Analyzing Moisture Availability

Moisture is an essential ingredient for the development of severe weather, and forecasters use a variety of resources to ascertain the amount of moisture available in the atmosphere.

Theta-E and Deep Moisture Convergence

Theta-E is meteorological shorthand for Equivalent Potential Temperature. Theta-E is the temperature that results when an unsaturated air parcel is brought dry adiabatically (by use of a Skew-T diagram) to 1000mb. Increases in dewpoint and temperature result in an increase in the value of Theta-E, therefore high values of Theta-E represent warm moist air. Since warm moist air is often associated with atmospheric instability, forecasters view regions of high Theta-E as ripe for the development of severe weather.

A review of the mean surface temperature on February 16, 2006 (below), shows that the lower Midwest was experiencing surface temperatures in the range of 15° to 20° C (60° F to 68° F), far above average for the middle of winter. In addition to unseasonably high temperatures, a vast region of high dew points was also present in the early afternoon of the 16th.

Mean surface temperature in degrees Celsius for February 16, 2006. Image provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado.

The chart of Theta-E at 11 AM EST on February 16th (below) displays a ridge of relatively high values stretching from the Gulf of Mexico into the lower Mississippi Valley. This warm moist air, sporting Theta-E values of 310-320° Kelvin was in stark contrast to the much lower values in the air mass behind the cold front sweeping southeasterly across Missouri. The stage was set for a clash between relatively cold, dry air behind the cold front and warm, moist air in front of it.

Surface Theta-E as of 16Z on February 16, 2006. Image from Cool Weather.

As the cold front approached southern Illinois and southeastern Missouri, forecasters at the SPC were analyzing the rate of change (advection) in Theta-E in the threatened area. A review of the chart from 20Z on the 16th (below) shows pockets of positive Theta-E advection (Theta-E was increasing) ahead of the frontal boundary. Positive values of Theta-E advection indicated that the air in the area was becoming both warmer and moister. Theta-E advection can also contribute to the destabilization of the air mass. The converging winds associated with the front are also clearly evident on the image.

Surface Theta-E Advection at 20Z on February 16, 2006. Image from the SPC.

Another method of determining the availability of moisture is to analyze a chart of deep moisture convergence such as the one below from 20Z on February 16, 2006. The values on the chart represent the result of a formula that combines the effect of moist air advection and converging winds. High values of moisture convergence correlate to areas of atmospheric instability and an increased potential for thunderstorm development. The highest values of moisture convergence were located in front of the approaching frontal boundary.

Deep Moisture Convergence at 20Z on February 16, 2006. Image from the SPC.

The warm moist air was delivered from the Gulf of Mexico by brisk southwesterly winds at 850mb (approximately 1,500 meters above the surface). The chart of mean 850mb vector winds on February 16th (below) shows winds speeds in excess of 45 knots (53.7 mph) over the mid-Mississippi Valley. There was no shortage fuel in the form of warm, moist air for the thunderstorms that developed in advance of the surface cold front.

Mean 850mb wind in meters per second for February 16, 2006. Image provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado.

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