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1925 tri-state tornado outbreak
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==Background== [[File:Tri-State Tornado trackmap (PAH).jpg|thumb|left|Track of the Tri-State tornado]] During a six-year review study of the Tri-State tornado published in 2013, new [[Surface weather analysis|surface]] and upper air data was obtained and [[meteorological reanalysis]] was utilized, adding significantly to knowledge of the [[Synoptic meteorology|synoptic]] and even [[Mesoscale meteorology|mesoscale]] background of the event. The late winter to early spring of 1925 was warmer and drier than normal over much of the central United States. There apparently was persistent [[Ridge (meteorology)|ridging]] in the western U.S. with a [[Trough (meteorology)|troughing]] pattern over the central U.S.{{sfn|Maddox|Gilmore|Doswell III|Johns|2013}} The [[extratropical cyclone]] that set the synoptic stage for the outbreak was centered over northwestern [[Montana]] at 7:00 a.m. [[Central Time Zone|CST]] (13:00 [[Coordinated Universal Time|UTC]]) on March 17. Meanwhile, a diffuse area of surface [[low-pressure area|low pressure]] was centered near [[Denver]], [[Colorado]], in association with a [[Trough (meteorology)#Lee trough|lee trough]]. [[Occluded front]]s extended from [[Hudson Bay]] southwestward into the northern Plains states and into the lee trough. The synoptic cyclone moved south-southeastward across the mountain states to eastern Colorado. A [[warm front]] stretched along the [[Gulf Coast of the United States|Gulf Coast]], separating warm, moist air from cool, showery weather with areas of fog that extended from [[Texas]] to the Carolinas. A well-mixed early-season continental tropical (cT) [[air mass]] existed over West Texas and northern [[New Mexico]]. To the east of this hot, dry air, buoyant maritime tropical (mT) air was [[Advection|advecting]] from the [[Gulf of Mexico]]. Simultaneously, a mid- to upper-level [[Shortwave (meteorology)|shortwave trough]] likely approached the northwest coast of the U.S. and moved rapidly through the persistent ridge then digging southeastward across the [[Great Basin]] and central [[Rocky Mountains]] and emerging in [[Eastern Plains|the Plains]] over Colorado. This initiated a "[[Colorado low]]" [[cyclogenesis]].{{sfn|Maddox|Gilmore|Doswell III|Johns|2013}} At 7:00 a.m. CST on March 18, the surface low-pressure area, at approximately {{convert|1003|hPa|inHg|abbr=on|lk=on}}, moved to far northeastern Oklahoma while the warm front shot north into the circulation where the front then extended eastward. A maritime Polar (mP) [[cold front]] draped southwestward across eastern Texas with a [[dry line]] forming directly to the south of the low. The open shortwave, likely somewhat negatively tilted, was continuing to approach from the northwest and an apparent [[outflow boundary]] moved just to the south of the [[warm front]] over northeastern Arkansas and northwestern Tennessee. Several weak pressure troughs were traversing the cool sector over the north-central U.S.. Surface temperatures in the warm sector near the dry line and warm front ranged from {{convert|60|β|70|F|C}}, and the [[dew point]] was {{convert|55|β|65|F|C}}, with higher values farther south and increasing over time as the deepening low-pressure area continued to pull up air from the Gulf of Mexico. https://www.weather.gov/pah/1925Tornado_wi#:~:text=In This resulted in unstable air and lower [[cloud base]]s, or low [[Lifted condensation level|LCL]] heights, which is favorable to [[tornadogenesis]]. From southeastern Kansas to Kentucky and Indiana, early morning showers and thunderstorms north of the low and warm front cooled and stabilized that air, retarding northward advancement of the front, and led to a sharp contrast in temperature from north to south. Such [[Baroclinity|baroclinic]] zones are also associated with tornadic storms. Ahead of the surface dry line, which are uncommon as far east as the Mississippi River,{{sfn|Duell|Van Den Broeke|2016}} an apparent "[[dry punch]]" of air aloft served to further increase [[Convective instability|instability]]. Concurrently, a [[capping inversion]] likely suppressed storms throughout the warm sector, leaving the Tri-State supercell undisturbed by nearby [[Atmospheric convection|convection]].