A Brief Climatology of Tropical Cyclones
in Texas (continued)

Rainfall.  By far, the most serious threat from a tropical cyclone to Texas residents is from flooding.  The worst aspect about tropical cyclones is that the weaker they are, the more efficient they can be at producing heavy rains and catastrophic flooding.  Systems with sprawling circulations also tend to make good rainmakers.  Claudette (1979), Allison (1989), and Charley (1998) stand out as recent examples of this fact.  Amelia (1978) shows that a system can meander around the state for over a week, creating headaches for residents anywhere between the High Plains and the Coastal Plain.  Any system, no matter its strength, can create major problems for the Lone Star State.

Tropical cyclones tend to exhibit different rainfall patterns, depending on the synoptic situation in which they are embedded, and the orography of the affected region.  Any pre-existing boundaries, such as frontal systems, and elevated terrain can focus higher rainfall amounts.  Outside of these effects, the maximum amount of rain is usually to the right of the storm track.  If the cyclone is a hurricane, the maximum will be near the coast, with lesser amounts inland.  Tropical storms usually have a double maximum in storm total rainfall; one will be near the coast, the other some distance inland (Schoner).  Weak tropical cyclones have produced some of the worst flooding in Texas History.  Below is a chart showing the highest rainfall amounts recorded for tropical cyclones and their remnants across Texas...only one of the heaviest amounts was from a major hurricane (Beulah in 1967).
 

In Schoner's paper on Texas tropical storm rainfall patterns, he notes a couple interesting exceptions for cyclones entering the coastline near and south of Matagorda Bay.  The two hurricanes in his 35-storm sample which struck Matagorda Bay have rainfall maxima just to the left of the track.  At a first glance, it is unclear why this would happen.  However, in a work by Gilbert and LaSeur prepared in 1956, an explanation becomes apparent.  While studying rainfall patterns associated with Hurricane Florence (1953), they noted a rainfall minimum east of the center, reminiscent of a dry slot around a mature, nontropical low.

In Texas, this situation would occur when a storm approached the Lower coast.  West winds through a deep layer of the troposphere wrap dry air around the south side of a modest to large-sized cyclone, initially.  This dry air comes from parcels which have descended adiabatically from the much higher terrain of northeast Mexico.  As dry air wraps around the east side of the center, residual moisture tends to pool on the north and west sides, leading to this uncharacteristic maximum just left of the track.

There are other impacts on the storm's structure due to this westerly flow off the continent.  In addition to the dry air intrusion, upwelling can occur as warm water near the coast is whisked away to the east and replaced by cooler water beneath the surface.  Major hurricanes such as Allen (1980) and Carla (1961) have weakened due to the decrease in latent heat produced, due to the aforementioned affects, effectively shutting down the heat engine tropical cyclones are known to be.  Also, Gilbert (1988) likely did not reintensify back to its prior category five status due to these affects.  Another important effect of dry air intrusion is a higher incidence of severe weather, such as microbursts, tornadoes, and hail.  Celia's microburst led to 170+ mph winds on the west side of the hurricane.  Beulah's 115 tornadoes may well have been due to dry air encircling the cyclone.

Benefits of tropical cyclones.  Of the 115 storms chronicled in this survey, ten are credited with alleviating drought conditions across the Lone Star State.  Without periodic invasions of tropical storms and hurricanes into Texas, summer rainfall would be a fraction of what currently falls near the coast.  This would be disastrous for cotton, corn, and rice grown statewide, as they are highly dependent on this added rainfall contribution.

Movement of tropical storms through Texas.  Two main tracks exist for tropical cyclones moving through the state.  Most enter from the southeast, and curve north and northeast through eastern and central portions of the state.  Storms that strike the lower coastline tend to stay on a more westward track, such as Corpus Christi's 1919 hurricane, Celia (1970), and Allen (1980).  This is due to the circulation around the perennial Mexican Plateau low/trough, which dominates the low level wind flow in south Texas through ten months of the year, including the hurricane season.  This low pressure area is present due to air at the same altitude surrounding the plateau being cooler than air on the plateau (the ground heats up the air more effectively).  This induces a "heat low" to form over northeast Mexico, similar to the way, but on a far smaller scale, than the low that develops over the Himalayas of southeast Asia, which creates epic monsoons in that portion of the world, and provides the mechanism for the creation of tropical waves over east Africa.

Early and late in the season, mainly June and October, storms can enter from the south or southwest, then move north and northeast across the Lone Star state.  Northerly and northeasterly tracks are induced by cold frontal intrusions that cause winds to become southerly across much of Texas during those months.  This explains why tropical cyclones from the eastern Pacific wield influence across Texas, even when their low level circulations have been destroyed by the Sierra Madre Occidental, the southern extent of the Rockies.  It also explains why there are very few October landfalls from Gulf cyclones, as very little of the coast lies north or northeast of the Gulf.

Fatalities.  At various places, in various ways, hurricanes have taken a number of victims from Texas.  In recent times, storms are taking far fewer lives due to education efforts from public and private sources, as well as timely warnings by the National Weather Service.  Unlike the old days, a hurricane will not hit the coast without the public being well aware of its presence, as much as a week ahead of time in some cases.  In many cases, two days of warning is as much as can be expected.
 

Long term trends/hurricane cycles.  Studies were made back in the 1950's by Dr. W. Armstrong Price on hurricane incidence along the Texas coast and the sunspot cycle. Regardless of whether or not it is due to sunspots or some other interannual climate cycle, using data back to 1829 that are periods in the climatological record of "hurricane-rich" and "hurricane poor" sets of years. A hurricane-rich set of year is represented by an average of 8 storms making landfall over an average of 10 years, plus or minus a couple of either. A hurricane-poor set of years is represented by an average of 2 storms making landfall over an average of 14 years, plus or minus a couple of either.

Using this pattern, he correctly predicted the hurricane-rich period he was entering in 1956 (it lasted from 1954-1971). Using this pattern, it is noted that the Texas coast has been in a hurricane-poor period since 1990. This would mean that at least one more landfalling hurricanes should be expected by around 2004. Thereafter, a hurricane-rich period would begin, lasting until approximately 2015, in which nearly eight hurricanes would make landfall. Remember, it only takes one hurricane making landfall in your location to create grief for you and your loved ones.