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FGUS76 KMTR 181912
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CAC041-055-097-190315-

Hydrologic Outlook
National Weather Service San Francisco CA
1112 AM PST Mon Nov 18 2024

HYDROLOGIC OUTLOOK
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
1030 AM PST Mon Nov 18 2024

PROLONGED MODERATE TO HEAVY RAINFALL WILL LEAD TO RAPID RIVER RISES
AND INCREASED FLOODING POTENTIAL

* WHATThe first significant atmospheric river of the season will
  impact the northern and central California region by midweek with
  impacts continuing into the weekend. Prolonged periods of moderate
  to heavy rainfall will lead to rapid rises on small creeks,
  streams and rivers as well as an increased flood risk.

A significant and strong atmospheric river will impact the region by
early Wednesday morning. Current model depictions show this system
will stall over the North Bay bringing a prolonged period of
moderate to heavy rain as well as strong winds to the North Bay.
Precipitation totals of 3 to 7 are expected Wednesday through
Sunday with up to 11 possible along the coastal range as this
system sits north of the Golden Gate for the better part of
Wednesday and Thursday before gradually dropping south across the
San Francisco and Monterey Bay Area on Friday and Saturday.

WPC (Weather Prediction Center) Day 3 Excessive Rainfall Outlook has
the North Bay in a Marginal (at least 5%) to Slight (at least a 15%)
chance of exceeding flash flood guidance for Wednesday into
Thursday. This system is tapping into very moist subtropical
moisture for an extended period of time which will allow for
moderate to heavy periods of precipitation over  several days.
Moderate to heavy rain will lead to rapid rises of area rivers,
streams, and creeks.  Localized ponding of water in low-lying or
poorly drained areas as well as localized flooding is possible due
to blocked culverts, drainages, and storm drains. Main stem rivers
are currently forecast to remain below flood stage, however the 10%
probability of exceedance (very low chance) show GUEC1- Russian
River at Guerneville may rise slightly above monitor stage over the
weekend. A few other rivers to keep an eye on that show higher
probability of reaching Action/Monitor or Minor flood stage
according to the latest CNRFC forecast are
CTIC1 - Laguna De Santa Rosa at Stony Pt Rd near Cotati has a 75%
probability of reaching Action/Monitor Stage and a 30% probability
of reaching Minor flood stage by Thursday.
MWEC1 - Mark West Creek at Mirabel Heights has a 50% probability of
reaching Action/Monitor Stage and a 20% probability of reaching
Minor flood stage Friday.
GEYC1 - Russian River at Geyserville has a 25% probability of
reaching Action/Monitor Stage and a 10% probability of reaching
Minor flood stage Friday.

* WHERESonoma and Napa Counties. Latest model guidance suggests the
  North Bay will receive the greatest accumulation of precipitation
  with the heaviest rainfall expected over the North Bay coastal
  range and mountains.

* WHENWednesday through Saturday. Moderate to heavy rainfall
  expected over the North Bay Wednesday and Thursday. The heaviest
  rainfall accumulation and periods of potentially intense rain
  rates are expected Wednesday and Thursday. On Friday the system is
  expected to begin moving south and east across the region with
  rain showers lingering over the North Bay and spreading south
  through Saturday.

* ADDITIONAL DETAILS...In addition to the heavy rainfall and
  localized ponding of water in low-lying or poorly drained areas,
  such as freeway offramps. Locally strong and gusty winds may
  result in downed trees, and power outages will be possible.

Details regarding this storm will likely change as the system nears.
Therefore, stay tuned to the latest forecast for the most up to date
weather information. Confidence is high that we will receive
rainfall from this system however, confidence is moderate with
respect to the exact location and timing of the most intense
rainfall.

$$

CW