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Research Projects » Hockley Downburst (April 2000)
Hockley Downburst
April 2, 2000
INTRODUCTION
On the early morning hours of April 2nd, a severe thunderstorm moved into northwest Harris County and produced large
hail and strong damaging winds in the Hockley area to the northwest of Houston. The downburst had wind speeds estimated between
110 and 120 mph which caused significant damage around the Hockley area. Mobile homes were damaged, powerlines were snapped,
and there were two injuries due to the storm. This summary will focus on the storm which caused the tremendous
straight line winds and the meteorological parameters which came together to produce such unusual and damaging weather.
SYNOPSIS
On Thursday March 30, an upper level low pressure system developed over the southwestern U.S. This scenario frequently
occurs during the cool season. What set this system apart from previous upper level lows was the associated low level moisture
fields, the position of the jet stream and the subsequent movement of the upper low. The upper level low drifted slowly south-
southwest Friday March 31 and Saturday April 1. Low level moisture which had been building over the western Gulf of
Mexico began to surge north late Friday night and Saturday. A weak warm front developed over the western Gulf and moved inland
across the Texas coast. As the upper level low inched closer, jet stream winds began to diverge over Southeast Texas. Air aloft
was being evacuated which required surface air to rise. Clouds increased rapidly on Saturday with a few showers developing by
late afternoon.
A vigorous disturbance embedded in the jet stream was visible in water vapor imagery. It was feared that this feature
would interact with the unusually high moisture levels, the warm front which was draped across Southeast Texas and favorable
upper level wind energy to produce a good chance of thunderstorms over the region Saturday night. A hazardous weather outlook
was issued by the Houston/Galveston National Weather Service office on Friday and again Saturday detailing the expected severe
weather threat. The first of two Severe Thunderstorm Watch boxes was issued by the Storm Prediction Center at 8 PM Saturday
night with the second watch issued at approximately 130 AM Sunday morning. The first of many Severe Thunderstorm Warnings
was issued at 1245 AM Sunday morning for San Jacinto and Polk counties. Additional thunderstorms developed west and southwest
of this area, quickly becoming severe with supercell characteristics. One of these supercells, which developed in western Waller
County, eventually became the devastating Hockley downburst.
At 132 AM, the Houston/Galveston National Weather Service issued a Severe Thunderstorm Warning for Waller and Harris
Counties. By definition, a Severe Thunderstorm Warning is issued when hail greater than 3/4 inch or wind gusts greater than 58 mph
are expected. Doppler radar indicated that this storm possessed severe characteristics as it approached Monaville in central Waller
County. The storm produce 3/4 inch hail in Monaville at 138 AM and wind damage along Highway 290 in the eastern half of the county
around 145 AM. The storm further intensified as it entered Harris County. Doppler radar indicated that the storm reached its greatest
intensity over extreme northwest Harris county between the communities of Hockley and Tomball around 2 AM CST (or 3 AM CDT).
The storm showed many characteristics which are commonly associated with downburst winds. The first was a strong
reflectivity core aloft. This is indicative of an extremely strong updraft. The second characteristic was a significant storm tilt
with height. This is indicative of strong inflow and potentially strong downdrafts. VIL (vertically integrated liquid measured in
grams per kilogram or g/kg) values are another tool used to determine the severity of a particular storm. The higher the
VIL value, the more likely it is that large hail or strong damaging winds will occur. Other severe thunderstorms early Sunday
morning which produced hail or strong winds possessed VIL values in excess of 55 g/kg or higher. This particular storm had VIL
values which approached 78 g/kg just prior to the downburst near Hockley. One of the benefits of Doppler radar is
the ability of the radar to measure wind speed and to detect rotation within a thunderstorm. Area radars did not indicate any storm
scale rotation with this cell.
Warning Products
Radar and Satellite Images
Damage Survey Reports
Prepared by NWSO Houston/Galveston Staff
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