{{sfn|Maddox|Gilmore|Doswell III|Johns|2013}} By 12:00 p.m. CST (18:00 UTC), the deepening surface low was centered over south-central Missouri, the shortwave axis was moving easterly and oriented over eastern Oklahoma, and the dry line was rapidly advancing eastward directly south of the low as the warm front, situated due east of the low, slowly shifted northward. Morning clouds cleared by midday across much of the Tri-State tornado's eventual path. A pronounced pressure trough extended northeast of the low and signaled its future track as a prefrontal trough formed southeast of the low ahead of the dry line. A bulge in the dry line may also have been forming slightly south of the low, and southerly to southeasterly surface winds were backing and increasing with time throughout the warm sector. The Tri-State supercell formed in a highly favorable area just ahead of the triple point where the cold front, warm front, and dry line met. The supercell initiated very near the surface low and moved east-northeastward, faster than the low, such that the storm gradually deviated east of the low's track. The supercell remained near this "sweet spot" for a prolonged period as it also traveled near the highly baroclinic warm front (likely just across the cool side of the boundary) for several hours.{{sfn|Maddox|Gilmore|Doswell III|Johns|2013}} [[File:Tri-State Tornado cyclone track map key.jpg|thumb|right|Tri-State tornado storm track and other tornadoes that day from ''[[Monthly Weather Review]]'', April 1925.<ref>[http://www.photolib.noaa.gov/htmls/wea00237.htm NOAA Photo Library]</ref> The information about the temperature, pressure, and other tornadoes may not be accurate.]] By 2:00 p.m. CST (20:00 UTC), the low was centered slightly south-southwest of St. Louis, Missouri, as the Tri-State supercell neared the Mississippi River. Other storms in the warm sector, removed from the Tri-State supercell, were initiating around 3:00 p.m. CST (21:00 UTC). Around 4:00 p.m. CST (22:00 UTC), the low's [[Atmospheric pressure|central pressure]] lowered to around {{convert|998|hPa|inHg|abbr=on}}, centered over south-central Illinois, as the supercell was moving into Indiana. This pressure is not particularly low compared to many other outbreak setups, but the [[pressure gradient]] was strong, which induced strong gradient winds and significant advection in the warm sector. A very strong [[low level jet]] was also in place just above the surface as winds veered with height, resulting in low-level curvature and long [[hodograph]]s. Strong [[wind shear]] thus existed, with pronounced directional shear likely in the vicinity of the warm front, with winds at the 700 hPa height level west-southwesterly around {{convert|70|mph|abbr=on}} and winds at the 500 hPa level about {{convert|90|β|110|mi/h|km/h|abbr=on}}. Theoretical hodographs returned estimated storm relative environmental [[Hydrodynamical helicity|helicity]] (SREH) values of 340 m<sup>2</sup> s<sup>β2</sup> in the vicinity of the Tri-State supercell track. Strong thunderstorms were now scattered throughout the warm sector and a line of severe thunderstorms was occurring near the dry line. The Tri-State supercell appeared to still be discrete and isolated, with a severe storm north of Cairo, Illinois, placed well to its south.{{sfn|Maddox|Gilmore|Doswell III|Johns|2013}} By 6:00 p.m. CST (00:00 UTC), the shortwave axis was over eastern Missouri and was lifting northeast. At 7:00 p.m. CST (01:00 UTC), the low was placed near [[Indianapolis, Indiana]], with numerous thunderstorms east and south of the low and a [[squall line]] moving into the southeastern U.S. Cold air advection behind the strong cold front fed into the cyclone as [[snow]] and [[Ice pellets|sleet]] fell from eastern Iowa to central Michigan. At 7:00 a.m. CST on March 19, the low was deepening and lifting rapidly northeastward into Canada.
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