Widespread significant severe weather possible tomorrow

Probability of severe weather within 25 miles of a point.
Hatched Area: 10% or greater probability of significant severe weather within 25 miles of a point. (More Info)

Update: The risk of severe weather in northern and central Indiana and northwestern Ohio tomorrow has increased from “slight” to “moderate.” Read more.

Update: National Weather Service to host live Twitter chat in advance of tomorrow’s severe weather. Read more.

There’s a fairly good chance of a widespread significant severe weather event across much of Indiana and nearby states tomorrow, Wednesday, June 12, according to various National Weather Service (NWS) sources, including the Day 2 Convective Outlook that the Storm Prediction Center (SPC) issued at 2:05 a.m. today. The area of risk is indicated on the map above, from this morning’s outlook. Notice especially the “hatched” area, which includes Fort Wayne and all 37 counties covered by the Northern Indiana NWS weather forecast office. That hatched area indicates a 10 percent or greater probability of significant severe weather within 25 miles of any point in the area.

The SPC defines “significant severe weather” as EF2 or stronger tornadoes, damaging winds with speeds greater than 75 mph, or large hail two inches or greater in diameter. So, the SPC is saying that the probability of at least oneof those things happening in our area tomorrow is 10 percent. While 10 percent might not seem very high, compare it to the probability of these things happening on any given day of the year. For example, if we get significant severe weather on average one day per year (and the actual average is probably lower than that), then the normal probability of significant severe weather is 1/365, or about three percent. So, if three percent is the normal probability, a 10 percent probability indicates that significant severe weather is more than three times more likely! That 10 percent looks a little more important now, doesn’t it?

Here are some details from the convective outlook:

After a round of thunderstorms late tonight and/or early tomorrow morning, few of which are forecast to be severe, new storms are forecast to develop by tomorrow afternoon. The initial afternoon storms will likely be supercells. Supercells often produce tornadoes, so a few tornadoes will be possible, along with large hail and damaging winds. The SPC expects the storms to eventually evolve into one or more bowing lines of storms, which will shift quickly east southeastward across the area. As this occurs, the threat for damaging straight-line winds is forecast to increase, along with large hail. The bowing band or bands of storms are forecast to reach the upper Ohio River Valley by late tomorrow evening, with ongoing and potentially widespread damaging winds.

I’ve noted in this blog in the past that sometimes information issued by the local weather forecast office seems to put less emphasis on severe weather risk than does the SPC convective outlook. For example, sometimes when the SPC calls for a slight risk of severe weather, the Hazardous Weather Outlook from the local office reads, “spotter activation is not anticipated.” That is not the casewith tomorrow!

The Hazardous Weather Outlook that the Northern Indiana weather forecast office issued at 5:14 a.m. today says the following about tomorrow: “There is a good chance for severe thunderstorms with large hail and damaging winds. An isolated tornado is possible as well.” In addition, it reads, “Spotter activation may be needed between late Tuesday night and late Wednesday night.” In addition, the Area Forecast Discussion that the local office issued at 4:05 a.m. today mentions a “potential high-impact event” Wednesday into Wednesday night.

So, if you’re a spotter, make sure you and your gear are ready. If you’re not a spotter, make sure you have fresh batteries in your weather alert radio and try to keep a television or radio playing during the day tomorrow, so you’ll know how the severe weather situation is progressing. As I have time, I’ll also post updates on this blog. The next time we’ll know more about tomorrow’s forecast will be around 1:30 p.m. EDT today, when the SPC issues an updated Day 2 Convective Outlook.