October 1998 Extreme Rains Over South Central Texas

A rain event that produced from 20 cm to over 75 cm of rain, October 17-19, 1998, occurred over a 17 countywide section of South Central Texas. Portions of a six-county area received over 50 cm of rain. This heavy rainfall event was a classic tropical/mid-latitide heavy-rain event. Synoptically, in a split flow aloft, a quasi-stationary, large-scale 700 to 300 mb trough was situated west of South Texas in the southern stream westerlies. Over the central and northern U.S., a progressive 700 to 300 mb trough was moving to the east. The strong low-level southerly winds that developed over South Central Texas from this synoptic pattern, tapped into deep tropical moisture to the south. The southwesterly winds aloft tapped into tropical moisture above 3000 meters from the Eastern Tropical Pacific Ocean. This pattern resulted in a nearly saturated atmosphere with precipitable water values from 5 to 6.25 cm.

The synoptic-scale pattern initiated favorable mesoscale ingrediants for heavy rain. From a mesoscale perspective, low-level warm air advection, low-level moisture convergence, orograhpic lift, radiational cooling of low-level cloud tops, and thunderstorm mergers initiated the convective complex in the pre-dawn hours of October 17. After several hours into the heavy-rain event, train-echo effects, latent heat feedback in a saturated atmosphere where little or no evaporation could occur, and merging of mesoscale boundaries combined to produce the extreme rain during the late morning and afternoon hours.

On the night of October 17 and early morning hours of the October 18, a slow-moving weak cold front moved across the area and helped initiate a new period of rainfall. After the cold front moved through the area during the day of October 18, rain subsided temporarily. On the night of the October 18 and early morning of October 19, the final period of moderate to heavy rain occurred. This last rain period was supported by lift ahead of and with the passage of a 500 to 300 mb trough moving across the area from the southwest, and by isentropic lift above the shallow cool front below 1525 meters. After the 500 to 300 mb trough passed the area, more stable enviromental conditions followed, as the cool dome of air deepened and a weak ridge developed at 500 mb.