US weather outlook highlights spring flooding risk

Enlarge / Below-average snowpack in the Sierra Nevada on March 3.NASA EO

The US National Oceanic and Atmospheric Administration put out its latest monthly update Thursday, this time focused primarily on the flooding risk around the US as spring arrives. The unusually warm winter has put the country in a bit of a split personality: some parts of the US are at risk for flooding, while others can expect worsening drought, instead.

With the notable exception of cooler weather in Alaska, the continental US ended up at the sixth warmest winter (December, January, February) on record. Although warmer temperatures meant below-average snowfall for a good portion of the country, the eastern half saw above-average precipitation overall—more of it simply fell as rain.

The western US, meanwhile, saw low precipitation in many areas, contributing to drought conditions. That includes California, where the snowpack is currently well below average despite a round of good snowfall this week.

  • This is how winter temperatures ranked in the lower 48. NOAA
  • And here's how wet or dry the winter was. NOAA
  • Drought status as of March 3. NOAA

Surface water temperatures in the Pacific Ocean have been stuck in neutral—neither El Niño nor La Niña—for the past year, and thats not expected to change in the coming months. That means it wont tilt the weather scales, leaving the US outlook subject to other weather patterns.

NOAAs outlook for April, May, and June is pretty much more of the same. Warmer-than-average temperatures are likely for most of the continental US—including Alaska this time. (Average, by the way, is defined based on 1981-2010 here.) For precipitation, a dry-west/wet-east pattern is still favored, which has obvious implications for flooding and drought.

This outlook is the product of pretty solid agreement in the long-range model forecasts, as well as expectations based on long-term trends. Low snow cover enables warmer temperatures in early spring, although wet soils in the central US could have the opposite effect. Unfortunately for Oregon, California, and southern Texas, dry soils are helping move the needle toward even more dry weather, as there isnt much water to evaporate into the air.

Where the soils are wet, that also means the risk for spring flooding is higher. Combined with the outlook for snowmelt and precipitation, NOAA is highlighting flooding risks for about a third of the country, centered on the Mississippi River. There is a possibility of moderate or major flooding—high enough to impact buildings—in several regions including the Dakotas and Minnesota, the southern Missouri River, the upper Mississippi River, and across Alabama, Georgia, and Mississippi.