FXUS66 KMTR 260132

Area Forecast Discussion
National Weather Service San Francisco Bay Area
632 PM PDT Wed Apr 25 2018

.SYNOPSIS...Cool, summer likely conditions will persist over the
region through Thursday with additional cooling expected Friday
into the upcoming weekend. There will be a slight chance of
showers Friday night into Saturday for the far northern portions
of the North Bay as the upper level system responsible for the
cool down pushes inland to our north. Dry conditions along with a
slight warming trend appears likely early next week.


.DISCUSSION...as of 01:45 PM PDT Wednesday...The marine layer
again impacted the region this morning with low clouds slow to
burn-off through the late morning. Temperatures range from the
upper 50s at the coast to lower 70s in our warmer inland locations
as well as higher elevations above the 1,800 foot marine layer.
Looking for similar conditions tonight into Thursday ahead of an
approaching mid/upper level trough.

Temperatures will trend a few degrees cooler on Friday and continue
to be below seasonal averages into the upcoming weekend in response
to the mid/upper level trough pushing inland. The core of the upper
level low will then push into the Pacific Northwest and northern
California late Friday into Saturday and bring a slight chance of
showers. Rain showers may develop as far south as the northern
portions of the North Bay during this time frame. However,
widespread rainfall is unlikely and most locations will remain dry,
especially south of the Golden Gate Bridge.

A slight warming trend is then likely early next week as the trough
axis shifts inland and a ridge of high pressure develops over the
eastern Pacific. However, the ridge is forecast to stay far enough
offshore to prevent significant warming. Ongoing dry weather
conditions are also expected in the extended forecast period.


.AVIATION...As of 6:31 PM PDT Wednesday...Ongoing lower level
atmospheric cooling can be seen affecting the characteristics of
the marine layer per visible satellite imagery early this evening.
A weak trough focused at 925 mb over the coastal waters is drifting
eastward, 925 mb cooling will be greatly uneven with the greatest
cooling tending to settle in over the South Bay to Central Coast
through tonight. Elsewhere a slower, but steady lower level cooling
will affect the marine layer overnight into Thursday morning as
well. In general, due to the proximity of a closed 500 mb low
approx 600 miles west of the Bay Area, waves of additional cooling
will continue to drift in from the west deepening to possibly mixing
out the marine layer Thursday. A negative tilt, amplifying mid level
trough arrives Thursday night which should further mix out the marine
layer. Weather conditions for this forecast period are thus mainly
favorable for the development of low stratus ceilings, not so much
fog due to a deepening marine layer; forecast confidence in the 00z
tafs is low to slightly more than moderate on ceiling bases, timing
and duration.

Vicinity of KSFO...The marine layer is deepening with stratus
accumulating along the San Mateo Coast. In the vicinity of KSFO
925 mb and 850 mb level temperatures cool to the point of mixing
out the marine layer possibly by late tonight, if not Thursday
morning. It's a complicated forecast with drying mixing into the
boundary layer, a further loss of the marine temperature inversion
through lower level cool air advection and then soon to arrive
nocturnal cooling. Inherited forecast looks good, only minor
adjustment made to the duration of westerly wind forecast this
evening. As advertised looks like good chances of MVFR cig
development no later than 07z, possibly a little earlier based on
recent satellite trend.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Being closer to greater cooling rates with
a 925 mb trough nearby the marine layer appears more chaotic in
structure, not very cohesive at the moment. Mixing and cooling is
tending to at least initially offset any traction the marine layer
stratus can make late this afternoon, however night-time cooling
and lower level cool air advection should result in stratus coverage
returning. Meso-scale forecasts are leaning toward high humidity
within the boundary layer tonight thus forecasts of MVFR look
good, tempo groups indicate start times in the 04z-08z time frame,
a bit broad brushed due to inherent forecast uncertainty as to
how/when the stratus will form. It's slightly more than moderate
confidence that MVFR cigs form tonight into Thursday morning.


.MARINE...as of 5:15 PM PDT Wednesday...Winds will remain generally
light over the coastal waters overnight, though locally breezy
conditions could develop near coastal gaps or prominent points
during the day on Thursday. Light to moderate seas will continue
through the forecast period with a mixed south and northwest swell
developing late in the week.






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