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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

                            
000
FXUS66 KMTR 171752
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1052 AM PDT Mon Jun 17 2019

.SYNOPSIS...Warmer temperatures are forecast for today and Tuesday  
as high pressure strengthens over California, and onshore flow 
weakens. However, warming at coastal areas will be limited due to 
persistent onshore winds. Seasonable temperatures, along with 
areas of night and morning low clouds, are forecast through the 
end of the week and into next weekend.

&&

.DISCUSSION...as of 08:40 AM PDT Monday...It's June so that can 
mean only one thing - night and morning clouds. Webcams and 
visible satellite show another well defined marine layer across 
the Bay Area. Depth of the marine layer is just tad more 
compressed today than 24 hours ago at a depth of 1,900-2,300 feet 
per KOAK sounding and Fort Ord profiler. Similar to yesterday 
patchy drizzle is being reported along the immediate coast with a 
few sites tipping a few hundredths. Interesting to note this 
morning are the temperatures above the marine layer. KOAK sounding
has temperatures at 900mb (3,000 feet) in the 80s, which is 
further confirmed by remote stations also reporting mild temps in
the 70s. Needless to say, what a difference if you're above the 
marine layer vs below it. 

Otherwise, the message for today will be a slight warming trend
thanks to building high pressure. Temperatures will be 5 to 10
degrees warmer than Sunday, especially across the interior. Highs
will be in the 80s to low 90s. One other interesting weather note
this morning is the warmth of the airmass. The airmass today will
be just as warm if not warmer than the airmass during the last 
heat wave. However, a big however, is the lack of offshore flow. 
Model guidance still suggests a weakening onshore flow, but the 
onshore flow never completely goes away. Therefore, the lingering 
night and morning clouds will keep heat in check. The warmest days
this week will be Tuesday and Wednesday.  

&&

.PREV DISCUSSION...As of 3:00 AM PDT Monday...Low clouds have developed
well inland, and into most valleys, once again this morning as 
the marine layer remains relatively deep (2000 feet currently at 
Fort Ord). Patchy drizzle is occurring in coastal areas, but
drizzle is not as widespread, or generating as much measurable
precipitation, compared to early yesterday morning.

The marine layer is expected to compress today as an upper ridge
over the Eastern Pacific starts to build inland across northern
California. Also, surface onshore pressure gradients are trending
weaker. These developments are expected to result in earlier
inland clearing of low clouds today as well as warmer 
temperatures. Today's high temperatures are forecast to be 5 to 
10 degrees warmer than yesterday in most areas, which would bring 
our temps back to about seasonal averages. Persistent onshore flow
and only partial afternoon clearing near the ocean will mean 
warming near the coast today will only be about 5 degrees, or 
less. The most robust warming will occur in the hills and inland 
valleys. The National Blend of Models (NBM) has been too warm over
the past few days, and this warm bias by the NBM appears to be 
continuing. Therefore, maximum temperatures for today, and through
much of the week ahead, have been adjusted downward from the NBM 
by an average of about five degrees. 

Another area where the NBM appears to be deficient is in how it's
handling marine layer clouds. Specifically, low clouds are more
widespread at present than indicated by the NBM, and the NBM is
indicating a significant reduction in night and morning low
clouds as the week progresses, something that doesn't appear to be
rooted in reality given the persistent onshore flow depicted by 
the models. Therefore, cloud cover has been increased, 
particularly in coastal areas, over what is forecast by the NBM. 

Warmer temperatures are expected to persist into Tuesday. In 
fact, we will likely see a few additional degrees of warming on 
Tuesday as H5 heights increase to 588 dm and 850 mb temps warm to 
25 deg C. However, temperatures at the surface are not expected to
warm anywhere near the record-breaking heat of a weak ago when 
offshore flow developed. By contrast, onshore flow is expected to 
persist this week which will keep surface temperatures in check, 
particularly in coastal areas.

An upper trough currently in the Gulf of Alaska is forecast to
slide to the southeast and into the Pacific Northwest by midweek.
This will likely result in slight cooling on Wednesday, and a few
additional degrees of cooling on Thursday as the trough digs into
the northern Great Basin, forcing the ridge to retreat offshore.
Temperatures are then forecast to warm slightly at the end of the
week and into the early part of the weekend. But beyond that, both
the GFS and ECMWF ensemble means indicate that an upper trough 
will be centered along the West Coast, which will likely mean 
cooler than normal temperatures late in the weekend and into the 
first part of next week. 

&&

.AVIATION...as of 10:52 AM PDT Monday...For 18z tafs. Stratus has
rapidly cleared from Bay Area terminals as northerly gradient is 
strengthening and inversions are compressing as high pressure 
builds. This is pushing last vestiges of stratus to the Monterey 
Bay region where clearing will be slow or non-existent for kmry. 
Expecting vfr through this evening for Bay Area terminals. 
Developing pattern suggests that stratus will have a harder time 
pushing inland tonight so Livermore and perhaps ksts will stay 
stratus free should northerly gradients persist. Have trended back
on cloud coverage for Tuesday morning given expected forecast of 
stronger northerly gradients and shallow marine layer.

Vicinity of KSFO...VFR this afternoon and evening. Expect sct
clouds to hold off until early Tuesday morning.

SFO Bridge Approach...Approach will be clear through tonight.
Impacts to approach should be short lived Tuesday morning with
light offshore flow aloft and shallow marine layer.

Monterey Bay Terminals...Clearing for ksns but clouds may linger
for kmry for much of the day. Taf will show just tempo scattered
through this afternoon. Shallow cloud layer will move back in by
early evening and linger overnight, likely through at least 18z
Tuesday for kmry.

&&

.MARINE...as of 10:44 AM PDT Monday...Building high pressure over
the Eastern Pacific will result in the strongest pressure
gradients setting up near Cape Mendocino. This will keep the
strongest winds over the northern and outer waters  while much
lighter winds and wind waves will persist across the  near shore
waters south of Pt Reyes through Friday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 3 PM

&&

$$

PUBLIC FORECAST: MM
AVIATION: RWW
MARINE: RWW

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