The science behind the deadly March 2 tornado outbreak
State College, Pa. - AccuWeather.com reports Friday, March 2, 2012 may be known as one of the worst tornado outbreak for early March on record. Eighty sightings of tornadoes were reported between 10 a.m. and 9 p.m. Friday. Friday's outbreak could include more tornadoes in one day than typically occur over the entire month of March in the United States.
A strong low-pressure system sent a blast of cold air into warm and humid air on Friday. Powerful winds aloft enhanced the threat for severe thunderstorms and tornadoes. A warmer-than-normal central Gulf of Mexico may have helped the extent of warm and humid air that surged north ahead of the cold, dry air.
The causes and effects are analyzed below.
7:15 A.M. FRIDAY MORNING
A low-pressure system was centered over southern Missouri and was forecast to move northeastward and intensify. A cold front stretched southwest from the low into northern Texas. Warm and humid air was surging north ahead of the low from the central Gulf Coast into the Tennessee Valley. Nashville, Tenn., reported a temperature of 65, already 11 degrees above their normal high for March 2.
Nashville was on the northern edge of widespread warm and humid air surging north ahead of the low. Warm and humid air is a key component to severe thunderstorm and tornado formation. A warmer-than-normal central Gulf of Mexico may have helped fuel the extent of the warmth and humidity.
The upper atmosphere featured huge amounts of wind shear, another key component to severe thunderstorm and tornado formation. Wind shear is a change in wind direction or speed from the winds at the lower levels of the atmosphere to the upper levels. The wind shear was detected by the fact of southerly winds in the lower levels of the atmosphere and westerly winds in the upper atmosphere ahead of the low pressure system.
A strong jet stream, with winds in excess of 100 miles per hour in the upper atmosphere, was in place as well.
Nearly every factor meteorologists look for when forecasting severe thunderstorms and tornadoes was in place.
1:45 P.M. FRIDAY AFTERNOON
The low-pressure system was stronger and centered over east-central Illinois with a cold front extending south from the low across southeast Missouri to central Arkansas and east Texas.
Warm and humid air continued to surge north, with widespread 70s noted from the Gulf Coast to Tennessee and Kentucky. Cold air was pressing eastward into this warmth with temperatures only in the 40s in eastern Missouri.
The clash between the cold and warm air is a classic setup for thunderstorms and provides lift in the atmosphere.
The first tornado for Indiana was reported only minutes earlier in Posey County, with an estimated size of the tornado being 300 yards wide.
Numerous tornadoes have already been reported since the morning from Alabama to Tennessee.
5:45 P.M. FRIDAY EVENING
The low-pressure system continued to strengthen and was centered over southwest Michigan. The cold front extended south of the low into central Kentucky to western Tennessee and northern Louisiana.
The instability of the atmosphere was near or just past the peak over the region. Widespread cold air pressing east was colliding with warm air that was fueled by daytime heating. Sufficient wind shear was in the atmosphere to allow the thunderstorms to rotate and spawn dangerous tornadoes.
The Storm Prediction Center in Norman, Okla., received approximately 17 reports of either tornadoes on the ground or damage already done by tornadoes over the past hour.
The radar mosaic showed severe thunderstorms along the cold front and individual thunderstorms ahead of the main line.
Severe thunderstorms and tornadoes were ravaging states from Ohio to Tennessee to Mississippi and Alabama.
9:30 P.M. FRIDAY NIGHT
The cold front was stretched from eastern Ohio south to eastern Tennessee and the southwest into central Louisiana. In wake of the cold front windy and chilly was settling into the ravaged Ohio and Tennessee valleys. The cold air filtering into the region stabilized the atmosphere, eliminating the threat for further thunderstorm development.
However, along and ahead of the cold front, heavy rain was pounding the central Appalachians while dangerous thunderstorms rumbled across northern Georgia to Louisiana.
Over 80 sightings of tornadoes were reported since 10 a.m., with unfathomable destruction reported in their wakes.
The front was forecast to continue pushing eastward through Saturday, prompting more risk for severe thunderstorms and tornadoes.