Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

000 
FXUS66 KMTR 222355
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
455 PM PDT Thu Jun 22 2017

...Hot weather continues inland this afternoon...

.SYNOPSIS...Heat Advisories and Warnings are in place today
through 9 pm for dangerously hot temperatures. A shallow marine
layer along the Central Coast will keep the Monterey Bay coastal
areas much cooler. Temperatures will still be hot across the
interior for Friday, just not as hot as this afternoon. Additional
cooling will occur over the weekend.

&&

.DISCUSSION...as of 01:35 PM PDT Thursday...Dangerously hot 
conditions are expected this afternoon parts of the area with 
Excessive Heat Warnings and Heat Advisories out. At noon hour, 
there were multiple coop stations in the interior East Bay 
Hills/Valleys that have exceeded the century mark, including 
Concord, Walnut Creek, Pleasanton, and San Ramon. These locations 
are all under an Excessive Heat Warning until 9 pm this evening as
temperatures rise to 105 to 107 degrees. It is not out of the 
question that some isolated locations reach 110 degrees in the 
interior East Bay or even the North Bay Mountains.

Virtually all major ASOS airports are reporting warmer conditions
than they were at this time yesterday. As of 1 pm, the North Bay 
is running anywhere from 5 to 15 degrees than 24 hours ago, the 
East Bay anywhere from 8 to 15 degrees warmer, and the South Bay 
is running about 5 to 15 degrees warmer. Overall, these 
temperatures will translate to afternoon highs that are anywhere 
from 15 to perhaps as high as 25 degrees above climatological 
normals for late June.

The culprits for this prolonged heatwave are two massive upper 
level highs that are residing over the Desert Southwest and 
several hundred miles off the northern California coast. Our area 
of California is currently straddled between these two high 
pressure systems. What does this exactly mean? Well, while it is 
very hot outside for interior areas, it would be even hotter if 
one of these high pressure centers were have picked up residence 
on top of our area.

The good news is that today should be the hottest day. The high 
over the Pacific is progged to move onshore toward the 
Oregon/Washington coast through Friday with 500 mb heights 
gradually falling over our area. This will translate to afternoon 
temperatures dropping anywhere from 5 to 15 degrees for tomorrow 
afternoon for interior areas. While temperatures will fall from 
today to tomorrow, we aren't completely free of any heat risks 
quite yet. In fact, temperatures will still be hot enough to 
warrant a heat advisory for the interior East Bay Hills/Valleys, 
interior San Benito County including Pinnacles National Park, and 
the Santa Lucia range in Monterey County. This advisory will begin
at 9 pm PDT Thursday (04Z Friday) as soon as the Excessive Heat 
Warning expires, and continue for 24 hours until late Friday 
evening.

On Saturday temperatures will perhaps drop another few degrees as
an upper level low begins to nudge toward the California coast. 
There could be some mid/upper-level moisture that advects from the
south/southeast that may bring isolated thunder to our east in 
the Sierra Nevada. Probability of precipitation may have to 
increase over our area if future model runs continue to push the 
moisture toward the Central Coast.

Early next week the upper low will push onshore and help to 
flatten the Desert Southwest ridge. Temperatures will finally 
return closer to normal with highs back to low 80s to low 90s for 
the interior East Bay and South Bay. There is even the possibility
that areas could be several degrees below normal by Tuesday.

Again, going back to today, dangerously hot weather is here for 
locations away from the coast. While a much welcomed cool down is 
will begin tomorrow, temperatures will still be elevated enough to
present heat impacts, especially to those sensitive to the heat. 
Take the necessary precautions to beat the heat: drink plenty of 
fluids, limit outdoor activity during the hottest time of the 
day, wear light-colored clothing, put on a hat. If you don't have 
air conditioning, consider going to a community cooling center, a 
shopping center/mall, or movie theater. If you're parked in the 
shade, or even if you're at the coast with outdoor temperatures in
the 60s: never leave children, pets, or adults alone in a parked 
vehicle. Stay safe, stay hydrated, and beat the heat.

&&

.AVIATION...As of 4:55 PM PDT Thursday...For 00z Tafs. Low 
clouds/fog had developed along much of the coastline by late 
afternoon Thursday and had begun to move locally inland. The 
marine layer remains shallow and so VFR conditions are expected to
continue at most terminals through Friday evening. The 
exception...IFR CIGS likely to develop at KSNS and KMRY during the
evening hours. IFR conditions also possible locally around SF Bay
by late tonight, but primarily at KOAK. VFR conditions likely to
redevelop at all terminals by 17Z Friday.

Moderate forecast confidence.

Vicinity of KSFO...Few-sct low clouds likely overnight. IFR CIGS 
not expected at KSFO but perhaps patchy IFR CIGS on eastern side 
of SF Bay near KOAK between 12Z-16Z. Patchy haze possible during 
the morning hours Friday. Moderate westerly winds this evening, 
gradually diminishing overnight.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...IFR cigs likely developing this evening,
initially at KSNS by 01Z and then at KMRY by 06Z. Local LIFR cigs
and MVFR vsbys late tonight and early Friday. Clearing expected by
17Z Friday. Light westerly winds this evening and again Friday
afternoon.

&&

.FIRE WEATHER...As of 4:06 AM PDT Thursday...The most critical 
areas this morning will be the hills of the North Bay, especially 
Napa county and around Mt St Helena where winds above 2000 feet 
will be northeast and gusting 20-25 mph. Humidity values are 
starting to drop down to 20-30% in the hills as of 3 am with hot 
temperatures in the 80s and lower 90s at this early hour. Expect a
hot and dry day all inland areas and hills today. Main concern 
this afternoon will be a transition to westerly winds should any 
new starts occur. 

Although onshore flow will return by tonight, the marine layer 
will remain shallow and the airmass aloft will remain very warm 
for a few more days. Thus, conditions in the hills will only 
gradually improve from Friday through the weekend. Much improved 
conditions are expected by early next week. 

&&

.CLIMATE...

Here is a list of record high temperatures

SF BAY AREA              Thursday
LOCATION.................Temp/Year   
 KENTFIELD................102/1989
 SAN RAFAEL................99/2006
 NAPA.....................105/1981
 SAN FRANCISCO.............97/1989
 SFO AIRPORT...............98/1989
 OAKLAND AIRPORT...........94/1957
 RICHMOND..................94/1989
 LIVERMORE................110/1961
 MOFFETT FIELD.............99/1989
 SAN JOSE.................101/1989
 GILROY...................110/1973

MONTEREY BAY AREA        Thursday
LOCATION..............   Temp/Year
 SALINAS...................89/1981
 SALINAS AIRPORT...........92/1947
 KING CITY................110/1929

&&

.MARINE...As of 04:39 PM PDT Thursday...As surface high pressure
lifts northward, winds will continue to  be light and southerly
this evening and through Friday. In  addition, seas will subside
through the remainder of the week and into the weekend. Winds are
expected to be moderate to strong  during the afternoon and early
evening hours around the Golden  Gate and Angel Island areas over
the next few days.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Heat Advisory...CAZ506-508-513-516
             Heat Advisory...CAZ518
             Excessive Heat Warning...CAZ507-510>512-517
             Heat Advisory...CAZ510-511-517
             SCA...SF Bay until 9 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 9 PM

&&

$$

PUBLIC FORECAST: Rowe
AVIATION: Dykema
MARINE: BAM
FIRE WEATHER: RWW


Visit us at www.weather.gov/sanfrancisco

National Weather Service Glossary of Abbreviations