Unless you live under a rock, you are aware of the tremendous storm system affecting the Great Plains region of the United States right now. Jonathan Erdman wrote in weather.com that unofficial low pressure records have already been tied or broken in Pueblo, Colorado, Alamosa, Colorado, and Clayton, New Mexico because of the storm some have named Ulmer. More are likely to come. At the time of writing, wind gusts exceeding category 1 hurricane strength were being measured in Denver (75 mph) and Lubbock, Texas. The National Weather Service -Lubbock tweeted,
Now this is impressive. The West Texas Mesonet Endee, NM site (East of Tucumcari) hit a peak gust of 83 mph at 1110am. We occasionally see thunderstorm winds this strong but it’s pretty rare in a straight line event. The Bootleg mesonet is sustained at 59 mph #lubwx
Because of how the storm appears in satellite imagery or these really cool websites like Windy.com, many people have called the storm an “inland hurricane.” While I get the comparison, it is technically incorrect to refer to this storm in that way.
The current storm is a good teachable moment about the difference between a hurricane and a mid-latitude cyclone. I will start with the mid-latitude cyclone, which is also referred to as an extratropical cyclone, baroclinic low, or low pressure system. NOAA’s website has a workable definition that I will build upon:
A mid-latitude cyclone is a synoptic (large-scale) low pressure system that forms along weather fronts in Earth’s mid-latitudes (usually between 30° and 60° latitude from the equator). In the continental U.S., these storms most often occur from late fall to early spring when the temperature contrast between warm and cold air masses is greatest. During winter, strong low pressure systems can produce a wide variety of weather impacts, including heavy snow, sleet, rain, and coastal flooding…As temperatures warm and summer arrives, mid-latitude cyclones become much less common over the continental U.S.
The primary energy driving a mid-latitude cyclone is the presence of contrasting air masses. The boundaries of such air masses are called fronts and indicate that such counterclockwise-rotating (in the Northern Hemisphere) storms are baroclinic systems. Mid-latitude cyclones tend to be more expansive systems and can last several days. They form through a complex set of processes involving convergence of low-level air and divergence of upper-level air. The North Carolina Climate Office points out,
The low pressure system forms to the east of the upper-level trough of the jet stream. Air rises in low pressure systems because of the convergence of air at the surface and diverging air aloft which forms clouds. For the cyclone to intensify, the diverging air aloft has to be greater than the converging air at the surface. This essentially pulls more air upwards and the surface pressure of the system drops, intensifying the cyclone. Latent heat is also released within the clouds of the low pressure system. This warms the air and causes instability which further intensifies the mid-latitude cyclone.
The current storm is located in one of the favored regions for mid-latitude cyclogenesis (formation of a cyclone), to the east (or leeward) of the Rocky Mountains. Other common regions for development include the Gulf of Mexico and the Gulf Stream area off the Carolina coast.
Hurricanes like mid-latitude cyclones in the Northern Hemisphere have a counterclockwise circulation rotation and the lowest pressure is typically found in the center of the storm. Unlike mid-latitude cyclones, hurricanes (also Typhoons and Indian Ocean Cyclones) derive energy from water vapor evaporating from warm ocean waters. There are no sharp differences in air masses of hurricane formation regions (a barotropic environment) and the storm has a warm core above the center. Hurricanes are also much more compact in size (below), and the volatility of their life cycle depends upon availability of warm water and low wind shear. More recently, studies have identified certain tropical cyclones or hurricanes that maintain strength or intensify over land. This typically happens if the environment over land is relatively barotropic and soils are sufficiently moist from previous rainfall. It has been called the “Brown Ocean Effect.”
While the focus herein is on differences, a storm can actually take on the characteristics of both types of cyclones. In 2012, Hurricane Sandy was called “Superstorm” because it transitioned from a system deriving its energy from the ocean to one that was feeding off of air mass contrast (mid-latitude cyclone). Unlike mid-latitude cyclones, hurricanes are not associated with wintry precipitation unless they transition to an extratropical system like Sandy.
The current storm in the Plains rapidly intensified through a process called “bombogenesis.” When a storm “bombs out,” the pressure falls 24 millibars or more over a 24 hour period of time. The term is typically associated with mid-latitude cyclones whereas rapid intensification (RI) is more aligned with hurricanes. RI is defined, according to a Climate Central article, as “strongest winds within the storm increased by at least 30 knots (about 35 mph) in 24 hours.”