Probabilistic Forecasting of Heavy Rainfall using Evolutionary Program Ensembles
Most heavy rainfall events in the midwestern U.S. develop from training but otherwise unremarkable convection. Past studies have established that training storms are often found on the cold side of a generally west-to-east quasi-stationary, frontal boundary and are typically characterized as nocturnal, elevated convection associated with a southerly low-level jet. The deep layer shear associated with these training systems is roughly parallel to the boundary. This indicates the importance of larger-scale atmospheric organization and suggests an opportunity to better quantify forecast uncertainty and to improve storm prediction. For example, the position of the training line will be sensitive to synoptic observational error, since relatively small spatial dislocations can determine whether or not a particular area will be affected and if so, how severely. The position error, in turn, depends on the dynamics of the stationary (usually, frontal) boundary and the low-level jet, each of which are likely to be highly sensitive to small errors in forecasts of the lower tropospheric wind field. The strength of the outflow boundaries from the convective storms themselves, which also govern storm evolution and movement, is a function of the energy available for storm development (convective available potential energy or CAPE). This can be assessed using available regional measures.
We use the unified precipitation dataset, with gridded precipitation at 0.25 x 0.25 degrees, for verification and apply the EP ensemble approach for the region from 40-45N and 88-92W (an area encompassing the region from central Illinois to central Wisconsin), for next day forecasts. Daily accumulated precipitation (P) for forecasts and observations are binned into five amount categories: P < 1.00 inches, 1.00 ≤ P < 1.50 inches, 1.50 ≤ P < 2.00 inches, 2.00 ≤ P < 3.00 inches, P ≥ 3.00 inches. Since a forecast of no measurable rain is relatively trivial, the training data are first screened to remove all observed days in which no measurable precipitation occurred. Input data to the EP include (1) maximum value in the time period of the regional average CAPE (obtained from RUC hourly analyses); (2) the time of the maximum CAPE; (3) regional average 0-6 km vertical wind shear (RUC) at the time of the maximum CAPE in (1); (4) regional average 700 hPa (mid-level) vector wind (RUC) at the time of the maximum CAPE in (1); (5) regional average 700 hPa (mid-level) vertical velocity (RUC) averaged over the time period; (6) regional average precipitable water obtained from RUC at the time of the maximum CAPE in (1); (7) the Model Output Statistics (MOS) QPF at a network of 16 regional NWS sites over each time period; and (8) the MOS QPF from the prior 1200 UTC forecast at the network of 16 regional NWS sites validating at the same time as the 0000 UTC MOS values. Additionally, measures of upstream, prior-day precipitation activity will be incorporated through rainfall measurements at specific sites. Preliminary results from this analysis will be presented.