Tragedy Strikes The Dauphin Island Race

Anatomy Of A Downburst

Thunderstorm downdrafts form when precipitation particles (water droplets, ice crystals, hail, etc.) become too heavy to be suspended by the storm’s updraft. The downdraft often mixes momentum from stronger winds aloft towards the surface, but typically this transfer of momentum doesn't account for the remarkable winds that characterize a downburst. The graph below (Figure 16) shows wind speeds from the surface to a height of 4,373 west of Mobile Bay, at 1:00 pm (1800Z). The winds barely reached 40 knots in the lowest 4,373 meters -- much less than the 60 to 70 knot gusts that would be observed as the severe thunderstorms crossed Mobile Bay.

Figure 16: Wind speeds from the surface to a heigh of 4,373 meters west of Mobile Bay at 1:00 pm (1800Z) on April 25, 2015.

As the precipitation particles fall, the melting of ice crystals and the evaporation of water droplets during descent serve to cool the air under the storm which frequently acts to dramatically increasing the intensity of the downdraft. Meteorologists refer to this region of cooler air under a thunderstorm or a cluster of thunderstorms as the cold pool. The strength of a cold pool is determined by its temperature (how cold it is), geographic size, and depth (height above the ground).

Evaporative cooling associated with a downdraft can be an effective method of creating, maintaining, and strengthening a cold pool and thereby increasing the potential for the thunderstorm to produce damaging wind gusts. Evaporative cooling is strongest when the downdraft passes through a layer of relatively dry air on its way to the surface. A layer (or layers) of dry air in an air column can be easily recognized by analyzing values of relative humidity. The graph below (figure 17) shows the derived relative humidity west of Mobile Bay, from the surface to a height of 4,373 meters, approximately 2 hours before the downburst swept across the Bay. (The full sounding can be viewed by clicking here.)

Figure 17:Relative humidity west Mobile Bay at 1:00 pm (1800Z) on April 25, 2015. Click here for an annotated version or here for an alternate view of relative humidity west of Mobile Bay, AL.

From the surface to a height of nearly 750 meters, relative humidity was high with values of approximately 80% or better. However, above 750 meters, the relative humidity quickly decreased, particularly in the layer between 1,000 and 1,750 meters, where the relative humidity was 45% or less (click here for an annotated graph). This layer of dry air promoted evaporational cooling and helped to strengthen the cold pool under the storm cluster as it approached Mobile Bay.

The much colder temperature associated with the passage of the cluster's cold pool was recorded by the automated weather stations surrounding Mobile Bay. For example, the graph of barometric pressure and temperature from the Middle Bay Light (Figure 18) shows a dramatic decrease in temperature and a rapid increase in barometric pressure marking the arrival of the downdraft shortly after 3:00 pm (2000Z).

Figure 18: Barometric pressure and air temperature from NDBC MBLA1 (Middle Bay Light) on April 25, 2015. Click here for a larger version.

While the temperature decrease associated with evaporational cooling is understandable, the corresponding increase in barometric pressure may be a little surprising. Although air is challenging to weigh in small quantities, air molecules have mass and, therefore, weight. Barometric pressure is simply the weight of the air molecules above a particular location. As the temperature of the air in the cold pool decreases, the barometric pressure rises, resulting in a bubble of higher pressure trailing the leading edge of the thunderstorm cluster. The magnitude of the pressure increase is related to the amount of evaporative cooling that has occurred under the system.

Doppler velocity imagery, a special type of imagery that displays the motion (speed and direction) of raindrops, hail, or other objects being carried along by the wind. The two-panel radar image from 2:19 pm (1919Z) below (Figure 19) displays base reflectivity on the left and base velocity on the right. The scale for the base velocity panel is located along the left side of image. The scale is in knots with 120 knots at the top, -120 knots at the bottom, and 0 located at the mid-point. Positive values of radar velocity represented by bright reds and yellows indicate outbound winds relative to the radar site, while negative winds represented by greens, blues and purple show were the winds are inbound (click here for an annotated example).

Special Note: Due to the curvature of the Earth and the behavior of radar beams, the height of the beam increases as the distance from the radar increases. Except for those thunderstorms located very close to the radar station, the Doppler velocity data does not indicate the speed of the surface wind.

Figure 19: Two-panel radar image from NWS Mobile showing base reflectivity (left) and base velocity (right) at 2:19 pm / 1919Z. Click here for an annotated version or here for an animation of images from 2:19 pm (1919Z) to 3:08 pm (2019Z).

A review of the base reflectivity and base velocity radar data from 2:19 pm (1919Z) to 3:08 pm (2008Z) suggests the approaching thunderstorms were producing strong downdrafts for at least 30 to 40 minutes prior to reaching Mobile Bay (click here for an animation of the data). This is particularly noticeable with the storms located northwest of the NWS Mobile radar station (labeled as KMOB on the imagery) early in the time period.

The base velocity panel of the 3:08 pm (2008Z) radar image (Figure 20) shows several areas of outbound wind (bright reds) representing speeds of 50 to 60 knots (click here for annotated version) between 750 and 1,000 above the surface just west of Mobile Bay associated with the downburst poised to blast across the Bay.

Figure 20: Two-panel radar image from NWS Mobile showing base reflectivity (left) and base velocity (right) at 3:08 pm / 2008Z. Click here for an annotated version.

The series of velocity radar images from 3:08 pm (2008Z) to 3:21 (2021Z) (figures 21 to 26) show the downbursts moving swiftly southeast across the Bay. One of the downbursts passed directly over the Middle Bay Light (NDBC MBLA1) where 63.4 knots was recorded.

Figure 21: Base velocity at 3:08 pm/2008Z from KMOB. (Larger version)
Figure 22: Base velocity at 3:10 pm/2010Z from KMOB. (Larger version)
Figure 23: Base velocity at 3:14 pm/2014Z from KMOB. (Larger version)

Figure 24: Base velocity at 3:16 pm/2016Z from KMOB. (Larger version)
Figure 25: Base velocity at 3:19 pm/2019Z from KMOB. (Larger version)
Figure 26: Base velocity at 3:21 pm/2021Z from KMOB. (Larger version)

Beginning at 3:14 pm (2014Z) and continuing for the next several minutes, a barrage of damaging wind gusts streaked across Mobile Bay. The array of automated weather stations monitoring conditions around Mobile Bay captured numerous very impressive wind gusts:

Storm- and hurricane-force winds reached nearly every corner of the Bay and left the fleet in tatters. There are no automated wave height monitors on the Bay, but several participants reported that the wave heights during the peak of the event reached six to eight feet. The one-two punch of extreme winds and high waves overwhelmed many boats, resulting in several capsizes and causing many participants to fall overboard. Tragically, six sailors would lose their lives during the onslaught.

A base reflectivity radar loop from 2:00 pm (1900Z) to 4:00 pm (2100Z) showing the approach of the thunderstorm cluster with an overlay of NWS warnings can be viewed here.