Major land-falling hurricanes as mesoscale convective systems: A paradigm shift for WFO operations

Traditionally, Tropical Cyclones (TCs) have been treated as synoptic-scale systems and primarily handled with long-fused warnings and subsequent weather statements. However, 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 NWS Weather Forecast Offices (WFOs) for the protection of lives during such events requires significant modification. Since the inner core of a TC is a mesoscale convective system (MCS), possessing a diversity of structure, 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 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 bring short-fused convective wind warnings to the forefront of WFO operations during major TC landfalls, advance hurricane preparedness, and improve societal response. 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.

To emphasize a greater need for mesoscale focus, several specific destructive wind impacts are recalled from the 2004 Hurricane season. Post event ground and air surveys reveal dramatic evidence of enhanced damage swaths associated with the passage of major hurricane inner rain-bands. Damage paths were comprised of nearly complete (inland) tree devastation adjacent to areas of minimal tree loss. Significant (coastal) structural damage was observed within widespread regions of lesser destruction. Also, destructive winds associated with major hurricane inner rain-bands led to the partial roof collapse of several large shelters occupied with hundreds of evacuees, in one case, jeopardizing the lives of 1200 individuals at a single location, well inland. Such extreme winds pose a very significant risk of casualties, especially if last minute protective actions are not taken.

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. In worst-case situations, a short-fused Extreme Wind Warning can be issued to further heighten awareness and to elicit specific reactions. 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 at a 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 urgently will prompt emergency managers, the media, and the general public to initiate immediate life-saving actions identical to the long-established and understood goal of the tornado warning. In major hurricane situations however, many more lives potentially reside in the path of the extreme winds, including many who may have evacuated from the coast to escape storm surge flooding.

The authors will present examples of MCSs and severe local storms associated with the inner rain-bands of recent major hurricanes. Potential negative impacts from these systems will be examined along with implications for enhancing TC preparedness, mitigation, and societal response. Most importantly, the authors would like to issue a challenge to the severe local storm community to expand research on TC inner rain-bands and their ability to generate severe convective wind storms. Traditionally, severe local storm research has not focused upon this issue, in part due to a broad assumption that the danger was an issue reserved for the tropical cyclone community. For example, consider the following statements from the 2001 AMS publication Severe Convective Storms: “…we choose to concentrate on the largest portion of the spectrum of MCSs, excluding, however, tropical storms” (Chapter 9, Mesoscale Convective Systems) and “I will not discuss the vigorous sustained winds that are found in the TC on the meso-β scale…” (Chapter 10, Severe Local Storms in the Tropics). In this era of increasing land-falling major TCs, it has become apparent that this research void must be overcome and preparedness knowledge advanced. There is much work to be done and benefit to be gained.