Wake Low Wind Event, May 3, 2007
Jim Wells, Kleberg and Nueces Counties
During the early morning hours of May 3, a weakening line of showers and thunderstorms moved from northwest to southeast across the Coastal Bend region. Within 10-20 minutes after the line of showers and thunderstorms moved east of locations across the southeastern Coastal Bend region, surface winds gusted between 50 and 60 mph. When winds reach 60 mph or greater, some minor damage can result, such as: breaking large tree limbs, downing small trees, and even roof damage to mobile homes. Winds 60 mph or greater are considered severe by National Weather Service standards.
Most people are probably accustomed to experiencing high winds within a strong or severe thunderstorm, tornado, or hurricane. However, the high winds that occurred during this event developed well behind the dissipating line of showers and thunderstorms. This phenomenon is known in meteorology as a "wake low" high wind event. The term "wake" refers to an area of lower pressure that forms behind, or in the wake of, the shower and thunderstorm activity.These events are considered quite rare and have been observed to occur approximately once every 2 to 3 years over south Texas.
Observed Winds and Pressures
For this case, the strongest winds began around midnight CDT at Alice, and then translated eastward into the Corpus Christi area shortly after 2 am. Strong winds finally moved over Bird Island and Bob Hall Pier on northern Padre Island around 3 am. The entire wind event persisted approximately 1 to 2 hours at each location, with the strongest wind gusts lasting for 20 to 30 minutes. Table 1 shows the peak wind gust and the time of occurrence for several locations across the impacted region.
In addition to the strong winds, observations from each location in table 1 clearly showed a sharp drop in surface pressure as the winds increased to their maximum value.In general, pressure drops of 7 to 9 millibars were recorded over a two hour period. This sharp pressure fall over a relatively short amount of time created the "wake low" region. Since air likes to flow from higher to lower pressure, these rapid pressure falls caused the air to accelerate toward the "wake low" pressure region. This accelerating air produced the strong winds. Click on the map locations below to view time series plots that show the relationship between wind speeds and pressures. Note how each location showed a very similar pattern, with the lowest pressure coinciding with the highest wind gust.
Doppler Radar Observations
Doppler radar velocity imagery also indicated max wind speeds in excess of 50 mph moving eastward from the Corpus Christi International Airport into Flour Bluff, then across Padre Island. The following is a Doppler radar loop of base velocity, which tracks the evolution and movement of the highest wind speeds.
The strongest winds diminished after a trailing mesoscale (meaning small-scale) boundary moved through the Coastal Bend. The following is a Doppler radar loop of base reflectivity, which shows the progression of the mesoscale boundary.
Some damage was reported by local media over Flour Bluff, such as shingles blown off roofs. No injuries associated with this event were reported through the time of this write-up. There are a few possibilities for why there were not more reports of damage with this event.First, the strongest winds occurred between midnight and 3 am, a time when most residents are sleeping. Second, the strong wind gusts came from the southeast, which is the prevailing wind direction for the Coastal Bend region. Therefore, structures, trees, etc. may have been hardened a little more against the stresses of wind from a southeasterly direction.