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Rainy Summer?
With the seemingly constant stream of moisture into most of the US this winter and spring, many are likely wondering what this moisture could suggest for this summer’s weather.
Fourth Wettest Winter
For the lower 48, this past winter, with the inclusion of March, has been the fourth wettest. (meteorological winter is December 1 – February 28). The only years wetter since the 1800s were 1982-83, 1997-98, and 1972-73[1].
Excess Moisture in Soil
The first effect of excess soil moisture is to keep summer temperatures down on average. The sun’s energy enters the ground and does two things: heats up the soil and evaporates water in the soil. If there is excess moisture in the soil, more energy goes toward evaporation and less to heating. This effect helps keep temperatures down if excess moisture is available, especially afternoon high temperatures. On the flip side, more evaporation leads to higher humidity. While afternoon high temperatures may be cooler, the discomfort of summer can still be there due to higher humidity[2] and warmer overnight low temperatures. The additional moisture in the atmosphere tends to lead to wetter summers.
Similar Wet Years
When comparing similar wet years this century, the summers of 2008 and 2017 were normal to cool and wet, while 2010 and 2016 were rather hot but still wet. The connection with dryness leading to heat is much more concrete—the only summers hotter than 2016 in this century were 2006, 2011, and 2012. These summers were preceded by drier than average winters. While summer temperatures are less than clear based only on wet winters, the odds of drought developing are lower than a typical summer.
El Niños
You may also notice a pattern in the winters wetter than the current one—they’re all El Niños. This is no coincidence, as El Niños tend to dump lots of moisture in the southern half of the US and the corresponding dry area in the Ohio Valley is small[3]. This winter was also an El Niño winter, but weaker and configured differently. The result was that the Gulf Coast was dry, but almost the entire rest of the country was wet[4].
Summer 2019
On average, the summer is likely to be rainier than usual for the US. This trend is significant for the Plains and Midwest, while the West looks dry as is typical in the summer. The current wet conditions alone aren’t conducive for temperature prediction but adding a continuing El Niño leads to a forecast of close to normal temperatures with some cooler areas in the Plains and Midwest. A warm lean is possible for the West, but not the excessively hot summer temperatures seen there recently. A few heat waves are still likely at times, but fewer in number and less severe than in recent years.
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1.CMB.Contact@noaa.gov. (n.d.). Climate at a Glance. Retrieved from https://www.ncdc.noaa.gov/cag/national/time-series/110/pcp/4/3/1895-2019?base_prd=true&begbaseyear=1901&endbaseyear=2000
2. Dry Times in North America. (n.d.). Retrieved from https://earthobservatory.nasa.gov/features/NAmerDrought/NAmer_drought_2.php
3. (2014, June 12). Retrieved from https://www.climate.gov/news-features/blogs/enso/united-states-el-niño-impacts-0
4. Enloe. (n.d.). National Temperature and Precipitation Maps. Retrieved from https://www.ncdc.noaa.gov/temp-and-precip/us-maps/3/201903?products[]=grid-ranks-prcp#us-maps-select
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