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What is it? The Dry Line
April 1, 2022 at 05:01 PM EDT
By WeatherBug Sr. Meteorologist, Chad Merrill
Deadly and destructive thunderstorms occur every spring across the southern Plains, but did you know besides fronts, another unique weather feature drives these major severe weather outbreaks?
The Dry Line is the boundary where moisture-starved Desert Southwest air meets warm, humid air streaming northward from the Gulf of Mexico. It typically develops in west Texas or the Texas and Oklahoma Panhandle ahead of low pressure centers that push east of the Front Range. The Dry Line migrates east with the low pressure sometimes pushing all the way to the Midwest and Deep South during major severe weather outbreaks.
The boundary develops as the hot and dry air from the northern Mexico plateau and the Desert Southwest spreads eastward. Dry air is much heavier than the more buoyant, humid air found over the rest of Texas and the southern Plains. When the two air masses clash, they set off explosive thunderstorms.
Where fronts can be easily discernable because they bring noticeable changes in temperatures, the only way to spot a Dry Line is to observe the dew point temperatures, which is a good indication of the air’s moisture content.
Ahead of a Dry Line, dew points can be in the 70s while behind it, dew points drop into the 30s. Similar to a front, cumulus clouds first appear in the afternoon along the Dry Line that quickly become cumulonimbus clouds and then produce thunderstorms. You’ll notice the Dry Line has moved east of your location because the air won’t have that “muggy” feeling after the storms have passed.
March and April are the prime months for Dry Lines to develop ahead of developing low pressure careening out of the Rockies into the Plains. However, they have been observed in the southern Plains as early as late winter. Large storm systems are needed to help push the dry, desert air into the warm, humid Plains, creating an atmosphere prime for the development of a Dry Line.
Outside of the spring season, the weather pattern doesn’t favor low pressure systems that sweep off the Front Range into the Plains.
Through the summer and early fall, the zone of upper-level winds, known as the jet stream, push north into Canada. This allows the storm track to shift north across the U.S.-Canada border. A more northern storm track keeps the Desert Southwest air from pushing into the southern Plains, thus Dry Line development ceases to exist.
Even if an out-of-season low pressure pushes out of the Southwest or Rockies during this time, there’s another factor inhibiting Dry Line formation. The Southwest U.S. begins to see increased thunderstorm activity in July that carries through early fall. Known as the Southwest Monsoon, the uptick in moisture from the tropical Pacific causes the normal arid Southwest air to become more humid.
A low pressure careening out of the southern Rockies this time of the year would push the humid, desert air into the southern Plains where it would meet warm, humid air trickling north from the Gulf of Mexico. The air masses would be quite similar in nature and a Dry Line would not develop.
The Dry Line can produce notorious thunderstorm outbreaks. One such example is the Tri-state tornado outbreak of March 1925. In this instance, the Dry Line moved from the Plains all the way into southern Illinois. Six hundred and ninety-five people died—a record for a single tornado—when an F5 touched down near Ellington, Mo., and dissipated near Petersburg, Ind. The “Super Outbreak” of April 3-4, 1974 produced 335 tornadoes in the Midwest, Ohio and Tennessee Valley and Southeast. A Dry Line in advance of a cold front pushed east from central Oklahoma and Texas to the Southeast and Midwest in this outbreak. Almost half of the deaths occurred in the Ohio Valley; the remaining deaths happened in the Midwest, Tennessee Valley and Southeast.
In more recent memory, a storm system swept from the Rockies to the central Plains and produced a Dry Line that helped trigger an F-3 tornado near Brady, Neb., that injured two people on May 17, 2000. Not be outdone, the deadly tornado outbreak in Moore, Okla., on May 20, 2013 was due to a Dry Line preceding a cold front that swept across the southern Plains. More than 200 died and 24 people were injured from an EF-5 tornado that pushed through Moore.
While the typical cold and warm fronts that are often talked about are important, the Dry Line can be another culprit for active weather. During the spring, try and listen to see if meteorologists use the word Dry Line. If it’s mentioned, that may be a notice to stay alert. Dry Lines can produce some of the most fascinating, yet dangerous weather.
