2 days ago
Why two tornado warnings near St. Louis came without a watch last week
ST. LOUIS – Twice last week, the National Weather Service issued tornado warnings in the St. Louis region without a tornado watch officially in place – a scenario that perhaps left some residents surprised or concerned.
On Wednesday, an EF-1 tornado tore through parts of Wildwood and Chesterfield. On Saturday, another EF-1 tornado struck just outside of De Soto city limits. In both cases, the tornadoes developed after a warning was issued, but without any prior tornado watch.
Fortunately, no injuries were reported from either tornado, but both storms caused various damages to homes, trees and properties.
After both tornados, many residents may wonder: How can a tornado warning be issued without a watch beforehand? After all, watches are typically meant to alert the public that conditions may support tornado development, giving people time to prepare. Warnings, by contrast, indicate a tornado is imminent or already happening, and more immediate action is needed.
While it's not exactly common to skip the tornado watch phase, it's not entirely unprecedented either.
During live coverage of Wednesday's storms from the FOX 2 studios, Extreme Weather Specialist Chris Higgins said of the sudden warnings:
'I know it's hard to believe this was a Level 1 risk out of 5, but that goes to show you that even a Level 1 risk can sometimes surprise you. And there's no question this was a tornado that sort of came out of nowhere, and the conditions that pulled it together happened very, very quickly over the course of less than 10s of minutes. More in the neighborhood of minutes or two minutes that it started to come together.'
Then, three days later, came another tornado-warned storm without a watch, marking two such instances over four days.
Higgins dove deeper into what happened with Saturday's tornado during a FOX 2 Weather Extra digital stream on Monday.
He explained that Saturday's storm developed from a weak surface low-pressure system moving from Kansas into Missouri. While it wasn't especially strong in terms of pressure or wind speed, it had enough low-level spin and wind shear to support rotating storms. That included mini-supercells, small, but potent thunderstorms capable of producing tornadoes.
The biggest reason a tornado watch wasn't issued? A discrepancy between local and national forecasting centers.
According to Higgins, the local National Weather Service office flagged concerns about possible tornado development well in advance and pushed for greater attention. However, the Storm Prediction Center, the national office responsible for issuing tornado watches, scaled back the outlined risk area the morning of the event and chose not issue a watch.
As the storms developed Saturday evening, meteorologists closely tracked rotation on radar, a key indicator of tornadic potential. Higgins said early radar scans showed some rotation, but not enough to immediately justify a severe weather alert.
'The National Weather Service at this point is aware that most of these storms are rotating, and I'd venture to say [before 7 p.m. Saturday], they're starting to draw up a possible tornado warning, but there's still the the rotation in the wind field here is still not supportive of a tornado,' Higgins said on Weather Extra.
'It's not overly alarming, but it's certainly enough that it's getting attention. But I would not be issuing a warning. If you issued a warning for either one of those cells all the time, you'd have hundreds of warnings and very few actual touchdowns. We're conscious of the fact that we want the weather service warnings to matter. They don't want them to issue and cry wolf because otherwise folks will not heed the warning.'
Higgins added that the tornado warning came about nine minutes before the actual touchdown, which is considered an adequate amount of lead time for sudden warnings.
FOX 2 also reached out to the National Weather Service for comment. Kevin Deitsch, a warning coordination meteorologist at the National Weather Service, answered five specific questions with the following remarks:
: Is it generally standard practice to issue a tornado watch before a tornado warning if there is any possibility of tornado threats for a particular day?
: 'Tornado Watches are issued in cases where conditions are becoming favorable for tornadoes over fairly broad areas. In these cases, the atmospheric conditions to produce tornadoes were marginal and very localized, therefore we did not feel a Tornado Watch over a broad area was necessary.'
What factor influence a decision to issue a tornado watch versus waiting on that and instead issuing a tornado warning without a watch?
'We look at the atmospheric ingredients conducive for tornadoes and how we expect storms to evolve as factors that go into issuing a Tornado Watch. The main reasons we did not issue a Tornado Watch on these two days were that the atmospheric conditions were only marginally supportive of tornadoes, and we did not deem it to be a widespread threat.'
Were there any specific challenges or uncertainties in forecasting last Wednesday or Saturday that affected why a watch was not issued?
'Marginal setups like that are challenging. You run the risk of issuing a Tornado Watch for millions of people, to have no tornadoes occur at all. Or in these cases, we didn't issue a Watch, but a couple brief, weak tornadoes did develop. The tornado in Chesterfield on Wednesday was about as isolated as they come, with it being the only tornado within about 1,000 miles of St. Louis.'
How is the National Weather Service looking to communicate with news agencies of the general public in cases where a storm may become tornado-warned without a tornado watch?
'We are constantly communicating to the TV meteorologists and public safety officials through our internal chat system. On each of those days, we gave a heads up that some of these storms were growing more capable of producing tornadoes, and we gave them a heads up on Tornado Warnings coming before they were officially issued. We also create weather packets for severe weather threats each day, which can be found at this link where we expect any threat of severe weather. These packets did highlight the low end threat of severe weather on those dates.'
Is there anything that could be different in the future with forecasting tornado threats after two severe storms that suddenly end up tornado-warned last week?
Dietsch: 'There is often a very low chance for tornadoes, but due to those low chances, tornadoes rarely materialize. If we were to issue tornado watches when there was any possibility of a tornado, we would have several dozen watches a year where no tornado develops. Instead, Tornado Watches are issued in cases where conditions are becoming favorable for tornadoes over fairly broad areas. In the recent cases [June 4 and June 7], the atmospheric conditions to produce tornadoes were marginal and very localized, therefore we did not feel a Tornado Watch over a broad area was necessary. The important piece is to take warnings seriously. Both of these tornadoes were warned in advance, thus as soon as folks get the warning, they should go to their safe space to shelter from the possible tornado.'
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