This is part two of a ‘thunderstorm climatology’ series that began with Europe in mid-March. The next one will be Oceania – yes, the whole continent!
If you want to find the largest, most violent thunderstorms on the planet (for whatever reason…), you’re best off roaming the central USA in the spring and early summer.
There and then, conditions are sometimes particularly conducive to powerful multicell or supercell thunderstorms somewhere in the region. There are multiple, interacting reasons for this, which I explored in detail last April.
On top of the typical thunderstorm risks, such storms can deliver giant hail (i.e. at least 1 inch diameter), severe straight line winds, or strong tornadoes.
Here and now, however, we’re focusing on the seasonal cycle of thunderstorm frequency. That includes non-severe (but still hazardous) ones. We’ll start at the quiet end: winter.
Generally, thunderstorms struggle to develop without much surface heating. During winter in the USA, surfaces rarely become warm enough away from the deep south. The sun’s rays just aren’t strong enough.
In the southwest, the main constraint differs; sea-level pressure (SLP) tends to be higher. This is due to high altitude westerly flow interacting with the Rocky Mountains. The air ‘piles up’ to the west. Some vacates north or south, the rest sinks toward the surface. The taller atmosphere and sinking motion both increase the SLP.
The latter is kryptonite to thunderstorms. They require moist, rising air to develop but under high SLP, not only does the air sink, it’s also very dry! The punchy subtropical sunshine may heat surfaces well, but that dry air and suppressed rising motion usually wins out.
In the southeast, lower SLP and a moisture supply from the Gulf of Mexico or neighbouring waters are enough to keep thunderstorms on the menu year-round.
Spring: Kicking Off
This season, lengthening days and strengthening sunshine enforce large changes.
Across the US, May average temperatures are some 5-10°C higher than the March ones. The biggest differences are seen well inland, away from the moderating effects of the oceans.
With greater heating of the land, air rises faster and further, supporting thunderstorm initiation.
However, it’s not enough by itself. There needs to be a good moisture supply too. Due to predominantly westerly flow aloft and the towering Rocky Mountains in the west, much of the USA must look south for that.
It’s not uncommon for warm, moisture-laden airmasses to surge well inland from the Gulf of Mexico. The typical reach of this is strongly imprinted onto the thunderstorm frequencies shown below.
A notorious thunderstorm hotspot spans from Texas to Kentucky to Mississippi. There, they tend to be not only most frequent, but most powerful too. This is when the tornado season hits it peak (especially April-May).
Elsewhere, most places see some increase in frequency, but there are notable exceptions.
Along the western coast, brisk sea breezes tend to develop by midday. This both cools the land and causes the rising air that thundestorms depend on to ‘lean’ inland.
In the far northeast, cooling breezes can come from several sources: the sea, the Great Lakes and eastern Canada. The restraints around New England are similar to those faced in ‘old’ England!
Summer: Prolific Coverage
Under the sizzling summer sun, there are few places in the country where thunderstorms remain rare. Away from the west coast, surface heating becomes so strong that it doesn’t take much moisture to facilitate some seriously sparky showers.
Compared with Europe, coverage of at least 8 thunderstorms per month is far greater. This is mainly due to lower moderation by large water bodies. With enough surface heating, strong uplift can outweigh a reduced moisture supply.
Unsurprisingly, thunderstorms are most frequent of all near the tropical waters. In parts of Florida, thunderstorms are an almost daily occurrence. This is not just down to heat and moisture: There are also sea breezes that often push inland from both sides and then collide in the middle. The thunderstorms follow the same pattern.
While they are at their most frequent and widespread, they’re not usually at their most powerful in summer. Due to changes in typical airmass characteristics, the ‘prime setup’ for enormous, tornado-producing thunderstorms (also covered in my blog piece severe US storms) is less frequent in summer than spring. So, you’re more likely to see a lightshow but less likely to encounter a strong tornado or giant hail.
Fall: De-Escalating Quickly
The mellow days of autumn bring a steadily relaxation of the thunderstorm risk. In September, the overall atmosphere, right up through the cloud level, tends to be at its warmest. This, despite surface temperatures starting to drop away in the north.
As that fall continues in tandem with weakening sunshine, sufficient uplift for thunderstorms becomes increasingly rare.
Compared to spring, autumn generally has a more stable atmosphere, due to fewer and weaker cold airmasses. The timing is also nearly 2 months further from the peak sunshine of mid-June.
This keeps the autumn monthly numbers 2-4 lower in the southeast US – but elsewhere, they’re very similar.
This is where moisture supply comes in, which is larger for two main reasons. The first is the warmer overall atmosphere, as it can ‘hold’ about 7% more moisture for every 1.8°F (1.0°C) rise in temperature. The second is deciduous plant leaves: They reach maximum coverage in Sep-Oct, transpiring some moisture into the air.
The Climate Trend
When it comes to weather systems and climate change, thunderstorms are among the most complex to resolve. Will higher atmospheric moisture capacity increase numbers or intensity, or will increased stability keep a lid on things? I tackled this question in greater depth back in July.
Now, though, MetSwift has a treasure trove of thunderstorm observation data that we can study (…evidently!). So, let’s take a quick look at what that reveals for the USA:
Comparing 2006-2020 with 1991-2020, changes are subtle, yet show clear regional differences.
The southeast is more or less ‘as you were’ all year round – but this is the exception. All other regions have experienced an appreciable increase in thunderstorm frequency during the hottest months; Jul-Sep. In the northern regions, May and June are also included.
This pattern has come correlation with the typical availability of moisture. Where that’s highest, the changes are least. An interesting connection that merits further investigation. In the meantime, I hope the thunderstorms behave well for you, wherever you may be… and whether you fear them or fancy them!
James Peacock MSc
Head Meteorologist at MetSwift