Mission of NOAA's NWS Hydrologic Services Program

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Mission of NOAA's NWSHydrologic Services Program

• To provide river and flood forecasts and warnings
  for protection of life and property
• Provide basic hydrologic forecast information for
  the nation's economic and environmental well
  being.

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1) Flash Flood Products

• Are issued for life-threatening floods which
  occur in lt6 hrs from causative event!!
• Floods which require immediate action to protect
  lives and property
• Dangerous flooding of streams, washes, alluvial
  fans (overland flooding) or urban areasdue to
  extremely heavy rain.
• Dam or levee failurescan happen anytime, even a
  clear day or night!
• Products
• Flood Watch (in Header) FFA
• Flash Flood Watch in Headline
• 6-48 hours ahead of event
• Original Follow-Ups
• Flash Flood Warning FFW
• Occurring or Imminent
• EAS!
• Flash Flood Statement FFS
• Follow-up to FFW.
• UGC Watch Zones Others Counties

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2) Forecast Point Flood Products

• Flood Products Which Include a Quantified Value
  (i.e. CNRFC forecast) and Impact Information
• For specific locations
• Applicable to specific reaches (area along river
  up and downstream of forecast point)
• For flooding which is expected to occur in gt6 hrs
  from causative event.
• May be in effect for same time as flash flood or
  areal flood products in the encompassing area.
• Point Flood Products Should Not Be Combined with
  Areal Products (except Flood Outlooks).
• Products
• Flood Outlook (ESF/PNS) 36 hrs 7 days
• Flood Watch (FFA) 6-48 hrs 72 hrs if likely.
• Flood Warning (FLW) lt24 hrs
• Flood Advisory (FLS) Nuisance Flooding
• Flood Statement (FLS) Follow-up to FLW
• Recommend EAS for 1st Major Flood Warning
• UGC Counties

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3) Areal Flood Products

• Issued for flooding which is
• Occurring or expected in a defined area such as a
  portion of a state, a group of counties, urban
  and/or small stream areas, areas along rivers or
  streams which threaten lives and property.
• Not appropriately covered by Flash Flood or
  Forecast Point Flood Products.
• For flooding which is expected to occur in gt6 hrs
  from causative event.
• May be in effect for same time as flash flood or
  forecast point flood products in the encompassing
  area.
• Areal FFAs FLWs Should Not Be Combined
  w/Forecast Point FFAs FLWs
• OK to combine Flood Outlooks.
• Products
• Flood Outlook (ESF/PNS) 36 hrs 7 days
• Flood Watch (FFA) 6-48 hrs 72 hrs if likely
• Flood Warning (FLW) lt24 hrs
• Flood Advisory (FLS) Nuisance
• Flood Statement (FLS) Follow-up to FLW
• Recommend EAS for 1st Major Flood Warning
• UGC Watch Zones Others Counties

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Flood OutlookGET READY!

• Flood Outlook (ESF PNS)
• Flooding possible in gt36 hrs to 7 days (issue
  with greatest lead time feasible!).
• Hazardous flooding event may develop.
• Intended to provide information to those who need
  considerable lead time to prepare for an event.
• OK to combine with areal flood outlook
• UGC Counties
• Update end w/another ESF PNS.
• Create w/AWIPS Formatter
• To get Flood Outlook info on REV Website map,
  always issue RNOSPSREV in addition to the
  RNOESFREV.

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Flood Watch GET SET!!

• Flood Watch (FFA)
• Flooding possible in 24 to 72 hrs (gt48 hrs only
  if gt50 probability use greatest lead time
  feasible).
• Expectation of a flood event has increased, but
  its occurrence, location, and/or timing is still
  uncertain.
• Intended to provide enough lead time so those who
  need to set their mitigation plans in motion can
  do so.
• UGC Counties (Zones for Areal)
• Dont combine w/Areal Flood Watch
• Update end w/another FFA.
• Create w/River Pro (GHG for Areal)
• Update Zones.

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Flood WarningGO!!!

• Flood Warning (FLW)
• Flooding forecast in lt24 hrs (up to 36 hrs, gt24
  hrs only if from CNRFC), or is already occurring.
• Update end w/FLS.
• Create w/RiverPro (WarnGen for Areal)
• UGC Counties

• EAS Recommended if First Warning for MAJOR
  Flooding
• EAS Not required for minor or moderate flooding,
  but may be used per FIC discretion or EM request,
  depending on circumstances.
• Update Zones.
• Dont combine w/Areal Flood Warning.

