The mountainous terrain of Vermont's Green Mountains includes steep slopes and numerous small river basins which result in rapid runoff of heavy rainfall. In many areas, streams and rivers can rise from normal flow to greater than flood stage in a matter of hours. National Weather Service (NWS) forecasters have learned river gauge monitoring alone is insufficient to provide adequate warning lead time, especially at headwater regions. Adequate warning lead times can only be achieved by accurately forecasting rainfall intensity and accumulation. When this information is combined with a knowledge of initial conditions, hydrological models, and forecaster experience, accurate river crest forecasts and timely flood warnings can be achieved.
Research and case studies suggest that the use of conceptual models is valuable in recognition of heavy rainfall events. Conceptual model parameters often linked to the occurrence of heavy rainfall include measures of absolute moisture, instability, and vertical motion. These parameters are useful in predicting rainfall intensity, but experience repeatedly demonstrates that the stationary nature of these parameters is often the key to rainfall accumulations that can lead to flooding. Life-threatening flash floods of small rivers and tributaries in Montgomery, VT in 1997 and Bristol, VT in 1998 resulted from intense surface-based convective rainfall. In these cases it was critical to predict the duration of vertical motion and regenerating convection in order to anticipate the rainfall accumulation and magnitude of the disaster. Persistence of frontogenesis in the mid-levels of the atmosphere can be a valuable tool for predicting the accumulation of stratiform or elevated convective rainfall.
On 11-12 June 2002, forecasters at NWS Burlington, VT used surface, radar and satellite observations, and operational Numerical weather prediction (NWP) of frontogenesis at 850 and 700 hPA and other essential parameters to successfully assess the intensity and duration of the rainfall event. Rainfall accumulations of greater than 4 inches within 18 hours were forecast for portions of northern Vermont. Countywide flood watches and warnings, and flood warnings for specific rivers, were issued with exceptional accuracy and lead time. Record floods were recorded along the Missisquoi River at North Troy, VT and East Branch of the Passumpsic at East Haven, VT, and were determined to have been events with return periods of greater than 100 years.
This presentation is a multifaceted look at this flood event. Flood climatology, topographic influences, and antecedent conditions for this event are first displayed. Surface, radar, and satellite observations, and NWP forecasts of mid-level frontogenesis and other parameters critical to the conceptual model for heavy rainfall are shown. Finally, rainfall forecasts are compared to observed rainfall, and flood watches and warnings are compared to hydrographs of river flooding.
Supplementary URL: http://www.erh.noaa.gov/er/btv/