Wednesday, June 12, 2013

Why The Plains Often Sees More Frequent Severe Weather Than Us

With tornadoes and severe weather in the news this week, one may begin to wonder why the Philadelphia area – and by extension, the Northeast – gets several weak tornadoes (like Monday’s EF-0 in Newark, DE) but does not get the kind of severe weather at the same frequency that is common in the Plains.

Most notable severe weather in the Plains, or at least, the storms that make the news nationally, are a storm type called supercells. Thunderstorms fall into three general categories, the single-cell, the multi-cell, and the supercell. The overwhelming majority of thunderstorms fall into the first two categories, but it is that third category that produces the largest hail, the strongest winds, and the most devastating tornadoes.

While all thunderstorms require three ingredients – moisture, lift, and instability – supercells require a fourth ingredient: vertical wind shear. And it just so happens that the Great Plains of the central United States are a perfect meeting place for all four of these parameters.

The Great Plains are located in a prime spot to receive very moist air flowing north from the Gulf of Mexico in the low levels of the atmosphere. Now the East Coast also receives moisture from the Gulf (as anyone who’s experienced a summer day in Center City can attest to), but the difference here is that on the East Coast, the moisture is not confined to the low levels of the atmosphere, which limits atmospheric lapse rates.

Let me explain. Dry air cools at a faster rate than moist air does. So on the East Coast, where we essentially have too much moisture, temperatures aloft do not cool as fast as they do on the Plains, where dry air in the middle levels of the atmosphere regularly advect off the higher terrain of the Rocky Mountains and cap the moisture below it. This region of dry air that is cooling rapidly is typically referred to as the Elevated Mixed Layer (EML). To even any semblance of an EML on the East Coast is extremely rare.

Those who are familiar with the parameter Convective Available Potential Energy (CAPE) will understand now why a big day on the East Coast may feature 1500 J/Kg of CAPE, whereas a big day on the Plains usually features CAPE values closer to 4000 J/Kg.

Higher CAPE means that air will travel upwards faster, and therefore also sink faster outside of the updraft, which leads to stronger straight-line winds.

In addition, very high instability (and associated colder air aloft than on the East Coast) also means that hail is typically much larger on the Plains than it is on the East Coast. This should make sense for two reasons. The first is that if the middle levels of the atmosphere are colder, hail will stay frozen longer as it falls. The second is that with high instability and associated fast updraft speeds, air that is rising in the storm will rise to higher levels of the atmosphere where it is even colder.

This covers wind and hail, but what about tornadoes?

As I already said, tornadoes require vertical wind shear. Vertical wind shear is the change in speed and direction of wind as one travels upward through the atmosphere. Typically (although not always) East Coast severe weather cases feature what is known as “speed shear,” – the increase of wind speeds from around 10 kts at the surface to around 40 kts at 500 mb – but lack “directional shear,” which is characterized by winds changing direction with height, typically in a veering pattern from southeasterly at the surface to southwesterly by around 700 mb.

A feature unique to the Central Plains of the United States is the Low Level Jet, which is an increase in southerly wind speed that occurs right before sunset and is most detectable at 850 mb. This Low Level Jet helps to transport more warmth and moisture northward for storms, but also significantly increase vertical wind shear, which explains why the central United States see so many supercells, and also explains why most strong tornadoes occur in the late afternoon, evening, and overnight.

This is not to say that high-end severe weather cannot occur on the East Coast. Violent EF-4 tornadoes have struck Worcester, MA (1953), Windsor Locks, CT (1979), and La Plata, MD (2002). However, these violent tornadoes are not nearly as common on the East Coast as they are on the Plains, which is why short-track EF-0 spin-ups, like Monday’s tornado near Wilmington, are the most common form of tornadoes in the Philadelphia area.

That said, with tomorrow's severe weather risk level at moderate, the potential for stronger tornadoes or damaging wind events does exist. We don't see these events with the same frequency as our friends in Oklahoma or Kansas do, but they can happen in the right setup.  With a strong upper level disturbance and the type of directional wind shear that supports tornadic activity, tomorrow is one of those setups that can produce stronger tornadoes around here. Again, it's more rare here than in the Plains, but it can happen.