000 
fxus66 kmtr 200044 
afdmtr 


Area forecast discussion 
National Weather Service San Francisco Bay area 
444 PM PST sun Feb 19 2017 


Synopsis...high impact event set to unfold during the next 24 to 
36 hours as a potent Pacific storm system and atmospheric river 
brings widespread rainfall and strong winds to the region. Periods 
of heavy rainfall are likely from late this evening through Monday 
evening, potentially resulting in flooding across portions of the 
region. In addition, southerly winds will increase late today and 
be locally strong and gusty through Monday evening. Widespread 
rainfall will then taper off late Monday night into Tuesday. 


&& 


Discussion...as of 03:00 PM PST Sunday... latest water vapor 
satellite imagery depicts a 1.30 - 1.70" tpw precipitable water 
plume extending from Hawaii to central California early this 
afternoon. This precipitable water plume qualifies as an 
atmospheric river given that forecast integrated water vapor 
transport values well exceed the accepted threshold values and 
then some. Model ivt values exceed 500 kg/MS (twice the accepted 
250kg/MS threshold), between 36-38n (roughly encompassing Big 
Sur to the north bay) which indicates the potential for excessive 
rainfall for our forecast area from this plume. Wpc (the nws' 
National forecast/modeling center) agrees with this assessment 
and have placed our local forecast area under a moderate risk of 
excessive rainfall, as well areas around Sacramento and the 
northern Sierra Nevada. 


As of now, only the lighter warm sector precipitation has advanced 
onshore ahead of the main moisture surge. Kmux radar is sensing 
light to moderate echoes of up to about 35dbz with these 
overrunning rain showers, which have already brought 0.75-1.30" 
to the coastal North Bay mountains. These rain showers initially 
began over the North Bay, but have now spread in coverage from the 
Oregon border in the north and Point Conception to the south. 
Satellite and forecast data have continued to suggest that the 
initial arrival of the main moisture surge would encompass a large 
stretch of coastal California, which reflects what is now being 
observed across the mosaic radar imagery, before narrowing into 
more intense band of extremely heavy rainfall later in the event. 


Forecast models have continued to struggle pinpointing the axis of 
heaviest precipitation with this particular atmospheric river, 
with various runs and models highlighting every location from Big 
Sur to the North Bay with intense rainfall. Model trends begin to 
emerge amongst the noise, which have given higher confidence than 
any single model alone could do. These trends suggest that 
for late this afternoon/this evening, rainfall will initially be 
advected inward on an east to west axis and widespread along the 
coastal ranges from Big Sur to the North Bay with no clear stand 
out. Next, the plume will begin to narrow and intensify between 
10pm and 4am overnight as it interacts with a descending low to the 
north and ridge to the south. As this band intensifies, the axis 
will shift from an east to west setup to southwest to northeast 
axis. This is important because it will likely enhance the 
orographics of this event and further increase anticipated rainfall. 
This narrow, intense band has been trending towards a landfall 
somewhere between the coastal Santa Cruz Mountains and North Bay, 
with a mean landfall zone over the i80 corridor (golden gate bridge 
to Sacramento to northern Sierra nevada). Confidence is moderate to 
high that the heaviest rain will fall somewhere in this zone. 


Forecast precipitation values have been wildly varying run to run 
and model to model, so confidence is not as high on exact storm 
total precipitation values. That said, the afternoon forecast 
package has been built around the most likely scenario previously 
discussed. Given that scenario, here are how the forecast values 
have played out, from least to most. Inland San Benito/Monterey 
County will be rain shadowed by the Big Sur range, 1.0-2.5". Santa 
Clara Valley, partially rain shadowed, 1.5-3.0". Inland East Bay, 
2.5-4.0". Inland North Bay and San Francisco Bay shorelines, 
3.0-5.0", locally higher if main intense rainfall band stalls 
aloft here. Big Sur and East Bay peaks and ranges, 4.0-6.0". 
Coastal North Bay 5.0-9.0". Coastal Santa Cruz and San Mateo 
ridges, 5.0-10.0, locally higher if main intense rainfall band 
stalls aloft here. Depending on how far north of south the main 
intense rainfall band wobbles over the next 24-36 hours, one or 
more of these locations could see locally higher or lower amounts. 
A Flood Watch is in effect for all areas in anticipated of this 
excessive rainfall through the coming days. 


