SPoRT Provides Forecasts of Hurricane Isaias' Landfall to NOAA's Hurricane Research Divison

Written by Patrick Duran
Aug 13, 2020

Each year, the Hurricane Research Division (HRD) of the National Oceanic and Atmospheric Administration (NOAA) conducts a field campaign to provide real-time observations to forecasters at the National Hurricane Center. The field program’s three aircraft collect data that are invaluable in improving the accuracy of hurricane forecasts. An important part of this field campaign is a daily weather briefing led by scientists from HRD, other federal agencies, and universities. This briefing is used to make important decisions regarding aircraft deployment and flight patterns.

SPoRT scientists led HRD’s daily weather briefings for the week of August 3-7. The briefings consisted of real-time analysis of satellite and aircraft datasets along with an interpretation of model forecasts. The beginning of the week included analyses and forecasts for Hurricane Isaias as it was off the east coast of Florida. While Isaias moved through the warm waters of the Bahamas on the weekend of July 31 to August 2, an eye formed once for a brief time, but decayed after only a few hours (Fig. 1). Over the next two days, Isaias’ eyewall exhibited multiple cycles of eyewall convective intensification and, at times, appeared to be close to closing off an eye once again. But each time, the eyewall convection on the southwest and southern side of the eyewall failed to persist.

Image 1

Fig 1. Radar reflectivity loop from the Bahamas depicting Isaias between July 31 and August 2. (Courtesy Brian McNoldy, Univ. of Miami, Rosenstiel School: http://bmcnoldy.rsmas.miami.edu/tropics/isaias20/Isaias_31Jul20_bahamas.gif )

This failure to organize was primarily caused by the presence of southwesterly vertical wind shear, which favors the formation of convection in the northeastern semicircle of the storm (“downshear” of the storm center) but suppresses convection in the southwestern semicircle (“upshear” of the storm center.)

Another factor that helped to prevent Isaias’ intensification over the warm waters of the Gulf Stream was very dry air in the mid-levels of the atmosphere. This dry air was well-observed by SPoRT’s Gridded NUCAPS product, which depicted relative humidity values below 20% over a broad area to the west and southwest of Isaias (Fig 2). The presence of dry air “upshear” of a tropical cyclone normally indicates that the environmental flow is acting to move the dry air into the storm, which further inhibits its ability to intensify.

Image 2

Fig 2. Gridded NUCAPS 500-mb relative humidity. At the time of this image, Tropical Storm Isaias was off the coast of Northeast Florida (indicated by the blue star). The region of dry air to the southwest and west of the storm is highlighted by the blue line.

The combination of vertical wind shear and dry air not only inhibited the storm’s intensification, but also reduced the storm’s impact on Florida by suppressing convection on the western side of the storm. Although Isaias’ center passed close to Florida’s east coast, relatively little rainfall fell on the peninsula because of this effect (Fig 3).

Image 3

Fig 3. Radar reflectivity image of Tropical Storm Isaias just east of Cape Canaveral, Florida. (Courtesy Mark Nissenbaum, Florida State University)

Finally, an interesting phenomenon observed in Isaias was a number of convective bands that propagated far from the storm as it moved through the Bahamas. Some of these bands can be seen on July 31 and August 1 in the first radar loop above, moving far northwest of the storm center. They also can be seen impacting Florida on August 1 and 2 (Fig 4).

Image 4

Fig 4. Radar reflectivity loop that shows convective bands propagating far to the northwest of the storm, impacting Florida. (Courtesy Brian McNoldy, Univ. of Miami, Rosenstiel School: http://bmcnoldy.rsmas.miami.edu/tropics/isaias20/Isaias_1-4Aug20_southeast.gif )

SPoRT scientists hypothesize that these bands could be similar to midlatitude squall lines that propagate along pools of cool, dense air near the surface. The dry air observed by NUCAPS to the west of the storm could have helped to maintain these cold pools, allowing the lines of thunderstorms to propagate far from the storm and affect Florida while the storm was still far offshore. SPoRT plans to investigate this further using satellite and surface-based observations combined with idealized modeling. Gridded NUCAPS was a highly valuable tool during the HRD map discussions, helping to characterize the dry air that prevented the storm’s intensification. It also shows promise in helping to diagnose structural changes like the development of squall lines on the periphery of a hurricane.

Back to Homepage

Technical Contact: Dr. Emily Berndt (emily.b.berndt@nasa.gov)
Responsible Official: Dr. Gary Jedlovec (gary.jedlovec@nasa.gov)
Page Curator: Paul Meyer (paul.meyer@nasa.gov)
Support     Disclaimer