An Examination of the Climatology and Environmental Characteristics of Flash Flooding in the Binghamton, New York County Warning Area

Stephen JessupM.S. Student

Dept. of Atmospheric ScienceCornell Univ.

Project Objectives

● Develop a long-term climatology of flash flood events for the BGM CWA.

● Identify any spatial differences in flash flood frequency and flood producing meteorological conditions across the CWA.

● Analyze a set of meteorological variables to quantitatively identify combinations of variables that are associated with flash flooding.

● Compare the conditions associated with flash floods to the conditions associated with non-events

Flash Flood Climatology

● Spatially: FF's most common in NY/PA border counties and in eastern NY counties

● Diurnally: – Peak in late afternoon/early evening

– Secondary max. in morning

● Seasonally:– Peak in summer (June max.)

– Min. in late fall/winter

18

66 8

7

7 6

8

5

16

16

16

13

4

92513

24

28 5 7

82

11

24.3

17.4

12.37.3

6.19.6

5.0

35.4

11.5

58.8

10.116.0

11.1

7.6

13.29.0

16.021.3

18.5

12.9

13.4

25.0

12.6

17.8

Flash floods per county area

0 EST

100

EST

200

EST

300

EST

400

EST

500

EST

600

EST

700

EST

800

EST

900

EST

1000

EST

1100

EST

1200

EST

1300

EST

1400

EST

1500

EST

1600

EST

1700

EST

1800

EST

1900

EST

2000

EST

2100

EST

2200

EST

2300

EST

0

2

4

6

8

10

12

Flash Flood Start Time, 1986-2003

Time

Perc

ent

of

Fla

sh F

loods

Mostly Fall/Winter/Spring

Mostly Spring/Summer

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec0

5

10

15

20

25

Number of Flash Floods per MonthP

erc

en

t of

Fla

sh

Flo

od

s

Antecedent Precipitation● Determined for one week (7 days) and one month

(30 days) prior to flash floods● Climatology for comparison

– Consists of all non-flood years (from 1986-2003) for each flash flood date

– To test hypothesis that floods tend to occur during periods of above-normal precipitation

● Flash floods tend to occur in anomalously wet periods

0-0.

25

0.26

-0.5

0

0.51

-0.7

5

0.76

-1.0

0

1.01

-1.2

5

1.26

-1.5

0

1.51

-1.7

5

1.76

-2.0

0

2.01

-2.2

5

2.26

-2.5

0

2.51

-2.7

5

2.76

-3.0

0

3.01

-3.2

5

3.26

-3.5

0

3.51

-3.7

5

3.76

-4.0

0

4.01

-4.2

5

4.26

-4.5

0

4.51

-4.7

5

4.76

-5.0

0

5.01

-5.2

5

5.26

-5.5

0

5.51

-5.7

5

5.76

-6.0

00

1

2

3

4

5

6

7

8

9

10

Precipitation Amounts for Flash Flood Events for a Full Year

Precipitation (inches)

Nu

mb

er

of

Eve

nts

Climatology vs. Flash Flood Antecedent Precip. (7-day)

0 1 2 3 4 5 6 70

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

7-day Antecedent Precip Cum. Dist.

ClimoFloods

Inches of Precip

Cu

m. F

req

.

Climatology vs. Flash Flood Antecedent Precip. (30-day)

0 2 4 6 8 10 12 14 160

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

30-day Antecedent Precip Cum. Dist.

ClimatologyFlood Events

Inches of Precip.

Cu

m. F

req

.

0 1 2 3 4 5 6 7 80

1

2

3

4

5

6

Storm total vs. Antecedent Rainfall

Weekly Antecedent Precipitation

Flood t

ota

l rain

fall

0 2 4 6 8 10 12 140

1

2

3

4

5

6

Storm total vs. Antecedent Rainfall

Monthly Antecedent Precipation

Flo

od

tota

l ra

infa

ll

Improving FF Forecasting Procedures for NWS BGM: Methodology

● Construct independent databases of flash flood and null event cases

● Determine meteorologically significant parameters and their values during events

● Find combinations of variables that improve predictability

● Plot composites to determine whether the synoptic situations of FF's and null events differ

