Did El Nino Put a Dent in the Major Western Drought?

As the epic El Nino of 2015-2016 continues to fade, this is a good time to analyze what impact it had on the multiyear drought across the western United States.  Many Westerners were excited by the prospect of significant rain and snow that the strong episode of El Nino had been expected to bring during the fall and winter months.  Although it did have a positive impact on parts of the drought-stricken west coast, it could have been more beneficial.

The U.S. Drought Monitor Index Map from last October showed how extensive the drought conditions were in the western United States.  Since the late fall and winter months are when the western United States expects to see additional rainfall and snowfall, that’s a good benchmark to show how extensive the drought conditions were prior to the arrival of the El Nino-influenced rainy season. 

Fast forward to the current Drought Monitor Index map and you can see there was significant improvement in the Pacific Northwest, which experienced a dramatic reduction in the expanse of drought conditions.  While some dry conditions currently exist in parts of Oregon and Montana, the extreme drought conditions are gone.  

There is a simple way to look at the impact the El Nino-influenced winter season of 2015-2016 had in this by comparing the rainfall between this past winter and the previous, non-El Nino winter of 2014-2015.  From November 1, 2014 through May 31, 2015, Seattle, Washington had a rainfall deficit of 2.72”, while from November 1, 2015 through April 30, 2016, Seattle had a rainfall surplus of 11.35”.  That’s a net swing of more than 14 inches of rain in just one year.  Meanwhile, Portland, Oregon, had a rainfall deficit of 5.41” from November 1, 2014 through May 31, 2015, while they had a rainfall surplus of 9.21” from November 1, 2015 through April 30, 2016.  That’s a difference of more than 14.5 inches of rain during the same period.  Such a dramatic difference between a non-El Nino season and a strong-El Nino winter shows what an impact the El Nino event had.

Areas farther south weren’t as lucky though.  While San Francisco had a rainfall surplus of less than one inch from November 1, 2015 through April 30, 2016, the cities of Los Angeles and San Diego have actually seen larger rainfall deficits this winter compared to the 2014-2015 winter season.  That explains why the drought conditions virtually disappeared in the Pacific Northwest while much of California and Nevada remain in extreme to exceptional drought conditions.

The significant El Nino episode this past winter helped enhance the precipitation in parts of the western United States, but it didn’t bring the entire west coast the rainfall it so desperately needs.  The southwestern United States can expect to see some isolated rainfall during the summer monsoon season, but no widespread rainfall is on the horizon.  That’s not good news as the dry summertime heat builds across the desert regions of interior California and the southwestern United States.  Serving to foreshadow the upcoming summer, Phoenix, Arizona saw its first 100-degree temperature of 2016 this past week when it was 102 degrees on May 4. 

Cinco De Mayo 2016: Chilly and Wet

Drier than average weather has characterized the Nation’s Capital for all of March and most of April.  However, since April 23 there has been a major shift in the weather pattern for the Mid-Atlantic Region bringing above average rainfall and below average temperatures.

  

There has been measureable rain at National Airport on ten of the last 13 days since April 23 – including our current streak of nine consecutive days.  That streak is poised to continue through Saturday and will put a significant dent in the rainfall deficit which had been 5.27” on April 30 (for the nine-month period that began August 1).  This sustained rainy period also helps build up water levels as the hotter summer months loom on the horizon.  Not since May 2003 have Washingtonians experienced a similar streak of wet weather during the spring in the Nation’s Capital.

During the last two weeks, there has been a great disparity in temperatures, including a four-day stretch where temperatures remained below 60 degrees in the Nation’s Capital (April 28 – May 1) – more characteristic of March.  On the flip side, the warmest day of the year occurred on April 26 when it was 86 degrees at National Airport.  Eleven of the 16 Mays since 2000 have included 90-degree heat in Washington, D.C., so the weather can heat up quickly this time of year.

People sometimes forget that May can be a streaky weather month in the Nation’s Capital.  Five of the last six Mays have been drier than average and the last six Mays have been warmer than average in Washington, D.C.  At the same time, three of the four wettest Mays to occur at National Airport have occurred since 2003 according to the National Weather Service. 

NOAA’s outlook for May calls for above average temperatures and an equal chance of above or below average precipitation.  So it’s important we make up as much of our modest rainfall deficit as possible during this wet stretch of weather.  That will put us in better shape with the summertime heat and humidity – characteristic of this part of the country – looming on the horizon.

What Can We Expect?

NOAA's temperature outlook for May

Although March and April were both drier than average in the Nation’s Capital, a major shift in the weather pattern has brought an extended period of unsettled weather to the Mid-Atlantic Region.  A total of 1.22 inches of rain has fallen in Washington, D.C. since April 27 which reduced the rainfall deficit that had grown to 3 inches just since March 1. 

Although there is no drought in the DC Metro Region, there has been some drier than average weather during the last few months.  Seven of the last nine months since last August have featured below average rainfall with a running deficit of 5.27” through April 30.  So, the rainfall that’s fallen the last few days has been quite beneficial for Washingtonians.

Like April, the month of May features a wide variety of weather in the Nation’s Capital.  The average high and low temperatures on May 1 are 71 and 52 degrees, respectively, and rise to 80 and 61 degrees on May 31.  Washington, D.C.’s hottest May daily temperature on record is 99 degrees, which occurred on May 31, 1991.  Its coldest May temperature on record is 33 degrees, which occurred on May 11, 1936.  Area residents should also be pleased to know that average highs will be at least 70 degrees in the Nation’s Capital from now through October 12. 

May is also the wettest month of the year in Washington, D.C. with 3.99” of rain on average.  However, five of the last six Mays have featured below average rainfall in the Nation’s Capital according to the National Weather Service.  Yesterday we received 0.48” of rain at National Airport, whereas that much rain didn’t occur until May 16 in 2015.  Concurrently, there is also a stretch of six consecutive warmer than average Mays in Washington, D.C.  Area residents may recall that May 2015 narrowly surpassed May 1991 as being the warmest on record.

A key factor that enhances May’s rainfall chances is the higher frequency of showers and thunderstorms that typically occur during the spring.  For example, a powerful EF5 tornado developed in Moore, Oklahoma as part of an outbreak of more than 30 tornadoes on May 20, 2013; and on May 8, 2008 when 10 tornadoes occurred from North Carolina to Maryland.

May has also featured cooler than average weather.  An example is May 2003, which was more than four degrees cooler than average in the Nation’s Capital and also significantly wetter than average.  NOAA’s temperature outlook for this May is for a 40% chance it will be warmer than average, but only a 50-50 chance for above average monthly precipitation.