A. Elevated Convection
Occasionally thunderstorms develop that have no obvious moisture or convergence source in the boundary layer over which they occur. These storms often form above the boundary layer along a frontal surface.
1. Colman (1990)
Colman (1990a), compiled four years of data and determined that most elevated thunderstorms occur between the front range of the Rockies and the Appalachians. They seem to peak in April and once again in September. Listed below are Colman's observations in his follow-up paper (1990b):
- Hydrostatic environment is stable and standard indices are little, if any, help.
- Strong frontal inversion.
- Stronger than normal shear.
- Mid tropospheric warm air advection.
- CSI may be a significant factor.
- Low level jet often acts as a convergence mechanism to help initiate thunderstorms above the boundary layer.
- Storms form in left exit region of the 850 mb wind maximum in cyclonically curved flow.
- Storms form in the right exit region of a 500 wind maximum in anticyclonically curved flow.
- Elevated thunderstorms form in a convectively stable environment and are most commonly a result of frontogenetical forcing in the presence of weak symmetric stability.
2. Grant (1995)
Grant (1995) investigated the atmospheric conditions during severe thunderstorm events that occurred north of an east-west oriented frontal boundary and came up with the following findings:
- Vast majority of reports were large hail (92%)
- The average distance north of the warm or stationary front was 143 miles.
- Environment is characterized by strong speed and directional shear in the lower and middle atmosphere. Average surface wind was from the east with an 850 mb wind from the south or southwest.
- Surface parcels were stable, but lifting 850 mb parcels produced an average CAPE of 700 J/Kg m, a Lifted Index of -3.1 and Total Totals of 52.
- 850 mb warm air advection and positive Theta-E advection were good forecast parameters.
- 500 mb jet was north of the severe weather area.
- Northeast quadrant of the 850 mb jet is a favorable location for severe development.
- Average cap for surface parcels was 2.7 C°.
- Cross sections of Theta-E showed the highest values and a decrease with height directly above the frontal inversion, suggesting convective instability.
Forecasters should look at individual soundings and recalculate stability indices above the frontal inversion. High mid level lapse rates (700-500 mb) > or = 6.5°C/km can support convection. Looking at non-traditional methods, such as analysis of gridded data or isentropic cross sections may be the key to improving the ability of forecasters to better anticipate elevated thunderstorm development (Mc Nulty 1993). Another excellent reference on elevated convection is Jungbluth and Kula (1997).
B. Dry Line
The dry line of west Texas, also called Marfa Front or Dew Point Front, can be an important severe weather feature to points much further east. A wide variety of severe weather can occur along and ahead of the dry line. Look for the following features:
- Warm dry intrusion from the surface to 700 mb
- Evaporative cooling
- Significant moisture advection through 850 mb
- Well defined upper level jet stream.
- If thunderstorms break through the cap, they will likely form in the maximum dew point gradient from dry to moist air.
- Formation along and 200 miles to the right of the upper level jet and from the maximum low level convergence downstream, to a point where the air is too dry to produce severe weather.
- Bulges in dry line. Convergence near the bulge can initiate severe weather.
- Storms form from late afternoon to mid evening, and normally last two to six hours. The activity may last longer if a dry line is driven by a 30 knot or greater jet from the surface through 700 mb (average layer wind speed).
- Favorable area for severe thunderstorm development is the triple point of the surface low, warm front, and dry line.
C. Northwest Flow Severe Weather
Northwest Flow (NWF) in the mid-troposphere has been noted by Miller to be responsible for the most destructive severe weather outbreaks of the summer (Johns 1982). Numerical models typically forecast limited precipitation with a large upper level ridge to the west of the forecast area. However, short waves, especially at 500 mb come off the front side of the ridge and produce large clusters of thunderstorms. Look for the following features:
- 500 mb long wave trough east (downstream) and long wave ridge west (upstream).
- 500 mb flow over the geographical midpoint is 280 degrees or greater.
- Troughs approaching the apex of the upper level ridge from 600- 400 mb.
- Optimum time is late afternoon to early morning hours.
- 77% of all NWF outbreaks include at least one tornado (Johns 1982). Outbreaks usually last 8 to 10 hours.
- NWF outbreaks are likely to repeat on several successive days (Miller 1972).
A Mesoscale Convective System (MCS) and the Mesoscale Convective Complex (MCC) produce a large spectrum of hazardous weather. This includes damaging winds, large hail, and tornadoes, but the most pronounced feature is torrential rainfall. Since heavy rain and flash flooding is the biggest culprit of these enormous features, an in depth discussion will not be included.
However, all operational forecasters should read or review Daly's (March 1998) two papers on MCS Propagation. Additional reading should include Rochette et al. (1996), Doswell et al. (1995), and Maddox (1980, 1979b).