Risk of severe thunderstorms, tornadoes tomorrow (even though it’s February)

Day 2 convective outlook map
Yellow area: 15% probability of severe weather within 25 miles of a point. Red area: 30% probability of severe weather within 25 miles of a point. Read about how the NWS defines the probabilities.

See an important update to this post.

The entire state of Indiana has a slight risk of severe thunderstorms between 7 a.m. EST Thursday, Feb. 20 and 7 a.m. EST Friday, Feb. 21, according to the Day 2 Convective Outlook that the National Weather Service (NWS) Storm Prediction Center (SPC) issued at 1:51 a.m. EST today.

As you can see on the map at right, the southern two-thirds of Indiana, plus adjacent parts of Illinois, Kentucky and Ohio, have a 30 percent probability of severe weather within 25 miles of a point, while the remainder of Indiana and adjacent areas of Illinois, Michigan and Ohio, have a 15 percent probability. The 30 percent area includes some of the southeastern counties of IMO SKYWARN quadrant two, including Blackford, Jay, and parts of Wells, Adams and Van Wert counties.

Why 15% and 30% are significant

It’s been months since I’ve written about convective outlooks, so here’s a reminder of what this all means.
In this case, the NWS defines “severe weather” as a tornado, damaging thunderstorm winds or wind gusts of 50 knots (58 mph) or higher, or one inch diameter hail or larger. The Day 2 Convective Outlook does not separate these, so a 30 percent probability indicates a risk of any one of the above types of severe weather. Also, risks of 15 and 30 percent might seem small, until you understand how low the probabilities normally are. For example, think of annual climatology. On how many days over the past year did at least one of the types of severe weather above occur within 25 miles of you? For illustration, let’s say that happened on 10 days in the past year. That would mean the normal probability for that area is 10/365, or 2.7 percent! A 15 percent probability, therefore, would indicate that severe weather is 5.5 times more likely than normal. A 30 percent probability would indicate that severe weather is 11.1 times more likely. You can read much more about these probabilities on the SPC website.

Brief summary of tomorrow’s weather setup

Here’s a brief summary of the outlook: A fairly widespread severe weather episode appears likely, primarily Thursday afternoon and evening and into the early morning hours of Friday. A strong cold front and associated, powerful upper storm system will move across the central U.S. and into the eastern U.S. By late afternoon, a nearly solid and fast-moving line of thunderstorms should stretch from the southern upper Great Lakes vicinity south-southwestward into the central and western Gulf coastal region. This line will bring a risk for fairly widespread damaging wind gusts. In addition, a few embedded tornadoes will be possible within the line — particularly within a zone in the mid-Ohio valley and mid Tennessee valley vicinity.

Plus, there’s a flood risk

Rainfall associated with a severe thunderstorm could increase the risk of flooding in northeastern Indiana, southern lower Michigan and northwestern Ohio. As the northern Indiana NWS office wrote in this morning’s Hydrologic Outlook,

The threat for flooding appears to be increasing in many areas, but is not certainty at this point. The severity and timing of flooding will depend on the amount of rainfall received Thursday, and the exact duration of warmer conditions, which remains uncertain. If you live in a flood prone area, now is the time to prepare for possible flooding. Stay alert to the latest forecasts trends over the next several days by visiting weather.gov/iwx.

The bottom line

How should you use the information in today’s Day 2 Convective Outlook? If you’re a SKYWARN storm spotter, check your gear (including communication equipment), gas up your vehicle and get ready for activation. Whether or not you’re not a spotter, make sure your NOAA weather radio is working (replace its batteries, if it’s been more than a year since you’ve done so), make a mental note to check your local forecast tomorrow morning (e.g. on www.weather.gov), and if the risk of severe weather remains in place, be sure you have a way to receive any watches or warnings that might come out. Above all, don’t assume that because it’s February, tornadoes and severe thunderstorms won’t happen!