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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 170459

Area Forecast Discussion
National Weather Service San Francisco Bay Area
959 PM PDT Thu May 16 2019

.SYNOPSIS...Showers will continue to diminish in coverage 
overnight. Mainly dry conditions are likely region-wide on Friday
ahead of our next storm system set to arrive Saturday. Another 
round of widespread rainfall is likely as a cold front sweeps 
across the region through the day with lingering showers lasting 
into Sunday. Wet and unsettled weather conditions may potentially 
continue into next week as well. 

&& of 9:59 PM PDT Thursday...No significant 
forecast updates planned for this evening. KMUX radar continues to
show shower activity but it's decreasing in coverage and 
intensity. Despite all the short term models showing activity 
ending do expect we'll see showers through much of the overnight, 
given the latest trends on the GOES-17 which continues to show 
cold cellular clouds just offshore. 

Friday is looking mainly dry. However cant rule out some towering
cumulus over the higher terrain that could generate some isolated
showers given sufficient low level boundary lower moisture along
with weak instability aloft as well as the strong May sun angle.

All eyes then on the Saturday frontal passage. NAM and GFS models
in solid agreement bringing a well organized cold front onshore
Saturday. Expect rain in the North Bay by morning then rapidly
shifting southward through the day. This type of northwest flow
system generally arrives earlier than later, so if anything would
be concerned that rain starts earlier on Saturday for the Bay 
Area and Santa Cruz region. Unlike the Thursday system, there 
should be sufficient cold advection and surface frontogenesis to 
get steady rains going for most areas as the front barrels 
through. Winds dont look too strong but a well noted wind shift 
and gusts to 45 mph will be likely as the front passes through. 
Precip should turn showery by Saturday evening. Lingering showers 
and even isolated thunderstorms on Sunday as the core of the upper
low passes over NorCal (but best t-storm chances will be north of
the Bay Area).

Showers ending by Sunday night with dry weather on Monday. Then 
yet another cold (but weaker) system arrives Monday night into 
Tuesday with another chance of showers. Then it looks drier but 
continued unseasonably cool the second half of next week.


.PREV of 1:42 PM PDT Thursday...Isolated to 
scattered showers continue to move over the region this afternoon 
as the core of a mid/upper level low pushes inland over northern 
California. Thunderstorms have been confined to the interior 
portion of the state as well as near the northern portion of the 
upper level low. However, cannot rule out an isolated thunderstorm
within the convection over our region through the afternoon 
hours. Thus, will leave a mention of a slight chance of 
thunderstorms in the forecast at this time.

Showers will being to diminish in coverage late in the evening as 
the upper low pushes further inland. This will allow for a weak 
ridge of high pressure to build over the region from late tonight 
into Friday, resulting in mainly dry weather conditions. This will 
also result in a slight decrease in cloud cover and a bit warmer 
daytime temperatures on Friday. 

The dry conditions will be brief as another storm system remains on 
track to impact the region this weekend. Forecast confidence is 
increasing with the next system to approach from the northwest 
Saturday morning with widespread rainfall spreading inland into 
Saturday night. PWAT values around 1.00" or locally greater will 
advect inland ahead of this system which should result in another 
impressive rainfall event for this time of year. However, overall 
dynamics and moisture plume look to be slightly weaker than the 
system that pushed across the region over the past 36 hours. Most 
urban locations can expect around 0.25" to 0.75" with lesser amounts 
in the typically drier inland valley locations. Totals upwards of 
around 1.00" cannot be ruled out for the North Bay while the coastal 
ranges and North Bay mountains could potentially pick up an 
additional 2.00" or so from Saturday into Sunday. In wake of the 
main frontal passage, showers will linger into at least the first 
half of Sunday as the core of the upper level system swings inland. 
In addition, there will be the potential for isolated thunderstorms 
as well Saturday night into Sunday. However, confidence remains low 
with respect to thunderstorm potential. 

While there remain some difference in the medium range operational 
models heading into early next week, the ensembles maintain an 
unsettled weather pattern. Therefore, expect chances of 
precipitation to continue through at least the first half next week 
as a series of upper level system drop southward along the West 
Coast. This will also keep temperatures below seasonal averages for 
most locations. Stay tuned... 

&& of 4:50 PM PDT Thursday...Scattered showers 
continue throughout the Bay Area this afternoon in the post-
frontal environment. SCT-BKN conditions with bases ranging from 
3000-5000 feet. Showers expected to diminish after 03Z but cigs 
could continue in and out through late tonight. West winds are 
gusting up to 25 kt and these will continue through 08Z but these 
winds should slowly decrease during the night. 

Vicinity of KSFO...SCT-BKN clouds in the 4000-5000 foot range
through tonight. Scattered showers diminishing after 03Z. West
winds gusting 22-25 kt slowly decreasing after 08Z. 

SFO Bridge Approach...Similar to KSFO except cloud bases may be a
little lower at 3000-4000 ft.

Monterey Bay Terminals...MVFR conditions with cigs around
3000-5000 ft with scattered showers. Showers ending at SNS after
03Z but continuing in MRY and the peninsula through 08Z. 
W winds gusting 18-22 kt through 03Z. 

&& of 09:41 PM PDT Thursday...High pressure off the
southern California coast will bring light to moderate northwest
winds through Friday with the strongest winds over the southern
waters. Winds will become southerly and increase Saturday as a low
pressure system approaches the far northern California coast. The
low will move inland Sunday morning with winds switching to
northwesterly behind it.


     .Tngt...SCA...Pt Reyes to Pigeon Pt 0-10 nm until 3 AM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 3 AM
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 3 AM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 3 AM
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Rough Bar Advisory for SF Bar until 3 AM




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