● Merge these into a forecasting methodology

Datasets● Warm-season flash flood cases (~May-Oct), n=51

– Separated by at least one week (7 days)

– Drawn from 1986-2003

● Warm-season heavy precipitation events, n=36

– At least 1” in one hour, at least 1.5” in six hours

– Separated from each other & FF's by at least one week

– Drawn from 1986-2003

● Random days representing same seasonality as FF's, n=51

– Random year (1986-2003) assigned to the date of each FF case

– Separated from each other & FF's by at least one week

● Watches/warnings that did not verify, n=17

– Separated from each other by at least one week

– Drawn from 1995-2003

Dataset Methodology

● NCEP Regional Reanalysis used as primary data source: 32 km, 3-hour resolution

● Three time periods used: time closest to the flood, two time steps of three hours prior

● Most variables averaged over quadrilateral area containing FF counties

– Area for prior time periods determined by backtracking four corners of this area using 850 mb wind

– 850 wind, storm motion vectors backtracked an extra timestep to reflect inflow

● Backtracking not used for several variables classified as synoptic; parameter representing large-scale field used instead

Highlights: Current BGM FF Checklist

● Winds/storm motion– Slow storm movement (MBE/Corfidi vector)

– Low level jet >= 20 kts

– 700 – 500 mb winds < 30 kts

– Weak mid-level (700-500 mb) shear

– Upper level divergence

● Atmospheric Moisture– Mean 1000-500 relative humidity >= 70%

– Precipitable water >=150% normal or >= 1.4 inches

BGM FF Checklist, continued

● Synoptic-scale features– Nearby surface boundary

– Low-level theta-e axis

– Upper level ridge axis

– 1000-500 mb thickness diffluence

● Other parameters– “Tall and skinny” CAPE

– Warm cloud depth exceeding 3-4 km

Summary: Results● Thresholds in checklist generally agree with FF results, but are

often exceeded in non-events

● Exception: Low-level jet apparently not as important for flash flooding, but more common for heavy rainfall non-events

● Measures of antecedent soil moisture a good first-guess criterion between both flood/heavy and flood/watch

● Properties of the 850-mb theta-e field differ in both flood/heavy and flood/watch comparisons

● Measures of 850-mb and 700-mb moisture (dewpoints and RH) differ for flood/heavy and flood/watch

● Notable differences in large-scale mid and low level wind patterns

● Notable differences in 850-500 relative humidity patterns

Flood

Watch

Heavy

Sea LevelPressure

850-mb wind speed

Expect strong LLJ

850-mb wind directionSome SE Mostly SW to W

850-mb wind speed vs. 850-mb wind direction

F = floodH = heavyW = watchR = random

SE mostly floods

NW often non-events Fast winds either

850 mbwind vector

Weak LLJ, convergence Stronger LLJ

Strong LLJ, convergence

Storm Motion Speed

Not necessarily slow storm motion

Storm motion direction

Primarily SSW to W

Mid-level (500 mb – 700 mb) Shear SpeedWeak shear

Mid-level (500mb-700mb) shear directionWeak directional shear

Mid level shear: direction vs. speed

Larger shearcan flood!

700 mbwind vector

Weak, convergence Strong

Strong, convergence

500 mbwind vector

Weaker Stronger

Stronger

250 mbwind vector

Precipitable water (% normal)

Weekly Ant. ppt. vs. Precipitable Water (% normal)

PrecipitableWater(anomaly)

Localized moisture Frontal signature?

Perhaps some of both?

850 mb Theta-e vs. weekly antecedent precipitation

Floods lower theta-e and wetter antecedent

CAPE

Long, skinny CAPE?

850 Wind Direction vs. 850 Dewpoint

Floods: lower 850 Td

850 mb RH

Floods have higherRH than heavy

700 mb RH

500 mb RH

Summary

● LLJ not as significant in FF's as in null events● Composites suggest theta-e ridging is less

significant in FF's than in null events● Atmospheric moisture greater and more localized

in FF's than in null events● Possible connection between antecedent soil

moisture and local maximum in moisture content during FF's?

● Caveat: small sample size, small spatial domain!

Acknowledgements

● COMET Outreach Project S05-52254 ● Art DeGaetano, Cornell University● Mike Evans, NWS Binghamton