Tropical Storm Fred originated as a tropical wave out of the west coast of Africa on August 5, 2021, travelling through the East Caribbean sea and making landfall as a tropical depression over Santo Domingo on August 12, 2021. As it moved northwestward, it weakened to a tropical wave as it passed over northern Cuba. However, the system re-organized into a tropical storm as it passed over the Gulf of Mexico, before heading northward for the Florida Panhandle. It made landfall on the Panhandle of Florida on August 16, 2021, at 20 UTC (4:00 pm EDT). This is when the storms maximum wind speed of 65 mph and minimum pressure of 992 mb were recorded. After making initial landfall, the system quickly weakened back into a tropical depression and eventually became an extratropical low after it propagated northeastward through Georgia before dissipating off the east coast of Massachusetts on August 20, 2021. Tropical Storm Fred resulted in 7 fatalities and $1.3 billion in damage with 36,000 customers without power in Florida. Some cities along the Panhandle coast reported between 8-12 inches of rain in 24 hours.
Figure 1: Projected Storm Track for Tropical Storm Fred, originating August 14 at 8:00 am to August 19 at 8:00 am. The symbols represent the classification of Fred as it moves into the Panhandle. Circulation symbol with “L” in the center over Eastern Cuba and Alabama represents Tropical Depression. Symbols along western coast of Florida signify Tropical Storm. “L” symbols over the southeast represent extratropical low
An interesting phenomenon of this system is its continuous reclassification due to the constant strengthening and weakening throughout its life span. Factors that can affect the structural integrity of a tropical storm include land interactions, sea-surface temperature, and vertical wind shear. Typically, a tropical system will weaken as it passes over land, as it is no longer taking advantage of warm moist ocean air. If the sea surface temperature is around 27 degrees Celsius or above, as was the case over the Gulf of Mexico, the tropical cyclone can strengthen.
The third factor is vertical wind shear, which is the difference in wind speed and/or direction between two atmospheric layers. For a tropical cyclone to strengthen, it needs to have no to “low” amounts of shear, as vertical wind shear can destabilize the center of a cyclone. This is because if the wind speed or direction in the top layer of an air column is inconsistent with the bottom layer, the system can become asymmetrical or lopsided. The shear scale is as follows:
“No Shear”: 0-5 m/s
“Low Shear”: 5-10 m/s
“Shear”: 10-15 m/s
“High Shear”: Greater than 15 m/s
Figure 3: 850-200 mb Wind Shear (m/s) plot for August 15, 2021. Darker blue colors represent lower shear values, warmer colors represent higher shear values. Wind vectors represent direction of wind between 850 mb and 200 mb.
Credit: NOAA Physical Sciences Laboratory
Figure 3 shows the wind shear between 850 mb and 200 mb. The tropical storm was located just south of the Florida Panhandle on August 15 (shortly before making landfall). This region has vertical wind shear values of 15 m/s which is a considerable amount of shear, enough to destabilize the structure of our tropical cyclone. The reason we do not see an immediate weakening of the tropical storm, and rather a strengthening, is because of the warm, moist air from the Gulf of Mexico fueling our system, which counterbalances the negative effects from vertical wind shear.
Satellite imagery can tell us what is occurring inside the eye of the system:
Figure 4: GOES-16 EAST Satellite Imagery Loop on August 16 from 11 UTC-19 UTC . Uses two channels: Visible “Red” and “Clean” Infrared Window. Measures cloud top brightness temperatures. Rainbow colors represent higher temperature values, black/gray colors represent lower temperature values.
Credit: National Weather Service
We observe on this loop that the center/eye of the storm has lower cloud-top brightness temperatures (as seen in the black color). Cloud-top brightness temperature tells us the temperature of the top of a cloud, and the rate at which it cools can provide information on updraft strength and convection. Fred’s center had a cloud-top brightness temperature of about -80 degrees Celsius just before landfall. In this area, we notice “bursts” of white color on the satellite loop. These are bursts of convection associated with the colder temperatures within the eye of the tropical storm, providing ample fuel to the system.
Radar Reflectivity data can be used to show precipitation intensity:
Figure 5: Radar Reflectivity Loop on August 15 from 12 UTC-14 UTC . Higher reflectivity factor (in dBz) indicated by warmer colors; lower values indicated by cooler colors.
Radar reflectivity represents the power returned to the radar after reflecting off of precipitation. This color bar shows dBz (decibels of reflectivity) values, which measure the strength of reflectivity, so a larger dBz value indicates stronger precipitation in that region. The maximum dBz value here is around 54 dBz associated with the orange/red color, which corresponds to where we saw intense precipitation.