Story Image: A thunderstorm near Childress, Texas. (VORTEX II via NOAA Photo Library)
The Dry Line is the boundary where moisture-starved Desert Southwest air meets warm, humid air streaming northward from the Gulf of Mexico. It typically develops in west Texas or the Texas and Oklahoma Panhandle ahead of low pressure centers that push east of the Front Range. The Dry Line migrates east with the low pressure sometimes pushing all the way to the Midwest and Deep South during major severe weather outbreaks.
The boundary develops as the hot and dry air from the northern Mexico plateau and the Desert Southwest spreads eastward. Dry air is much heavier than the more buoyant, humid air found over the rest of Texas and the southern Plains. When the two air masses clash, they set off explosive thunderstorms.
Where fronts can be easily discernable because they bring noticeable changes in temperatures, the only way to spot a Dry Line is to observe the dew point temperatures, which is a good indication of the air’s moisture content.
Ahead of a Dry Line, dew points can be in the 70s while behind it, dew points drop into the 30s. Similar to a front, cumulus clouds first appear in the afternoon along the Dry Line that quickly become cumulonimbus clouds and then produce thunderstorms. You’ll notice the Dry Line has moved east of your location because the air won’t have that “muggy” feeling after the storms have passed.
March and April are the prime months for Dry Lines to develop ahead of developing low pressure careening out of the Rockies into the Plains. However, they have been observed in the southern Plains as early as late winter. Large storm systems are needed to help push the dry, desert air into the warm, humid Plains, creating an atmosphere prime for the development of a Dry Line.
Outside of the spring season, the weather pattern doesn’t favor low pressure systems that sweep off the Front Range into the Plains.
Through the summer and early fall, the zone of upper-level winds, known as the jet stream, push north into Canada. This allows the storm track to shift north across the U.S.-Canada border. A more northern storm track keeps the Desert Southwest air from pushing into the southern Plains, thus Dry Line development ceases to exist.
Even if an out-of-season low pressure pushes out of the Southwest or Rockies during this time, there’s another factor inhibiting Dry Line formation. The Southwest U.S. begins to see increased thunderstorm activity in July that carries through early fall. Known as the Southwest Monsoon, the uptick in moisture from the tropical Pacific causes the normal arid Southwest air to become more humid.
A low pressure careening out of the southern Rockies this time of the year would push the humid, desert air into the southern Plains where it would meet warm, humid air trickling north from the Gulf of Mexico. The air masses would be quite similar in nature and a Dry Line would not develop.
The Dry Line can produce notorious thunderstorm outbreaks. One such example is the Tri-state tornado outbreak of March 1925. In this instance, the Dry Line moved from the Plains all the way into southern Illinois. Six hundred and ninety-five people died—a record for a single tornado—when an F5 touched down near Ellington, Mo., and dissipated near Petersburg, Ind. The “Super Outbreak” of April 3-4, 1974 produced 335 tornadoes in the Midwest, Ohio and Tennessee Valley and Southeast. A Dry Line in advance of a cold front pushed east from central Oklahoma and Texas to the Southeast and Midwest in this outbreak. Almost half of the deaths occurred in the Ohio Valley; the remaining deaths happened in the Midwest, Tennessee Valley and Southeast.
In more recent memory, a storm system swept from the Rockies to the central Plains and produced a Dry Line that helped trigger an F-3 tornado near Brady, Neb., that injured two people on May 17, 2000. Not be outdone, the deadly tornado outbreak in Moore, Okla., on May 20, 2013 was due to a Dry Line preceding a cold front that swept across the southern Plains. More than 200 died and 24 people were injured from an EF-5 tornado that pushed through Moore.
While the typical cold and warm fronts that are often talked about are important, the Dry Line can be another culprit for active weather. During the spring, try and listen to see if meteorologists use the word Dry Line. If it’s mentioned, that may be a notice to stay alert. Dry Lines can produce some of the most fascinating, yet dangerous weather.
Story Image: A thunderstorm near Childress, Texas. (VORTEX II via NOAA Photo Library)