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Forecasting the Extreme Event
Gregory Waller
Service Coordination Hydrologist
NWS – West Gulf River Forecast Center
Meteorology and Hydrology “Clues”…
The WGRFC AreaDiverse Water Issues
402,000 mi2 total area 87,000 mi2 in MX (Rio Grande headwaters)320 forecast points, 15 major river systems
Over 65,336 miles of rivers (U.S. only)Snowpack Water supply
• Hill Country Hydrology• Flash flood threats• Rapid river responses• Cycles of Flood/Drought
Now…The Hydrologic Cycle
Courtesy of the USGS Water Science School
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Perspective
More Perspective
The Vocabulary• Public vs. “High End User”
Images courtesy of Jeff Lindner, Harris County FCD
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ACTIONImpact: Water is over the banks and into the flood plain, but not a
threat to structures or roadways. Some action may be required
such as moving farm equipment or increasing awareness
MINORImpact: Typically water is impacting areas inside of flood plain which
can vary by location. Some low water crossings covered by water,
agricultural flooding, water approaching public areas (parks, sidewalks
etc.). Areas frequently flooded can expect to be impacted
MODERATEImpact: Water now reaching areas only impacted by significant rain
events. Structures can be inundated, several roads covered with
water, water may cut off certain areas, widespread agricultural flooding
MAJORImpact: Water is near the highest its ever been representing rare
flooding and significant widespread impacts. Most roads will be
covered by water in the area cutting off if not completely flooding
subdivisions, rivers can be several miles wide in areas. Homes and
structures underwater, bridges inundated and in danger of being hit by
debris. Impacts may be greater than ever experienced.
BELOW CRITERIAImpact: Water is within the banks of the river with no impacts to
the surrounding area. Flow speeds may still be high during rainfall
or releases which could impact recreational activities
Understanding River Forecast Categories
* NWS generally defines a river flooding as the point at which higher flows produce impacts. Common impacts are road closures, parks inundated, and water in structures. River flooding does not have to be associated with other flood threats (flash, areal, coastal).
National Weather Servicewww.weather.gov/wgrfc
Low Flow No Flooding ACTION STAGE
(AS)
MINOR
(MIN)
MODERATE
(MDT)
MAJOR
(MAJ)
River is within
banks, no
impacts
Buildings
threatened, roads
flooded and
impassable
River is higher,
possibly out of
banks
Impacts to
secondary roads
and lowlands near
floodplain
Significant impacts
to buildings,
structures and
roadways
Impacts to water
supply and
intake/outflow
structures
Only daily
operational points
will have forecasts
available
Flood Statements,
River Forecasts
Begin
Flood Warnings,
River Forecasts
Flood Warnings,
River Forecasts
• Evacuations may be
necessary
• Seek higher ground
• Potentially safer to
shelter in place
• Turn around don’t
drown!
• Homes and buildings
could be cut off as well
and should prepare
• Turn around don’t
drown!
• Stay away from
river’s edge as flow
speeds could be high
• Higher flows
• River could be out of
banks
• River can be running
higher than “normal”
so caution should still
be used
• Agriculture industry
will have to prepare
• Reservoir levels for
water storage
monitored
RECORD
(REC)
Impacts beyond
anything
previously
experienced
Flood Warnings,
River Forecasts,
Flood of Record
Statements
• Seek safety or
higher ground by
any means
• Impact areas
should be avoided
Flood Warnings,
River Forecasts
Only daily
operational points
will have forecasts
available SIGNIFICANT FLOODING
More Vocabulary
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Even More Vocabulary
• Flash Flood Emergency– By definition, the NWS issues a “Warning” when conditions are
life threatening it could kill you
– But with the “Emergency”…this time I really mean it
• Notice “Flash Flood”– Not river flood
• NWS uses Minor, Moderate, Major, Record for river flooding– Is that enough?
• Steered away from “analogs”– If you are expecting a record, something no one has ever seen,
then how do you expect them to compare to a past event…
When To Go “Whoa!”
• A good clue…– Know your environment going in…
• Honestly…soil moisture is not a large component on the extreme events (it is for the more typical flood events)
– When the atmospheric parameters are expected to “be outta whack”
– Look for something 2 standard deviations off…• Can be forecast height, lift
• For severe weather…CAPE or helicity
• For flooding…look at the PW’s
Precipitable Water
• Atmospheric River
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Precipitable Water
• Atmospheric Sounding
Sabine River 2016 –LCH Sounding
March 10, 12Z Sounding 55 mm 2.13 inches!
https://www.spc.noaa.gov/exper/soundingclimo/
The Obvious
• Harvey, Michael
– Rule of thumb for tropical activity
– Storm total precipitation take 100 and divide by the forward speed of the storm
– Storm moving 10 miles per hour…100/10 = 10 inches
• What if the storm doesn’t move (Allison 2001)
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NOAA Atlas 14
• https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html
NOAA Atlas 14
• Much needed
• Previous update was my age…(let that sink in)
• Coordinated, intensive effort (USACE, NOAA, and others)
• Tidbit…USACE notes…
Going Into The Event
• Has it been wet?
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Last Thing…
• It matters with forecast confidence…
– Consistency
– Multiple models show the same thing
• Hydro and meteorological
– Do they show it consistently over time
Confidence? • Consistency
Satellite
Chiclets!!!
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Stay Away From Ensembles!
• Ensembles should be used to figure out areal extent, rainfall amounts, and confidence…!
• Ensembles, by their nature, smooth out extremes…
Explain Please!
#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11
1.15 2.55 6.35 6.00 3.30 1.00 0.75 7.15 2.25 1.95 3.10
Model QPF…
Ensemble QPF Average = 3.23 inches
One Really Last Thing…
• If you are REALLY nerdy…
– Learn the forecasters, learn their trends and biases
– Some are conservative… (if they are forecasting a big event…look out!)
– Some are aggressive (he has accurately predicted 5 of the last 2 events)
Father Time…old NWR story…!
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Finally…Know Your Source
– Forecasters are second guessed almost as much as head football coaches
– Know who is providing the forecast
• Beware of social media “alarmists” and “wish-casters”
• If it sounds far-fetched, double check with another trusted source…
• (watch out for that “@wallywx92 guy”…)
Trusted sources… can be anyone in the Weather Enterprise…
Gregory WallerService Coordination HydrologistNWS West Gulf River Forecast [email protected]