Southern Climate Monitor June 2019 | Volume 9, Issue 2 In This Issue: Page 2-5: The Biggest Rain Events Ever Page 6-7: NOAA’s Hurricane Season Outlook Page 8-9: Managing the Costs of Disasters Page 10-11: Evaluating Heat Related Illnesses Page 12-13: 20 Years Later: The May 3-4, 1999 Southern Plains Tornado Outbreak Page 14-16: Are there trends in the heaviest hourly periods with rainfall? Page 17: About SCIPP Team: Barry Keim Page 18: From Our Partners The Southern Climate Monitor is available at www.srcc.lsu.edu & www.southernclimate.org Feature The Biggest Rain Events Ever John Nielson-Gammon, Texas A&M University, Texas State Climatologist While Hurricane Harvey was intensifying in available to dispose of all the water, massive the Gulf of Mexico, it was pretty obvious that flooding resulted. somebody was going to get a ton of rain. As it turns out, the highest rainfall total credibly I wondered, how does Harvey stack up against measured by a rain gauge was 60.58” in the biggest rain events ever documented in the Nederland, in the southeast corner of Texas United States? And which events were they? (Blake and Zelinsky 2018). That’s five feet of Do they normally happen in the southern rain.* United States? Or were we just lucky? As Harvey was happening, I had a hunch that To figure out the total amount of rain in a it could break some multi-day rainfall records. given area, I needed what are called rainfall So I looked the records up. It turns out there isn’t an official list list of multi-day rainfall records for the United States. There was, however, an official record for greatest rainfall total from a tropical cyclone, set a long time ago on the Hawaiian Islands, which Harvey handily beat after a few days. Within the year, another storm hit the Hawaiian Islands with an even larger rainfall total than Harvey, so the record’s back where it came from. That much rain in a short period of time will overflow the rain gauge, but that’s only a problem for weather data geeks like myself. The bigger problems happen when the heavy rain falls over a large area. And then keeps falling. A map of total rainfall from Harvey is truly impressive. At its normal flow rate, it would have taken the mighty Mississippi River three days to drain all the water dumped on Harris County during Harvey. Since there wasn’t a Mississippi River Figure 1: Integrated Harvey rainfall analysis by Applied Weather As- Southern Climate Monitor sociates, based on integrated radar and gauge measurements. June 2019 | Volume 9, Issue 2 2 “analyses”: maps of the distribution of rainfall. Fortunately, such analyses are produced During the road- and dam-building eras of the by several groups. I made use of analyses early to mid 20th century, special maps were from the Northeast Regional Climate Center produced after every big rainstorm. Engineers that use radar and rain gauge data over the needed to know how much rain could fall over central and eastern United States over the past a given area, and how big a storm could be 15 years and rain gauge data over all of the expected during the lifetime of the road or United States going back to 1950. dam. So there’s a nice collection of analyzed heavy rainfall events from which I can crudely With all these analyses, I had access to data estimate things like how much rain fell over from all storms since 1913 in the lower 48 three days in a 10,000 square mile area. states as well as a handful before 1913. I even had multiple analyses for some storms. And The need for such analyses continues, but often, those analyses did not agree. now humans can combine with computers to do the work much more efficiently. The One notable case was a slow-moving tropical result is a “gridded” analysis of precipitation, storm that hit southern Louisiana in August meaning that average precipitation totals are 1940 (Roth 2010). Several stations in the area estimated on a regularly-spaced grid, like the recorded more than two feet of rain in just a squares formed by a screen. This made my couple of days. My analysis from AWA had job much easier: I could just figure out how the Louisiana 1940 storm rivalling Harvey many squares it took to make 10,000 square for a two-day event over 1,000 square miles miles and calculate the amount of rain over (Figure 2). But the Livneh analysis had only that many squares. One of the companies I about half the rainfall amount, making the worked with on the Harvey rainfall analysis, Louisiana storm irrelevant. Applied Weather Associates (AWA), happened to have an archive of analyses that they’d Seeing this, I decided to look at the data for performed of some big events of the past, in myself and do my own analysis. An online order to estimate the risk of extreme rainfall in data interface was frustrating, because most particular locations. of the data was missing during what should have been the heaviest part of the storm. Another use for analyses of rainfall is to understand impacts on soil moisture and plants. In Biggest Rainstorms Ever (that we know of) recent years, the impact of climate T10 change on soil moisture and WA, Jan 1935 plants has become an important T15 OR, Oct 1950 issue, so a team from the western 2 OR, Dec 1964 T5 CA, Dec 1955 United States (Livneh et al. 1913) Harvey 2017 T10 CA, Feb 1986 Floyd 1989 developed a set of temperature LA, Aug 1940 Claudette 1979 and rainfall analysis going back Florence 2018 T10 CA, Dec 1966 all the way to 1915. While they Georges 1988 TX, Jun 1899 8 analyzed everything, not just the AL, Jul 1916 3 heavy rain events, I could pull TX, Oct 1994 out the heavy rain analyses and Beulah 1967 T10 make use of them. 4 9 T10 1 T5 T15 The Livneh analyses include data 7 through the year 2011, so I still needed more recent analyses. Figure 2: Map of rankings of biggest known rainstorms in United States history, according to the number of times they appear in the top five all- Southern Climate Monitor time lists for a range of number of days and area sizes. June 2019 | Volume 9, Issue 2 3 Digging into the raw archive, I found that this All other second-place events were along the data was present but had been flagged by the Gulf or Atlantic coasts: Hurricane Georges quality control algorithm for being suspicious. in 1998 (Mississippi, Alabama, and Georgia), The Livneh analysis had apparently accepted Hurricane Floyd in 1999 (North Carolina to the quality control flag and tossed out the Delaware), and Hurricane Florence in 2018 suspicious data. Since all the one-of-a-kind (the Carolinas), in addition to the 1940 event data had been ignored, the resulting analysis in Louisiana. Expand the list to top threes was not of a one-of-a-kind rainstorm. and we only need to add three more storms: a California atmospheric river in 1955, I was able to alert the data archivers of Hurricane Beulah in 1967 (Texas) and the this issue, so they went in and manually 1899 Hearne storm (also in Texas). overrode the quality control algorithm. When The map shows the locations of all storms that future scientists analyze the event using the made at least one of my eighteen top 5 lists, computerized data, they’ll have access to all along with ranking on the “appears on most of it. top 5 lists” list. Harvey, of course, appears on all 18, so it’s #1. A poor little storm like With analyses compiled and reconciled, it was Claudette in 1979 only shows up on one list, time to do the rankings. I looked at areas so it ranks near the bottom of this highly ranging from 1,000 sq mi to 50,000 sq mi and selective group. Claudette, by the way, held event lengths from 2 days to 5 days: 18 different the record for greatest one-day precipitation rankings in all. And in every single category, total in the United States until yet another Harvey was the winner! In some categories, Hawaiian storm last year eclipsed it. Harvey produced 30% more rainfall than its nearest rival. Just about every storm on this list was associated with a tropical storm or hurricane, When it gets interesting is with the Avises of except for the west coast ones. Tropical the rainfall world, those who come in second storms and hurricanes aren’t guaranteed to even though they try harder. Most often second produce record amounts of rainfall, but they was the “Christmas Flood”, an atmospheric certainly make it possible. river event that hit Oregon and California in December 1964 and easily defeats all others So if you want lots and lots of rain, you’ve (except for Harvey) for events lasting five days got two options: the Gulf or southeast Atlantic (USGS 1964). Atmospheric rivers are long, coasts in summer or early fall when a tropical thin tongues of atmospheric moisture that cyclone hits, or the Pacific coast in winter emanate from the tropics and subtropics and when an atmospheric river hits. And don’t produce lots of rain when they make landfall. forget your chaise lounge.** Southern Climate Monitor June 2019 | Volume 9, Issue 2 4 Footnotes *Here’s an amazing fact for the next time you’re short on conversation topics in a bar.