“Very large hail” is expected in the southern Plains on Tuesday afternoon as a low-pressure system drives east through northern Texas and southern Oklahoma. Tornadoes are possible, but they aren’t the main threat for cities at risk, which include Abilene and Wichita Falls in Texas, and Oklahoma City.

If you live in this area, you may want to park your car in a garage or car port now.

Weak thunderstorms that began Tuesday morning will strengthen and move east into the evening. They may begin as severe isolated storms, but the moisture content is so high and the atmosphere so unstable that we suspect they will quickly grow into large clusters of storms that will generate torrential downpours and very large hail. It also seems likely that wind gusts will reach a damaging threshold in some storms, which will lead to power failures.

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The Storm Prediction Center increased the risk category to “moderate” this afternoon, which is second-highest on the scale, because of the threat of large hail. A few tornadoes are likely, they say, but that’s actually not the greatest concern in this event — in all likelihood, hail is going to do the most widespread damage:

Relatively high confidence in the coverage and location of storms warrants an upgrade to a hail-driven moderate risk from the Big Country into southwest OK. A few tornadic storms appear probable in this similar corridor. However, the temporal overlap of discrete cells with enlarging hodographs may be relatively short in the open warm sector.

Translation: The hail threat is obvious, and a few tornadoes are likely, but forecasters don’t expect them to be very strong or widespread because they won’t be in the right environment for long enough.

Low-level hodographs will be quite enlarged along the warm front, but should be coincident with modest low-level lapse rates and predominant cluster to linear mode.

Translation: Closer to the warm front, there will be more than enough “spin” in the atmosphere to generate tornadoes, but this region doesn’t have the moisture and the strong rising motion to get the storms going in the first place.

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Large hail is likely in storms in which the air is rising fast but the atmosphere doesn’t have much spin. That seems to be what will set up over Oklahoma and Texas on Tuesday.

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The rising motion is quantified in a lapse rate — how many degrees a small bubble of air will cool as it rises. In Tuesday morning’s weather balloon soundings, the lapse rate was as high as 9 degrees of cooling per kilometer. Generally, storms can form in a lapse rate as low as 5. Once it gets to 9.5, they are all but guaranteed.

Anytime there are severe thunderstorms, there is going to be a risk of tornadoes. Unfortunately, one of the great unknowns of meteorology is exactly how and why a tornado forms. What we do know is that all the ingredients need to be in the same place at the same time, and it doesn’t look as though Tuesday day fits the bill.

The severe storm threat will move east into the lower Mississippi Valley on Wednesday.

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