Many people labor under the misconception that storm chasing begins and ends in the month of May when, in fact, this is not the case. It is generally true that, on average, the severe storm activity shifts northward as summer approaches. However, the severity does not generally decrease until mid-summer when the jet stream usually weakens and retreats into Canada. Many times supercells and tornadoes intercepted during the late spring and summer can make your chase year successful, ask any veteran chaser. The statistics definitely bear out that any time period from early May through the end of June is an equally valid time to chase.
Consider a few points when deciding whether to chase during the “early” or “late” tornado season:
- During the early spring, cold fronts sometimes sweep through the plains removing most of the low-level moisture so crucial for severe storm development. It can take several days for sufficient moisture to return in the wake of a strong cold front leaving chasers “high and dry” for the duration. If the “wrong” weather pattern is in place, one could suffer through several such cold air outbreaks earlier in the spring. The result would be very little severe weather activity in April and May. A few years where this occurred were 1983, 1992, 1993 and 1997 (and even so, there were several big tornado episodes during May of these years). Cold air mass intrusions like these decrease in both frequency AND intensity as the spring matures resulting in more potential chase days. Furthermore, in the late spring and early summer (June through July) weak cold front passages actually create an environment very favorable for supercell (rotating) storms in the high plains (the higher terrain of eastern Colorado, Wyoming and western Nebraska) a day or two after the front moves by. In fact, such a scenario is the reason why so many tornadic storms occur in this region from June through July and often through the remainder of the summer months. Email a few storm chasers and ask them where they like to be in June and July!
- The ultimate source of the low-level moisture so crucial to severe storm development and intensity, the Gulf of Mexico, is much warmer in late spring than earlier in the year. Hence, much more moisture is readily available for storms in the late spring and this moisture can easily take up residence in the high plains, northern/central plains and Midwest where the high-altitude westerly winds are prevalent by June (these west winds at high levels of the troposphere are also very important for severe storm formation and can sometimes disappear prematurely; see below).
- Since the prevailing westerly winds (often associated with the jet stream) at middle and high altitudes tend to weaken during late spring, storms move more slowly thereby making them much easier to intercept. During the early and middle spring tornado outbreaks, these winds can exceed 90 mph resulting in storms racing northeastward at 40-60 mph. This means you may not be able to catch a storm on such a day or, if you position yourself just right, you might get just one shot as the storm races by. Moreover, since weather systems are moving from west to east more slowly in June, often times it is possible to see severe storms in the same area for several days in a row. Of course, sometimes the jet stream takes a long hiatus in June or even reverts to an early spring-like pattern, shutting off the June tornado spigot.
Are there disadvantages to chasing in the late season? Sure! Here is a table which lists the primary advantages and disadvantages of Early versus Late season chasing:
Early Season: April 1 through May 31
|Jet stream is usually strong leading to more frequent tornado outbreaks.
|Jet Stream may contort itself into a configuration unfavorable for storms and drive cold fronts into the gulf.
|Often time most severe thunderstorms occur within a few hour drive from Oklahoma City.
|Many storm chasers take their annual vacations in May, so extreme traffic congestion occurs around storms.
|The Mexican plateau and the higher terrain of New Mexico are not that hot, so "capping" is frequently less of a problem.
|Sometimes there is too much thunderstorm activity (inhibits tornado formation) in one area due to a lack of a cap
|"Sloshing dryline" sets up many years in the Texas Panhandle resulting in classic supercell events.
|Some years, the dryline never makes it out of New Mexico until June. Active dryline years results in a record number of chaser's jockeying for position.
Late Season: June 1 through July 31
|Jet Stream is weaker so storms move slower and are easier to catch than their early season counterparts.
|Jet stream may disappear completely meaning nothing but uninteresting summer thunderstorms.
|Can have several days of great storms in the high plains meaning little travel. Less chasers are out meaning no traffic congestions.
|Roads are sparse in the high plains. There are usually several areas which look equally favorable making for difficult forecasts. More and more chasers are finding the remote areas now!
|Often there is tremendous instability (energy for storms to grow- related to low-level moisture availability).
|Sometimes the cap is much too strong so you just get a sunburn!
|Some of the most prolific Texas Panhandle tornado events have occurred in June.
|It is easy to miss the transition from southern plains to northern plains resulting in a "big" busted chase or two as the activity shifts northward in early to mid June.
- The cap is difficult to explain, but briefly, it is frequently associated with a layer of relatively warm and dry air formed over the elevated deserts (2-4 km above mean sea level) of New Mexico, Arizona and northern Mexico during springtime as the sun heats this region. Because it’s so hot at its base, this layer of air is characterized by convective overturning, which homogenizes – or mixes the layer – through its depth. Thus, this airmass is often called the “elevated mixed layer” (or EML). The prevailing southwesterly winds often blow the EML out over the lower terrain of the Great Plains and Midwest. This provides an effective barrier to thunderstorm updrafts but this barrier is sometimes overcome if the low-level moisture, warming and lifting is great enough. When this happens, big severe storms develop.
- The dry line is intimately related to EML. It is the leading edge of the desertified air mass AT THE GROUND and is often a key player in severe storm formation in the plains. The dry line exhibits a “diurnal cycle” (night and day) whereby it moves eastward during the day (due to earthward mixing of high-level westerly momentum within the EML) and retreats at night. Such a cycle often repeats itself for days at a time, thus the term “sloshing dryline”