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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

                            
000
FXUS66 KMTR 191753
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1053 AM PDT Fri Apr 19 2019

.SYNOPSIS...A cooling trend begins today, though with inland 
temperatures still well above seasonal norms. More significant 
cooling is then expected over the weekend as onshore flow deepens 
and strengthens and an upper level low pressure system passes 
through. Can't rule out a chance of some sprinkles or even light 
rain, especially Saturday night into Sunday morning coast and 
coastal hills south of the Golden Gate. A renewed warming trend 
appears in store for the first part of next week.

&&

.DISCUSSION...as of 08:30 AM PDT Friday...The region is situated
between a downstream ridge over the great basin, a 574dm 500mb 
upper low 400 miles to the southwest of Point Conception, and a
broader trough over the Gulf of Alaska. A shallow marine layer
developed along the coast and pushed inland through low lying
coastal areas this morning, bringing areas of patchy dense fog,
isolated pockets of drizzle, and relatively warmer morning
temperatures (given the insulating effect of the cloud cover). In
addition to the low level clouds, satellite imagery shows
additional mid to high level clouds moving from south to north
across the region ahead of the low to the southwest of Point
Conception. 

For today, the two upstream disturbances will advance on the
region, bringing enhanced onshore flow, thickening cloud cover,
and cooler temperatures throughout the atmosphere. The downstream
ridge that brought warm temperatures yesterday will bring another
day of unseasonably warm temperatures to the most extreme inland
location for one more day before completely exiting the area. 

By tonight and into tomorrow morning, the disturbances move over
California, the broader trough over NorCal and the other upper low
across SoCal. This will bring an increase in low to mid level cloud
cover, cooler temperatures across the region, and increased 
onshore winds. Though most of the chance of rain will evade the
San Francisco Bay Area and Central Coast, we could see some light
drizzle fall along the coast tonight into tomorrow morning. 

The current forecast looks on track, so no changes are planned
this morning. Please see the previous discussion for more
information on the current forecast package that was issued
earlier this morning. The main forecast challenge for today will
be reevaluating potential for any precipitation tonight into early
tomorrow morning and the magnitude of the upcoming warming trend 
expected next week.

.PREVIOUS DISCUSSION...as of 02:53 AM PDT Friday...The taste of 
summer- like warmth will be short-lived as stratus and onshore 
flow takes hold. Overnight satellite imagery shows widespread 
stratus over the coastal waters and move locally inland, 
especially the Salinas Valley. Fort Ord profiler puts the marine 
layer depth at near 600-800 feet. Given the shallow nature fog is 
likely near the coast. Monterey, Salinas, Half Moon Bay and 
Watsonville are all reporting fog, some of it dense at times. 
Therefore, forecast will include patchy dense fog over the waters 
and the immediate coast this morning. Low clouds and fog will roll
back to the coast by mid morning. It's possible that some coastal
locations will hold onto low clouds through out much of the day. 
The more established marine layer will result in much cooler 
temperatures across the entire Bay Area today. The biggest 
cooldown will be seen near the coast and Bay Shoreline today with 
a 10 to 15 degree drop at some locations. Interior locations will 
be cooler as well, but more likely in the 5 degrees cooler. Highs 
will be in the 60s at the coast and 70s to mid 80s inland.

An approaching trough and associated cold front will help to
further deepen the marine layer Friday night/Saturday morning.
Clouds and periods of drizzle have been added to the forecast.
Enough moisture may be present Saturday afternoon/evening to allow
for a few scattered showers with the frontal passage. Latest
models keep the best chance for showers N and E of the Bay Area.
However, hi-res models do show some showers over Santa Cruz Mts
and Big Sur Coast. Therefore, decided to add some shower activity
Saturday afternoon/evening. Confidence is low at this point, but
coverage may need to be expanded later.

Cooler weather will linger into Sunday behind the departing front.
By Monday upper level ridging will re-build over the state
starting another warming trend. It looks like this warming trend 
has the potential to be warmer than yesterday, especially across 
the interior. Current forecast will have a few low 90s by 
Wednesday across the interior. Weak onshore flow will keep coastal
area in check, but still above normal for mid April.

The extended is very low confidence as the longwave pattern is
completely opposite depending upon model of choice. The GFS shows
a 552 DAM low and ECMWF has a 579 DAM ridge. Therefore, no really
change to previous forecast for next week.

Side note and more interesting piece of Bay Area history - 
Thursday was the 113th commemoration of the 1906 earthquake that 
devastated San Francisco. Many buildings completely collapsed, 
but one of the few buildings still standing included the Mills 
Building. The Mills Building housed the U.S. Weather Bureau (now 
NWS) at the time. Climate record logs for the day note - Records 
destroyed by fire and earthquake. Many weather climate records for
much of Northern California were lost on that day.

&&

.AVIATION...as 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.

&&

.MARINE...as 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 
weekend.

&&

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

&&

$$

PUBLIC FORECAST: DRP
AVIATION: RGass
MARINE: RGass

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