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

National Weather Digest, Dec, 2003 by Joseph Rogash

d. Type IV pattern

Six flash flood cases (13%) occurred with the Type IV or "westerly flow" pattern. Figure 10 depicts the typical surface conditions which, as with most other flash flood situations, includes a thermal trough through western Arizona. However, a weaker surface trough is usually aligned south or southwestward through south-central or southwestern New Mexico, with high pressure centered over the central and southern Rockies between the Arizona and New Mexico troughs. In three of the six cases, the New Mexico trough was also collocated with a dry line. To the east, the westward portions of the Bermuda high extend across southern Texas, not unlike the flash flood patterns discussed previously. Easterly and southeasterly surface winds associated with this pattern again transport Gulf of Mexico moisture into the CWA, with dewpoints above 55[degrees]F over the lower elevations. Thunderstorm initiation is more favorable along the surface trough where low-level convergence and boundary-layer forcing of upward vertical motion is strongest.

The middle and upper troposphere are characterized by a westerly or southwesterly flow across the southern Rocky Mountains, with a short-wave trough or closed low embedded in the mean flow, approaching southern New Mexico and western Texas. The quasi-geostrophic forcing (via differential positive vorticity advection) along and east of the trough axis combines with low-level forcing induced by the surface trough and orography to initiate and sustain deep convection. More organized severe weather may also accompany thunderstorms with the Type IV pattern due to relatively strong low- to mid-tropospheric wind shear and relatively dry air at midlevels.

6. Discussion and Conclusion

Flash flood-producing convection poses an increasing threat to both life and property across the southwestern United States, despite the region having a desert or semiarid climate. However, forecasting heavy rains is especially difficult for this area due to the irregular terrain, the lack of data and the resultant poor performance of numerical models in determining the potential for heavy rainfall. There remains a need to determine environments conducive for flash flooding in order to assist forecasters with short-range excessive-rainfall prediction.

This study investigated flash floods over south-central and southwestern New Mexico and far western Texas for a 31-year period to determine climatological and meteorological aspects of flash floods over the region. It was determined that the overwhelming majority of cases occurred in the summer season during the afternoon and evening hours, in associated with the southwestern United States monsoon. Some common ingredients include an air mass that was usually at least moderately unstable and with a moisture content well above the climatological average; mean MUCAPES were 1500 J [kg.sup.1] and mean precipitable water amounts were 1.3 in. (33 mm) or 160% of normal. Cloud-layer winds were usually light (average 14 kt [7 m [s.sup.-1]), suggesting an environment favorable for slower moving thunderstorms with minimal entrainment.