Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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FXUS66 KMTR 251149

Area Forecast Discussion
National Weather Service San Francisco Bay Area
349 AM PST Sun Feb 25 2018

.SYNOPSIS...Dry weather and slightly warmer temperatures are
expected today. A cold front is forecast to sweep from north to
south through the region late Sunday night and Monday morning,
bringing a brief period of rain. Scattered showers are then
expected Monday afternoon and evening along with locally breezy
conditions. A stronger and wetter storm system will likely bring
widespread precipitation to the region Wednesday night and
Thursday, with showers continuing through at least Friday.


.DISCUSSION...As of 3:10 AM PST Sunday...Skies are mostly clear
early this morning and winds are generally light. Early morning
dewpoints are running as much as 15 degrees higher compared to
the same time yesterday morning. This increased moisture is
helping to maintain more mild temperatures overnight compared to
last night. At 2 am temperatures ranged from the upper 30s to mid
40s in most areas. These temperatures were as much as 11 degrees
warmer than at the same time yesterday morning. Only the coldest
inland valleys are expected to see minimum temperatures near the
freezing mark by sunrise.

No significant changes are expected in our weather today compared
to yesterday. Winds will be a little lighter today and afternoon
highs slightly warmer. Even with slight warming today,
temperatures will remain a bit cooler than normal.

Early morning satellite imagery shows another cold weather system
dropping south into the Pacific Northwest. The models remain in
good agreement regarding the timing and precipitation potential of
this system as it moves south into our area. A cold front is
expected to spread rain across the North Bay late tonight, across
the rest of the San Francisco Bay Area during the early morning
commute on Monday, and then into the Monterey Bay Area by mid/late
morning. Unlike the past couple of systems that produced spotty
precipitation, this system is forecast to generate widespread
rainfall late tonight and Monday morning. However, the front will
move through quickly and so overall rainfall amounts will be
light. Most models forecast about a quarter of an inch or less
across our region. After the front moves through, scattered
showers are expected Monday afternoon and evening, mainly across
the southern and eastern portions of our area.

The incoming system isn`t expected to be quite as cold as those of
last week, but still cold enough to drop snow levels to around
3000 feet as the front moves through. Snow levels after frontal
passage will likely drop as low as 2000 feet. However, most
precipitation with this system is expected to occur with the
front. Higher peaks in the Bay Area may see light snowfall
accumulation. Locations above 3000 feet in the North and East Bay
Hills and Santa Cruz Mountains may pick up as much as an inch of
snow. Higher elevation locations above 4000 feet, such as Mount
Hamilton in Santa Clara County and the Santa Lucias in Monterey
County, may accumulate up to 2 or 3 inches of snow by late Monday.

Northwest winds will increase behind the cold front on Monday
and locally breezy conditions will develop by afternoon, mainly
near the coast. Winds will then veer to the north on Monday night
and remain gusty in the hills from Monday night into Tuesday.
Although the airmass following the cold front will be quite cold,
with 850 mb temps forecast to drop as low as minus 4 deg C, winds
on Monday night should provide enough mixing to keep temperatures
from falling below freezing except perhaps in the most wind-
sheltered valleys. Winds will subside by Tuesday night and
overnight lows that night will likely be colder than Monday night,
especially across eastern and southern portions of our area.

By the middle of this week the models show a cold system moving
southeast across the Gulf of Alaska. Unlike the Monday system,
this system is expected to tap into a plume of moist air flowing
over the ridge along 150W. The models are in relatively good
agreement in bringing a robust front through our region between
late Wednesday and late Thursday, with widespread moderate to
locally heavy rain rates. A cold upper trough is then forecast to
slowly move across our area after frontal passage and maintain
shower activity through Friday and possibly into the early part of
next weekend. Preliminary rainfall totals during the second half
of this week are in the 0.5-1.5" range with perhaps up to 2" in
the coastal mountains. If these projected rain totals verify, this
would be the most rain our region has seen from a storm system
since early January.

&& of 3:49 AM PST Sunday for 12Z TAFs. A challenging
TAF package with timing the onset of increasing low clouds
associated with the next system. VFR conditions are expected to
prevail at terminals for most of the day on Sunday before clouds
increase between 2,000 to 5,000 ft AGL from north to south. Expect
most terminals to transition from VFR to MVFR in the evening and
continuing overnight into Monday. Rain chances are also introduced
at the tail end of the TAF period for the northern terminals.
Winds will remain west to northwest during the day on Sunday with
the wind-prone terminals reaching 15 to 20 kt in the afternoon
(higher gusts possible).

Vicinity of KSFO...VFR conditions expected to prevail through most
of the day on Sunday with clouds gradually increasing by the
afternoon with west to northwest winds 15 to 20 kt. Expect VFR
conditions to eventually transition to MVFR ceilings sometime
after 00Z to 03Z Monday. Light (and possibly moderate) rain will
reach the terminal around/after 10Z Monday. Overall medium
confidence, but low confidence on the arrival time of the BKN low
clouds and precipitation.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR conditions are expected for most of
Sunday before increasing low clouds bring in MVFR ceilings at both
KSNS and KMRY. Offshore winds Sunday morning will transition
onshore in the late morning/early afternoon, and may become breezy
at times.



A few locations may be near record low temperatures Sunday morning
and possibly Tuesday morning next week. Temperatures will be mainly
below late February normals through early next week, but increasing
clouds, showers, and breezy winds with a low pressure system arriving
later Sunday into Monday should help keep lows above records on Monday.

Location          Sunday          Tuesday

SF Bay Area

Healdsburg        29 in 1920      25 in 1962
Santa Rosa        24 in 1915      24 in 1962
Calistoga         29 in 1966      24 in 2012
Kentfield         29 in 1920      26 in 1962
San Rafael        26 in 1904      29 in 1962
Napa              30 in 1962      28 in 2011
San Francisco     38 in 1887      38 in 1962
SFO Airport       35 in 1953      32 in 1951
Half Moon Bay     33 in 1996      31 in 2011
Oakland Airport   33 in 1953      31 in 1962
Richmond          36 in 1962      34 in 1962
Livermore         28 in 1942      25 in 1962
Moffett Field     34 in 1953      30 in 1962
San Jose          28 in 1894      28 in 1893
Gilroy            31 in 1965      24 in 1962

Monterey Bay Area

Santa Cruz        30 in 1911      25 in 1962
Salinas           32 in 1997      25 in 1962
Salinas Airport   25 in 1935      29 in 1962
Monterey          35 in 1956      27 in 1962
King City         23 in 1974      23 in 1962


.MARINE...As of 2:23 AM PST Sunday...Moderate northerly winds
will continue over the coastal waters as high pressure resides
over the eastern Pacific. By Monday and Tuesday, a storm system
will drop south from the Gulf of Alaska and bring with it a
renewed opportunity for strong winds (potentially gale-force),
fresh swell, and rain to the waters.


     .Tday...SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Mry Bay from 1 PM




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