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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 192348

Area Forecast Discussion
National Weather Service San Francisco Bay Area
448 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 4:45 PM PDT Friday...Approaching upper trough
from the northwest has increased the onshore gradient and deepened
the marine layer. This will bring a return of MVFR cigs to the 
SFO Bay Area reaching the East Bay first then spreading over the
rest of the bay. SFO and the approach should stay VFR through the
airport evening rush before cigs move in later tonight. 
Cannot rule out patchy drizzle as well early Wednesday morning. 

Vicinity of KSFO...MVFR cigs after 07Z. Possible drizzle with
reduced vsbys to 5-6 miles after 12Z. Clearing late in the 
morning Saturday with westerly winds gradually increasing on 
Saturday. Gusts to 25-28 kt Saturday afternoon.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...An early return of IFR cigs is expected
as stratus covers the MRY Bay. Cigs could come in as early as 02Z. 
Patchy drizzle possible. 

&& of 04:33 PM 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 weekend.


     .Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm
             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




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