U.S. Severe Convective Storm Season

22 March 2019

MATT CELLO - CERMS Team Intern, JLT Re, Chicago, IL
JOSH DARR - Senior Vice President and Lead Meteorologist, JLT Re, Chicago, IL

A Near Record Slow Start to the Season

The start of the severe convective storm season was put on delay as winter persisted across much of Central U.S. through April. Extended late winter cold and dry air flowing out of Canada into the U.S. resulted in an unfavorable atmosphere for severe weather formation. For example, the first tornado in Oklahoma did not occur until the last week of April. Lack of moisture, lift and instability squelched any major outbreaks during a traditionally active month.

Thereafter, the jet stream pattern across North America flipped quickly over a ten day period from a winter to summer pattern. The transition period between these extreme temperature regimes produced several billion dollar severe weather outbreaks across the U.S., widespread in nature that extended from the central to eastern portions of the country. Interestingly, this rapid shift in the jet stream in early May can be tracked back to a split of the Polar Vortex several months earlier, in February. The split brought forth an exceptionally stormy March along the Eastern Seaboard, followed by entrenched anomalous cold across North America in April relative to the rest of the Earth. The summer pattern, well established by mid-May, suppressed thunderstorm activity in the center of the hail and tornado belt. Upper levels of the atmosphere were too warm, or ‘capped’, hindering thunderstorm development in what is usually the most active month of the year. The warmest May on record was recorded across multiple states in the eastern two-thirds of the U.S., indicative of the summer conditions lacking the ingredients for severe thunderstorm activity.

Figure 1: The evolution of the slow start of the severe thunderstorm season. A) The polar vortex split of late February 2018 set in motion a colder than average spring. (Source: earth.nullschool.net) B) Aerial depiction of March storm progression over several weeks that reinforced colder bias for the following weeks into April. (Source: RAMMB/CIRA/NOAA) C) Northern hemisphere temperature anomalies in mid-April, with North America featuring the coldest temperatures compared to average, prohibiting severe thunderstorm development in the heart of the tornado season. (Source: ClimateReanalyzer.org / Climate Change Institute, University of Maine) D) Statewide temperature ranks, showing all states above average, much above average, or record warmest in May when the jet stream flipped dramatically, also decreasing severe thunderstorm formation. (Source: NOAA/NCEI)

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Not All Thunderstorm Perils Were Equal in 2018

An interesting aspect of 2018 was the dichotomy between the three sub-perils of hail, tornado and straight line winds. The charts at the top of Figure 2 highlight hail and tornado storm reports on a cumulative and daily basis, relative to normal for the U.S. Due to hail and tornado being heavily weighted to the months of March through June, the maps in the bottom row highlight the hail and tornado reports for each state relative to the normal count in the heart of the season. The amount of blue and grey shaded states on the maps visualizes the relative infrequency of tornado and hail events in 2018 compared to the long-term average of each state.

Figure 2: Daily count and cumulative total of hail (upper left) and tornado reports (upper right) and maps of the cumulative departure of hail (lower left) and tornado reports (lower right) by state from March through June. (Source: NOAA/SPC/JLT)

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For hail, the most notable region that experienced well below average frequency of hail reports was the Great Plains, from Texas north to Nebraska and even into the Northern Plains. In fact, every state east of the Continental Divide experienced below average hail report totals. The below average tornado count was most prominent in the Southern Plains, however several states, namely Louisiana, saw above average counts of tornado storm reports this season.

Figure 3: Daily count and cumulative total of straight line wind storm reports and the cumulative departure wind storm reports by state from March through June (Source: NOAA/SPC/JLT)

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In contrast to hail and tornado, straight line wind reports are close to the national average in 2018. While a slow start was also seen in the first four months of the year, by mid-May a significant uptick in wind reports occurred as the late winter pattern switched to early summer. The wind report deficit closed to near average and maintained a near normal level of activity through the rest of 2018. On a regional basis, the Northern Plains and Southeast saw an above average tally of wind reports during the first half of the calendar year. A late June series of mesoscale convective systems developed north and east of a heat wave across the Southern Plains, setting in motion multiple thunderstorm clusters originating in the Northern Plains and tracking into the Southeast, culminating in the largest single wind report day of the entire year.

Key Events Driving Insured Severe Thunderstorm Losses

convective stormThe topics of weather volatility and potential climate influences are most often associated with periods of high activity. Large thunderstorm cat events, like Joplin and Tuscaloosa in 2011, or high frequency years, such as 2013, are recent examples of research on potential climate influences on frequency and severity of severe convective storm events and seasons. However, this past year illustrates that weather volatility related to climate influences can also facilitate periods of storm infrequency or lower than normal activity. Understanding these patterns over time, and the potential for climate influences to widen both sides of the frequency/severity distribution, is pivotal for future forecasting and ultimately estimating potential for loss.