Patterns of Historic River Flood Events in the Mid-Atlantic Region

Richard H. GrummNOAA/NWS Weather Forecast Office, State College, Pennsylvania

and Charles Chillag

NOAA/NWS Middle Atlantic River Forecast Center, State College, Pennsylvania

Contributions byAlaina MacFarlane and Ron Holmes

Motivation

• Ability to compare and rank flood events– For impacts and Federal disasters– Learning from the past to gain knowledge

• Understand flood events– Patterns and conditions for flooding– Education forecasters, users, students

• Knowledge and data should be F U N

Methods and Data

• Mid-Atlantic River Forecast Center Flood data– From archive (points/stages) and research– Extensive dataset with pre-20th Century examples– Based on points over flood and ranked

• Climate data rainfall observations– Where how much when.

• Re-analysis data – Reconstruct the cases 20th Century, NCEP/NCAR, CFSR

20th Century Re-analysis• Used for cases prior to 1949

• 24 pressure levels including 10 hPa

• 6-hourly data

• netCDF or plot-4-U

20th Century Re-analysis site

MARFC Flood Power RankingsIs a arbitrary value weighted according to flood severity

• Simple Method to rate flood based on– Number of points raw number bias

• Power Ranking based on severity/Type of Flood:– minor (1) – moderate(5) – major (10)– unknown (1)

Points Minor Moderate Major Unknown Ranking Event17 5 5 5 2 82 Example

140 34 47 58 1 850 Jan 1996120 20 19 81 0 925 Agnes 1972109 34 43 32 0 569 Lee32 10 11 11 0 175 May 194617 2 7 8 0 117 May 1924

Flood Data Display and Access main access site

• Extensive database with pre-20th Century cases

• Top floods of all time and Month– Sortable by number, categories, and rankings

• Event summaries

Top 20Flood Events

Ranked by points over flood stage

January record

by Events

andthen

Power ranking

May Rankings

1946 event is shown

later

June Events

September Events

Case Example

• Flooding Event of 26 May 1946– 20th Century Re-analysis Example

• Wet month with several day wet period

• Sunbury, PA wet May

May 1946provides summary of event type and flood data

Flood StatisticsYear Count Month Count Total Minor Moderate Major Missing Power

Ranking4 of 8 2 of 2 32 10 11 11 0 175

Weather Summary

See May 21, 1946 flood. The ridge retrograded about 24 May and a surge of high PW air came up the coast. It was a textbook case with a quasi east-west boundary and easterly

flow setting up over the Mid-Atlantic region with a deep southerly jet into it. Heavy rainfall was with strong easterly flow along the boundary and a surge of high PW up the

coast. The low was a southern stream low. Similar to the pattern for the March 2010 Boston/RI flood event in many respects. Probably a Frontal to synoptic transition. – (Rich

Grumm National Weather Service State College, PA)

Additional InformationMARFC Power Ranking is (Minor = 1 - Moderate = 5 - Major = 10 - Missing = 1)

NOAA Daily Weather Maps LinkNOAA Northeast Regional Climate Center Link

NAME TYPE Total Precipitation SUNBURY COOP 11.80SELINSGROVE 2 S COOP 11.32LANSFORD COOP 11.15TAMAQUA 4 N DAM COOP 10.96WATROUS 9 S LEE FIRE COOP 10.88

QUAKERTOWN COOP 10.64LANSFORD FOREMANS SH COOP 10.58

TAMAQUA COOP 10.53JIM THORPE COOP 10.29MONROETON 2 S COOP 10.23TOWER CITY 5 SW COOP 10.19FREELAND COOP 10.19SUNBURY AP WBAN 10.14BERWICK COOP 10.11GREENWOOD RSVR COOP 10.09GRANTVILLE 2 SW COOP 10.04NEW CASTLE 1 N COOP 9.96RETREAT 1 SW COOP 9.90KANE 1 NNE COOP 9.89ALBION COOP 9.88RENOVO COOP 9.82GOULDSBORO COOP 9.70PLEASANT MT 1 W COOP 9.61LEHIGHTON COOP 9.60CANTON 1 NW COOP 9.59NEW BLOOMFIELD COOP 9.52WILKES BARRE COOP 9.51BEAR GAP COOP 9.50EAGLES MERE COOP 9.46Williamsport Area ThreadEx 9.45WILLIAMSPORT WBAN 9.45NEWBURG 3 W COOP 9.45

May 1946Rainfall

For Month

over Pennsylvania

Sunbury Rainfall May 1946

• A wet period

• Wettest May at Sunbury

• Several days of rainfall

• Antecedent conditions played a role. – Some flooding 21 May in NY 1 point!

Date Rainfall1946-05-01 0.001946-05-02 0.001946-05-03 0.001946-05-04 T1946-05-05 0.091946-05-06 0.001946-05-07 0.021946-05-08 0.661946-05-09 0.001946-05-10 0.001946-05-11 0.021946-05-12 0.161946-05-13 0.921946-05-14 0.001946-05-15 0.671946-05-16 0.001946-05-17 0.231946-05-18 0.341946-05-19 0.451946-05-20 0.001946-05-21 1.541946-05-22 0.291946-05-23 0.001946-05-24 0.001946-05-25 0.001946-05-26 1.441946-05-27 1.531946-05-28 3.221946-05-29 0.221946-05-30 0.001946-05-31 0.00Sum 11.80Average -Normal 3.72

The Pattern for the Event

• 20th Century re-analysis data– 250 hPa heights and anomalies sharp wave– 500 hPa heights and anomalies cut-off– High PW East-west then more north-south– LLJ• Easterly flow north of frontal boundary• Southerly flow in warm sector (+5s)

• Textbook P A T T E R N

End of May Pattern

• Large ridge 20-26 May over northwestern Atlantic

• The East Coast and Mid-Atlantic had wet period– Some location had wettest May on record– Sunbury showed wet period

• Persistent pattern then big rain Flood

Event Types Emerge

• Strong south-north PW surges– With strong LLJ– Maddox Synoptic Pattern– Ridge to EAST often critical

• Strong Frontal Systems with easterly flow

• Tropical Systems– With Maddox-Frontal often record events

• Lesser seen cut-off low events

Cut-off Events

• Lack the high PW air

• Slow moving

• Cold core

• Instability driven? There are fewer of these and typically not many points and low power rankings.

Patterns with CFSR Cases

Key Issues and follow-ons

• Data base exists to rank and sort floods

• Not all the floods have been characterized– The data exist to accomplish this back into the 19th

Century– Good learning and teaching tool– Good basic student research Project

• Could be semi-automated?

Summary

• Ability to compare and rank flood events– For impacts and Federal disasters– Learning from the past to gain knowledge– Power rankings are helpful

• Understand flood events– Patterns and conditions for flooding– Education forecasters, users, students– Use our knowledge to improve pattern recognition and

perhaps better identify Extreme Weather Events (EWE)