National Weather Service Raw Text Product

Displaying AFOS PIL: AFDMTR Received: 2018-01-14 01:37 UTC

FXUS66 KMTR 140137

Area Forecast Discussion
National Weather Service San Francisco Bay Area
537 PM PST Sat Jan 13 2018

.SYNOPSIS...A ridge of high pressure over the region will maintain 
dry and mild conditions through the remainder of the weekend. 
Unsettled weather conditions are then expected to return early in 
the workweek as a storm system approaches the coast. Periods of 
rain/rain showers are then expected through late week and 
potentially into next weekend as a series of storm systems impact 
the region.

&& of 01:46 PM PST Saturday...Another mild day 
across the Central Coast where upper 60s and lower 70s are being 
reported. Meanwhile, cooler conditions persist in the North Bay 
Valleys in response to early morning fog and cloud cover that has 
been slow to burn-off. Otherwise, mostly clear skies prevail over 
the region with low clouds trapped in the Central Valley. Fog will
likely be patchy over the North and East Bay Valleys overnight, 
yet should be less widespread compared to previous mornings. 
Temperatures on Sunday will be similar to those today with 
widespread 70s across the Central Coast and mostly 60s elsewhere 
as high pressure remains over the region. 

The forecast models continue to show the mid/upper level ridge over 
our region will shift inland for the start of the workweek ahead of 
an approaching mid/upper level system. The latest thinking is that 
light rain will move into the North Bay around or after sunrise 
Monday morning and then spread slowly south and inland through 
Tuesday. The main mid/upper level low pushing this system inland 
will lift northward off of the Pacific Northwest Coast early in the 
week. As a result, should expect minimal impacts from this storm 
system with the deeper moisture and upper level lift staying to our 
north. Nonetheless, 0.25" to 0.75" can be expected across the North 
Bay from Monday into Tuesday with potentially upwards of 1.00" in the 
coastal ranges north of the Golden Gate Bridge. To the south, look 
for amounts of around 0.10" to 0.25" with upwards of around one-
third inch for the Santa Cruz Mountains. As normal, inland valleys 
such as the Santa Clara and southern Salinas Valley may see lesser 

After a few lingering showers Tuesday afternoon, dry conditions are 
forecast to return briefly to the region through Wednesday morning. 
By late Wednesday, another storm system is forecast to approach the 
Pacific Northwest with a leading frontal boundary expected to bring 
light rain to the North Bay. The mid/upper level trough is forecast 
to deepen slightly as the core of the low stays off of the British 
Columbia coast through late week. This should result in light 
rain/rain showers over much of the region from Thursday into early 
Friday. Rainfall amounts from this system look to be similar to 
those from the system earlier in the week. However, given this is 
still several days out, expect these values to be modified in the 
coming days. It is worth noting that the late week system will 
likely advect colder air into the region from the north. Thus, 
lingering post frontal showers are likely into Friday with a 
noticeable cool down region-wide. 

While the region may experience a period of dry conditions next 
weekend, the forecast models appear to maintain an unsettled 
pattern. While the medium range models differ some, they generally 
show another mid/upper level trough impacting the Pacific Northwest 
and California coast late next weekend into the following week. This 
thought is also depicted in the Climate Prediction Center's 6-10 
Day outlook with our region in the 50% to 60% probability of above 
average precipitation. 

&& of 5:37 PM PST Saturday...VFR except IFR cigs and 
MVFR in haze/mist continue near the eastern boundary of the forecast
area due to Tule fog. At the coastline locally hazy conditions are
reported as strong temperature inversion bases vary from 1,300 feet
to sea level. 

The WMC-SFO and SAC-SFO pressure gradients are 10.6 mb and 2.4 mb
respectively. Valley winds are for the most part either calm or 
light easterly while locally winds are gusty from the east over 
the East Bay Hills, San Francisco Peninsula (GG Bridge ENE gusting
to 27 knots for example), over the coastal waters per buoy reports
and latest weather obs from the Farallon Islands. The 00z Oakland
sounding showed a temperature inversion based at 2,000 feet so there
may be some lingering lee of Sierra Nevada mountain wave forcing in
spots over the Bay Area occuring as well as some late day contribution
from thermally direct Valley breezes; similar to a sea breeze circulation,
but mild Bay Area temperatures are in contrast to cooler Central Valley
temperatures. With onset of evening cooling over the Bay Area this should
become less of a factor in winds through tonight. Visible imagery shows
low clouds retreating over the ocean as warmer air shifts sea-ward and
cirrus clouds arriving from the west. 

Inherited tafs detail things very well. LLWS possible for a while longer
this evening vicinity KOAK. NAM forecasts a weakening offshore gradient
and wind later tonight into Sunday, and the strong January mid-upper
level ridge steadily weakens into early next week. 

Vicinity of KSFO...VFR. Light winds. FEW-SCT high clouds. 

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. FEW-SCT high clouds. Winds light
near KMRY but very breezy through the Salinas valley including
near KSNS. 

&& of 3:51 PM PST Saturday...Generally light offshore 
winds today, with locally stronger winds just offshore of any low 
lying coastal gaps, such as the Golden Gate gap, San Bruno Gap, or
near Point Reyes. Winds turn southerly and weaken slightly later 
on Sunday. Southerly winds strengthen Monday primarily from Pigeon
Point and northward. A large, long period swell arrives early 
next week.





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