all h-ll tends to break out. Because storms need fuel, instability, and spin. And Gulf moisture is the magic ingredient that, when combined with an unstable air mass, produces hen-egg hail and tornadoes.
I seem to have spent my live alternating between living east and west of the 100th meridian, or roughly the 20″ rainfall line. West of this line the average precipitation is less than 20 inches per year and farming without irrigation can be rather chancy. The mixed-grass prairie shifts into short-grass steppe until the plains bump into the Rocky Mountain foreland. To the east of this invisible division is the Mississippi Embayment, tall-grass prairies, the land of tall corn and fat pigs, and humidity.
Even west of that magical line, however, when the Gulf opens up, we brace for impact.
I recall a number of summer afternoons when I’d stand by the tarmac at the airport, looking at the huge storms billowing up to the east, blindingly white in the afternoon sun, and thinking “poor shnooks are going to get pounded.” And at least twice, an hour or so later I’d notice it getting kinda dark, and a familiar smell in the air, and the wind going fitful, then calm. And I’d look to the west-southwest, gulp as a mass of blue-black spat lightning down, and squeak, “Oh sheepdip!”
If you look at a map of North America, you will see that the Rocky Mountains block much of the moisture from the Pacific from reaching the central and southern Great Plains. The High Plains are even more prone to this rain shadow, although we do get part of the so-called “summer monsoon” in most years, which draws on water from the Baja. It requires moisture from the Gulf of Mexico to really give the High Plains much precipitation,though, so we don’t start taking forecasts of rain seriously until we get a south or southeast wind. Even then, if high pressure forms a “lid” on top of us, all we get is sticky heat and fond wishes. I’ve flown into Amarillo on days where the lid capped out at over 20,000 feet and the high for the day was 110 F. Not fun. Until the storms blow up hard enough to break that cap, in which case you get the monsters that graze the stratosphere and drop cabbage sized hail.
Most people are used to the little thunderstorms that bubble up, make some noise, drop rain and perhaps pea-sized hail, and fall apart, choked off by their own downdrafts. (The outflow of cool air blocks the inflow of fresh, warm air and robs the storm of its fuel.) These “popcorn showers” form when you have unstable air (hot and wet) but no sheer in the atmosphere. Things start getting interesting when you combine warm, wet, bubbly air with winds that change direction over height. I can pretty much guarantee that when the wind at the surface is from the west, say, but I’m watching clouds race past going north-east (i.e. pushed by a southeast wind) there are going to be moments of extreme interest later that day. That wind sheer starts the rotation within the storm that takes an updraft, starts it rotating, tips it over, and turns it into a tornado. It also keeps the storms moving, so that once they reach the downdraft stage, the bulk of the storm continues moving and is not choked off by the downdrafts. That is when you get the self-propagating super cells that continue after sunset. Ideally, a bunch of these monsters will merge, form a line (“lining out”) and the threat for huge hail and tornadoes will turn into “mere” flood warnings.
Where I currently live, the most exciting afternoons and evenings come about when we have open gates to the Gulf, a dry line, a cold front, and a low in the middle. On a weather chart it looks like a triangle, with the low pressure core at the apex, and the cold front and the dry-line (where the winds change and desert air hits tropical air) forming the legs below. You have instability (the low), energy (the Gulf moisture), and a trigger (the cold air pushing under the warmer air to lift it). We call that a triple-point and it brings strong mixed emotions. We always need the rain. We never want giant hail or tornadoes. I’ve watched storms form, observed their anvils sprawling across the sky to pick up all the colors of sunset, and gone home to discover that a chunk of a small city got torn up by an F-3 tornado. Thanks be, most storms that do drop tornadoes do so over empty (relatively) ground, so things like Omaha (1918, 1975), Wichita Falls, Wichita, Pratt, Pampa, Moore OK, Oklahoma City, Greensburg KS, Tuscaloosa AL, Joplin MO, Spencer SD, are the exceptions.
And so the season goes when the Gulf gates open.