Meteorological aspects of south-central and southwestern New Mexico and far western Texas flash floods

National Weather Digest, Dec, 2003 by Joseph Rogash

Abstract

Deep convection that produces excessive rainfall and flash flooding poses a threat to lives and property over south-central and southwestern New Mexico and far western Texas, primarily during the summer monsoon season. Forecasting these phenomena are difficult across this region due to the irregular terrain, the sparse data and the relatively poor performance of numerical models in the prediction of heavy rain across the southwestern United States. This study, therefore, examines meteorological aspects of flash flood-producing convection for this area over a 30-year period.

Climatologically, it was found the vast majority of flash floods coincided with the southwestern United States monsoon season from late June through early September, during the afternoon and evening hours. The air mass for most events exhibited at least moderate instability, moisture contents well above normal, and low cloud-layer wind speeds. There were four distinct large-scale patterns that were associated with flash flood events, but a common feature was the presence of a surface thermal trough or "heat low" covering western Arizona, southeastern California and northwestern Mexico. The thermal trough supports a low-level easterly or southeasterly surface flow favorable for the advection of abundant moisture from the Gulf of Mexico into the region. In almost half of all cases, a weak surface front or trough appeared to play some role in storm initiation. While there was more variability in the large-scale middle and upper-tropospheric patterns, deep convection frequently developed near an advancing upper-level short-wave trough and/or in the left front or right rear quadrants of upper tropospheric jet streaks. Because these forcing mechanisms may be poorly defined or located in data sparse areas, close examination of satellite images is important in their detection.

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1. Introduction

Although the climate of southwestern New Mexico and far western Texas is considered to be semi-arid or desert, during the warm season the region frequently experiences deep convection with attendant heavy rainfall and flash flooding. This is mainly due to seasonal changes in the circulation across the southwestern United States during the early summer. Usually during late June into early July, the prevailing westerly flow that transports drier air masses into the region retreats northward, while the warm surface temperatures induce a broad area of low pressure across the surface of southern California, western Arizona and northwestern Mexico (Tang and Reiter 1984). This pattern evolution periodically supports the transport of moisture into Arizona, southern New Mexico and far western Texas, typically from late June into early September. Thus, the region is considered to have a monsoon period over the summer months (Adams and Comrie 1997; Wallace et al. 1999), with thunderstorms becoming relatively frequent.

As discussed by Doswell et al. (1996), flash floods occur within environments having certain characteristics or "ingredients" favorable for excessive precipitation. These ingredients include a high moisture content, a convectively unstable or buoyant air mass, a mechanism to lift the air mass to its level of free convection, and cloud-layer wind and moisture profiles unfavorable for processes that reduce precipitation efficiency, such as entrainment. Such environments can even develop over the semi-arid or desert regions of the western United States, particularly during the summer months (Maddox et al. 1980).

A number of studies have explored deep convection and heavy rain events over the southwestern United States (e.g., Hales 1974; McCollum et al. 1995; Maddox et al. 1995), but these investigations have been primarily concerned with convection over Arizona. In contrast, little formal research has addressed flash flood-producing thunderstorms over southwestern New Mexico and far western Texas, a region within the County Warning Area (CWA) of the El Paso National Weather Service Forecast Office (NWSFO; actually located at Santa Teresa, New Mexico [KEPZ], shown in Fig. 1). As will be demonstrated, heavy rain and flash flooding also pose major concerns and present significant hazards to residents in this particular area, especially in far western Texas, where the El Paso metropolitan area is located. The danger is expected to worsen in the coming years as the population continues to increase and the area undergoing urban development expands.

As this paper will explain, the meteorological patterns associated with flash flood events over the El Paso NWSFO CWA (henceforth designated as EPZ CWA) can have distinct differences from patterns associated with heavy rainfall over Arizona. For example, whereas the moisture source for the Arizona monsoon is primarily the Gulf of California (Hales 1974), in the lower boundary layer at least, moisture fueling flash flood-producing convection over the EPZ CWA comes most frequently from the Gulf of Mexico. Thus, forecasting techniques derived from previously cited papers will have limited applicability. Precipitation forecasts from numerical models also provide little practical assistance to operational meteorologists, due in part to the irregular terrain and the lack of available meteorological data over northern Mexico and the eastern Pacific, especially data related to moisture and winds aloft. Studies by Junker et al. (1992) and Dunn and Horel (1994) have illustrated the poor performance of NOAA/NEWS National Centers for Environmental Prediction (NCEP) models in predicting heavy rainfall over the southwestern United States.