An Examination of the Climatology and Environmental Characteristics of Flash Flooding in the Binghamton, New York County Warning Area Stephen Jessup M.S. Student Dept. of Atmospheric Science Cornell 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

Flash floods per county area

Mostly Fall/Winter/Spring Mostly Spring/Summer

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 ) 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

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

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

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 ● 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 ● Random days representing same seasonality as FF's, n=51 – Random year ( ) 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

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 ( mb) shear – Upper level divergence ● Atmospheric Moisture – Mean 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 – 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 relative humidity patterns

Flood Watch Heavy Sea Level Pressure

850-mb wind speed Expect strong LLJ

850-mb wind direction Some SE Mostly SW to W

850-mb wind speed vs. 850-mb wind direction F = flood H = heavy W = watch R = random SE mostly floods NW often non-events Fast winds either

850 mb wind vector Weak LLJ, convergenceStronger 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 Speed Weak shear

Mid-level (500mb-700mb) shear direction Weak directional shear

Mid level shear: direction vs. speed Larger shear can flood!

700 mb wind vector Weak, convergenceStrong Strong, convergence

500 mb wind vector WeakerStronger

250 mb wind vector

Precipitable water (% normal)

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

Precipitable Water (anomaly) Localized moistureFrontal 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 higher RH 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 S ● Art DeGaetano, Cornell University ● Mike Evans, NWS Binghamton