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

Abstract

of great depth convection that produces excessive rainfall and flash flooding artificial positions a threat to lives and peculiarity over south-central and southwestern novel Mexico and far western Texas, primarily during the summer monsoon season. Forecasting these phenomena are difficult across this region owed to the irregular terrain, the sparse data and the relatively poor performance of numerical archetypes in the prediction of heavy rain across the southwestern United States. This close attention therefore, examines meteorological aspects of flash flood-producing convection for this area above a 30-year period.

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

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1 Introduction

Although the climate of southwestern of recent origin Mexico and far western Texas is considered to be semi-arid or wild during the warm season the region not seldom experiences deep convection with attendant heavy rainfall and flash flooding. This is mainly owed 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 run that transports drier air masses into the region retreats northward, while the warm surface temperatures induce a broad area of depressed 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 novel Mexico and far western Texas, typically from late June into early September. Thus, the region is considered to have a monsoon period above the summer months (Adams and Comrie 1997; Wallace et al. 1999) with thunderstorms becoming relatively frequent

As discussed through Doswell et al. (1996), flash inundations occur within environments having certain characteristics or "ingredients" favorable for excessive precipitation. These ingredients include a high moisture satisfied a convectively unstable or buoyant air mass, a mechanism to lift the air mass to its horizontal of free convection, and cloud-layer wind and moisture profiles unfavorable for processe that restore precipitation efficiency, such as entrainment. of the like kind environments can even develop above the semi-arid or desert regions of the western United States, particularly during the summer month (Maddox et al. 1980)

A number of studies have explored down-reaching convection and heavy rain circumstances over the southwestern United States (eg Hales 1974; McCollum et al. 1995; Maddox et al. 1995) on the other hand these investigations have been primarily bear uponed with convection over Arizona. In contrast, little formal research has addressed flash flood-producing thunderstorms above southwestern New Mexico and far western Texas, a region within the shire Warning Area (CWA) of the El Paso National Weather Service Forecast Office (NWSFO; actually located at Santa Teresa, of recent origin Mexico [KEPZ], shown in Fig. 1) As will be demonstrated, heavy rain and flash flooding also posture 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 awaited to worsen in the coming years as the population continues to increase and the area undergoing urban unravelling expands.

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