Super outbreak anniversary brings slight to moderate risk of severe weather

Probabilities map from Day 2 Convective Outlook
Probabilities of severe weather with 25 miles of a point, 8 a.m. EDT April 3 to 8 a.m. EDT April 4. Hatched area: 10% or greater probability of significant severe weather (see text below) within 25 miles of a point.

Tomorrow is the 40th anniversary of a super outbreak of tornadoes … an outbreak that caused massive damage in Monticello, Indiana and Xenia, Ohio, among many other places.

Ironically, some of the same areas that were affected by the 1974 outbreak have either a slight or moderate risk of severe weather, including tornadoes, on the anniversary.

Most of Indiana has a slight risk of severe weather between 8 a.m. EDT tomorrow (April 3) and 8 a.m. EDT Friday, according to the Day 2 Convective Outlook that the National Weather Service Storm Prediction Center (SPC) issued at 1:58 a.m. EDT today. The slight risk area also includes parts of Ohio and several states to the south and west. The slight risk area includes all or part of the Indiana counties of Huntington, Wells, Adams, Blackford and Jay, as well as other Indiana counties to the west and south of those. It also includes part of Van Wert County, Ohio. As of this morning, the slight risk area does not include the Indiana counties of Allen or Whitley, or counties to the north and west of them.

As you can see on the probabilities map above, north central and northeastern Indiana counties in the slight risk area have a 15 percent probability of severe weather occurring within 25 miles of a point. You can see an excellent explanation of severe weather probabilities on the SPC website. Day two outlooks lump all types of severe weather together, so these probabilities indicate that any of the following could occur: large hail, damaging wind, or tornadoes.

What’s particularly interesting about this morning’s day two outlook is that it indicates an area in which there’s a 10 percent or greater risk of significant severe weather. The SPC defines “significant severe weather” as two-inch diameter or larger hail, 65 knot (75 mph) or stronger winds or tornadoes capable of producing EF2 or greater damage. Because there’s less than a one percent probability of any of these events on a normal day, a 10 percent or greater probability represents a much greater risk than normal.

The area with increased risk of significant severe weather is indicated on the map above by the black loop and hash marks. It includes much of central and southern Indiana, including the Indianapolis, Terre Haute and Evansville areas. It also includes the southern halves of Illinois and Missouri, western halves of Kentucky and Tennessee, northwestern Mississippi, and all of Arkansas.

Here’s a summary of what SPC meteorologists wrote in the textual part of this morning’s Day 2 Convective Outlook: The potential exists for numerous severe storms Thursday with damaging wind, large hail and tornadoes possible, especially from the lower- to mid-Mississippi Valleys into the western Tennessee and Ohio valleys.

Surface-based thunderstorms will probably form across the lower- to middle- Mississippi Valley Thursday afternoon. These storms may initially be discrete in character, and conditions will become increasingly favorable for supercells capable of large hail and tornadoes. This activity will eventually spread eastward into western portions of the Tennessee Valley during the evening and overnight with a continued severe threat.

In the Ohio Valley region (including Indiana), the primary severe threat should develop Thursday afternoon and again overnight as deep layer winds strengthen ahead of a mesoscale convective system that is expected to evolve upstream across the mid-Mississippi Valley region. Damaging wind and a few tornadoes will be the primary threats

Our next look at the severe weather picture for tomorrow comes midday today, when the SPC will issue an updated Day 2 Convective Outlook. Even more specific information will appear in a Day 1 Convective Outlook due out early tomorrow morning.