3/28/20: A severe weather day for the Midwest

Beginning yesterday, the Storm Prediction Center began issuing ominous warnings for today’s severe weather in northern Illinois. Most of the ingredients were already in place: a warm front pulling humidity from the south, a very strong mid-level jet from the southwest, and a steep lapse gradient above the cap. A practice round of storms sprang up across central Illinois last night, wetting the land with brief bouts of heavy rain and small hail. This morning, the SPC issued a rare “Particularly Dangerous Situation” tornado watch, with a moderate (15%) chance of local tornadoes depicted in the 12Z convective outlook.

Categorical outlook issued around 7am for 3/28/2020, above, along with a significant hatched area of heightened tornado risk centered around northwest Illinois, below. (NWS)

Warnings this serious only happen a few times a year for the entire country, isn’t it too early in the season to have tornadoes that far north? A Midwestern severe weather outbreak at this time of year isn’t without precedent. In fact, one happened on this exact date, March 28th, 1920. The first of the infamous “Palm Sunday Outbreaks” brought an estimated total of 37 tornadoes to Illinois, Indiana, Michigan, Wisconsin, Ohio, Alabama, and Georgia. Without radar or broadcast news coverage, over 380 people were killed. In today’s terms, that’s a staggering toll, more fatalities than every tornado outbreak since 2012 combined. Thankfully, our long-term and short-term warnings have come a long way.

The New London, CT newspaper from the following day offered its three cents on the outbreak: imagine the calls that were made to all of the different regional authorities to create this report. The headline underestimates the death toll by about half because information just didn’t travel very fast in 1920.

In a stroke of good fortune, today’s setup did not turn into the historic outbreak that many expected. Lingering morning showers cooled the surface layer, then cloud cover prevented insolation to the land surface. For topographical heterogeneities to play a role in tornado formation, differential heating at the ground level is practically a requirement. I ran my computational model for the northern Illinois, even exaggerating the sun exposure over the domain. Didn’t matter, the low-level environment was not energetic enough to explode through the mid-level cap. Thus, the parent thunderstorms that eventually formed lacked the energy or rotation to sustain long-track tornadoes, instead dropping a few brief twisters downwind of the Rock River and Illinois River valleys.

There were larger tornadoes that hit Jonesboro, Arkansas and Waterloo, Iowa; though outside the hatched area, these places were still under a severe weather watch. Both areas received several hours of sunshine before the storms rolled in, so perhaps the forecasting lesson is to never underestimate the impact of cloud cover on suppressing severe weather. As I incorporate more complete cloud cover data into my program, I will be very interested to examine the effects of land surface shading on atmospheric flows. In the ongoing quest to determine why specific thunderstorms develop tornadoes and others don’t, there may be insights in the sunlight.

Traveling amid COVID-19

Well, here we are. I’m back in Oklahoma, and the world is very different now. I’ve been in self-quarantine for about 5 days, sleeping, eating, and working from home. There is a remote chance that I was exposed to the virus during the last week of my trip, so strict isolation is the appropriate response, for others’ sake. First, a tour guide at Kualoa Ranch on Oahu began experiencing symptoms and submitted for a COVID-19 test last Wednesday. My cousin and I toured the ranch last Thursday. The positive test result only came back on Sunday night (Hawaii sends their sample kits to the mainland for confirmation), marking the first community transmission in the state. It is unlikely that we encountered the virus here: the contagious tourist was long gone, the infected tour guide stayed home, and the ranch is enormous. But during the week or so between the first transmission and the positive test, I continued visiting restaurants, museums, and bars in Honolulu, which is disconcerting, even with the extra hand-washing.

From a risk modelling perspective, the delayed onset of symptoms – and the consequent delay of information – is the defining challenge in tracing the viral spread. The number of confirmed cases can only be used as an indicator, not least because some cases inevitably go untested. Where we have no data, the instinct of the analyst is to estimate. Oahu hosts tens of thousands of tourists every week, but there was no chain reaction of infections on Kualoa Ranch or in Waikiki. An asymptomatic transmission, presumably rare. It boosted Hawaii’s count to 8, but that data reflects only reported illness cases from 5 days ago. Extrapolating with the exponential growth rate and the assumptions that 20% of cases are asymptomatic and 50% go untested, there were likely 20-30 COVID-positive people in the state at the time of my departure, the majority oblivious to their condition.

In the spirit of present news reflecting the reality of 5-7 days ago, I’ll tell you about my experience flying home on Monday-Tuesday. The airport atmosphere was noticeably tenser than usual, even in Honolulu with its pleasant open-air terminals. The line for security was long, but only because most passengers were standing 6 feet apart from one another. Unfortunately, the social distancing broke down before I even boarded the plane. About 20% of people seemed unperturbed by the inherent risk of the situation, including the girl behind me who crowded my bubble from the ticket queue all the way to our seats. And the rest of the passengers seemed resigned to the fact that distancing would be impossible on a full flight. As far as I could tell, I was the only one to disinfect my seat and tray table before sitting down, and I was among a minority wearing a mask. All in all, I came in direct contact with about 5-8 people, most of whom appeared oblivious or nonchalant about the heightened risk of exposure. Multiplying my risk by the number of people I encountered while traveling, I calculated my risk to be that of encountering any of about 100-200 people out of about 500,000 – about the same odds of running into someone I know in Oklahoma City. Over the span of a week, that’s not an incredibly likely occurrence but certainly can’t be ruled out.

There may be some who think my response is excessively paranoid. The truth is, it’s scary to do the math on how many people we are susceptible to catching a virus from or spreading a virus to. That’s why social distancing is so important for everyone, why self-imposed quarantines are so important for those with known contact with an infected individual. Even with the delayed symptoms, if everyone cut off their social connections for 14 days, the virus would run out of people to infect. No need for panicking or hoarding, but everyone would need to participate. China has only a couple of isolated new cases per day because they mandated a state-wide quarantine. While that level of disruption may not be permissible here in the United States, we should do our best to follow CDC recommendations, to at least flatten the curve if we can’t scale it down entirely. Our neighbors who are elderly, have preexisting medical conditions, or work in the healthcare field depend on it with their lives.

Edit, March 2021: Most of this post is irrelevant conjecture from before the airborne transmission dynamics were understood; heck, the general public wasn’t even instructed to wear masks until early April. But I’m leaving it up as a window to a fascinating and scary time to be alive.