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Writer's pictureBryan Mroczka

"Unprecedented Rainfall: Analyzing the Sarasota Florida Flooding and Historic Rainfall Event of June 11, 2024"

Even Eddie Rabbitt would not have loved this particular rainy night. During the evening of June 11th, 2024, while most of the headlines were focusing on the flooding occurring farther south toward Miami and Fort Lauderdale, the Sarasota, FL vicinity experienced a historic and highly localized flood event of its own.


ComposiNWS Composite Radar Loop Covering the Entire Flooding Event
Image 1) Composite Radar Loop Covering the Entire Flooding Event

The Flooding Event Unfolds:


Periods of showers and thunderstorms had been common around Sarasota on the 11th, as the weather pattern was certainly unsettled, but the complex of training thunderstorms that moved overhead and ramped up between 4 and 5 PM EDT was something entirely different. Torrential rainfall continued unabated for several hours, before finally exiting eastward and inland shortly after 8 PM EDT. In this relatively short amount of time, most of the Sarasota area saw between 5 and 10 inches of rainfall. Combine the rainfall from this particular storm with other rounds of rainfall on the 11th, and a few localized spots in the Sarasota vicinity saw amounts topping 11 inches for the day. Image 1 shows the composite reflectivity loop of the entire flooding event from the NWS KTBW radar.


Doppler Velocity Loop from NWS TBW Radar
Image 2) Doppler Velocity Loop from NWS TBW WSR-88D Radar

The main culprit for the flooding was the presence of a compact, and nearly stationary area of low pressure spinning over the Gulf of Mexico, just west of the Sarasota County coast. Image 2 shows a doppler radar velocity loop from the KTBW radar, located in the upper right corner of the image. The cooler colors (greens->blues) show winds blowing radially toward the radar site, and the warmer colors (reds->orange) show winds blowing radially away from the radar.



Knowledge Nugget - Doppler Radar:


Doppler radar beam sampling only a portion of the true velocity when the target is moving in a direction the creates an angle > 0 from the radar beam
Image 3) Doppler radar sampling a target not moving along the radar beam. credit: NWS Jetstream Website

Doppler radar measures the wind speeds, or velocity component along a radial, or along the straight-line beam emanating outwards from the radar site. Therefore, the more perpendicular the direction the wind is blowing in relation to the radar beam/radial, the lower percentage of the actual windspeed is detected. A wind direction exactly perpendicular to the radial will measure a wind speed of zero, while a wind blowing in a direction exactly along the radial (either toward or away from the

radar) will measure 100% of the actual windspeed velocity. In the example shown on Image 3, the angle between the radar beam (blue arrow) and target movement (red arrow) will result in approximately 80% of the true velocity being detected and displayed by the radar.


In image 4, we utilize these doppler principles to analyze the thunderstorm complex setup. To the southwest of Sarasota, strong winds are detected blowing southwest to northeast, or toward the radar, over the Gulf of Mexico. However, if we look from around the city of Sarasota and northwestward to Longboat Key and beyond, we see mostly light gray colors. These light gray colors represent lighter wind speeds detected by the radar. The winds in this region are not actually as light as shown, but are blowing from the southeast to northwest, or a direction that is nearly perpendicular to the radial radar beam. As discussed above, this results in a detection of weaker winds than are actually occurring, but allows us to determine that a substantial wind shift has occurred in this region, compared to areas farther south/southwest.

The abrupt change in wind direction from southwest to east-south-east/southeast resulted in a "convergence zone", where air essentially "piles up" and is forced upwards. The rising air causes and maintains the thunderstorms along the vicinity of this convergence zone. An attempt to articulate the radar interrogation and spatial wind evolution described above is drawn on Image 4.


Meso-low and convergence zone forcing the flooding storms.
Image 4) Meso-low and convergence zone forcing the flooding storms. Dashed blue line shows the position of the convergence zone extending east form center of low pressure. Arrows indicate approximate direction of wind flow.


