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Research Projects » Drought Analysis of Southeast Texas (1998)
DROUGHT ANALYSIS OF
SOUTHEAST TEXAS: PAST AND PRESENT
The rainfall deficit over Southeast Texas received some much needed relief in
June. At Houston's Intercontinental Airport, over eight inches of rain (8.38)
fell in June. This total was 3.42 inches above the normal June monthly average.
However this did little to relieve the drought-like conditions over the area.
The year-to-date rainfall total is 12.38 inches, which is 10.20 inches below
normal. In comparison, over the first six months of 1996, more than two-thirds
(67.7%) of the yearly precipitation fell in the month of June.
However, Houston was not the only location that received a disproportional amount
of rain in June. Twelve stations across Southeast Texas reported more rain in
June than in the previous five month period. Six of the twelve locations were
in and around the Houston Metropolitan Area. Another five locations were located
along the coastal plains in the Colorado and Lavaca-Navidad River Basins, with
one lone station in the Brazos Valley. Even with the widespread and generous
rainfall for June, every reporting station in the Southeast Texas Area continued
to show below normal annual rainfall totals.
Even with the ten inch deficit of rain at Houston's Intercontinental Airport,
the first six months of 1996 were not at record breaking dry level. However, 1996
does rank in the top ten for the driest January to June periods ever. The driest
occurred in 1917 when only 8.84 inches of rain fell (June 1996 recorded 8.38 inches by
comparison). The lowest yearly total occurred in 1917 when 17.66 inches fell. On
our current pace, 1996 would put Houston's Intercontinental Airport in the middle of
the pack when comparing the driest years.
Houston may be dry, but other areas across Southeast Texas have been hit harder. Sealy
and Madisonville both reported rainfall accumulations at about one-fourth of their normal
amounts. Less than five inches of rain has fallen at Sealy in the first half of the year
(less than one inch per month).
What conditions must be present to generate a drought over Southeast Texas? The
most significant factor for 1996 was the conditions in the upper levels of the
atmosphere. Over much of this year, a strong upper level ridge anchored itself
over the southwestern U.S. initiating several weather related patterns over
Southeast Texas. First and foremost, upper level highs inhibit thunderstorm
development, effectively capping off the atmosphere. Also, any disturbances in
the atmosphere will tend to track around the edge of an upper level ridge (high).
With the position of the upper level ridge over the southwest and with Southeast
Texas remaining on its eastern periphery, many possible weather events slid north
or east around the ridge and missed Southeast Texas. By comparison, areas of
Louisiana have received significantly more rainfall over the calendar year 1996
than areas across the border in Texas.
The question arises: How does a drought end? For Southeast Texas, droughts are
typically broken (if at all) in one of two ways: significant rainfall may be
generated either by local convection (thunderstorms) or tropical cyclones. Due
to the close proximity in relation to the Gulf of Mexico, it would seem intuitive
that the most likely resolution to a drought would be a tropically driven event
event (depression, storm, or hurricane). But the facts are surprising: Through
the period of May to September, the most advantageous time to relieve a
winter/spring drought, the greatest number of rain events are produced by sub-
synoptic scale convective systems such as seabreezes, outflow boundaries, and
weak upper level disturbances. Since 1900, there have been 33 monthly rainfall
totals in excess of 10 inches between May and September. Tropical cyclone
events, which bring widespread rains, generated only eleven of the 33 events.
Convective events (which tend to be less widespread goegraphically) generated
another 21 events. The remaining case (in 1958) was the result of ordinary sub-
synoptic scale convection in conjunction with the tropical rains associated with
Hurricane Ella, but neither the local convection or the tropical cyclone could
be fingered exclusively to the drought relief. The large rainfall accumulations
for June 1996 were also the result of local convective systems. Thus by a margin
of two to one, Southeast Texas drought relief in the late spring and early
summer will be the result of local convective systems rather than tropical
cyclones.
The ten driest January-June periods tend to follow this typical pattern. If the
drought has broken at all, it has usually been by convective systems: On three
separate occasions (1917,1901, and 1988), the dry weather pattern continued
throughout the year. 1954 had some tropical influence in breaking the drought
and in August 1915, a hurricane struck the upper Texas coast bringing an end to
drought conditions. The other four years (1937,1916,1971, and 1933) had the
drought conditions broken by convective means (1996 is the tenth year with the
outcome obviously in doubt). Less than one-quarter of the dry January-June
periods were rescued by tropical activity.
Please forward all questions/comments/suggestions concerning this paper to:
James Maxwell or
Greg Waller.
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