Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

FXUS66 KMTR 212139

Area Forecast Discussion
National Weather Service San Francisco Bay Area
239 PM PDT Sat Jul 21 2018

.SYNOPSIS...Seasonable conditions will remain in place through
Monday. High pressure will build over the region for the middle
part of the week bringing hot temperatures to interior areas and 
in the mountains.

&& of 01:57 PM PDT Saturday...A few clouds continue
to stream over the Bay Area and Monterey Bay regions this
afternoon as seen on visible satellite, while day time high temps
continue to warm to seasonable norms. Sunday and Monday will
continue to reflect normal July conditions with low clouds and fog
along the coast overnight and near normal temps across the region
each day. Monsoonal moistures remains to our east, mainly
impacting the Sierras.

The area of forecast for this afternoons forecast package is the
heat that will creep into the area Tuesday through Thursday. 500
mb high pressure over the 4 corners region will drift to the west
the next couple days, then set up shop near the CA/AZ border. As 
it makes the shift the high pressure will strengthen to near 599
dam, ridging into the Bay Area region and causing subsidence in 
the air column. As this pattern sets up looking for a quick heat 
up on Tuesday and remaining into Thursday. Since our area of 
responsibility will be on the edge of the heat dome, the main 
areas of focus will be for the SE portion of Monterey County, most
of San Benito County, and the eastern portion of Santa Clara 
County. It will also cook for the far NE portion of Napa County 
Tuesday and Wednesday. Onshore flow is expected to remain in place
and keep the remainder of the Bay Area only a little above 
seasonal averages. We will be watching how the winds are shaping 
up over the coming days. If the winds calm on Tuesday/Wednesday 
afternoon, effectively eliminating the onshore push, temps could 
ramp up in the Bay Area core. Models are not indicating this 
currently, but bears watching. With this synoptic scale pattern 
change coming and the ridge bringing in 850 mb temps around 25C, 
we went ahead and took the preemptive step of issuing a heat 
advisory with significant lead time. The heat advisory will start 
around 11 am on Tuesday and run through Wednesday at 9 pm. Due to 
the warm airmass aloft, the overnight lows in the hills and 
mountains will be rather high, likely in the 70s and low 80s. 
Valleys will dip into the 50s and 60s, so the heat recovery 
overnight will be for lower elevations. This is why the heat 
advisory covers mainly elevated terrain. If models hold true, the 
heat advisory may need to be extended into Thursday, as it looks 
much like the setup taking place on Tuesday.

Friday will bring a noticeable change as the dome of high 
pressure is forecast to shift to the east again due to low 
pressure moving into the Pac NW. This pattern will help to shift 
the heat out and bring the region back into seasonable norms.

Overall...typical July conditions Sunday/Monday followed by an
interior heat up Tuesday through Thursday with a week punctuated 
by a return to seasonal norms.


.AVIATION...As of 10:16 AM PDT Saturday...for 18z tafs. Almost a
repeat of the last few mornings, all terminals have become VFR.
Expecting VFR this afternoon with a few high clouds overhead. Some
gusty winds will be possible this afternoon with onshore flow.
Expect cigs to move in again overnight with lower confidence for

Vicinity of KSFO...VFR. Some patchy cigs will likely develop again
around the terminal, but conf is too low to add to taf. Light 
Ne winds this morning increasing to westerly around 15 kt this 

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR through this afternoon. Low cigs
return early this evening.

&& of 01:57 PM PDT Saturday...A weak low pressure system
off the coast will produce light to  moderate winds over the
coastal waters this weekend. Northwesterly winds are then forecast
to increase through much of the coming  week. Two long period
southerly swells will impact the region this weekend and early
next week.


     .Tngt...SCA...SF Bay until 10 PM
 .Tue/Wed...Heat Advisory for zones 511/516/517/518




Visit us at

National Weather Service Glossary of Abbreviations