The Extreme Precipitation Blog has been quiet this 2017-18 winter, and for good reason: much of the U.S. has simply not experienced significant heavy precipitation throughout the winter. Until, that is, the Mississippi and Ohio River Valley floods of mid-to-late February 2018. In this blog, we will share our preliminary analysis of the recent heavy rain that stretched from Texas to Michigan and re-evaluate our first-ever Extreme Precipitation Outlook, issued back in November 2017.
The Lower Mississippi & Ohio Valley Floods
After a dry-to-average January across much of the South and Ohio Valley regions, many locations were running below average precipitation for the winter (Figure 1). The cool-and-dry pattern continued into early February, until the middle of the month, when a very warm, summerlike high pressure system established itself off the Southeast coast, ushering in summerlike temperatures across the South. As southerly and southwesterly winds carried warm air northwards across the region, they also began to pick up moisture from the eastern Pacific Ocean and the Gulf of Mexico (Figure 2a). By February 20, 2018, atmospheric moisture content had built up to the highest levels ever recorded in the winter months over places like Illinois (Figure 2b)!
With record amounts of moisture in place from Texas to Michigan and the strong ridge over the southeast stuck in place, all of the ingredients were in place for a major flooding rain event. On February 19, 2018, a surface cold front began pushing into the region from the northwest, kicking off a wave of heavy rain from Missouri to Michigan. As the front slowly pushed southeast, its progress was impeded by the massive high pressure parked over the south, and it eventually stalled near a Houston, TX to Cleveland, OH line. For the next three days, waves of showers and thunderstorms developed over Texas, Arkansas, and Louisiana (with freezing rain and sleet at times on the northwest flank!). Each wave moved northeastward, spreading heavy rain across much of the south-central U.S. and on towards the Great Lakes. After five incessant days of rain, total precipitation really began to add up: our preliminary MetStorm® analysis indicates that as much as 13.81″ of rain fell across parts of Arkansas (Figure 3) during a 5-day period. Even more impressive is the extent of heavy rain: areas receiving more than 4″ of rain in 5 days spanned the area from near Dallas, TX, to Louisville, KY, with spotty areas as far north as SW Michigan seeing as much as 6″! According to our Average Recurrence Interval (ARI) analysis, the maximum 24-hour rainfall in parts of the region corresponded to nearly a 1-in-100 year event (Figure 4).
While the flooding rains seen in the mid-South this February were extreme, they weren’t entirely unexpected. Time for a look back at our winter 2017-2018 Extreme Precipitation Outlook.
Winter 2017-2018 Outlook in Review
Back in November, we issued our first-ever seasonal extreme precipitation outlook in the hopes of highlighting areas that we felt were more at risk than usual for heavy rain this winter. We used an analogue forecast approach, which compares observed weather patterns from similar years to extrapolate what the near future might hold. On the whole, we highlighted two main regions as locations to watch for extreme precipitation: the Pacific Northwest/Northern Rocky Mountains, and the Great Lakes states. Figure 5 shows what our analogue average (the five most similar years blended together) precipitation anomaly looked like, compared to the observed percentage of normal for December through the end of February.
To first order, our forecast was reasonably successful. It generally captured the potential for dryness in the southern Plains, desert Southwest, and Florida, and correctly highlighted the northern Plains and parts of the Great Lakes as likely to be on the wet side. Our first region to watch for extreme precipitation, the Pacific Northwest and Northern Rockies, was a bit of a mixed result: parts of Montana and Wyoming got hammered with repeated heavy snow events, receiving as much as 200% to 500% of normal precipitation over the winter! However, the coastal Pacific Northwest received anywhere from 10% to 50% less than average precipitation this winter.
The really interesting part of this forecast verification is the wet weather in the Great Lakes and mid-South. Much of the Great Lakes region was wetter than normal this year, although that was largely not a product of extreme precipitation events so much as frequent smaller events. Much of the precipitation has also been in the form of snow, limiting the immediate hydrologic impacts. However, the area of wetter-than-average conditions that stretches from extreme northeast Texas to Ohio was actually drier than normal for much of the winter. In the southwestern part of this region (TX, AR, LA), this was anticipated by our forecast; for the northeastern part, we expected better chances of precipitation. However, the late February storm that produced so much flooding boosted rainfall totals all through this corridor, eventually resulting in a wetter-than-normal winter overall for this area. The heavy rain that occurred in the Ohio Valley lines up very nicely with our expectations; it just so happens that this area of heavy rain extended somewhat further southwest than the “average” analog year would have suggested.
Overall, the precipitation pattern across the U.S. this year was well-anticipated by our forecast. The 19-24 February 2018 wide-spread flooding rain event was a great example of the type of event that we anticipated may be more likely than normal back in November: “La Niñas are also associated with more low pressure systems crossing the Ohio Valley, meaning higher potential for widespread heavy rain and snow exists.” While the presence of La Nina conditions in the eastern Pacific Ocean perhaps made atmospheric patterns somewhat more predictable than average this winter, we look forward to trying our hand at seasonal forecasting again in the future. As always, be sure to subscribe to our blog and check back with us regularly for more extreme precipitation analyses.