Composite means and anomalies of meteorological parameters for summertime flash flooding in the National Weather Service Eastern Region

Flash flooding is a common occurrence in the National Weather Service Eastern Region (NWSER). During a recent 10-year period (1996 to 2005), around 500 to 1500 flash flood events were reported each year. Of these, on average about one-half (47 percent) occurred during the climatological summer months of June July and August. Diurnally, summertime flash flooding is most likely from mid-afternoon through early evening, compared to a more balanced diurnal distribution at other times of the year.

To further study the meteorology of summertime flash floods, the NWSER was broken up into four sub-regions: roughly speaking, "New England", "Mid-Atlantic", "Ohio Valley" and "South". Analyses of surface and upper-air meteorological parameters were examined for "significant" flash-flooding days (roughly 10 or more events) in each sub-region during June July and August. Days with named tropical systems in or near the NWSER were excluded. Composite mean and anomaly charts were created from NCEP/NCAR Global Re-analyses and from the NCEP North American Regional Re-analyses, accessed through NOAA-ESRL web interfaces. Variables examined included pressure-height, zonal and meridional winds, temperature and specific humidity and other derived variables.

The resulting composites show mostly positive anomalies in moisture and meridional wind and negative anomalies in pressure-height, with some interesting exceptions. Synoptic patterns such as surface low pressure, mid-level troughs and upper-level jet streaks, are most similar for the New England and mid-Atlantic sub-regions, somewhat similar but generally weaker for the South, but distinctly different for the Ohio Valley. Time-lagged mean and anomaly charts show patterns evolving over time in a way that could be useful for anticipating flash flood events. Pattern differences among ER sub-regions, as well as the ER-wide results mentioned above, will be illustrated and described further in an extended abstract and in the conference presentation.