The disturbance over the northern Gulf spawned late last week by the leftovers of an old frontal boundary is moving inland over south Texas this morning between Brownsville along the U.S./Mexico border and Corpus Christi. As we anticipated in yesterday’s newsletter, the disturbance made an attempt at further organization in the overnight hours and while coming ashore, but the last gasp wasn’t enough to meet the criteria of a full-fledged tropical depression.
Nevertheless, winds piling up along the coast around the low-pressure area are enhancing convergence and increasing heavy rainfall inland over south Texas. Grounds are parched here from the ongoing drought and for now guidance indicates the rain rates, despite being an impressive two to three inches per hour, should remain below what may otherwise necessitate the issuance of flash flood watches. Still, despite the dry preexisting conditions and need for rain, isolated flash flooding is possible through the lower Rio Grande Valley and Coastal Bend today from the slow moving system.
For those who follow the tropics closely (which likely includes everyone reading this far), it may not always be immediately clear why an area of low pressure is or – in the case of the Gulf disturbance – isn’t designated a tropical depression or storm. Two of the key criteria for tropical cyclone designation (the umbrella term for tropical depressions, tropical storms, and hurricanes) are both a closed surface wind circulation and a well-defined center. By yesterday morning, a low orbiting satellite about 500 miles up passing over the disturbance measured surface winds indicating a closed circulation. You can see this if you follow the direction of the winds on the satellite picture below.
Still missing, however, was a well-defined center to the swirling winds. Generally, forecasters consider a defined circulation to be when the major axis of the low-pressure center (the longest diameter of the oblong center) is less than 75 nautical miles and when the ratio of the major to minor axis (the shorter diameter of the oblong center) is less than two. In the case of the Gulf disturbance yesterday, the major axis of its circulation center was roughly 100 nautical miles across and the ratio of the major to minor axis was greater than two (95 divided by 40 or about 2.3), so on both accounts it failed to meet the standard of a well-defined center.
Although these requirements sometimes feel overly strict, they are rooted in science and important measures of the overall health of a developing system. Without a well-defined, closed circulation and organized deep convection (also missing yesterday), the processes that lead to further strengthening can’t begin in earnest. The official designation of a tropical cyclone by NHC is an important step, but the major ingredients were missing with this one.
Meanwhile, we expect yet another unusually quiet week in the Atlantic with no major disturbances on the horizon, an uncommon but blissful sight for middle August.