We will begin with an overview of surface and upper level analysis charts valid for 8 a.m. and 8 p.m. on May 13 th across North America. Although it will take a few minutes to review all of these charts, we believe a good starting point for this tornado case (or any forecast process) is to first strive to obtain a good general sense and feel for the overall weather pattern. After a certain comfort level is attained with the “big picture”, we will get into much more detailed surface analysis, soundings, hodographs, assessment of instability, and mesoanalysis at the meso and micro scales.
Surface Weather Map at 8 a.m. E.D.T. 5/13/80
A stationary front was in place across the northern portions of Illinois, Indiana, and Ohio. In addition, a wave of low pressure was evident along the front over northwestern Ohio. Although it is not indicated on this analysis, another significant wave of low pressure was located over eastern Iowa (this is clear on a more detailed 8 a.m. surface analysis over the Midwest region, to be presented later). The cold front extended from the low in eastern Iowa south through Missouri.
850 millibar temperatures 8 a.m. E.D.T. 5/13/80
A thermal trough at 850 mb was located across the Northern Plains states while strong warm air advection was occurring over the southeastern United States. Lower Michigan was situated right in the middle of the tight thermal gradient.
850 millibar winds 8 a.m. E.D.T. 5/13/80
A wind max of 40 to 45 knots was located over the lower Ohio valley.
500 millibar heights 8 a.m. E.D.T. 5/13/80
The 500 millibar trough axis was located over the upper Midwest.
500 millibar wind speeds 8 a.m. E.D.T. 5/13/80
One speed max on the order of 70 to 75 knots was located over the New England region and southern Quebec. More importantly, a 65-knot speed max was located over Missouri and southern Illinois. A coupled jet structure between the left exit region of the MO/IL jet streak and the right entrance region of the jet streak over New England is evident. The coupling of the circulations associated with these two jet streaks was likely a substantial contributor to large scale vertical motion field that set the stage for thunderstorm development on 13 May 1980.
250 millibar wind speeds 8 a.m. E.D.T. 5/13/80
A very impressive 120-knot jet max was located over Missouri and poised to move into Illinois, Indiana and Ohio. This placed Lower Michigan in the favorable left exit region of the jet. This strong upper level jet was an important aspect of the environment on 13 May 1980. The strong upper level jet is an indication of strong deep shear that can contribute to development of supercell thunderstorms. In addition, the upper level divergence associated with this jet streak would help to steepen mid level lapse rates resulting in an environment more conducive to the development of strong thunderstorms. This jet structure is very similar to that associated with other significant tornado outbreaks in southern Lower Michigan
Surface Weather Map at 8 p.m. E.D.T. 5/13/80
By 8 p.m., the cold front had moved through the Grand Rapids, MI county warning area.
850 millibar temperatures 8 p.m. E.D.T. 5/13/80
The strong 850 mb temperature gradient was still very pronounced at 8 p.m. 850 mb cold air advection had not begun yet over Lower Michigan.
850 millibar winds 8 p.m. E.D.T. 5/13/80
850 millibar wind fields were not too impressive. A max of 30 to 35 knot winds extended from the Lower Ohio Valley, east to the mid Atlantic region.
500 millibar heights 8 p.m. E.D.T. 5/13/80
The 500-millibar trough axis had made some eastward progress and extended from near Sault St. Marie back to the west across northern Wisconsin.
500 millibar wind speeds 8 p.m. E.D.T. 5/13/80
A 500 mb wind max of 65 to 70 knots was located over Ohio and Pennsylvania.
250 millibar wind speeds 8 p.m. E.D.T. 5/13/80
A 250 mb jet max of around 115 knots extended across Indiana and Ohio. 250 mb wind speeds to 100 knots were analyzed across far southern Lower Michigan.