Environmental conditions favorable for the initiation of nocturnal convection over the eastern plains

Forecasting nocturnal convection in the Northern Plains has always been a challenge. Large hail, damaging winds, flash flooding and even tornadoes can be associated with storms that develop after sunset. Barring the development of a large convective complex, the usual result is a relatively small area of heavy rainfall and severe weather surrounded by large areas with little convective activity. For forecast information to be useful to the public at these late hours, forecasters need to provide long lead time with watches since television and radio warnings and even sirens are less likely to be effective after 2200 LT. The general tendencies of the Northern Plains nocturnal maximum have been well documented, but synoptic and mesoscale processes that support local variations have not been well researched. Regional climatologies have shown that the primary time of thunderstorm reports and summer precipitation events are near local midnight in eastern South Dakota. Cloud-to-ground lightning data have shown that the maximum occurrence of lightning varies across South Dakota from about 1800 LT in the Black Hills to about midnight in the eastern portion of the state. While some have speculated that nocturnal thunderstorms in eastern South Dakota initiated over the Black Hills and east slopes of the Rockies during the late afternoon, others have found that the area to the east of the Missouri River into Minnesota and Iowa essentially have the same late evening to midnight maximum, indicating this region could be one of new development. An analysis of severe weather hours across this area shows where there is a narrow peak of occurrence for tornadoes between 1700 LT and 2100 LT, the reports of large hail and damaging wind have broader peaks that extend past midnight. A small number of events continue through the early morning hours and after sunrise.

A basic climatology of the initiation of nocturnal convection was done using National Lightning Detection Network (NLDN) data. Cloud-to-ground lightning data were examined to find days when convection initiated over portions of eastern South Dakota, southwest Minnesota, northwest Iowa, or northeast Nebraska between 2100 LT and 0900 LT. A second set of cases were identified when convection occurred during the day but ended shortly after sunset. When convection was determined to initiate within the study area, the date, hour, location of convection initiation, and if severe weather accompanied the convection was recorded. Composite maps of geopotential height, temperature, wind, and dew point were computed for 12 h prior, 6 h prior, and the initiation time of convection. A similar composite was constructed for cases when convection occurred during the day in the study area, but ended by 2300 LT. Finally, two subsets of composites were done for non-severe and severe nocturnal convection cases. From this data, comparisons are made between the surface and upper atmospheric pattern in order better predict the time and location of elevated convection initiation.