Storm-Relative Velocity loop during the flooding event
Image 5) Storm-Relative Velocity loop during the flooding event

The slow motion of the low pressure and convergence zone maintained the region experiencing the heaviest rainfall for several consecutive hours.

As we will see in the statistics section below, the greatest totals of rainfall occurred in a localized area from near Downtown Sarasota out to Lido Key and northern portions of Siesta Key. The rainfall outside of this localized area was still extremely heavy, even for Florida standards, but measurements inside this region were astonishingly rare, with recurrence or return intervals into the hundreds of years.



Storm Relative Velocity Image at 7:12 PM EDT on June 11th, 2024.
Image 6) Storm Relative Velocity Image at 7:12 PM EDT on June 11th, 2024.

What seems to have set these locations apart from other nearby regions, was their proximity to a persistent mid-level mesocyclone. Mesocyclone is just a fancy meteorological word for organized rotation within the storm. Rotation can enhances the strength of the updraft, causing more rising air, and heavier precipitation to fall in the vicinity. A loop of the storms-relative doppler velocities, showing this mesocyclone/rotation is shown in Image 5, and a single shot of the rotation may be viewed in Image 6. When you factor in the duration that this enhancement was present, to the already favorable setup for heavy rainfall over this region, it becomes apparent why the very heaviest rainfall totals occurred where they did.



A Deep Dive Into the Event Statistics:


The rainfall event on the evening of June 11, 2024 around Sarasota, FL was extraordinary by any measure, and this part of the blog will attempt to quantify the event based on all available data and reliable sources. Many stories and reports were quickly filed in the first 24 hours following this event that painted a picture of a "once in a thousand years storm". This headline grabbing value likely had roots in an unofficial rain gauge value approaching 12 inches from a very localized spot on Siesta Key. The unofficial report was then blanketed to the entire Sarasota area, which did not accurately depict what truly occurred that evening for most of those impacted. While not diminishing the extreme flooding that did occur, and the terrible and lasting property damage that resulted, expect for the one unofficial Siesta Key gauge report, the nature of the event spatially around Sarasota is generally a bit more modest.


Sarasota Florida 6-hour Rainfall Total During the Evening of June 11, 2024
Image 7) 6-hour MRMS Q3-Pass 2 Maximum Rainfall Total During the Evening of June 11, 2024

If we want to take a glimpse at the greatest abnormality of this event and longest potential recurrence/return intervals for this type of rainfall around Sarasota, the best measure from this particular event, is the 6-hour rainfall totals between 4 PM EDT and 10 PM EDT. Image 7 is taken from the 6-hour Multi-Radar-Multi-Sensor (MRMS) Q3 rainfall total analysis, which combines radar-derived rainfall totals with reliable rain gauge observations. As mentioned earlier this the blog, most of the area around Sarasota, FL experienced between 5 and 9 inches for this period, with a small area over the adjacent barrier Keys approaching 10 inches.


Sarasota Florida 6-Hour Rainfall Total Return Interval In Years from event on June 11, 2024
Image 8) 6-Hour Maximum Rainfall Total Return Interval In Years

If we take the same 6-hour rainfall total map from Image 7 and apply NOAA Atlas 14 Point Precipitation Frequency (PF) Estimates values to the shading, then the result is shown in Image 8. A majority of the Sarasota vicinity saw rainfall during this period with a return interval of between 10 and 100 years, while the downtown area and out to Lido / Siesta Keys, the return intervals jumped to between 200 and 500 years. Disclaimer: 1) NOAA Atlas 14 PF values can vary slightly from location to location across small geographic areas, and this map is intended to provide the viewer with a reasonable meteorological extrapolation from rainfall to recurrence interval. 2) Values are intended to represent land areas only, The values over the open Gulf of Mexico waters are more questionable, due to lack of historical observational data. Plus, these values are "less" important", as it is kind of hard to flood the ocean.



