Sunset over Washington, D.C. May 24, 2015 ( Jim Havard via Flick r)

Even though it seems like we’re already well into summer as we wrap up one of our warmest May’s on record, summer – by its meteorological definition – doesn’t actually start until Monday.

After a brutal 3-year stretch of summers from 2010-2012, our last two summers have been pretty reasonable by D.C. standards. Can we make it 3 in a row?

We think we can, by and large.

Although we’re calling for a slightly warmer than normal summer on balance, we expect this summer to more closely resemble the last two rather than the record hot summers of 2010-2012.  It should feel fairly comfortable compared to that oppressive three year stretch, particularly in August.

But we still anticipate plenty of D.C.’s trademark heat and humidity, especially in July. Compared to last summer, we think it will be a notch warmer, with more 90 degree days.

As for the summer’s first month – June, while we doubt the unusually warm May portends a particularly hot June, we do predict the month to be slightly warmer than average.

Unfortunately – unless you really like long stretches of occasionally oppressive heat and humidity – we predict our warmest month relative to average to be July. Considering that is our warmest average month anyway, July could be pretty nasty with a reasonable chance of a medium to long-lasting heat wave (8+ days), and perhaps a 100-degree day or two.

However, we forecast the August that follows to be another cool one compared to normal. It has a chance to be cooler than 2013 and rank among our coolest in over 10 years. This should act to shorten the feel of what would otherwise be a very long summer, considering our early start.

Caveats: When putting together a summer outlook we’re less likely to see the signals for extreme warmth or cold (relative to average) that we sometimes see in advance of winter. As such, our outlook is of low-medium confidence. This kind of seasonal forecasting is experimental and errors are certainly possible.

Summer outlook overview

Temperatures

We expect that this summer will finish with temperatures slightly above normal.

Precipitation

We predict that precipitation will finish right around normal.

Outlook details

Temperatures

  • June: 1 to 2 degrees above average
  • July: 2 degrees above average
  • August: 1 degree below average

Overall: Around 1 degree above average

Number of 90-degree days for June/July/August: 28-34 (Normal is 31)

Number of 100-degree days: 1

Longest Streak of 90+: 8-10 days

Methodology;

The following factors were given the most consideration in preparing the outlook. It should be noted that any one factor does not necessarily correlate with a particular kind of summer (e.g., warm, cool, dry or wet).

El Nino

We are currently experiencing a weak to moderate El Nino event that officially started in March.


Sea surface temperature difference from normal. Above normal temperatures in the central and eastern Tropical Pacific indicate El Nino conditions (NOAA)

We expect moderate El Nino conditions to gradually strengthen throughout the summer.

[Models and experts lean toward strong El Nino forecast for the fall]

El Nino has a very weak correlation with slightly cooler temperatures in our area when considering summer as a whole. Though in July, there is actually a weak correlation to warmer than normal conditions.

[Current El Niño improves odds for average-to-cool summer in Washington, D.C.]

Pacific Decadal Oscillation and Arctic Oscillation

The PDO (Pacific Decadal Oscillation) has been in a positive stretch for well over a year now, and there is no reason to believe it won’t average positive this summer.


The Pacific Decadal Oscillation has moved into its positive phase over the last year (Japan Meteorological Agency)

The AO (Arctic Oscillation) has averaged positive for almost 6 months now, and we expect that to continue this summer. Both a positive PDO and a positive AO have a weak correlation with warmer than normal summer conditions in our area.

Miscellaneous

  • We are experiencing one of our warmest May’s on record, with temperatures running over 6 degrees above normal.
  • The Southern Plains have been getting hammered with heavy rains lately. While it is unclear if this will continue, such a pattern would help to mitigate hot air masses. Often in very hot summers with extreme heat, the Plains and Southeast are dry.

Precipitation difference from average last 30 days shows tremendous moisture surplus in the Southern Plains (High Plains Regional Climate Center)
  • 9 out of the last 10 summers have been above normal. It is hard to bet against a warmer than average summer, especially with D.C.’s urban heat island effect consistently elevating overnight lows, even in air masses that are yielding average afternoon highs.

Analog years

The analog years, or the years where some of the above factors were present to some extent, are: 1930 and 1982.

The weather during these summers was given some loose consideration in formulating our outlook.

CWG’s 2014 summer outlook (post-summer evaluation)
CWG’s 2013 summer outlook
(post-summer evaluation)
CWG’s 2012 summer outlook (post-summer evaluation)
CWG’s 2011 Summer Outlook (post-summer evaluation)
CWG’S 2010 Summer Outlook (post-summer evaluation)
CWG’s 2009 Summer Outlook (post-summer evaluation)