Forecast Diagnostics to Improve Tropical Wave Forecasting at WFO San Juan

Tropical waves are the primary weather modulator for Puerto Rico during the convective months from June through October. A tropical wave passage across Puerto Rico often results in heavy rainfall which is accompanied by strong gusty winds across the San Juan forecast area. The rainfall and gusty winds create hazards for both the marine community and residents on the local islands. The impact of tropical waves on the local forecast area makes it necessary for the National Weather Service (NWS) Forecast Office in San Juan to accurately track tropical waves. By NWS definition, a tropical wave is a trough or cyclonic curvature maximum in the trade wind easterlies. These features have several sources; and are primarily of African origin, but can also originate from upper tropospheric trough reflections, or even the midlatitudes. At times, these features can be subtle and difficult to discern via satellite or traditional wave tracking methods, especially when the features are weak or lack active deep convection. These subtle features pose a significant challenge for the forecasters in San Juan, and since they are difficult to track can result in significant forecasting errors. Recent diagnostic tools, developed at the University at Albany, in collaboration with the UK Met Office, provide an objective method to identify synoptic scale easterly waves in gridded data sets using the 700 hPa wind field. These diagnostic tools have been used the past few convective seasons in Puerto Rico, and have proven to be useful to determine tropical wave origins and tracks.

This study will highlight one event, which occurred in June 2007, where a tropical wave resulted in portions of Puerto Rico receiving over 5 inches of rainfall. The diagnostics indicate that this particular event was ultimately the result of a weakened midlatitude trough from the eastern Atlantic. This feature tracked around the periphery of the subtropical high, and then moved westward in the tradewinds. Along the path across the Atlantic, this feature remained subtle as it was overlaid by the leading edge of a Saharan air layer (SAL) plume, and deep convection was limited. This situation made tracking this feature challenging using traditional analysis techniques, whereas the new diagnostics allowed us to coherently follow this feature across the Atlantic basin. When the feature approached the Lesser Antilles, it moved from under the SAL and interacted with an upper level trough. At this point convection in the tropical wave increased, and resulted in a significant rain event when it tracked across Puerto Rico. This presentation will demonstrate how these new diagnostics can be used to augment more traditional forecasting tools and methods in order to reduce the forecaster's workload and improve the quality of the WFO analyses and forecasts.