Image 9) one hour radar derived rainfall total from NWS KTBW between 6:38 PM EDT and 7:38 PM EDT
Image 9) one hour radar derived rainfall total from NWS KTBW between 6:38 PM EDT and 7:38 PM EDT

One Incredible Hour at KSRQ Airport:

While not coming in with a rainfall total for the event near the top of the charts for the Sarasota vicinity, Sarasota-Bradenton Airport (KSRQ) did experience one particular hour during this storm that was truly "record breaking". The rain roughly between the period 6:30 PM EDT and 7:30 PM EDT was intense, to say the least. The official ASOS at KSRQ record a phenomenal 3.93 inches during that one hour period. This total toppled the previous hourly rainfall record by nearly an inch. Normal amounts of rainfall at KSRQ for the entire month of June, in a region that is used to heavy summer rainfalls, is only around 7 inches. In this particular case, more than half that amount occurred in only one hour. Once again, while astonishing in terms of rainfall amounts for a single hour, the 1/1000 event that was immediately pinned to this observation was somewhat exaggerated, with the true recurrence interval between 50-100 years.


Image 9 shows the 1-hour radar-derived rainfall totals from the NWS KTBW radar. The darker magenta shading represents totals of 2-3", which are unusually high hourly values, even for Florida, without the understanding the the radar was underestimating the rainfall rate. The underestimation is understandable in this scenario, as the environment was straddling between the normal "Convective" Z/R relationship often utilized by Florida NWS offices to derive rainfall totals during the warm season, and the "Tropical" Z/R relationship. The Tropical Z/R will result in slightly higher totals over time, but is normally reserved for tropical cyclone operations, when that particular Z-R relationship is more certain to be the correct choice.


Below in Table 1 we summarize the maximum rainfall recurrence / return interval experienced at specific locations around Sarasota, FL for various time periods during the evening of the 11th.



1 Hour

3 Hour

6 Hour

Downtown Sarasota

50

100

200

Lido Key

50

100

200

Siesta Key

50

100

200-500

Sarasota-Bradenton Airport (KSRQ)

50-100

10

5

Table 1) NOAA Atlas 14 Point Precipitation Frequency (PF) Estimates  recurrence / return intervals (in years) for selected locations around Sarasota, FL utilizing MRMS Q3 multi-sensor analysis and measured rainfall gauge observations.


The rainfall event on the 11th of June 2024 is one that will not be forgotten anytime soon by the residents of Sarasota, FL. Many impacted by this storm continue the process of cleanup, trying to return their homes and businesses to a sense of normalcy. We wish all those impacts a speedy recovery and a chance to dry out, while dealing with a hurricane season that threatens the potential for more inclement weather ahead.


This case also underscores that necessity to research and acquire a full forensic meteorology understanding of a weather event's scope before assigning statistics and associated terms that may be incorrect, or at the very least unrepresentative of the larger region impacted.


At Streamline Weather Consulting, LLC we specialize in researching and producing forensic meteorology reports related to just these types of impactful weather events. Our clients often utilize these reports for legal purposes, or for the engineering preparation of infrastructure designs and improvement. Visit streamlineweather.com to learn more about how we can assist your business or firm uncover those detailed site-specific weather conditions for events such as the Sarasota Florida flood, or prepare for the inevitability of future inclement and impactful weather. Thanks for reading!


Supplementary Graphics:


In this supplementary section of this blog we show the MRMS Q3 rainfall total analysis and corresponding return interval maps, similar to the 6-hour maps, shown earlier, but now for the maximum 3-hour rainfall totals and maximum 12-hour rainfall totals surrounding the event.


3-Hour Maximum Rainfall Totals
Image 10) 3-Hour Maximum Rainfall Totals

3-Hour Maximum  Rainfall Total Return Interval In Years
Image 11) 3-Hour Maximum Rainfall Total Return Interval In Years


12-Hour Maximum  Rainfall Total
Image 12) 12-Hour Maximum Rainfall Total


12-Hour Maximum  Rainfall Total Return Interval In Years
Image 13) 12-Hour Maximum Rainfall Total Return Interval In Years

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