An upper-level low pressure center will move over Central Arkansas this afternoon. This will occur as temperatures warm to close to 70°. The upper low is a pocket of cool air several thousand feet above the ground. Placing colder air over warm surface air will increase instability. And high instability is one of the ingredients for severe weather. This is going to be true, especially in the eastern part of Arkansas where the Storm Prediction Center is forecasting a Slight or Level 2 severe weather probability (15% chance of severe weather happening within 25 miles of any single point).
Hail is the biggest threat
An unstable atmosphere increases the potential for air to rise faster. If a thunderstorm forms in this atmosphere, it may have a strong updraft transporting the warm and humid air from the surface into the cold air way up high. The air temperature around three miles high will be around 0°F. And it will be even colder higher up. Hail will form in this environment and have the opportunity to be suspended for a period of time thanks to the strong updraft. When the updraft cannot support the weight of the hailstones suspended in the cloud, the hailstones will fall to the ground. Hail, if any, should not be bigger than quarter to ping-pong size hail. It is for the hail threat alone, why the Storm Prediction Center increased their risk assessment from Marginal/Level 1 to Slight/Level 2 Thursday morning.
Damaging Wind is the second highest threat
Strong updrafts will often yield strong downdrafts. In other words, what goes up must come down. The downdraft portion of a storm is where the rain fall and also the hail. If there is very heavy rain and some hail falling, the storms will be capable of producing brief damaging wind gusts. The more mass that gets suspended within the cloud the stronger the downdraft will be, and hence, the higher the damaging wind threat will be.
Tornado threat is very low
Very low does not mean zero. Like large hail and damaging wind that can result from storms that form in an unstable atmosphere which allows strong updrafts and downdrafts, tornadoes do too. However, tornadoes need an atmosphere that has a significant wind direction change as you go higher up from the ground, particularly from the southeast at the ground to the southwest or west in the highest levels. That setup really won’t be present today. So, tornadoes will be highly unlikely, but not impossible. If there are any tornadoes, they will be weak and brief.
Storms will likely start to form around 2:00 p.m. and start becoming strong around 3:00. Around 4:00, Severe Thunderstorm Warnings may be issued. Click through these slides below to get an idea on the timing, placement, and intensity of the storms that may form this afternoon.