The focus during tropical cyclone events is typically on high winds, coastal storm surge, heavy coastal and inland rainfall, and possible flooding. However, if one is located far enough away from the storm’s circulation, atmospheric processes may lead to less rainfall. Such is the case for southwest Florida during the past few days. And while rainfall didn’t vanish entirely from the southwest Florida area, it certainly dropped off dramatically.
Fig. 1 shows the upper level wind flow as determined by weather satellite and radiosonde (weather balloon) data. Computer programs can track cloud elements and compute their motion (and, hence, winds). Combining radiosonde wind and temperature data and infrared satellite cloud temperature measurements, computer programs can assign winds into specific altitude bands.
Late on Jun. 21, 2017, it is easy to see this altitudinal variation near Tropical Storm Cindy (located just offshore from the Texas-Louisiana coast). Winds shown in green show a counterclockwise (low-pressure) circulation. Mid-altitude winds are tagged to be between 20,000 feet to 25,000 feet above ground level or at pressure altitudes of 350mb to 500mb. Pressure is measured in millibars (mb) and decreases as one goes higher in the atmosphere (due to less air above).
To the north and east of Cindy’s mid-altitude circulation, higher altitude winds (shown in blue) actually spin slightly clockwise (indicating a high-pressure system). This differential circulation pattern is often found in well-developed tropical storms and hurricanes.
This upper-level circulation leads to a ridge or high-pressure system to the east of the storm’s circulation. In this high-pressure system, sinking air dominates. This is in contrast to the rising air motion (and associated clouds and precipitation) within Cindy’s circulation. Note that precipitation can still extend quite a distance from the storm’s center, but is mostly confined within the low- and mid-level counter-clockwise circulation pattern (Fig. 2). As a result of these upper and lower level circulation considerations, southwest Florida experienced less cloudiness and less shower and thunderstorm activity during the past two days.
As Cindy landfalls, weakens and moves to the northeast (see Fig. 2 – the elongated shape of the storm’s circulation is oriented in the direction of future movement), the upper ridge over Florida will slowly weaken and a more usual daytime shower and thunderstorm pattern will return to southwest Florida.
© 2017 H. Michael Mogil
Originally posted 6/22/17
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