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Flood AdvisoryMinor Flooding Expected Be
Careful Out There!

• A Flood Advisory (FLS) may be issued
  (forecasters discretion) for Mainstem River
  Forecast Points, If
• Forecast is for minor flooding only no
  significant threat to life/property
• If flooding which is a threat to life property
  is expected (gt0.5 above flood stage), a flood
  warning (FLW) must be issued!!!
• NO further rises are expected thereafter
• Rises mainly from snowmelt no heavy rain in
  forecast
• Dont combine with Areal Flood Advisory
• UGC Counties
• Update end w/FLS.
• Create w/RiverPro On NWR.

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Flood Statement News About Flood Situation

• Flood Statements (FLS)
• Give the current status of previously issued NWS
  Flood Warning or Advisory.
• No significant change to previously forecast
  crest stages (lt0.5) or timing (lt6 hrs) expected.
• Issued at least twice per day when minor flooding
  is occurring/expected.
• Issued 3 to 4 times per day when moderate to
  major flooding is occurring/expected.
• Create w/RiverPro
• UGC Counties
• Dont combine Point w/Areal Flood Statements.

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Hydrologic StatementsCurrently, It Is Not
Expected to Flood, but Significant Rises are
Expected

• Hydrologic Statements (RVS)
• Issued when river forecasts have been prepared
  for rivers in the HSA, or to disseminate
  information on significant hydrologic conditions.
• Rivers not currently expected to flood, but
  significant rises (above Monitor Stage)
  expected).
• Create w/RiverPro
• UGC Counties
• On NWR

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River Stage

• Level of the surface of the water at a given
  location (in a river, stream or lake) above a
  datum. Not the depth above the bottom!
• Datum Usually an arbitrary point which has a
  measured elevation above MSL.

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Monitor Stage

• The stage at which initial action must be taken
  by concerned interests
• Livestock movement, removal of equipment from
  lowest overflow areas, or general surveillance of
  the situation.
• May produce limited overbank flows sufficient to
  cause minor flooding of low-lying lands and local
  roads.
• Typically 75 of flood flow.
• If reached, a state of readiness must be
  maintained by those concerned about possible
  flooding on that stretch of river.

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Flood Stage

• Stage set for a particular river gage where water
  will just begin to spill out of banks and cause
  flooding problems somewhere on that stretch of
  river or stream.
• If Flood Stage is forecast to be exceeded or is
  actually exceeded, the NWS will issue a Flood
  Warning for that stretch of river.

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Minor, Moderate, Major Flooding

• Minor Minimal or no property damage, but some
  public inconvenience.
• Moderate Inundation of secondary roads movement
  of property to higher elevations necessary to
  avoid damage. Evacuation of lowest portions of
  floodplain necessary.
• Major Extensive inundation and property damage.
  Evacuation of people, livestock, inventory,
  equipment necessary. Most primary secondary
  roads closed. Impacts to transportation, power,
  communications.

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Antecedent Soil Moisture Conditions

• The degree of wetness of the soil at the onset of
  a rainfall or snowmelt event.
• A primary component of the percentage of rainfall
  or snowmelt which will run off.

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Cubic feet Per Second

• A unit of measurement of discharge equal to
  streamflow of one cubic foot of water per second
  past a given point.
• What most streamflow in the USA is measured in,
  and what the NWS forecastsand then usually
  converts to stage via a rating.
• 1 cubic foot 7.5 gallons, or about the same
  volume as 4 basketballs.
• So, 10,000 cfs 40,000 basketballs full of
  water per second!

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Flood Frequency

• Statistical frequency, expressed in years, at
  which a flood of a given magnitude should, on
  average be equaled or exceeded, given a record of
  past flood events.
• Equal to 100 divided by the percent chance that a
  flood will happen any year.
• Example If a certain flood has a 2 chance of
  occurring any given year, its frequency would be
  100/2 50 yr flood.
• As the occurrence of floods is random in time, a
  flood of any frequency may occur in any year or
  in successive years.
• A flood of a certain frequency will change as
  more flood record evolves over time.

