Short-term and long-term flood potential for Washington State

1
Short-term and long-term flood potential for
Washington State
National Weather Service Brent Bower, Royce
Fontenot, Marilyn Lohmann, Steve King, Joanne
Salerno, Andy Bryant 08 February 2008
2
Outline

• Background information
• Monitoring River Levels
• Advisory, Watch and Warning Definitions
• WFO boundaries (public forecast boundaries)
• Flooding Types (East side vs. West side)
• Current Conditions
• Short Term/Long Term outlooks
• Impact Summary

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Overview

• Mountain snowpack greatest since 1998-1999
• 110-160 of normal
• Enhanced mid to low elevation snowpack
• Moderate to strong La Niña to persist through the
  spring season ? favors wetter than normal
  conditions
• Active pattern expected for the next couple of
  weeks
• Generally mountain snow and lowland rain
• Brief periods of warmer temperatures and mountain
  rain

4
Monitoring River Levels

• NWS, USGS, State and Local gages for Washington
  (300)
• Update frequency 15min - 4 hours
• Active monitoring via Hydrographs
• Threshold Alarm

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Advisory, Watch and Warning Definitions
Flood State-ments
Flood Warning/Advisory
Hydrologic Outlook
Flood Watch
72
48
24
0
Increasing confidence event will occur
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NWS/WFO Boundaries
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Flooding Types

• Typical Washington Flood Season
• Western Washington Oct/Nov through March
• Eastern Washington Oct/Nov through March, and
  April through June (snow melt)
• Western Washington Floods
• Heavy rainfall is the primary forcing mechanism
• Impact of snow melt on Western Washington
  Flooding is secondary
• Snow melt does not increase the odds of
  significant flooding occurring, but rather can
  increase the severity
• Eastern Washington Floods
• Forcing mechanism can be snow melt and or heavy
  rainfall

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Current Conditions and Short Term Forecast

• Snow map (NWRFC) (NOHRSC)
• Washington Stream Flow Conditions (USGS)
• Active weather pattern expected for the next
  couple of weeks (link)

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Short Term

• Enhanced mid and low level snow pack can be an
  important factor if coupled with heavy rain.
• Western Washington
• Chehalis, Skokomish ? heightened risk for minor
  flooding
• Eastern Washington
• Spokane through Palouse (Hangman Creek, Palouse
  River and many of the tributaries and small
  streams throughout the area) ? minor flooding
  expected

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Long Term

• Western Washington
• None at this time, typically Western Washington
  flood season winds down by end of March and is
  not driven by snowmelt
• Eastern Washington
• Generally expecting high flows east of the
  Columbia River
• Area is more sensitive to precipitation events
  given expected high flows.
• Potential for flooding on
• Palouse River
• Stehekin River
• Klickitat River

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Summary

• Short-term
• Active period for the next 10-15 days
• Generally lowland rain and mountain snow
• Brief periods that are warm enough for mountain
  rain
• No blockbuster systems at this time
• Long-term
• No concerns for Western Washington at this time
• Above normal mountain snowpack definitely a
  concern for Eastern Washington
• As always please stay tuned for updates...

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Questions