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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 192146

Area Forecast Discussion
National Weather Service San Francisco Bay Area
246 PM PDT Fri Apr 19 2019

.SYNOPSIS...A pair of upper lows will impact California through
the next 24 hours bringing a cooling/moistening trend, a slight
chance of drizzle along the coast overnight, and increased onshore
flow through tomorrow. A renewed warming trend appears in store 
for the first part of next week.

&& of 02:45 PM PDT Friday...California is currently
positioned between 3 synoptic scale features-- downstream, the 
high pressure ridge that brought unseasonably warm temperatures 
yesterday and upstream, a pair of two upper level disturbances.

The influence of the ridge waned across the majority of the 
region by this afternoon, however, a handful of locations in the 
extreme interior continue to run a few degrees warmer today than 
the same time yesterday. That said, most urban areas are running a
few to several degrees cooler in response to advancing pair of
upper lows. Compared to 24 hours ago, temperatures inland are
running 3 to 6 degrees cooler and along the coast anywhere from 10
to 20 degrees cooler. This temperature drop is not too surprising
given that the latest satellite imagery shows a deck of marine 
stratus banked up along and just inland of the coastal interface. 
Most locations seeing the largest drop were at least mostly sunny 
this time yesterday while they are in the thick of the cool/moist 
marine air mass this afternoon. In addition, the approaching lows
have brought a marked increase in the onshore flow early this 
afternoon, becoming breezy, with wind speeds of up to 20mph 
versus 5 to 10mph yesterday.

The first upper low to impact the region is now around 200 miles
to the southwest of Point Conception (off of Santa Barbara
county). This feature is already helping to deepen the marine
layer that is present along the coast, enhance onshore flow, and
spread a fractured deck of mid to high level clouds northeastward
across the state. Forecast models show this more petite upper low
sliding nearly due eastward across southern California and into 
Arizona through the next 18 to 24 hours. As this occurs, the
marine layer will deepen further, air mass temps will drop 
somewhat, and we could even see some periods of drizzle overnight
tonight along the coast and higher terrain (mainly San Mateo 
coastal range/Santa Cruz mountains/Big Sur range/coastal Monterey 
Bay). Model ensembles suggest amounts will struggle to push beyond
a trace of precipitation, though the wetter spots could see a 
hundredth or two squeezed out of the marine layer if they are 

The second upper low to impact the region is descending
southeastward out of Gulf of Alaska early this afternoon and is
expected to move into California overnight tonight through
Saturday morning. This particular upper low will be preceded by a
cold front that will bring increasingly breezy to gusty onshore 
winds tonight into early Saturday, a thicker deck of mid to high 
level clouds, and another slight chance of drizzle along the coast
through Saturday morning. Again, precipitation amounts with the 
second system will struggle to break out beyond a trace to a few 
hundredths and even then, primarily along the coast. The bulk of 
the precipitation from this system will be caught instead by the 
Sierra Nevada to our east. It is not too unusual for our spring 
systems to go this way. 

Tomorrow will be cool, breezy, and cloudy for most urban areas.
Temperatures will range from the mid 50s to the low 60s along the
coast, throughout the 60s inland, and into the low 70s for the
extreme inland locations. These values represent readings that are
slightly below seasonal norms. Onshore winds will be breezy,
ranging from 10 to 20 mph but may gust up to 25 to 30 mph in the 
windier locations (such as the San Bruno gap, Golden Gate gap, 
etc). Some afternoon clearing in the clouds is possible in the
wake of the front, however, post frontal instability and a
lingering marine layer will keep some clouds in the vicinity.

A warming/drying trend will then begin Sunday and peak mid week
next week as high pressure gradually builds into the state.
Current data shows that this warming trend has the potential to be
slightly stronger than the one that peaked yesterday, with highs
pushing into the low 90s in the warmest spots. Most locations will
be warmest on Tuesday though the extreme inland locations may peak
on Wednesday as the ridge axis shifts inland. Afternoon highs will
be 12 to 16 degrees above seasonal normal mid week next week 
given the current forecast data. Stay tuned for more information 
about the upcoming warming trend into next week.

&& of 10:50 AM PDT Friday...Mainly VFR conditions 
have returned over the terminals with low clouds lingering 
offshore. Expecting onshore flow to increase this afternoon with 
continued high clouds streaming inland across the region. Abundant 
boundary layer moisture and approaching mid/upper level trough 
looks to bring an early return of MVFR-IFR ceilings back to the 
San Francisco Bay and Monterey Bay terminals in the 02Z-06Z 
timeframe which will likely persist into Saturday morning. Cannot 
rule out patchy drizzle as well early Wednesday morning which may 
further worsen flight conditions. Confidence in low ceilings 
developing over KSTS and KSJC is lower. 

Vicinity of KSFO...VFR conditions are forecast to persist through
the afternoon with an increase in westerly winds. Looking for an
early return of stratus late in the evening and into the
overnight. Cannot rule out patchy drizzle early in the morning
with low clouds slow to dissipate on Saturday. Westerly winds 
gradually increase on Saturday with gusts up to around 25-30 kt. 
Moderate confidence.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR conditions through the early
afternoon with an early return of stratus by late afternoon/early
evening. MVFR-IFR conditions likely overnight, potentially lower
at times with the possibility of patchy drizzle. These conditions
will likely persist into Saturday morning. Moderate confidence.

&& of 10:50 AM PDT Friday...Northwest winds will 
increase over the northern waters and near the coast today through
Saturday as high pressure develops off of the Pacific Northwest 
coast. A moderate northwest swell will also prevail through the 


     .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm from 9 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 9 PM
             SCA...Pt Arena to Pt Reyes 0-10 nm




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