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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 180949

Area Forecast Discussion
National Weather Service San Francisco Bay Area
249 AM PDT Thu Apr 18 2019

.SYNOPSIS...Warmer temperatures are forecast for today, 
especially inland areas where widespread highs in the lower to mid
80s are expected. A cooling trend is then expected to begin 
Friday and continue into the weekend as onshore flow redevelops 
and strengthens. This will be followed by renewed warming for the 
first part of next week. No precipitation is expected through the 
forecast period.


.DISCUSSION...As of 02:42 AM PDT Thursday...GOES satellite fog 
product reveals mostly clear skies around the Bay Area overnight 
with a few exceptions: developing stratus over the coastal 
waters, patchy low clouds developing along portions of the 
immediate coastline and patchy fog/low clouds North Bay Valleys. 
Current forecast will have some patchy low clouds/fog possible 
early this morning, but impacts will be minimal. 

The bigger weather story for the day will be the noticeable warm
up. Synoptically speaking, a large upper level ridge currently
along the West Coast will strengthen as it moves eastward. The 
building ridge can be seen by looking at increasing 500 mb heights
and warming temperatures at 850 mb. Observed highs on Wednesday 
were upper 50s to upper 60s near the coast and 70s to near 80 
inland. Generally speaking a modest 5 to 10 degrees of warming
will be seen today with a few spots approaching 15 degrees of
warming compare to Wednesday. In other words, highs in the 60s to
mid 70s along the coast and widespread 80s inland. It's possible
that a few interior locations in Monterey/San Benito Counties may
eclipse 90 degrees. Temperatures this warm are about 5 to 10
degrees above normal for mid April. One limiting factor for really
warm weather is the lack of offshore flow and continued onshore
flow, especially near the coast. From a heat risk standpoint,
there is enough cooling (relief) Thursday night and Friday that
impacts will be minimal. However, today could be the warmest day
so far this year for some locations so some precaution should be
used - stay hydrated, monitor activities during peak heating of 
the day, take breaks, etc.

The warm up will be short-lived as the ridge moves east, an upper
level low undercuts the ridge across SoCal and low level onshore 
flow ramps on Friday. A general cooldown of 3 to 5 degrees is 
expected on Friday with a slight increase of early morning 
clouds/patchy fog. Not a full force summer stratus push just yet, 
but more of a preview. 

An even bigger cooldown is expected on Saturday as an upper level
low swings through NorCal bringing increasing clouds, cooler 850 
mb temps and possible precipitation to CA. 00Z model suite
continues to show mixed results on potential precip impacting the
Bay Area on Saturday. Current forecast will remain dry at this
point as best chance still remains N and E of the forecast area.
Will keep a close eye on this and precip may added to the forecast
later. High temperatures will be cool and mostly in the 60s an
lower 70s. Interesting to see the temp swing from above normal on
Thursday to below normal for Saturday. 

Similarly to the initial warm up, the cooldown will be short-
lived as well. High pressure is forecast to re-build over the 
region Sunday into Monday. The warming trend is forecast to 
continue well into next week. If some of the medium range models 
and ensembles are correct the second round of warming may be even 
warmer than the Thursday warm up. Interior locations may hit the 
upper 80s to lower 90s next Wednesday and Thursday. 

Longer range forecast indicate that the wet season may be ending 
and the dry season is beginning. The latest 8 to 14 day CPC 
outlook has much of NorCal drier than normal through May 1.

&& of 10:40 PM PDT Tuesday...Upper level ridge will 
be moving over the area tonight and Thursday. Surface high 
pressure west of San Francisco with a weak ridge building into the
Oregon coast. This will keep the stratus patchy and limited 
mainly to the MRY Bay Area and around the Golden Gate which 
includes OAK. Currently there are patches of low clouds along the
coast and Monterey Bay.

Vicinity of KSFO...VFR through at least 12Z. Stratus around the
Golden gate could possibly spread briefly into SFO between 12-17Z
but given that it remained VFR this morning and that the marine 
layer will be shallower is less likely that stratus
gets to SFO. 

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Currently only a few clouds over the bay
but this is an indication of moisture in the area. Expecting 
IFR conditions at MRY 09Z-16Z and 12Z-16Z at SNS.

&& of 10:33 PM PDT Wednesday...High pressure off the
California coast will keep light to moderate northwest winds
through Thursday night. The high will shift north and build into
the Pacific Northwest on Friday resulting in  increasing winds
over the northern waters that will last through the weekend.





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