Predictability of an Extreme Coastal Precipitation Event in New York City Using the New York State Mesonet

As an application of numerical weather prediction in the coastal regions of New York State (NYS), we examine the predictability of the impacts of the remnants of Hurricane Ida in New York City (NYC). An important element of this work has been the use of high-quality local observations for both data assimilation to reduce the errors in the initial state, and to assess the fidelity of the model results. This has included data from both public and private sources. The former has typically been conventional observations available from NOAA MADIS. Private data have included those from our own efforts with micronets as part of lake watersheds observing systems. However, there typically remains a significant gap at the intermediate, mesoscale in most regions. The NYS Mesonet (NYSM) deployed and operated by the State University of New York at Albany was designed specifically to address that observational gap for that state. Therefore, we have enabled automated utilization of near-real-time conventional observations from the NYSM for our operational execution of a customized version of the community WRF-ARW weather model.

We evaluate the skill of this model for an extreme precipitation event. On Wednesday, 1 September 2021, the remnants of Hurricane Ida led to record-breaking rainfall in the New York City metropolitan area, including over 200mm in only a few hours, resulting in subway platforms and streets looking like rivers, and over a dozen fatalities from drowning. For the first time, the National Weather Service issued a flash flood emergency for New York City, “This is a life-threatening situation,” the emergency read. “Seek higher ground now!”

We will present results with two types of model configurations focused on the Atlantic Ocean coastal region in NYS, including data denial experiments with the assimilation of conventional observations from the NYSM. One is targeted for the NYC metropolitan area at 667m horizontal resolution with coarse coverage of NYS in the outer nests. The other covers all of NYS and the surrounding region at 1km horizontal resolution. We will illustrate how localized characteristics of extreme events are shown by data from the NYSM. We also evaluate how such data can be used to assess and improve this model. In addition, we will present an overview of the case study event and the approach to the modelling.