National Weather Service Raw Text Product

814 
FXUS66 KMTR 141230
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
430 AM PST Sun Jan 14 2018

.SYNOPSIS...A ridge of high pressure over California will 
maintain dry and mild conditions today. Rain chances will develop
in the North Bay by Monday afternoon and across most of the 
region by late Monday night. A period of dry weather is forecast 
from Tuesday afternoon through Wednesday. Periods of rain/rain 
showers are then expected from Wednesday night through the end of
the week, with more rain possible by late next weekend.

&&

.DISCUSSION...As of 3:25 AM PST Sunday...Easterly winds continue 
to advect low level moisture from the Central Valley into portions
of the SF Bay Area early this morning. Widespread low clouds and 
patchy fog are currently begin observed in the North and East Bay.
Based on the current WRF, expect those low clouds and fog to be 
slow to clear today, especially in areas near the Delta. 
Elsewhere, skies are currently partly cloudy as a band of high 
clouds moves over California. High temperatures today are expected
to be similar to those of yesterday, with 50s in areas where low 
clouds and fog are slow to clear, low to mid 60s elsewhere across 
the SF Bay Area, and upper 60s to mid 70s from Santa Cruz and 
southern Santa Clara counties southward.

IR satellite imagery currently shows a frontal system offshore 
along 130W. This system is forecast to dissipate well before 
reaching California as it encounters the ridge currently in place 
along the West Coast. The ridge will weaken enough, however, to 
allow a second system to sweep across northern California late 
Monday and Monday night. Light rain is expected to begin in the 
North Bay by late Monday afternoon, and then spread south and east
across all but the far southern portion of our forecast area 
Monday night and early Tuesday. This system is expected to be 
quite weak and move through relatively quickly. The models have 
recently been trending drier with this system, especially the 
ECMWF which now forecasts no more than a quarter of an inch of 
rain by midday Tuesday. The 00Z GFS and 00Z WRF both forecast 
similarly meager rainfall amounts. The 06Z NAM is somewhat wetter,
forecasting as much as 0.65" in the North Bay through Tuesday. 
Both the GFS and ECMWF indicate that all rain will end by midday 
Tuesday while the latest NAM forecasts isolated light rain 
continuing into Tuesday afternoon.

Dry conditions are expected for most areas from Tuesday afternoon
through Wednesday as a shortwave ridge develops temporarily over
California. 

The next system, due to begin producing rain in the North Bay by 
Wednesday night, is forecast to be wetter than the Monday night 
system. This second system is also forecast to dig farther south
with widespread rain expected to spread across all of our 
forecast area on Thursday and Thursday evening. Scattered showers
are then likely from late Thursday night through Friday evening 
as a cold upper trough digs southeast across California. Snow 
levels are forecast to drop as low as 3000 feet, or locally lower,
on Friday and Friday night and a dusting of snow is possible on 
higher peaks across our region. Rainfall amounts from Wednesday 
night through Friday night are currently forecast to generally 
range from a half inch to an inch. Locally gusty winds are also 
possible on Thursday, but not of advisory level.

In the longer range, the GFS has been forecasting that a stronger,
windier, and much wetter system will impact our region sometime 
between next Sunday and Tuesday (21st-23rd). However, the ECMWF 
has consistently shown a less impressive system compared to the 
GFS, and that's especially true with the latest (00Z) ECMWF. 

&&

.AVIATION...as of 4:30 AM PST Sunday...IFR cigs were being
reported in the South Bay on the 12Z METARs and have been reported
at times at SQL and SFO as well. This will have impacts on the
approach to SFO through 17Z possibly a little longer. IFR cigs
also at LVK and STS this morning with low vsbys at STS. 

Vicinity of KSFO...IFR cigs through 16-17Z. Light east winds
becoming variable in the afternoon. 

SFO Bridge Approach...IFR cigs through 17Z possibly as late as
18Z. 

Monterey Bay Terminals...Drainage wind through the Salinas Valley
will keep VFR conditions. 

&&

.MARINE...as of 04:23 AM PST Sunday...High pressure over Idaho
will result in gusty easterly winds near coastal gaps through
today. Winds will decrease and turn  southerly tonight. Southerly
winds will increase Monday as a frontal system approaches. A large
long period swell will impact the coastal waters later today
through Tuesday. A larger swell will impact the coastal waters
later in the week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...High Surf Advisory...Sonoma County coast to Monterey County coast

&&

$$

PUBLIC FORECAST: Dykema
AVIATION: W Pi
MARINE: W Pi

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