Jason Elliott NWS Baltimore/Washington Media Workshop March 29, 2013 Pictured: Gunpowder Falls near Parkton, MD

 It was a quiet year…  Drought over much of the country  No real spring flood  …until Sandy…  Conditions have turned wetter both locally and in other parts of the country since late October 2012

 2012 had the fewest flood-related fatalities in 50 years! (28…only 3 east of the Mississippi) Data Source: NWS Hydrologic Information Center Note: These data do not include surge-related flooding, like Sandy or Katrina.

 Flash Flood Event: ◦ Baltimore County 8/26/12 L H

 Flash Flood Event: ◦ Berkeley Springs WV 9/1/12 Photo: WHAG-TV Viewer Submission

 Flash Flood Event: ◦ Frederick MD 9/18/12 Photo: Frederick News-Post

 Major Flooding (worst since 1996) ◦ US 15 – primary north/south route – flooded Photo: Loudoun Co. Sheriff

 29 stream gauges with established flood levels exceeded their flood stages in Sandy. Forecast Points in Flood (12) Potomac River at Brookmont (Little Falls), MD Wills Creek near Cumberland, MD Seneca Creek near Dawsonville, MD Conococheague Creek at Fairview, MD Monocacy River near Frederick, MD Cacapon River near Great Cacapon, WV Goose Creek near Evergreen Mills (Leesburg), VA Opequon Creek near Martinsburg, WV Shenandoah River near Millville, WV Potomac River at Point of Rocks, MD Rappahannock River at Remington, VA Antietam 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

 “River Assessment Group” formed in 2011  Goal to determine flood risk & levels at non-forecast locations  Over 150 sites visited to date

 More sites with flood stages  Better anticipation of, and (especially) knowledge of length of flood  Longer (but increasingly targeted) flood warnings

 Moderate Flooding ◦ Stranded utility truck Photo: Leesburg Today

 Major Flooding ◦ MD 26 flooded near river; Gambrill Mill (NPS) flooded. (at peak, water was up to seat of bench) Photo: NPS

 “Minor” Flooding ◦ Water surrounded a house and covered Leiters Mill Road enough to strand this car. ◦ Flood stage here was revised downward.

 Minor flooding…but significant impact! ◦ Many nearby tributaries that we do not yet have a handle on Photo: WTOP

 Freshwater flooding is handled by polygon- based warnings, except for river forecast points ◦ This will change!  The polygons will still be larger than the affected area but 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

 In the mid-Atlantic region, our flood forecasts are usually % 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?

  Ensembles offer ranges of possibilities based on different computer model changes  The official forecast is not depicted on this graph!

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

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

 Zooming in on a county helps us to know which basins / parts of counties are affected and which will not be.

 DP precip estimates are bad outside the bright band in the winter. ◦ Consistently too high

 On the plus side, the DP precip products are higher-resolution so the beam blockages are not as pronounced…

 Jason Elliott Senior Service Hydrologist