the
coastline that may bring precip if they cross over a terminal. But
confidence being any rain would be light and short duration, and
therefore was not mentioned in the TAFs. Winds will remain light
through the evening before becoming locally breezy Friday
afternoon.

&&

.MARINE...as of 09:39 PM PST Thursday...As sea states continue to
gradually ease with the subsiding swell overnight, the small craft
advisories previously issued will be  allowed to expire. Moderate
winds out of the northwest will continue through the night and be
breezy at times. The next change in winds be Friday night into
Saturday as the next weather system approaches the Bay Area. It
will shift the winds to the south and increase to strong gales by
Saturday morning. Strong gale  conditions are forecast to persist
through Saturday and slowly  ease on Sunday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Frost Advisory...CAZ505-510-513
             Freeze Warning...CAZ506-507

&&

PUBLIC FORECAST: Dykema
AVIATION: DK
MARINE: DK

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