Mesoscale Evolution of an Extreme Precipitation Event over Long Island on 13 August 2014

On 13 August 2014, a historic heavy rainfall event impacted highly urbanized central Long Island. Many locations in western Suffolk County received nearly 12” (~305 mm) of rainfall in approximately four hours, with Islip MacArthur Airport receiving a record-breaking 24-h total of 13.57” (345 mm). The main objective of this project is to explore the mesoscale evolution of this flooding event, and the mechanisms for the heavy rainfall. Another goal is to determine whether a mesoscale model run at high resolution can realistically simulate the evolution and magnitude of this flooding event, and explore some reasons why most ensemble members using different physics and initial conditions struggled to produce the magnitude of this event. The Weather Research and Forecasting (WRF v3.5.1) model was used for these simulations down to 1-km grid spacing for an 18-h prediction starting at 0000 UTC 13 August 2014. Those WRF simulations initialized using the 0.5-degree Global Forecast System (GFS) analysis and 6-hourly forecast grids provided the most realistic WRF predictions for this event. Numerous physics were tested, but the control (best) run uses the WSM6 microphysics, RRTM longwave radiation, RRTMG shortwave radiation, ACM2 PBL, and KF cumulus schemes (for grid spacing > 5 km).

A realistic WRF member at 1-km grid spacing was obtained, in which over 285 mm of precipitation fell in a narrow band at about the same locations as observed. We will highlight some of the low-level forcing and vertical circulations associated with the rainfall in this successful run and compare them with observations. Both radar and surface observations as well as 1-km WRF simulations illustrate a weak meso-low near Long Island that enhanced the low-level convergence, upward motion, and rain rates over this region. The mean storm motion was parallel to the orientation of the front, resulting in cells “training” over the same location for a few hours. The importance of latent heating and cooling on this evolution explored using WRF runs in which these processes are turned off in the model. Many WRF members using different NCEP analyses as initial conditions and physics underpredict the precipitation by a factor of two. We will show that the size of the nested domain has a significant impact on the evolution of the precipitation and the subsequent evolution of the intense precipitation band. A larger explicit precipitation domain (3-km domain) results in more spurious model convection forming to the south along the warm front during the first 6 hours of the simulation. This in turn perturbs (weakens) the low-level jet helping of transport moisture and instability to the location where cells initiate and grow over Long Island.