NWS Flooding 101

Jason ElliottNWS Baltimore/Washington

Media WorkshopMarch 29, 2013Pictured: Gunpowder Falls near Parkton, MD

2012 – Year in Review

It was a quiet year… Drought over much of

the country No real spring flood…until Sandy… Conditions have turned

wetter both locally andin other parts of the countrysince late October 2012

2012 Year in Review 2012 had the fewest flood-related fatalities in

50 years! (28…only 3 east of the Mississippi)

19931995

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20092011

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Flood Fatalities & Damage

FatalitiesDamage

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Data Source:NWS HydrologicInformationCenter

Note: These datado not includesurge-relatedflooding, likeSandy or Katrina.

Year in Review Flash Flood Event:◦Baltimore County 8/26/12

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Year in Review Flash Flood Event:◦Berkeley Springs WV 9/1/12

Photo: WHAG-TV Viewer Submission

Year in Review Flash Flood Event:◦ Frederick MD 9/18/12

Photo: Frederick News-Post

Year in Review – Goose Creek in “Sandy”

Major Flooding (worst since 1996)◦US 15 – primary north/south route – flooded

Photo: Loudoun Co. Sheriff

Year in Review – “Superstorm Sandy” 29 stream gauges with established flood levels

exceeded their flood stages in Sandy.Forecast Points in Flood (12)Potomac River at Brookmont (Little Falls), MDWills Creek near Cumberland, MDSeneca Creek near Dawsonville, MDConococheague Creek at Fairview, MDMonocacy River near Frederick, MDCacapon River near Great Cacapon, WVGoose Creek near Evergreen Mills (Leesburg), VAOpequon Creek near Martinsburg, WVShenandoah River near Millville, WVPotomac River at Point of Rocks, MDRappahannock River at Remington, VAAntietam Creek at Sharpsburg, MD

Non-Forecast Points in Flood (17)Monocacy River at Bridgeport, MD

NF Shenandoah River at Burnshire Dam, VA

Cedar Run near Catlett, VA

NW Branch Anacostia River near Colesville, MD

Patterson Creek near Headsville, WV

Broad Run at Sterling, VA

Battle Run at Laurel Mills, VA

Beaverdam Creek near Mountville, VA

NF Goose Creek near Lincoln, VA

Licking Creek at Pecktonville, MD

Rock Creek at Sherrill Drive, Washington, DC

Hazel River at Rixeyville, VA

Catoctin Creek (VA) at Taylorstown, VA

Antietam Creek near Waynesboro, PA

SF & NF Catoctin Creek (VA) near Waterford, VA

Georges Creek near Westernport, MD

How did we know?

“River Assessment Group” formed in 2011

Goal to determine floodrisk & levels at non-forecastlocations

Over 150 sites visited to date

Where we’ve been…

Results

More sites with flood stages Better anticipation of, and

(especially) knowledge oflength of flood

Longer (but increasinglytargeted) flood warnings

Beaverdam Creek / Loudoun County, VA Moderate Flooding◦ Stranded utility truck

Photo: Leesburg Today

Monocacy River / Frederick, MD Major Flooding◦MD 26 flooded near river; Gambrill Mill (NPS)

flooded. (at peak, water was up to seat of bench)

Photo: NPS

Antietam Creek / Leitersburg, MD “Minor” Flooding◦Water surrounded a house and covered Leiters Mill

Road enough to strand this car.◦ Flood stage here was revised downward.

Western Branch Patuxent / Upper Marlboro, MD Minor flooding…but significant impact!◦ Many nearby tributaries that we do not yet have a handle on

Photo: WTOP

River Flooding Freshwater flooding is handled by polygon-

based warnings, except for river forecast points◦ This will change!

The polygons will still belarger than the affected areabut more targeted◦ (example: the polygon won’t

include Middleburg for a Potomac flood)

Just a first step…◦Ultimately we would like to warn based on dynamic

inundation areas – but we’re not there yet

Determining a range of possibilities In the mid-Atlantic region, our flood forecasts

are usually 90-100% predicated on rain which has not yet occurred.

This means both the forecast rainfall and the river forecast can change significantly before the event is ongoing.

We give you our “most likely” forecast, but what if you want to know a range of possibilities?

River Forecast Ensembles http://www.erh.noaa.gov/mmefs Ensembles

offer ranges of possibilities based on different computer model changes

The officialforecast isnot depictedon this graph!

River Forecast Ensembles Important Note: The ensembles utilize the

GEFS, NAEFS, and SREF. (i.e., no ECMWF). So if we are favoring the Euro, so the official forecast could be outside the range of any of the ensembles.

Training on use of the ensembles is available:http://www.erh.noaa.gov/mmefs/RecordedTraining/

Once the rain is falling, you should use the official forecast instead of an ensemble range.

Flash Flooding We utilize the Flash Flood

Monitoring & Prediction System (FFMP) to monitor estimated precipitation vs. Flash Flood Guidance.

Flash Flooding Zooming in on a county helps us to know

which basins / parts of counties are affectedand which will not be.

Notes about dual-pol precip DP precip estimates are bad outside the bright

band in the winter.◦ Consistently too high

Notes about dual-pol precip On the plus side, the DP precip products are

higher-resolution so the beam blockages are not as pronounced…

Questions? Jason Elliott

Senior Service Hydrologistjason.elliott@noaa.gov