Treating hurricanes as Mesoscale Convective Systems - A paradigm shift for WFO landfall operations
Scott M. Spratt, NOAA/NWSFO, Melbourne, FL; and B. C. Hagemeyer and D. W. Sharp
To provide necessary lead-time for coastal evacuations and to account for inherent track/structure uncertainties, Tropical Cyclone (TC) watch/warnings are typically issued well in advance of landfall for large coastal expanses. However, as the direct impacts reach land, National Weather Service (NWS) local Weather Forecast Offices (WFOs) traditionally have treated TC's as synoptic scale systems, typically issuing infrequent weather statements, often lacking urgency and geographic/temporal specificity.
Since the inner core of a TC is a mesoscale convective system (MCS), possessing a diversity of embedded features, treating its passage as a severe local storm event instead of part of a synoptic-scale system will allow for an enhanced short-fused focus on the most life-threatening convective wind impacts. To fully implement this concept, a change in the basic assumptions within the ruling theory of applied science needs to occur, i.e. a shift in the governing paradigm is needed. Adopting this philosophy would help to 1) bring short-fused extreme wind warnings and frequent impact updates to the forefront of WFO operations during significant TC landfalls; 2) advance hurricane preparedness efforts; and 3) improve societal responses. Operational warning strategies would be employed similar to those utilized during significant extra-tropical convective wind events, concomitant with adjustments to hurricane preparedness and mitigation strategies. With the recent onslaught of major land-falling hurricanes along the United States coastline, it has become increasingly obvious that the current operational approach employed by WFOs during such events requires a series of critical modifications to more effectively safeguard the public.
Therefore, a new paradigm for WFO operations is recommended during times of hurricane landfall to raise the likelihood of final life-saving protective actions by those exposed to the maximum wind risk. Careful radar analyses can reveal significant details within TC core regions which can also be used to issue frequent and detailed mesoscale statements, providing a nearly continuous stream of severe weather information for individuals within the path of the most intense inner rain-bands. A new official NWS short-fused warning, the Extreme Wind Warning, will provide WFOs with a crucial tool to further heighten awareness and to elicit specific reactions during especially high impact events. Since continuous and rapid evolution of the inner rain-bands occur, such warnings typically would not be issued for individual embedded transient cells, but rather for longer lived inner rain-bands on a county or sub-county scale, likely containing winds over 100 knots (115 mph), capable of producing a rapid onset of destructive winds over a small region. Receipt of such a warning will urgently prompt emergency managers, the media, and the general public to initiate immediate life-saving actions identical to the long-established and well understood goal of the tornado warning.
To emphasize the greater need for a mesoscale focus, several high profile hurricane wind events from recent seasons are examined. In one case, destructive winds associated with the inner rain-bands of a major hurricane led to the partial roof collapse of a large evacuation shelter, jeopardizing the lives of 1200 individuals at a single (inland) location. Another situation occurred as a result of an unprecedented (and under-forecast) rate of TC development at landfall. Although the system achieved only minimal hurricane strength, many residents were caught off-guard and ill-prepared for the resulting wind impacts. Such wind events pose a very significant risk of casualties, especially if last minute protective actions are not taken. An aggressive and proactive use of frequent and timely mesoscale updates by WFOs to communicate specific wind impacts, locations and durations, as well as a judicious use of Extreme Wind Warnings will further enhance and elevate the established long-fused warning process. Adopting these approaches will reduce human casualties by specifically targeting individuals within the most dangerous wind impact zones, during the final moments before onset, with urgent advice to shelter accordingly.
Disclaimer: Readers are advised that the stated positions of the authors do not necessarily reflect those of the National Weather Service or its parent agency (NOAA).
Extended Abstract (980K)
Session 11C, Tropical Cyclones at Landfall
Wednesday, 30 April 2008, 1:15 PM-3:00 PM, Palms H
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