Reflections on the April 26–27 outbreak
There are aspects — meteorological and otherwise — about what happened April 26 into the early morning hours of April 27 which are notable beyond that event. In today’s viral age, that outbreak is now so last month and so many news cycles ago, but what did all of the data, which took a while to acquire, show?
Was the initial reaction in the blog/Twitter-sphere of the outcome being a forecast bust valid?
It all started
With this forecast from the Storm Prediction Center (SPC) a week in advance for Tuesday April 26 into early April 27…
And then the next day, the O-word…
By then, these forecasts had already created a buzz in social media.
Fast forward to the evening of the 26th and into the next morning, when there was a perception of an epic bust, with reports of only five tornadoes and no or minor impact from them. Show’s over, move along, nothing to see here. Disappointment was even expressed. (As if the weather not being more capable of death and destruction should ever be disappointing.)
Bust?
So was it? Well, things didn’t turn out as generally anticipated — but in many ways, and contrary to the prevailing opinion and premature proclamation, the outcome actually exceeded expectations.
What primarily led to the original perception was that thunderstorms along and near the dryline during the afternoon and early evening were unable to become the type of supercells that produce tornadoes which are high-end strength, long-track, and/or easy to be visually documented by storm chasers, like one last week in Kansas which had a 26-mile path and a peak rated intensity of EF4.
Fomenting that impression was the fact that one of the watches issued was a “PDS” (particularly dangerous situation) tornado watch, and the result did not meet the guidelines for such an issuance.
The PDS watch, especially in the context of official convective outlooks having never utilized the highest risk category (rather, the misleadingly named “moderate” risk, the next-to-highest one), led to legitimate questions being raised about its issuance, and no doubt significantly contributed to the negative impression.
Interestingly, though, in looking at the entirety of the forecast for that particular watch, the remainder of it, other than EF2+ tornadoes, verified.
And during the latter part of the time period for which that watch was issued, this ensued:
Conventional wisdom may be that when discrete cells “line out,” i.e. coalesce into a squall line, the tornado potential decreases, but in this case, it increased — a lot. In fact, that quasi-linear convective system (QLCS) of which the radar loop was part produced nearly 30 tornadoes, along with widespread straight-line wind damage from a serial derecho.
BTW, FWIW, here is an example of a model forecast for composite parameters of tornado ingredients which correctly anticipated the afternoon lack and later evening escalation.
Even without any EF2+ twisters, one wonders what the reaction would have been, by the public in the path of all the letters on this map, not just meteorologists, if that watch had been issued for a location/time just a bit northeast/later.
Of course, that’s hypothetical, but the point is that while the watch did not verify in a key aspect, it’s not necessarily as simple as being a categorical bust, as opposed to something in between.
Outbreak?
There is no universally-accepted, official threshold of what constitutes a tornado or severe weather outbreak. In fact, as a peer-reviewed paper stated in its abstract:
“The notion of an ‘outbreak’ of severe weather has been used for decades, but has never been formally defined. There are many different criteria by which outbreaks can be defined based on severe weather occurrence data, and there is not likely to be any compelling logic to choose any single criterion as ideal for all purposes.”
And from its conclusion, “…we wish to reemphasize that while we have developed a ranking scheme for severe weather outbreaks, we have not claimed the scheme is intended to define [their italics] an outbreak of severe weather.”
Originally, many years ago, the minimum number of tornadoes was considered to be five or six, but that is thought to be outdated.
Notwithstanding that, after the initial dearth of tornado reports on April 26 and early on the 27th and sentiment that a tornado outbreak had not occurred, now we’re up to 39 confirmed tornadoes. [Note that there were other severe thunderstorms and tornadoes from a separate event later in the day on the 27th and into that night.]
By any reasonable standard, that’s a tornado outbreak. (A relatively low-ranking one given the total path length and intensity, but an outbreak nonetheless.)
Preliminarily that’s the largest number of tornadoes for a single day-night event of this spring (the only bigger one was the winter outbreak on February 23).
Not only was it also an overall severe weather outbreak too, consider this:
- By far the largest total number of daily “filtered” severe (wind/hail/tornado) reports so far this year, 496 (second highest was 376 on February 24).
- Neither of the last two springs (2015, 2014) had any days with 400+; last year, the only day with more reports than this year’s April 26 outbreak was the July 13 derecho, and there were no such days in the whole previous year.
- By far the largest number of severe (1"+ diameter) hail reports so far this year: 197.
- Three derechos.
3 derechos
The occurrence of three separate derechos within the same day-night outbreak is exceptional.
In addition to the one noted above, earlier in the day a summerlike progressive derecho blasted ESE from Missouri to the Ohio Valley along a warm front. And later, shortly after midnight a line of thunderstorms developed west of Austin and then blasted east all the way into Louisiana, with enough severe wind and damage reports to meet the criteria for another derecho.
EF0
Interestingly, there was a preponderance of EF1-rated tornadoes (27 of the 39), and fortunately no EF2+ much less EF4 or 5.
In the wake of the outbreak, there was even sarcasm expressed about EF0 tornadoes.
But these people weren’t laughing that night, and to this woman and loved ones it doesn’t matter whether this tornado within that final derecho was an EF4, like that one in Kansas last week which thankfully killed nobody, or an EF0.
There’s also the issue of legitimacy of EF scale rating precision — for example whether any of the EF1s could actually have been EF2s and whether a tornado near Bells, Texas with a longer track than most during the outbreak would have been rated higher than EF0 if its path had not been mainly over open land.
The viral age
All of this is representative of much to be digested and understood in regard to this weather event, both scientifically and culturally.
Meteorologically, exactly why were the late afternoon supercells unable to sustain, and why did everything else seem to “overachieve”?
Those initial Day 6–7 outlooks by SPC nearly a week in advance were absolutely justified, and note that they, and ones on subsequent days, didn’t even specify a tornado outbreak per se, rather a severe weather event/outbreak.
Along the way, between then and the outbreak, were examples of sensationalism — there’s a difference between stating and overstating the potential — and in turn that led to cases of overreaction. Seems to be a lot of difficulty in calibrating response to weather, too often overdoing or underdoing it, and whether tornadoes, hurricanes, snow, floods or other phenomena.
Weather predictions can involve probability, meteorology/science can be complicated, analyzing information can take time. Yet we seem to live in an increasingly all-or-nothing (Hype! Bust!), reactionary, perception-is-reality, out-of-context, rush-to-judgment, short-attention-span, breaking-news world, lacking room or patience for complexity, gray areas, nuance, balance, objectivity.
Caveat emptor.
