NCWF-2 Probabilistic Forecasts

This paper describes the National Convective Weather Forecast – 2hr (NCWF-2), a software system that automatically combines meteorological observations from radar and lightning data, feature detection algorithms, and Rapid Update Cycle (RUC) numerical weather prediction output to provide routine (every 5 min) 1 and 2 hr probabilistic nowcasts of convective storm location. NCWF-2 follows the operational NCWF. The current operational version of NCWF shows the convective hazard detection field and a binary forecast of storm location with a one hour lead. Problems associated with NCWF include, long lead-times before the first extrapolations are provided, extrapolation vectors turning on and off if storms are near the threshold size limit, no extrapolation on small storms that do not meet size thresholds, and sporadic motion vectors especially in the southern regions of the country where steering flow winds are weak. Many of the problems associated with NCWF-1 motion vectors are directly addressed in NCWF-2 by improved quality control and use of the RUC steering level winds. In addition to problems with the motion vectors, the NCWF-1 nowcasts are limited because of uncertainty in the polygon shapes and inability to forecast initiation or forecast regions of change (growth and decay). The NCWF-2 uses probabilities to quantify uncertainty inherent in the forecasts. Probability maps are calculated based on the spatial distribution of VIP level 3 or higher around a point in Lagrangian space. Trends of these probability maps are calculated using a weighted linear fit of a mean value within a given radius circle over a given period. The trends toward lower probabilities are applied to the probability maps to signify areas that are dissipating. The RUC data are used to capture regions favorable for new storm development and growth. A low probability region is expanded around small radar detected cells that are located with these growth regions. . NCWF-2 is developed by the National Center for Atmospheric Research with prime funding from the Federal Aviation Administration and as part of the Aviation Weather Research Program’s Convective Weather Product Development team that includes collaboration by MIT Lincoln Laboratory, NOAA National Severe Storms Laboratory, and NOAA National Weather Service Aviation Weather Center (AWC).