The Influence of Cold Internal Boundary Layer Decoupling
on the Landfall of Tropical Storm Josephine, October 07, 1996
Patrick Welsh, Chris Herbster, Pablo Santos, and Kent Kuyper
A unique interaction between Tropical Storm Josephine and a local Northeaster over North Florida had a major influence on the precipitation, flow structure, boundary layer wind damage, and tornado generation associated with the storms landfall. This case study examines the process of boundary layer decoupling of the storm wind field from the surface terrain as a key feature in assessment of several unique and initially puzzling aspects of this storm.
An evaluation of the RUC and Meso-Eta operational model gridded fields and initial analysis was compared with both high resolution (4 km) mesoscale MM5 model output and measured or inferred vertical atmospheric structure from local and remotely sensed data.
Due to the strong thermal boundary layer cap and the thickness of the stable boundary layer, rather than displacing the existing flow field, the tropical storm was only able to slowly erode this layer. During the period of this erosion process, tropical storm force winds did not reach the surface, while the resulting lift from overrunning may have enhanced the rainfall in the area.
Additionally, the evidence that this boundary layer may have influenced tornadogenesis is presented to assist researchers and forecasters in future tropical storm decoupled flow cases.
The evening of October 7 , 1996, Tropical Storm Josephine made landfall in North Florida. Surface winds did not reach sustained tropical storm force across much of the area, but extensive flooding occurred across the area. These events were the result of the interaction between the warm tropical air mass associated with the storm and a cool and stable air mass in place over the region for several days prior to Josephine's landfall. A strong baroclinic zone formed rapidly as the storm approached the coast decoupling the tropical storm from the surface and cutting off its moisture supply.
2. Storm Background
Late on the evening of October 7, 1996, Tropical Storm Josephine made landfall in the Big Bend Area of North Florida. Hurricane warnings were posted across the coastal areas of the Northeastern Gulf of Mexico, while Tropical Storm Warnings were posted for the entire Northeast Florida Coast. Inland, the high wind warnings for most of the interior sections of North and Northeast Florida and Southeast Georgia were not verified by observations. The highest gust reported at the NWS JAX Office was 40 mph on the 7th and 47 early on the 8th. The average 2 minute wind speed peaked at 31 mph on both days
Near record amounts of rainfall were reported with WSR-88D rainfall estimates reaching above six inches in parts of North and Northeast Florida during the afternoon and evening of October 7. Overnight, spotter reports across Northeast Florida and Southeast Georgia were indicating as much as 10 inches of rain. At the National Weather Service Office in Jacksonville, FL, a total of 6.14 inches were recorded during the 7th. This combined with several days of rainfall that preceded the passage of Josephine accounted for a total of 11.46 inches of rainfall during the month of October, making it one of the wettest Octobers on record. By the morning of the 8th, both Black Creek and the St Marys River rose above flood stage. Extensive street flooding was also widely reported in communities throughout Northeast Florida and Southeast Georgia.
While Tropical Storm Josephine was developing over the southwest Gulf of Mexico, northeaster-like conditions developed across Southeast Georgia and Northeast Florida with prevailing Northeast winds at 15 to 20 mph with gusts over 30 mph in place for 3 to 4 days. These conditions were responsible for developing a cold boundary layer across the region with temperatures into the 50s and 60s across Central and South Georgia as Josephine approached the coast. Due to strong stratification, these winds remained in place until eventually eroded away by Tropical Storm Josephine. After 03Z on 08 October, the stronger surface winds and warm surface temperatures of the tropical storm were finally observed in the inland areas of the state. Figure 1 shows a surface analysis from the Meso Eta Model valid at 03Z on 10/08. It shows 1000 mb winds, isobars, and temperature with temperatures between 60 and 80 degrees shaded. The tight temperature gradient across North Florida and Southeast Georgia and the frontal kinks in the isobars, suggest an extratropical system, and the possibility that this system was extra tropical even before landfall.
Figure 2 is a cross section of winds, theta surfaces, and relative humidity from central Georgia to central Florida. This cross section shows a warm frontal structure with the cooler airmass located from Gainesville, FL northward into Georgia. Wind fields show a northerly component over GA and extreme north Florida with warm southerly winds flowing over and "mixing out" the top of the cold shallow boundary layer; slowly eroding the stable boundary layer. The isentropic countours indicate the stability of the interface, and are similar to a frontal overrunning situation.
This cold stable boundary layer interface across the region was responsible for decoupling the existing tropical storm force winds from the surface (fig. 2), and for providing isentropic lift resulting in the copious rainfall observed across the region.
Although not shown here, VIS and IR imagery from the daylight hours on 07 October showed dry air entrainment into the storm. These and additional evidence from other sources will be presented to bolster the case for stratified decoupling of the Tropical Storm from the surface, including reflectivity, precipitation, and velocity imagery from WSR-88D Doppler radars from NWSO Jacksonville, FL and Moody AFB in Valdosta, GA and MM5 mesoscale model results. The impact of this layer on mesocyclone and tornado formation was complex. Although mesocyclones formed both north and south of the boundary, tornado touch downs only occurred south of the boundary, north of the boundary numerous funnel clouds were reported but no tornado touchdowns. We believe the cold stable layer decoupled the tornadic activity from the surface preventing actual touchdowns in the cold layer region.
The model, surface, radar, and satellite data reviewed in this study are remarkably consistent in indicating that Tropical Storm Josephine was evolving rapidly into an extratropical system on October 7,1996 as it made landfall. A cold internal boundary layer across the region was responsible for decoupling of the tropical storm from the surface moisture source, preventing the strong winds from reaching the surface, influencing the tornado development, and producing the extreme rainfall reported.