4/23/2019

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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 Centergreg.waller@noaa.gov