A Climatology of Flash Flood Events in the NWS Eastern Region Alan M. Cope National Weather Service Mount Holly, New Jersey

Part 1: All FF Events Spatial and Temporal Distribution of Flash Flood Events

Methodology Do a climatological study of flash flood events only, not warnings or verification. Download data for all flash flood events in NWS Eastern Region, , from the Verification/Storm Data “Stats on Demand” web site. Stratify data by year, time of year, time of day, sub-region, WFO, etc. Plot the results…

Flash Flood Events by CWA,

Flash Flood Events by County,

Ohio ValleyNew England SouthMid Atlantic

Part 1 Conclusions Total number of FF Events in NWS-ER varies, from year to year, by a factor of 3 to 4. FF Definition change in 2001 significantly reduced the number of cool season FF Events. Total number of FF Events varies by a factor of 10 or more from one CWA to another in ER. A large majority of FF events occur during the warm season (May through September). Most FF events occur from 1900 to 0300 UTC, especially during the three summer months. In the Ohio Valley, FF events peak in June; in the South, the peak is in August-September.

Part 2: Summer FF Events Composite Charts: Means and Anomalies

Methodology Looked at FF events in summer months (Jun, Jul, Aug). Looked for "significant" days with >= 10 FF events in each sub-region (>=8 in South and New England). Removed days with a named/organized tropical system anywhere in ER. For multi-day events (<=3 days apart), used only the one day with most FF events. Also noted start time (1 st FF event) to nearest 3 hours. Some days were "significant" in more than one sub-region. Created composites from NCEP/NCAR global re-analysis and NARR data sets, via NOAA/ESRL web sites.

Ohio Valley (20 Cases) Z Z Z # Z Z Z # Z Z Z Z Z Z Z Z Z Z Z Z Z Z New England (17 Cases) Z Z Z Z Z $ Z Z Z Z $ Z Z Z Z Z Z Z Z South (18 Cases) Mid Atlantic (19 Cases) Z # Z Z # Z $ Z Z Z Z $ Z Z Z Z Z Z Z Z Z Z Flash Flood “Big Day” Cases

Ohio Valley New England SouthMid Atlantic NWS-ER “Sub-Regions”

Ohio Valley New England SouthMid Atlantic MSLP Composite Mean

Ohio ValleyNew England SouthMid Atlantic MSLP Composite Anomaly

Ohio ValleyNew England SouthMid Atlantic 500mb Height Composite Mean

Ohio ValleyNew England SouthMid Atlantic 500mb Height Composite Anomaly

Ohio ValleyNew England SouthMid Atlantic 700mb Specific Humidity Mean

Ohio Valley New England SouthMid Atlantic 700 mb SH Composite Anomaly

Ohio ValleyNew England SouthMid Atlantic 925mb V-Wind Composite Mean

Ohio ValleyNew England SouthMid Atlantic 925mb V-Wind Composite Anomaly

Ohio ValleyNew England SouthMid Atlantic 250mb U-Wind Composite Mean

Ohio ValleyNew England SouthMid Atlantic 250mb U-Wind Composite Anomaly

Ohio ValleyNew England SouthMid Atlantic 250mb Total Wind Composite Mean

Part 2 Conclusions Composite analyses show different signals for each NWS-ER “Sub-region”. Ohio valley sub-region seems most different, likely due to being west of the Appalachian Mtns. Negative height anomalies are found to the west of each sub-region, except Ohio Valley. Positive moisture anomalies are found over each sub- region. Positive v-wind anomalies are found over or near each sub-region. Upper-level jet streak pattern indicates favorable position for large-scale upward motion.

THE END Expanded version of Part 2 is at ftp.werh.noaa.govftp.werh.noaa.gov /share/PHI/FF_NWS-ER_Study.ppt