April 3 Severe Weather Bust

Earlier this week, model guidance began hinting at the possibility of severe weather to occur yesterday. Closer to the event, confidence was increasing that a cold front would provide enough lift to produce a line of severe storms with a few supercells popping up ahead of the line. A favorable environment for damaging winds, hail, and isolated tornadoes was expected and we began to forecast the severe weather potential for yesterday.

My forecast yesterday afternoon continued to stress that severe weather was likely to occur yesterday afternoon and evening:

“Isolated supercells containing hail, damaging winds, and  tornadoes will be possible across southern  Kentucky ahead of the cold front today.”

“The other threat for severe storms is expected to occur between 4-7 PM as a broken line of strong to severe thunderstorms associated with the cold front will begin to push east across the area. These storms will be more widespread with the potential for damaging winds, large hail, and isolated tornadoes.”

How did this forecast turn out? There were zero tornado reports across the entire country yesterday and there were only four reports of severe weather within the state of Kentucky (1 hail, 3 damaging wind). The closest severe weather reports to southern Kentucky occurred across northern Tennessee.

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Storm reports occurring on April 3, 2015 (Courtesy: SPC)

I think everyone can agree this qualifies as a busted forecast.

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h/t giphy.com

So what happened to yesterday’s severe weather potential? Wind profiles were very impressive yesterday with 55-65 knots of bulk shear and 20-30 knots of wind shear in the lowest kilometer of the atmosphere. Temperature lapse rates were high enough to warrant concern for severe weather and a cold front was expected to provide enough lift to initiate storms.

While all of these conditions were favorable for severe weather, there were some key ingredients that were missing yesterday. A large area of moderate rainfall raced from west to east across Kentucky during the mid-morning hours yesterday. Cloud cover from this area of precipitation was slow to clear and all of Kentucky saw significantly less sunshine than was expected.

Abundant sunshine can help the atmosphere to become unstable. Instability describes the ability of air parcels to accelerate upwards after being lifted and higher instability values indicate a greater potential for severe weather to occur. One variable used to indicate instability is CAPE. Without much sunshine, CAPE values topped out at around 500-750 J/kg across southern Kentucky yesterday. Just for some perspective, we typically see CAPE values approach 4000 J/kg during the summer months in this region.

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Archived MLCAPE valid at 2pm CST yesterday (Courtesy: SPC)

My forecast yesterday assumed that storms would be able to thrive in a low instability environment due to favorable wind shear. This assumption may have held true if there had been a cap, or presence of convective inhibition, yesterday. A cap serves as an “atmospheric lid” that can prevent air parcels from rising above a certain height and thus, prevent thunderstorm development.

A cap can also enhance severe weather potential because it will hold off convection until the cap breaks. With no storms occurring before the cap breaks, when thunderstorms do break the cap and began to rise vertically they will be able to use all of the untapped instability. On a day like yesterday when instability was already limited, a cap would have helped to ration the fuel thunderstorms needed. Instead, we saw scattered showers and storms all afternoon which meant that none of this instability was conserved.

By the time the front pushed east across the state, the atmosphere was completely worked over across Kentucky and there was just not enough fuel to produce severe thunderstorms. While yesterday’s forecast ended up being a complete bust, rest assured that every forecaster learned from what occurred. The next time a low instability/high wind shear severe weather event occurs, we will be able to use what we learned yesterday to better forecast the severe weather potential (or lack of).