Forecast Performance of an Operational Mesoscale Modeling System for Tropical Storm Irene in the New York City Metropolitan Region

On 27-28 August 2011, Tropical Storm Irene severely impacted the northeastern portion of the United States, including New Jersey, Connecticut, Vermont, and the New York City metropolitan area, five days after being initially classified as a Category 1 Hurricane. In preparation for the expected landfall many municipalities as well as private and public sector agencies deployed resources based upon anticipated hurricane intensity, although the storm had weakened to a tropical storm prior to landfall. The storm produced three to almost ten inches of rainfall in the New York City metropolitan region with rainfall totals from six to twelve inches in the Hudson Valley through New England. Wind gusts ranged from fifty miles per hour to higher than seventy miles per hour near the coast with forty to fifty miles per hour inland. Extensive flooding and infrastructure damage resulted from the intense precipitation and strong winds. Widespread power outages resulted with nearly one million people affected; some were without power for two weeks.

In our continuing work focused on providing weather sensitive business solutions, IBM's “Deep Thunder” service provides operational forecasts twice daily for areas of southeastern New York State and northern New Jersey. With an operational history that spans more than a decade, producing one- to three- day model-based forecasts at one to two kilometer resolution, the overall model configuration has evolved and improved over time to reflect improvements in NWP model capability as well as computational efficiency. Over the past several years, the system has focused on producing 84-hour predictions updated every 12 hours. The NWP component is derived from a configuration of the WRF-ARW (version 3.1.1) community model. It operates in a nested configuration, with the highest resolution at two km, utilizing 42 vertical levels. The configuration also includes parameterization and selection of physics options appropriate for the range of geography within the domain from highly urbanized to rural. This includes WSM-6 microphysics (explicit ice, snow and graupel), Yonsei University non-local-K scheme with explicit entrainment layer and parabolic K profile in the unstable mixed layer for the planetary boundary layer, NOAH land-surface modelling with soil temperature and moisture in four layers, fractional snow cover and frozen soil physics, Grell-Devenyi ensemble cumulus parameterization, and the 3-category urban canopy model with surface effects for roofs, walls, and streets.

Given the model length and frequency, the system produced six operational forecasts that covered the period prior to landfall and the impact in New York and New Jersey. The system exhibited proficient skill in forecasting regional as well as local scale impacts of Irene with significant lead time.. In particular, with the model run initialized at 12 UTC on 26 August 2012 and those produced afterwards forecasted Irene to weaken and make landfall as a tropical storm.

In order to evaluate the quality of the forecasts produced by Deep Thunder at a storm-scale and its potential skill, we compare the model results with observational data and other available forecasts as well as the operational availability of specific forecast products. Such performance is examined by considering forecast timing, locality, and intensity of the storms impacts as well as through the utilization of traditional and spatial verification methodologies.