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

National Weather Digest, Dec, 2003 by Joseph Rogash

4. Thermodynamic and Vertical Wind Profiles

From the constructed proximity soundings, critical data related to instability, moisture and wind were obtained and are presented in Table 2. Mean values of MUCAPE and best lifted index are 1500 J [kg.sup.1] and -5[degrees]C, respectively. Only six (13%) flash floods occurred where MUCAPES were less than 1000 J [kg.sup.-1], while only two events (4%) developed where MUCAPE exceeded 3000 J [kg.sup.-1]. Thus 40 cases (83%) of the events evolved within an air mass considered "moderately unstable," according to the criteria used by most operational meteorologists. It is speculated that one reason very few events occurred within a more highly unstable air mass is because such environments often include a mass of drier air (and attendant dry adiabatic lapse rates) in the middle troposphere, which would favor greater entrainment and a reduction in precipitation efficiency. In addition, high CAPE is associated with very intense updrafts that can decrease the precipitation efficiency of convection by reducing the residence time of water substance in the updraft. Other studies of flash floods (e.g., Maddox et al. 1980; Rogash 1988) have also suggested heavy rain events usually develop in an environment of weak to moderate instability.

Table 2 also shows abundant moisture present within the flash flood environment, with a mean (and median) PW value of 1.3 in. (33 mm). No flash floods were reported where the PW was less than one inch, and 90% of the events developed where the PW was at least 1.2 in (30.5 mm). On average, flash floods occurred where the PW was 160% of climatological normals. In particular, moisture content in the lower boundary layer was high, with surface dewpoints of at least 55[degrees]F (13[degrees]C) in a large majority of cases. Finally, both the mean and median K index values were 38[degrees]C, indicating ample instability and moisture availability for heavy rainfall in the majority of cases (Funk 1991). For a large majority of cases (81%), the K index was at least 35[degrees]C.

An examination of cloud layer winds shows average speeds were rather light at 14 kt (7 m [s.sup.-1]) with cloud layer winds less than 20 kt (10 m [s.sup.-1]) for 36 of the cases. This can be significant for several reasons. First, lighter cloud layer wind speeds indicate a propensity for slower-moving storms, allowing for an individual storm to drop more rainfall over a limited area. Second, lighter wind speeds within the cloud layer reduce the potential for entrainment or the evaporation of water droplets. This is especially important if the atmosphere surrounding the cloud has low relative humidities. Finally, stronger flow aloft can transport water droplets further downstream where they may evaporate elsewhere or fall over a broader area. Therefore, weaker flow generally contributes to higher precipitation efficiencies (Doswell et al. 1996) by allowing water vapor that enters the storm updraft a higher probability of condensing and falling to the ground in a relatively limited area, especially if the storm exhibits slower movement.