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Hydrograph

• A graph showing river stage and/or stream flow
  with respect to time.

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Six Largest Floods on the Truckee River at Reno
Since 1950(Flood Flow 10,500 cfs)

• December 23, 1955.....20,800 cfs
• November 21, 1950.....19,900 cfs
• February 1, 1963......18,400 cfs
• January 2, 1997........18,200 cfs
• December 31, 2005..16,400 cfs
• February 17, 1986........14,400 cfs
• All between 50 and 100 year events
• 1 to 2 chance any year

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Six Largest Floods on the Truckee River at Vista
Since 1963 (Flood Flow 6,800 cfs)

• February 1, 1963......18,900 cfs
• January 2, 1997........18,500 cfs
• February 17, 1986........16,100 cfs
• December 31, 2005..13,700 cfs
• December 23, 1964..11,700 cfs
• January 14, 1980.. 9,970 cfs
• All between 25 and 100 year events
• 1 to 4 chance any year

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Key Ingredients for Flooding in the Reno
Hydrologic Services Area (HSA)(orConditions
to Watch Out For)
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Excessive Rain Over a Large Area, Especially at
Higher Elevations

• Truckee River Floods Usually Begin in the Sierra
• Whats Excessive Rain??
• Storm Totals gt8-10 rain at Tahoe City/Truckee
  (6000-6500) gt1 rain at Reno (4400) 10 to
  1 ratio!!
• High Elevation Rain Rates
• gt1/3hrs, gt2/12hrs , gt3/ 1 day
• Butcan be much less (i.e., half above amounts or
  less) if
• Soils are saturated,
• Rain on ripe (ready to melt) snowpack (ripe snow
  gt0.4 in water per inch of snow),
• Rain on steep, lightly vegetated, impermeable
  surfaces

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Tahoe City Precipitation Totals Six Largest
Reno Floods Since 1950
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2) High Snow LevelsFlood Producing Runoff

• The Higher the Snow Level, the Larger the Area
  Contributing to the Flood
• Generally, Snow Levels above 7500 are considered
  High and can cause flood problems
• If temperatures are higher than 37 deg at Tahoe
  City/S Lk Tahoe or 45 at Reno during a major
  storm, high flood potential on Truckee R!
• Representative Elevations on Truckee Basin
• 8200-9000 Mt. Rose Ski Area
• 6500-8500 Alpine Meadows/Squaw Valley
• 7200 Donner Summit
• 6300 Tahoe City
• 5900 Truckee
• 5200 Farad
• 4900 Verdi
• 4400 Reno
• Heavy rain with relatively low snow levels (i.e.
  5500-6500) can cause flooding on tributaries
  (e.g. Steamboat Ck., N Truckee Drain), and
  urban/small stream flooding in valleys.

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Temperature at Tahoe City During Five Largest
Truckee River Floods
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Other Conditions Which Have the Potential to
Cause or Aggravate Flooding on the Truckee

• Large low elevation snowpack (over 6 in. below
  6000 ft), esp. if ready to melt (gt0.4 inch water
  per inch of snow).
• If not ready to melt, snow is able to absorb a
  lot of rain before melting, like a sponge.
• Ice and hard packed snow act as an impermeable
  surfaces and increases runoff.
• River/stream levels already high or rapidly
  rising.
• Saturated or Frozen Soils.
• Long duration Moderate (0.1 to 0.3 in/hr) to
  Heavy (gt0.3 in/hr) rain.

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Other Conditions Which Have the Potential to
Cause or Aggravate Flooding on the Truckee

• Channel Capacity
• Some portions of a rivers channel have
  significantly lower capacity than others
  therefore, flooding may occur at lower flows on
  some portions of a river than others
• e.g., channel capacity (flood stage) at Truckee R
  _at_ Reno is 10500 cfs at Vista it is 6800 cfs.

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Other Conditions Which Have the Potential to
Cause or Aggravate Flooding on the Truckee

• Reservoirs
• If reservoirs have flood control storage
  allotted, and are able to be operated during
  winter storms to control downstream flows, they
  can greatly decrease flood severity (e.g.
  Prosser, Boca, Stampede)
• However, once reservoirs with flood control
  storage fill, they loose their flood control
  capability.