An unusually active April weather pattern existed for Oklahoma during the week of April 20th through April 28th, 1912. During three of those days, Oklahomans witnessed multiple devastating tornadoes across the state from Jackson County, OK to Kay County, OK. Those tornadic days included April 20th, April 25th, and April 27th, which added an exclamation point to the end of a very destructive and deadly week. The April 27-28, 1912 tornado outbreak rivals the more recent May 3-4, 1999 Oklahoma tornado outbreak in terms of the number of strong to violent tornadoes, the amount of destruction to communities, and, sadly, the number of deaths.
So, what caused these three historic days of tornadoes? Unfortunately, surface and upper air observations were very limited in 1912. In fact, there were no radars, no soundings, no automated surface observations, and no satellites. Back then, data was limited to human observed surface variables at select locations across the United States. The weather observers measured temperature, pressure, wind speed and direction, and rainfall amounts. From this limited atmospheric data, the Weather Bureau, which was part of the U.S. Department of Agriculture, created daily hand-drawn surface analysis maps. Later, during World War II, the Army Air Forces reanalyzed these Weather Bureau maps. These reanalyzed analyses will be used to ascertain the characteristics of the rest of the atmosphere so that a weather synopsis can be obtained for each of the days.
April 20th: On April 17th, cold, dry air encompassed the Central Plains with the cold front positioned well south of Texas out over the open waters of the Gulf of Mexico. In order for storms to have the chance to develop, the assumed warm, moist air south of the cold front needed to advect back north into the Southern Plains. On the morning of April 19th, a surface low pressure system centered over the Four Corners region of the Southwest United States began the task of advecting that warm, moist air back north into the Southern Plains. Warm, moist air is associated with instability, which is the energy source for thunderstorms.
By the morning of April 20th, the surface low pressure system moved east-northeastward into South Central Colorado. This indirectly implies the mid- and upper-level winds aloft (5-10 km above ground level) were directed from the west-southwest. Combined with southeasterly winds at the surface, it is evident that plenty of vertical wind (directional and speed) shear was in place for severe thunderstorms. Even though no consistent moisture measurements existed in 1912, the southeasterly surface winds across the Southern Plains hint at the continual advection of the warm, moist air out ahead of the analyzed surface cold front, which extended southward from the low pressure system across Eastern New Mexico. Although a dryline wasn't analyzed, the convergent winds ahead of the cold front in the Texas and Oklahoma Panhandles indicate the potential that one existed on the morning of the 20th. Either way, the cold front and/or dryline provided the necessary lift needed for the development of thunderstorms later in the day from central South Central Kansas southward through Central Oklahoma and into North Central Texas.
The combination of shear, lift, instability, and moisture (SLIM) associated with this day lead to three separate areas (South Central Kansas, Central Oklahoma, and North Central Texas) of strong to violent tornadoes in the Southern Plains. The next two cases were rarer and differ in one major aspect-a warm front provided the necessary lift needed for thunderstorm development.
April 25th: After the passage of the cold front on April 20th, surface high pressure dominated the Central Plains through April 23th. However, on the morning of April 24th, the high pressure center moved southeastward and was centered over the Southeast. Since surface air flows clockwise around a high pressure system, the Southern Plains started to receive return flow from the Gulf of Mexico, which is a source of warm, moist air.
The morning of April 25th revealed a broad surface low pressure system situated over the Western Plains after being located west of the Continental Divide on the previous day. Also, a warm front had developed on the northern edge of the presumed moisture return across Central Oklahoma and Northeast Texas. By mid-afternoon, the warm front was most likely stretched across North Central Oklahoma and South Central Kansas. Along and just north of the warm front, the surface winds were from the east-southeast, yielding strong vertical wind shear since the winds aloft were likely from the west-southwest due to the approach of an upper level trough. Once again, all of the ingredients (SLIM) came together for tornadic supercells to develop just like on April 20th, except this time they formed across South Central and Southeastern Kansas and North Central Oklahoma.
April 27th/28th: During the night of April 25th and the morning of April 26th, a cold front pushed through Oklahoma leaving northwest winds and cooler air in its wake. However, the cold front stalled out in Central Texas forming a southwest-to-northeast oriented stationary front, and by the morning of April 27th, the stationary front pushed northward as a warm front across Southeast Oklahoma and North Texas out ahead of the upper-level trough and the next surface low pressure system. Jumping "forward in time" to the morning of April 28th, the surface low pressure system was located over Western Oklahoma with a cold front extending southeastward across South Central Oklahoma and a warm front extending eastward across Northern Oklahoma. Thus, the warm front moved from around the Red River Valley to Northern Oklahoma over the course of 24 hours. This warm front provided the lift needed for multiple rounds of tornadic thunderstorms during the day and night of April 27th.
South of the warm front, the surface winds were southeasterly, and north of the warm front, the winds were northeasterly. That surface wind field coupled with strong westerly/southwesterly upper-level flow produced the strong vertical wind shear needed for the tornado outbreak. Along and south of the warm front, moisture and instability were likely abundant, so the necessary ingredients (SLIM) undoubtedly existed for tornadic supercells across Western and Central Oklahoma on April 27th.
With such sparse data from April 1912, little can be factually said about the state of the atmosphere during these three tornado episodes, and many assumptions were made with the above weather synopsis. Additionally, smaller scale features, which could have been the main reason for where and when storms and tornadoes developed on those three days, were unresolvable 100 years ago. In fact, meteorologists, both forecasters and researchers, still struggle with this problem even with a denser observation network. Even though the exact details on those three days in April 1912 are unknown, the fact still stands that several violent tornadoes, especially on April 27th, occurred in historic fashion and will not be forgotten.