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Air masses clash over the central U.S.

urface weather map, early on Apr. 27, 2014.  Color codes are: red (warm); blue (cold); orange (dry); green (moist).
urface weather map, early on Apr. 27, 2014. Color codes are: red (warm); blue (cold); orange (dry); green (moist).
Plymouth State University

Meteorologists monitor air mass movements and transitions because these are often a clue to impending weather events. An air mass is a large volume of air that has similar characteristics across its areal extent. Typically, frontal formations and movements (fronts are the weather map features that separate air masses) showcase when and where significant air mass clashes and resulting weather will occur.

Early this Sunday morning (Apr. 27, 2014), the atmosphere across the central U.S. is primed for significant air mass interaction. Warm, humid air from the Gulf of Mexico is streaming northward, while colder Canadian air is starting to head south. Dry, desert air from the southwest U.S. is also entering the mix, heralded by a dry line (not strictly a front in terms of temperature, but definitely a dividing line between air masses of vastly different dew points). These three air masses are on a collision course as winds circulate around a storm system in southwestern Nebraska.

Perhaps most telling about how out of balance these air masses are involves a quick look at how weather variables have changed in a short period of time. At Oklahoma City, OK, for example, the 24-hour temperature and dew point changes are striking. Early this Sunday morning, the temperature is already 73 degrees at Oklahoma City. The dew point, sitting at 65 degrees, is 26 degrees higher than at the same time 24 hours earlier. Lubbock, TX, located in the northwest part of the state, showed a dew point swing from 50 degrees early yesterday to 17 during the late afternoon hours (15 hour time difference). With temperatures well up in the 80’s, the relative humidity at Lubbock bottomed out at 6 percent.

At upper levels, a negatively-tilted trough is moving out into the Plains states. This orientation (from northwest to southeast, as opposed to the more usual northeast to southwest) enhances upper level wind flow divergence, a factor in generating strong to severe thunderstorms.

So, given the upper level wind dynamics, low-level winds, temperature and moisture parameters and an unstable atmosphere across the region, it is no surprise that NOAA’s Storm Prediction Center (SPC) is concerned about a significant severe weather outbreak (see link for yesterday’s story). In addition to large hail and strong thunderstormwinds, this weather situation is favorable for the formation of some large, strong, long-lived tornadoes.

The outbreak really kicks off today and continues for the next several days across the Deep South. Yesterday was the teaser day with just a few reports of severe weather. Early this morning, significant thunderstorms had developed across parts of southwest Oklahoma and central and eastern Kansas. By later in the week, the system is expected to finally exit the Carolina coast.

Heavy rainfall and potential flooding remain a risk factor, as well. Three to five inch rainfall totals (with locally higher amounts) are anticipated across much of the same region that is expected to receive severe weather. Even well to the north of the severe weather zone, one to three inches of rain can be expected over a five-day rainy period.

© 2014 H. Michael Mogil

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