reThought Flood

What Did We Learn From 2023 Events?

What did we learn from 2023 events?

Looking back, 2023 was in some senses an easy year for flood followers. The usual suspects of flooding were relatively quiet. We saw some flooding levels in the upper Mississippi in the spring, but no major flood events there. The Missouri basin was unusually quiet. The Gulf Coast saw only one tropical storm make landfall in south Texas. That’s like next to nothing! 

What happened? Didn’t climate change bode major weather catastrophes?  

Although the usual suspects were spared, we don’t have to look far to see that other places got more than their fair share of floodwaters, even more than their historical fair share. On top of that, certain nail-biting threats had everyone on edge. 

Snow Pack

From November 2022 into April 2023, the Sierra Nevada mountains in California got loaded with snow. So did the northern and southern ranges and the Cascades. California snowpack was measured at 10-1/2 ft in early April at South Lake Tahoe, with a Donner-esque high of 19.4 ft. at Panther Meadow east of Fresno. The total snowfall is identified as the second-largest on record. Distribution was uneven, with the north just reaching double the historical average, but with parts of the south at as much as four times historical. Flooding was guaranteed. 

Fortunately plenty of flood-control water storage is available. But the largest storage reservoirs are in the north, so the south suffered more. The snowpack had the California Department of Water Resources (Cal DWR) – and insurance companies – on tenterhooks over how quickly the melt would happen, because the speed of melting determines how quickly downstream flooding occurs. 

For the most part Cal DWR managed it well, drawing down the reservoirs as quickly as possible while managing inflow and additional precipitation. Levee breaches did occur, and residents of the Tulare Lake basin were reminded why their dry land was named a lake. But where did all this precipitation come from? 

Atmospheric rivers

2023 weather reports were all abuzz with the stories of “atmospheric rivers.” These are super-soaker linear flows of water-laden weather fronts coming in from the Pacific Ocean. They slam into California, with the Sierras acting like a squeegee. As many as thirty-one atmospheric rivers were identified between October and March. They contributed not only to the snow pack, but to common fluvial (riverine) and pluvial (flash) flooding as well. For the most part California commercial and residential areas were spared a major catastrophe, except for the levee failures near Wilton; residents of Pajaro were not so lucky. Overall though, management of the flood control systems worked wonders over what might have happened.   

Atmospheric rivers continue to rise up in the Pacific. 2024 will be an adventure for sure.

 Hurricanes, as always?

As noted, the Gulf Coast was spared this year. Meteorologists will ponder and conjure various reasons, but it’s all just down to the odds. Meanwhile, the west coast of Florida, and Tampa Bay in particular, were threatened by Idalia, which could have become “the big one” for low-lying Tampa-St. Pete Hillsborough County. This was starkly reminiscent of 2022’s Ida, and even 2020’s Sally, so nothing really new for this area. Overall though, 2023 ranked as fourth in history for named storms. Only Idalia was a land falling hurricane, but it could have been worse. It landfell in a very thinly populated area of the Florida Gulf Coast.

Let’s not forget Lahaina. While the Hawaiian city was destroyed by wildfire, the event was likely exacerbated by the low pressure of Hurricane Dora to the south. If a wet system blows into that side of the island before vegetation is restored, watch out for mudflows. Finally, surprising Hilary traveled north to visit the already-flooded Tulare Lake, although it was tardy to mate up with the springtime events.  

Rainfall = Flash Floods

2023 will be burned (soaked?) into memory in Michigan, Vermont, and Massachusetts due to individual intense pluvial events. Rainfalls of almost 5 inches fell in one day in very localized areas south of Detroit. Vermont saw the most widespread flooding in their narrow valleys, with up to 7 inches over three days wreaking havoc, including the entire downtown of the state capital. Leominster, Massachusetts received over 9 inches in one day, in a steep small area, eroding riverbanks and foundations, and overtopping (and nearly failing) a local dam. New York City had streets, tunnels, and basements filled again by more than 12 inches of rain in September. 

While none of these amounts were particularly unusual as a monthly total, what may be noted in the 2023 experience is the focused intensities of the downpours. When heavy rains fall in small areas, the infrastructure is overwhelmed. Collapsing riverbanks and moving debris obstruct normal flow paths, and create new floodways and unexpected flows. The future of risk assessment must continuously consider the potential dynamic impact of the developed environment. 

What’s next?

NOAA calls it an “outlook”, appropriately so. There is no such thing as prediction in flood or weather, other than conditional statements like “if X inches of rain fall then water will rise to [some level]”. Forecasting is always a probability business, and any long-term forecast is more climate science than a predicted estimate of what will actually happen. What we can observe right now is that the California snowpack is slightly above average in the north, average in the central, and below average in the south. The Missouri River contributing snowpack is near median levels.  

All eyes are on El Niño, which as of 14 December is expected to continue through the Northern Hemisphere winter. El Niño occurs when temperatures across the equatorial Pacific are above average, and it is strong this year, possibly the warmest since recording began in 1950. 

Climate scientists know that El Niño is associated with a strong southern jet stream. We humans then infer that the southwest will be wetter, the mid-west drier, and the central and northwest warmer. If the late spring brings a La Nina, we can see a flip in the wetness. But that is a very big uncertain “if”.  

Hurricane forecasters pop up like daffodils in the spring, with the usual variations in bragging estimates, but with no probabilities of landfalls that can be considered realistic. What we can say is that the atmosphere and the oceans continue to warm, and that both contribute energy to the tropospheric weather engine.  

We can also be confident that good risk assessment of local exposure and conditions is the best defense against flooding. Rains and floods will come. Where the water actually goes, and how much damage is incurred, depends on the individual risk, and the measures taken to protect it.  

  

Ed Haas

Ed Haas joined reThought Insurance in October 2019 as property risk and CAT modeling consultant. Ed was formerly with Marsh Risk Consulting for over twenty years as Senior VP, with a focus on natural hazard modeling for flood, wind, earthquake, and other perils. Work included managing data for modeiing, interpreting results, and applying them to insurance and risk management programs. The work included site assessments at a wide variety of unique properties throughout the US and internationally. Ed was the Risk Consulting leader in the Marsh Real Estate Practice and served as lead risk advisor to the largest clients. Responsibilities included insurance program design in regard to selection of limits and deductibles, as well as designing and managing appropriate risk control programs. Specification for new construction with regard to CAT perils was a key contribution to these clients, Ed’s career started with FM Global, and has held P&C broker’s and professional engineer licenses.