Thousands of warnings for severe weather and flooding are issued by the National Weather Service during the busy months of spring and summer. Although primarily a warm-season phenomenon, these warnings trickle out even in the heart of winter. The weather never sleeps, and sometimes you wonder whether the good folks at the National Weather Service (NWS) forecast offices do, either.
Since I last examined storm warning data provided by the NWS two years ago, new ways to show off what they indicate have emerged. Given more years of data, we now have a more comprehensive view of when and where extreme weather has most frequently affected the Lower 48 states in recent times, which coincides with when the NWS began standardizing dissemination of these lifesaving warnings on Twitter.
Five years of warnings in one chart
The chart below presents all of the first-issue warnings (not including updates) for severe thunderstorms, flash floods and tornadoes disseminated by the Weather Service since 2014 and through the middle of 2018.
What is immediately noticeable is that 2017 was a low point in this stretch for warnings. The year was fairly quiet overall for severe weather, particularly during the typical peak season in later spring. Even the lackluster performance of the 2018 tornado season — one of the quietest on record — is masked a bit by plentiful severe thunderstorm warnings.
That red line for tornado warnings is rather flat in 2018, and never rises to past years’ numbers.
Flood season peaks now, and the Gulf Coast is frequently a target
Although flooding does not tend to generate the headlines of a tornado outbreak, in many years flooding is a leading cause of weather fatalities and economic losses. The map below shows flash flooding warnings have concentrated in the south-central states, which routinely draw tropical moisture from the Gulf of Mexico.
You may have also noticed in the chart above that the flash flood warning peak comes later than the severe thunderstorm and tornado peak. This is largely because, as you get deep into the warm season, much of the country experiences occasional thunderstorms, whereas spring tends to feature the most tornadoes. Also, the jet stream is at its weakest over the United States later in the summer into early fall, so storms generally move slower and unload excessive rain for a longer duration. Finally, the North American monsoon and Atlantic hurricane season peak in late summer and early fall, which brings flooding to the Desert Southwest and, in some years, the Gulf Coast and eastern United States.
Severe thunderstorm season is ongoing and can touch much of the nation
While severe thunderstorm warnings are a frequent part of life in many parts of the country, there is still plenty of confusion about what makes a storm severe. The NWS defines a severe storm as one “producing hail that is at least one inch in diameter or larger, and/or wind gusts of 58 mph or greater, and/or a tornado.” They continue, “although lightning can be deadly, the NWS doesn’t use it to define a severe thunderstorm.”
Curious whether there are any signals in the warnings issued, I mapped and analyzed them based on the severity of the projected hazards.
Below, I present warnings issued based on their maximum expected winds. It’s probably no surprise that the highest winds in warnings focus on the Plains and Midwest, places that generally receive violent storms with greatest consistency.
The map of severe thunderstorm warnings based on their maximum expected hail size reveals baseball-size and larger hail are most common in the Plains, and especially the Northern Plains. Every now and then, giant hail migrates out of those zones, as well.
While they don’t cover as much territory as severe thunderstorm warnings over time, tornado warnings have touched much of the Lower 48 over recent years.
But not all tornado warnings are equal. The National Weather Service in recent years introduced impact-based warnings, which elevate certain tornado situations above others. When there is a confirmed tornado visually, or a significant tornado is expected based on radar confirmation, the NWS may issues a particularly dangerous situation (PDS) tornado warning. It may also declare a tornado emergency when a destructive tornado is headed toward a densely populated area.
The map below shows where PDS tornado warnings and tornado emergencies have been issued over the past two years.
Extreme wind warnings
An eyewall of a powerful hurricane can have effects akin to those of a strong tornado. In these cases, a special kind of warning known as an extreme wind warning is deployed. These warnings are issued when winds are anticipated to reach or surpass 115 mph in the near term.
Because of the inactivity of Atlantic hurricane seasons before 2017, there were no deployments of this warning in recent years. Then the extreme 2017 Atlantic hurricane season changed that in a big way.
That year, extreme wind warnings were declared when Harvey struck Texas, Irma blasted the U.S. Virgin Islands and Florida, and when Maria plowed into Puerto Rico and the island of Vieques.
One is the loneliest number
When I first analyzed warning data two years ago, I examined warnings issued for a population of zero. They exist and are not as rare as they might seem.
We like numbers, or in this case lack of numbers, and a warning for no one is a fun idea. But what about a warning for one person? They’re even more rare …
The NWS has issued more than twice as many severe weather warnings (for floods, thunderstorms and tornadoes) for no people (415) than for one person (192).
Whether the NWS issues a warning for one person or, in the case of a recent severe thunderstorm warning for a record population of 18,142,215 in the New York City area, they are critical for protecting life and property. Since I produced my first analysis, the NWS has increased the breadth of its warning dissemination on Twitter. It introduced flash flood warnings in July 2016, and an increasing number of local offices are sharing warning information on their feeds. This is surely a positive development.