In addition to these heavy rains, this system will also generate 
strong southerly winds through the duration of the event. The 
strongest winds are anticipated to arrive Monday afternoon into 
evening, before tapering off overnight into Tuesday. A Wind 
Advisory is effect for all areas tonight into early Monday, with a 
High Wind Warning in effect for most coastal and higher elevation 
locations by Monday afternoon. The strongest winds from this 
system are not expected to be as strong as our last windy system 
on Friday. 


Given the wet antecedent conditions from weeks of wet weather, 
and the anticipated heavy rainfall and accompany wind forecast, 
except to see widespread impacts in various forms. Excessive 
rainfall falling on already saturated soils will runoff moreso 
than be absorbed, which will lead to widespread areal flooding 
from pooling of water in low lying areas, and rapid rises on 
creeks, streams, and rivers. These saturated soils will also 
respond to the additional rain in the form of rock, mud, or 
landslides, especially in steeper terrain. These slides could 
block, damage, or destroy roadways. Furthermore, even if the winds 
are not as strong as the last system, they will be sufficient to 
knock over additional weakened trees who are rooted in saturated 
soils, which could lead to power outages, blocked roadways, or 
blocked drainages. Be sure to plan ahead if travel is necessary 
and check the latest Road conditions available from caltrans 
websites. Turn around, don't drown, flooded areas can be 
deceptively deep. 


The core of the upper low associated with this feature will shift 
inland early next week, leading to an increased chance of 
thunderstorms, primarily for the North Bay. Drier weather is then 
anticipated for later Wednesday, Thursday, and into early Friday. 
Model solutions diverge late in the week into early next week, but 
there it seems more likely than not another system will roll 
through our area by next weekend. 


&& 


Aviation...as of 4:15 PM PST Sunday...MVFR cigs. Areas of light 
rain will continue into early evening. Latest models show that 
by 08z a narrow plume of moderate to heavy rain will move into the 
sfo Bay area. Hard to pinpoint where this plume is aimed at but 
all terminals should expect to see some moderate rains. Rain will 
taper off late Monday morning as the plume moves inland but a 
surface low will pass closest to the area Monday afternoon as it 
lifts northeast. This will keep showers going through Monday 
night. Southeast wind gusts increasing to 25-30 kt after 06z as 
the plume of moisture arrives. Winds shift to south to southwest 
Monday morning then increase in gusts to 35-40 kt Monday afternoon 
due to the close proximity of the low. 


Vicinity of ksfo...MVFR. Light rain today becoming moderate 
after 09z. Southeast wind gusts to 25 kt after 06z increasing to 
30-35 kt after 18z. 


Sfo bridge approach...similar to sfo. 


Monterey Bay terminals...VFR/MVFR. Light rain tapering off after 
13z as moisture plume shifts north. Southeast winds gusting to 25 
kt in the Salinas valley becoming more widespread after 06z. 
Low level wind shear (llws) developing during the evening. 


&& 


Marine...as of 01:40 PM PST Sunday...an approaching storm system 
will bring gusty southerly winds to the coastal waters through 
Monday with gale force winds developing near the coast. Marginally 
large swell Monday over the southern coastal waters will result in 
hazardous seas. 


&& 


Mtr watches/warnings/advisories... 
... Watch...entire San Francisco and Monterey Bay areas 
Wind Advisory...entire San Francisco and Monterey Bay areas 
glw...pt Reyes to Pigeon pt 0-10 nm 
glw...Pigeon pt to pt pinos 0-10 nm 
glw...pt pinos to pt Piedras Blancas 0-10 nm 
Small Craft Advisory...sf Bay 
Small Craft Advisory...mry Bay 
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm 
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm 
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm 
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm 
Small Craft Advisory...rough bar advisory for sf bar 


&& 


$$ 


Public forecast: drp 
aviation: west pi 
marine: Sims 




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