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

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FXUS66 KMTR 240128

Area Forecast Discussion
National Weather Service San Francisco Bay Area
628 PM PDT Wed May 23 2018

.SYNOPSIS...Seasonably cool weather conditions along with overnight
and morning low clouds will continue to impact the region through
late week. An upper level system will then approach the coast late
Thursday and bring a chance of rain showers to much of the region
into Friday. Showers may linger into Saturday morning before a
warming and drying trend returns late in the holiday weekend.

&& of 01:46 PM PDT Wednesday...Low clouds continue
to impact the northern two-thirds of the region this afternoon
with persistent onshore flow. This has resulted cooler conditions
compared to yesterday over portions of the North Bay while areas
further south are a few degrees warmer. In addition, seeing breaks
in the cloud cover over the Pacific this afternoon.

With all this, expecting little change overall in the pattern
heading into tonight and Thursday morning ahead of an approaching
mid/upper level low pressure system. Look for coastal drizzle to be
a bit less widespread overnight into early Thursday morning with
continued cool weather conditions. Low clouds will likely fill back
in over the ocean and spread inland during the overnight hours as

Rain showers are forecast to develop late Thursday night and spread
inland Friday as the mid/upper level system pushes inland. There is
also a non-zero chance of high-based thunderstorms across portions
of the region late Thursday night into Friday morning as moisture
and instability increases in the 500-700 MB layer. However, have
left out mention of thunderstorms in the forecast as confidence is
low at this time and will needed to be monitored in the next 24
hours. Showers will remain possible into Friday night before
diminishing Saturday morning as the upper level system shifts inland.
Overall, rainfall amounts will range from a few hundredths to one-
tenth of an inch in most locations. Locally higher amounts will be
possible in any heavier showers that develop while the showery
nature of this system may leave some areas with no measurable
rainfall at all.

A warming and drying trend is then forecast into Sunday with
temperatures back to around seasonal averages by the Memorial Day
Holiday. This will be the result of a building ridge of high
pressure along the eastern Pacific. Another mid/upper level trough
is then forecast by the middle part of next week that would cool
temperatures once again and potentially enhance overnight/morning
coastal stratus.


.AVIATION...As of 6:28 PM PDT Wednesday...Areas MVFR ceilings and
good visibilities are likely to continue tonight into Thursday
morning. Lower to mid level thermal ridging embedded in steadily
weakening 500 mb heights tonight will transition to deeper layered
thermal and 500 mb height troughing Thursday and Thursday night.
High boundary layer humidity remains in place tonight and Thursday
with low clouds gradually filling back in tonight; hints of some
very weak convective appearance (strato-cumulus vs stratus) were
seen in the low clouds today, increasing mid level cooling could
cause more strato-cumulus to cumulus clouds Thursday.

Rippling, lee side waves, due to strong and gusty westerly winds
is showing up in late day visible imagery over the East Bay;
mesowest obs showing gusts up to 30-35 mph in the East Bay
Hills/Mountains. Surface winds are also gusty in the Delta.
Onshore flow is mostly contained under a subsidence inversion near
3,000 feet (thermal ridging above this height) per 00z Oakland
upper air sounding.

As mentioned in the public discussion, convective parameters steepen
a bit more by Thursday night with upper level troughing. Too early
to include VCTS in tafs at this time, but isolated to widely scattered
convection leading to isolated t`storm development is something to
keep in mind, a possibility either Thursday afternoon and/or Thursday
night. 700-500 mb layer instability steepens a bit, with a swath
of drier mid level air arriving from the southwest Thursday, but
not much of anything seen in surface based mucape on today`s 18z NAM.

Vicinity of KSFO...Some erosion of low clouds can be seen on the
visible imagery and KSFO web cam, clouds have more of a strato-cu
appearance to them this afternoon. WRF model leans heavily with
high relative humidity in the boundary layer thus forecast for
MVFR ceiling tonight looks good, but could stay scattered for a
while this evening. Bank of low clouds over the coastal waters is
filling in per recent visible satellite imagery, onshore winds
should usher low clouds inland tonight and Thursday morning.
Thursday is a transition day to upper level troughing, low clouds
should lift to scattered coverage with late day and/or evening
shower possible.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...MVFR ceilings tonight and Thursday
morning. Partial clearing Thursday afternoon and evening, upper
level troughing will swing in from the southwest possibly bringing
late day and/or evening shower.

&& of 4:43 PM PDT Wednesday...Northwest winds will
remain light across the waters today and tonight. Winds will turn
southerly tomorrow morning as an upper low and weak surface cold
front approach from the west. This will bring showers to the area
tomorrow and Friday. Winds will turn back out of the northwest
once the upper low moves through late Friday. Winds across the
waters will also increase and remain gusty through the holiday
weekend. Long period southerly swell will persist through the end
of the week. Periods and swell heights will gradually lower over
the next few days before a second long period, although weaker,
southerly swell moves in towards the end of the weekend.





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