Flash Flood and Debris Flow in California, March 20 to 22, 2018

Area Forecast Discussion
National Weather Service San Francisco Bay Area
206 PM PDT Tue Mar 20 2018

.SYNOPSIS...A series of storm systems and atmospheric river aimed
at the central and southern California coast will produce
widespread rainfall across our region today through Thursday.
Periods of heavy rain are possible, mainly along the Big Sur
Coast, as well as locally strong southerly winds and isolated
thunderstorms. Showers will linger on Friday and Saturday with dry
conditions returning by Sunday.

&&

.DISCUSSION...as of 02:00 PM PDT Tuesday...Afternoon radar
imagery and satellite imagery pretty much says it all,
precipitation has spread across the entire Bay Area. Rainfall
amounts since this morning have been highest over the higher
mountains of the Big Sur. Six hour rainfall amounts have been just
over 0.5" along Big Sur with a few hundredths to two tenths
everywhere else. Given the cloud cover/rain temperatures have
remained on the cool side and mostly in the 50s. Did lower some of
the forecast max temps for today.

For the rest of today and into this evening - latest hi-res models
continue to advect in some much higher PWATs translating to
increased rainfall rates. Blended Total Precipitable Water
satellite imagery SW of the Big Sur ranges from 1.5-1.7", which is
over 200% of normal. Taking a closer look at the AR Monitoring
system at Pt Sur indicate a real ramp up of integrated water vapor
(IWV) and upslope IWV flux from 22-03Z. That being said, rain will
be heaviest during that timeframe. North of Monterey, rain will
increase as well, but not as intense at the coastal mountains.
Impacts could be ponding on roads and slower commute times this
afternoon/evening. After about 03-05Z it looks like the main
moisture plume drifts southward leading to decreasing rain rates
and possibly more showery precip late tonight early Wednesday. It
should be note that guidance continues to show a pulse of
moisture interacting with a weak shortwave over the North Bay this
evening/overnight, which may result in some enhance precip
activity.

Wednesday is still forecast to start off with less rainfall, but
still light to moderate for many locations. Given the ample low
level moisture and decent southwesterly flow orographically
favored spots may see moderate to possibly heavy rains at times.
As the main low/trough off the coast begins to march eastward rain
is expected to increase in short order late Wednesday
afternoon/evening. As this happens, high PWATs to the south will
slowly begin to creep northward with greatest impact over the Big
Sur Coast again. Additionally, NAM/GFS/WRF all show instability in
the afternoon with a few hundred CAPE. Will keep thunder in for
the afternoon/evening hours. One limiting factor could be the
amount of daytime heating with cloud cover.

The increasing PWATs will gain additional support as the main
upper trough moves to the coastline late Wednesday night/early
Thursday. This period will be the greatest concern for any
localized flooding due to saturated ground from today's rain and
additional heavy rain. This timeframe will also be the most likely
period for rapidly accumulating rainfall. Moderate to heavy
rainfall will diminish gradually during the day Thursday as a cold
front begins to move through. The cold front will provide a
renewed chance for thunderstorms Thursday afternoon/evening.

Besides the rain impacts, wind will also impact the region. Winds
will increase late tonight/early Wednesday from the south. Winds
will peak Wednesday night into Thursday with gusts of 30 to 45
mph. Strongest along the coast and higher peaks around the Bay
Area. Still on the fence for a wind advisory, but if one is issued
it would be Wednesday night.

Rainfall totals have not changed much earlier forecast. Amounts
are still 0.50" to 1.50" in most urban areas with the lesser
amounts in rain shadowed valley locations such as San Jose to
higher amounts along the coast and in the North Bay. Look for
1.00" to 3.00" inches in the coastal mountains of the Bay Area
with isolated higher amounts upwards of 4.00" or more in the Santa
Cruz Mountains. The greatest rainfall totals will be along the
coastal ranges of the Santa Lucia Mountains where 3.00" to 6.00"
will be likely with isolated amounts approaching 10.00". For what
it's worth one hi-res model shows 16" for Big Sur Coast, but that
is an outlier. The greatest potential for small stream flooding
and mudslides, rock slides and debris flows will be along the Big
Sur coast.

Scattered showers will be possible Friday into Saturday as a weak
upper level trough moves over the region. Drier weather is
expected Sunday into Monday with warmer temperatures.

&&

.AVIATION...As of 11:09 AM PDT Tuesday...For 18z tafs. Low
confidence forecast through the next 24-36 hours as an area of low
pressure approaches from the west/southwest. Ceilings remain
above 3000 feet across the district, as of 1730z. Do expect some
gradual lowering to MVFR cigs through the day, but low confidence
on exact timing of MVFR cigs. TAFs may have to be amended
periodically through the day. KMUX radar has majority of the rain
south of San Jose, with a general movement towards the ENE/NE. It
appears Bay Area and North Bay terminals will see a break in the
rain in the next few hours. Hi-res models has rain moving back
into the Bay Area by late afternoon or early evening.

Winds are expected to be generally light out of the
south/southeast this afternoon, then increasing this evening and
becoming gusty overnight for many TAF sites. Did include LLWS for
KSTS, KSFO, and KOAK starting around 02-03z as guidance suggests
winds will increase 30-35 kt between sfc-2000 ft through about 12z
Wed.

Vicinity of KSFO...Low confidence VFR to MVFR later this afternoon
or evening. Rain likely through day and evening and may be
moderate to heavy at times. May have to adjust CIGs/rain through
the day to reflect observations, if needed. Light
southerly/southeasterly winds through the afternoon, increasing
this evening and overnight, around 15 kt. LLWS possible this
evening through early Wednesday morning.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Low confidence VFR to MVFR by this
afternoon or evening. Light rain to moderate rain through the day,
which may lower cigs/vsby to MVFR/IFR at times. Generally light
south/southeast winds this afternoon, increasing through the
Salinas Valley this evening. Model guidance continues to show
wind gusts in excess of 20 kt for KSNS this afternoon, and
perhaps stronger overnight tonight.

&&

.MARINE...as of 10:28 AM PDT Tuesday...Southerly winds will
increase through the day as low pressure approaches from the west.
Expect rain on and off throughout the week with periods of heavy
rain likely along with a slight chance of thunderstorms Thursday
afternoon. Increasing mixed swells will develop today through
Thursday resulting in hazardous seas.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm

&&

PUBLIC FORECAST: MM
AVIATION: BAM
MARINE: BAM/AS

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