Mayport, FL Tornado - July 14, 2007
Write-up by Andrew Shashy

During the afternoon hours of July 14, 2007, an F-0 tornado formed near Mayport, FL that rapidly developed near the coast.  The tornado climatology for the NWS Jacksonville county warning area (CWA) typically reaches a peak from March to April, with second maximums from June to July and also in September. The first peak in tornado activity is mostly due to squall line thunderstorms that push into the warning area with the latter one near the peak of hurricane season partly due to tornadoes associated with rainbands from hurricanes.  Records from the National Climatic Data Center show that from 1950 to 2006 that the Jacksonville CWA averages about 10 tornadoes per year.  This particular case occurred when there was minimal vertical shear with fairly typical summer thunderstorm activity but moisture and instability for this mid July event was slightly above average. 

The 12Z surface map shows the forecast area was north of a ridge of high pressure with a cold front located over central Georgia (fig. 1).  Sounding parameters indicated a very moist and unstable atmosphere with surface based cape (convective available potential energy) of around 3500 J/kg and lifted indices near -7C (fig. 2).  Using modified temperatures in the upper 80s and dewpoints in the mid 70s from around 1300 LDT (about an hour before the tornado occurred), the cape was on the order of 4000 j/kg and lifted indices around -8C.  The updated Storm Prediction Center (SPC) outlook, issued early in the afternoon, called for a slight risk of severe weather over the forecast area with damaging winds and marginally severe hail the main threats. Scattered to numerous thunderstorms developed by early afternoon and became strong to severe intensity over Northeast Florida. Thunderstorms increased in intensity as the approached the coastline with the most severe one moving into the Mayport and Atlantic Beach area of Northeast Florida around 1350-1400 LDT.

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Figure 1 Figure 2

Due to the nature of this tornado and atmospheric conditions, this tornado event can be classified as a landspout.  Landspouts are tornadoes that form along pre-existing boundaries as horizontal circulations are stretched and tilted vertically within the updraft region of rapidly developing thunderstorms.  Such tornadoes develop where low level instability can be maximized allowing the low level parcels to rapidly accelerate upwards. Based on temperatures and dewpoints at the time around 1300 LDT, low level (0-3 km) cape was on the order of 200-250 J/kg, which is considered rather high for low level cape.  This low level cape is highlighted in red in figure 3.  Landspout F scale ratings are almost always F-0 or F-1 range with rare occasions reaching F-2 intensity. According to military observers, the tornado initiated on the Mayport Naval Base around 1757 LDT, crossed through portions of the Mayport ship basin, and then dissipated around 1759 LDT.  The path length was about half a mile and its width was estimated about 100 feet.  A building had 12 windows blown out and suffered some roof damage. A baseball field also had damage to the bleachers and fencing with some other recreational equipment blown around or damaged.

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Figure 3

Radar Imagery

Radar images show rapid development as thunderstorm activity moved eastward into a more humid airmass as the sea breeze raised dewpoints into the mid 70s right along the coast (fig 4).  A few minutes before the tornado developed, the storm relative velocity image (fig. 5) indicated increasing inflow off the Atlantic of 20 to 30 mph (shown in green) while inland a few miles west southwest of Mayport winds were 30 to 40 mph outbound (red and pink).  This created sufficient low level horizontal shear to create a favorable environment for vertical circulations to develop underneath the severe thunderstorm over Mayport Naval Base.  Due to radar sampling limitations and the small size of the tornado, these events can prove hard to detect.

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