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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

                            
000
FXUS66 KMTR 211737
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1037 AM PDT Wed Aug 21 2019

.SYNOPSIS...High pressure over Central and Southern California
will kick off a warming trend through Thursday and will compress 
the marine layer along the coast leading to significantly less 
night and morning low clouds. Seasonable temperatures are forecast
through the upcoming weekend, along with continued dry conditions.

&&

.DISCUSSION...as of 09:10 AM PDT Wednesday...The marine layer
struggled overnight and into this morning -- bringing clear to
mostly sunny conditions to most regions. A combination of being
partially mixed by a trough to the north and compressed by a
retrograding ridge aloft and to the south has kept marine stratus
mostly in check this morning. Satellite imagery shows mostly clear
skies across the SF Bay Area while the remnants of the compressed
marine layer/otter eddy are bringing some low clouds to the 
Salinas valley and lip of the Monterey Bay. 

For today and the coming days, a modest warming trend is forecast in 
response to a retrograding ridge. Seasonal temperatures are expected 
near the coast while inland areas could rise to 3 to 8 degrees above 
normal. Coastal highs in the upper 60s to mid 70s are expected, 
while areas in proximity to shorelines will rise to the upper 70s to 
mid 80s. Extreme inland areas, such as the North/East Bay valleys, 
southern Santa Clara valley, and inland Monterey/San Benito counties 
are forecast to hit the upper 80s to mid 90s. Some passing high 
clouds are expected through the day, increasing the farther 
northward you are and through the course of the day, as a 
consequence of the low pressure trough digging southward later today 
and into tonight. These high clouds could suppress temperatures in 
the North Bay by a few degrees which they will make up tomorrow. 

Tomorrow will be similar to day but with less overall trough 
influence -- fewer clouds, a compressed marine layer, and slightly 
warmer temperatures inland, particularly for the extreme North 
Bay that will be under some cloud cover today.

No significant change is planned for the overnight forecast
package. Please see previous discussion for more detail on the
current forecast. Main forecast challenge of the day will be what 
impacts, if any, we may see as the moist, unstable remnants of a 
tropical depression potentially move over the area mid week next 
week. 

.PREVIOUS DISCUSSION...as of 3:53 AM PDT Wednesday...An 
interesting weather pattern is currently in place over the west 
coast with California nuzzled between a ridge of high pressure to 
the east and a ridge of high pressure to the west while a trough 
of low pressure passes through the region. High pressure at the 
surface presently centered over the Four-corners region compressed
the marine layer along the coast resulted in little to no marine 
layer overnight. Currently only a few patches of stratus are 
visible by satellite over the coastal waters and locally inland 
over coastal hills and peaks. Gradients have seen little change 
overnight with the northerly gradients remaining around 3 mb while
the onshore gradient from SFO to WMC weakened slightly to around 
2 mb. 

Model guidance continues to depict a warming trend through
Thursday before temperatures plateau maintaining seasonably warm
temperatures across the region through the remainder of the week and 
through the weekend. 850 mb temperatures will warm to around 22 C by 
this afternoon then cool on Thursday in response to the trough 
aloft. However strong ridging at the surface appears to counteract 
any cooling resulting in relatively little change through the rest 
of the forecast period. Mostly clear skies will also aid this 
warming trend to get off to an earlier start today and Thursday.

.PREV DISCUSSION...as of 9:53 PM PDT Tuesday...The boundary layer
winds will turn more northerly on Weds which will induce some 
adiabatic warming. Model guidance suggests many of the North and 
East Bay valleys will warm into the upper 80s and lower 90s on 
Weds which will be as much as 10 degrees warmer than what was 
observed on Tuesday. 

One item to watch on Weds night is some potentially gusty 
northerly winds in the hills. 00z NAM has 20-30 kt winds at 925 mb
(around 2500 feet) from Sonoma to Monterey late Weds afternoon 
into Weds night. Not expecting any wind advisories but could be a 
brief fire weather concern.

Pattern looks to continue with above normal/warming trend for
inland areas Thursday and Friday as the ridge builds over the
region with potential for the thermal trough to straddle the coast
at times keeping marine stratus to a minimum. Long range trends
remain warm and dry. Will be watching the tropical activity as it
heads north from Baja next week. Current ECMWF would bring
remnants of our coast by about next Thursday...a long ways off.

&&

.AVIATION...As of 10:37 AM PDT Wednesday...For 18Z TAFS...
VFR expected today. Patchy low clouds persist over the Monterey
Bay. Northwest winds this evening will strengthen around 2000 
feet AGL creating some potential for low level wind shear but 
magnitude too low to include in TAFs at this time. These winds are
also likely to keep stratus development tonight reduced.

Vicinity of KSFO...VFR. West winds increasing after 21Z to around
20kt with gusts near 30kt expected.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR Today. Onshore winds 10-15 kt this 
afternoon. Patchy IFR/MVFR cigs returning overnight tonight. 

&&

.MARINE...as of 8:39 AM PDT Wednesday...Light to moderate 
northwest winds will persist across the coastal waters through 
mid-week as high pressure sits over the eastern Pacific and low 
pressure develops over central California. Locally gusty coastal 
jets are forecast south of Point Arena, Pigeon Point, and Point 
Sur that will create hazardous seas conditions for smaller 
vessels. Breezy afternoon and evening winds are also forecast over
the San Francisco Bay as well as over the Monterey Bay. Mixed 
seas will continue with shorter period northwest waves at around 8
to 10 seconds, a light longer period west swell, and a light 
southerly swell.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...SF Bay from 12 PM
             SCA...Mry Bay from 12 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm from 12 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm from 12 PM

&&

$$

PUBLIC FORECAST: DRP
AVIATION: Sims
MARINE: Sims

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