Below is the Daily Surface Weather Map, valid 7 AM EST on May 27, 1973.
The tornado roared into Brent just over 13 hours later.
Many thanks to Bill Murray, Weather Historian and regular contributor to the ABC 33/40 Weather Blog in Birmingham, AL, for his in depth analysis below the map.
Analysis of Map by: Bill Murray
Weather maps on the morning of Sunday, May 27, 1973 had an unusual look to them for so late in the spring. There was a very strong upper level cut off low over Kansas. At the surface, a double-barreled low had centers over southeastern Nebraska and a second over Central Missouri. Barometers near the center were lower than 982 millibars, quite impressive for late May. A cold front trailed from the Missouri low southeastward into Central Arkansas and eastern Texas.
Thunderstorms were active in the warm sector of the strong surface low. One line of thunderstorms extended across Central Kentucky and Tennessee. Another squall line extended from western Tennessee into northwestern Mississippi and northern Louisiana.
The week had been marked by a series of low pressure troughs sliding over the top and down the front side of a developing upper level ridge over the Pacific Coast. The last ten days of May are typically prime time for severe weather over the southern Plains, but the week leading up to the 27th had been very slow in terms of reports. Between the 21st and 25th, there had been just 41 tornadoes across the country, which is not unusually low, but only eleven of them had come from the Plains.
Despite the lack of tornadoes, weather researchers from the National Severe Storms Laboratory had enjoyed a successful tornado intercept on Thursday, May 24th, however. They scored a major victory in understanding tornado formation when they caught a major F4 tornado on their powerful Doppler radars from its inception to its dissipation west of Oklahoma City. Unfortunately, it destroyed much of the town of Union City, but the data allowed researchers to identify the Tornado Vortex Signature, a feature that would eventually lead to vastly improved tornado warnings.
Severe weather had ramped up considerably on Saturday the 26th however, as an outbreak of thirty-four tornadoes killed twelve people and injured 365. Five people were killed near Keefeton, Oklahoma and another three members of a family were killed on a lake in Sedgwick County, Kansas. Late that afternoon, a tornado roared into Jonesboro, Arkansas, doing $37 million in damage. The swarm of tornadoes was the result of the developing surface low over Kansas, being powered by a negatively tilted 500 millibar trough extending from the Rockies into the Plains states.
On the morning of the 27th, a warm, moist airmass covered the state of Alabama. At 6 a.m., it was 72F at Birmingham, with a dewpoint of 69F. There was a breezy southerly wind averaging 15 mph. The combination of the warmth, humidity and strong southerly winds gave the air a “tornado day” kind of feel.
Digging into the upper air sounding obtained from the morning sounding at Montgomery shows even more important clues that a significant severe weather event was likely that day.
When the balloon was launched from Dannelly Field at 12:00 GMT (7 a.m. CDT), the surface temperature was 75F with a dewpoint of 70F. While those values are not especially high for late May in Central Alabama, they are impressive given the dynamic late season storm system that was approaching.
The high surface dewpont indicated that very moist air was in place, a key ingredient for thunderstorms. Other signs were a high 850 mb dewpoint at 850 millibars (15.8F) and a Precipitable Water value of 1.62 inches. While not a record for Central Alabama, that PW value is 2 standard deviations above the mean, indicating unusually moist air.
But a quick look at the K-Index showed something interesting. It was 11.70, below the threshold (30) for heavy rain. A low K-Index value indicates the presence of dry air at 700 mb, indicating an enhanced potential for severe weather. In fact, there was a significant layer of dry air above 775 millibars on the sounding. The Total Totals index was 52, indicating thunderstorms, some possibly severe.
Further indication of the instability could be seen on the Lifted Index, which was already -5.75C that morning (moderately unstable). CAPE values were already moderately unstable as well, running 1407 j/kg on the sounding. Adjusting those value for the projected highs in the upper 80s would yield extremely unstable conditions by afternoon. It would indeed reach 90F that afternoon at Montgomery. More persistent rainfall during the midday hours would retard the instability values a little further northwest where the tornado would form, but they would be plenty high for severe thunderstorm updrafts.
Widespread showers and storms would form by late morning over Central Alabama, as all that instability was virtually uncapped. The convective inhibition on the Montgomery sounding was just -16 j/kg, meaning it would take almost no heating to get convection started.
The Wet Bulb Zero height was around 2,400 meters, or about 7,875 feet, which is prime for large hail development that can still reach the ground.
One important consideration for significant tornadoes across Central Alabama is a low Lifted Condensation Level. That morning, the LCL at Montgomery was in the 551-meter range, which is very low. With heating, that value would rise to around 820 meters by afternoon. Recent research by the National Weather Service in Birmingham has shown that the magic number for this parameter for significant tornadoes across Central Alabama is around 800 meters.
The Bulk Richardson Number (BRN) was 17.8, indicative of an environment where supercell thunderstorms would form. While 0-6km bulk shear values were not overly impressive (36 knots), they would really escalate by late afternoon, approaching 50 knots, allowing for nicely tilted updrafts.
Backed surface winds meant there was plenty of low level shear, with southeasterly surface winds near the ground and strong southwesterly winds just above the surface.
The result was waves of showers and thunderstorms that pushed across the state during the afternoon hours. One long-lived supercell thunderstorm would produce the F-4 tornado that remained on the ground for 139 miles across Central Alabama, heavily damaging the WSR-57 radar installation southwest of Centrevile and Brent and killing seven people along its path.