fxus66 kmtr 201816 

Area forecast discussion 
National Weather Service San Francisco Bay area 
issued by National Weather Service Los Angeles/Oxnard California 
917 am PDT Wed Jun 20 2018 

Synopsis...not much change in weather conditions through 
Thursday, then a pronounced warming trend for the end of the week 
as high pressure builds in and winds turn more offshore. A return 
to cooling and redevelopment of the marine layer appears in store 
for Sunday into the first part of next week as flow shifts back 


Discussion...as of 9:15 am PDT Wednesday...a good day to do some 
computer work at National Weather Service Bay area with a classic June gloom weather 
pattern. The marine layer rises up to about 1700 feet at sfo 
(according to aviation data) which is exactly the same as at lax 
this morning, over 330 miles away. Low clouds and patchy fog cover 
most coastal areas and extend all the way into the southern 
Salinas valley. The high pressure ridge over the eastern Pacific 
is rather strong though and low clouds will retreat by afternoon, 
but coastal areas will be the cool spot compared to inland areas. 
Gusty northwest winds over the nearshore waters will combine with 
a 6-7mb onshore pressure gradient for locally gusty winds into the 
sfo Bay region this afternoon. Insignificant changes in the upper 
level pattern tonight into Thursday should mean a similar marine 
layer result for the next 24-hours. 

Synoptically-speaking, an upper level ridge continues to build 
with its center located just off the Baja California coast. Over 
the next 24 hours, a shortwave trough will ripple through this 
ridge's outer periphery and clip northern California. While no 
precipitation is expected with this shortwave, this should limit 
substantial warming for the North Bay this afternoon. Southern 
portions of the County Warning Area such as the southern Salinas valley and 
interior San Benito counties will probably be too far south to see 
any influence from this shortwave and would be the most likely 
locations to experience 3-6 degrees of warming from yesterday to 

The shortwave will quickly push east toward the northern rockies 
by Thursday. For the San Francisco Bay area and central California 
coast, not expecting much change from Wednesday to Thursday for 
afternoon highs as the upper level ridge flattens. The ridge won't 
stay flattened for long as models agree that broad high will 
build offshore, pushing 500 mb heights over the Bay area anywhere 
from 590 to 592 dam by Saturday. At the surface, this will result 
in hot conditions, mostly for interior locations, where highs are 
expected to soar to the middle/upper 90s to the triple digits. 
Locations along the immediate Pacific coast should remain 
protected from these hot temperatures with just enough of an 
onshore flow. This all changes as you head inland and/or up in 

The big question at this point in time is exactly how hot it will 
get along the sf Bay shoreline for Saturday. One tool forecasters 
use for determining the potential outcomes is through ensemble 
forecasting. An ensemble is comprised of ensemble members 
(anywhere from a dozen to several dozen, depending on the model), 
each of which produces a slightly different forecast solution. The 
spread of these forecast solutions is valuable to meteorologists 
as we can see the potential spread through all the ensemble 
members. Now, when looking at the European model (ecmwf) ensemble prediction system 
(eps) for saturday's heat event, some of the hottest ensemble 
members -- at sjc, for example -- do exceed 100 deg f. The current 
forecast package for Saturday does bump up maximum temperatures a 
bit along the sf Bay shoreline and the South Bay, though 
confidence still remains on the low side in regard to these exact 
values as there remains a rather large spread in possible 
outcomes. The potential heat risks based on this forecast does 
introduce widespread moderate and isolated high risks across the 
North Bay East Bay interior, South Bay, and interior Monterey + 
San Benito counties. Still a bit early to launch any headlines, 
but if the current forecast holds, we'd likely be looking at heat 
advisories for the warmest locations. Overnight low temperatures 
will remain elevated in the thermal belts with some locations 
at/above ~ 1000 ft elevation likely to only cool to the mid/upper 
60s to 70s. 

Another thing to mention for the late Friday to Saturday timeframe 
is the potential for gusty winds in the higher elevations. The 
GFS does advertise north to northeast winds for the north and East 
Bay hills/mountains that could produce elevated fire weather 
concerns. Will continue to monitor these forecast trends, 
especially as we approach the event when the higher resolution 
models are able to capture this with finer detail. 

Models for Sunday suggest the start of what would be a welcomed 
cooling trend across the interior. The latest GFS run even brings 
in what could be a southerly surge along the Big Sur coast, 
pushing north toward the Santa Cruz and San Mateo coasts through 
the day. Additional cooling is expected for much of the region -- 
especially interior locations -- heading into the beginning of 
next week. 


Aviation...as of 11 am...clouds scattered out from much of the 
interior. However, stratus continues to impact some coastal 
airfields, including kmry, as of 18z. Clouds will continue to erode 
from interior areas through 19-20z with stratus expected to remain 
along the immediate coast. Onshore flow this afternoon will increase 
winds to around 12-15kt, with locally higher gusts. Stratus will 
push back in again tonight, but confidence is low on exact timing 
and coverage at this time. Thus we have opted to stick with a 
persistence forecast. 

Vicinity of ksfo...VFR into the afternoon. Onshore winds around 20 
kt, with gusts near 30 kt this afternoon and evening. MVFR/IFR cigs 
likely again tonight, best guess around midnight, give/take a couple 
of hours. 

Sfo bridge approach...VFR into the afternoon. MVFR/IFR will return 
late this evening into tonight. 

Monterey Bay terminals...IFR ceilings through the early afternoon 
with periods of brief scattering possible. VFR possible by 21z but 
confidence is moderate. West/northwest winds 10-15 kt this afternoon 
and early evening. IFR/LIFR cigs return this evening, around 02-03z. 


Fire weather...as of 3:30 am PDT Wednesday...a pronounced 
warming and drying trend is forecast for the end of the week as 
high pressure builds in and winds turn offshore. Saturday 
presently looks to be the hottest day, with poor overnight 
relative humidity recoveries for both Friday night and Saturday 
night. Locally gusty north to northeast winds are possible in the 
north and East Bay hills Friday night and Saturday morning. A 
return to cooling and redevelopment of the marine layer appears in 
store for Sunday into the first part of next week as flow shifts 
back onshore. 


Marine...as of 08:36 am PDT Wednesday...a 1023mb high pressure 
was located about 700nm west of Monterey Bay and will continue to 
strengthen today resulting in increasing northwest winds, which 
will prevail through much of the upcoming weekend. Sustained gale 
force winds are expected along the Big Sur coast, south of Point 
Pinos, this afternoon and evening. Gale force winds will also be 
possible for the northern outer waters, north of Pigeon Point, 
Friday evening through Saturday morning. More details will be 
addressed in future updates. Steep fresh swell from the northwest 
winds will create hazardous seas into the weekend, particularly 
for operators of small craft. 


Mtr watches/warnings/advisories... 
... Bay from 12 PM 
Small Craft Advisory...mry Bay from 12 PM 
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm from 12 PM 
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm from 12 PM 
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm from 12 PM 
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm until 2 PM 
glw...pt pinos to pt Piedras Blancas 0-10 nm from 2 PM 
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm 
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm from 2 PM 



Public forecast: Rowe/boldt 
aviation: kml/BAM 
marine: BAM 
fire weather: blier/Rowe 

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