(For the record, last summer finished 0.6 degrees warmer than normal; June and July were warmer than normal and August was cooler than normal)
We think we’ll hit 90 slightly more than last summer, when we had only 23 90-degree days (June through August), but that should be offset by some relatively “cooler” nights. More likely than not, we’ll avoid any 100-degree days.
Overall, we think this summer will be quite manageable by D.C. standards. If we can survive some of those oppressive mid-summer days (they always happen), we think there is a shot we could be rewarded with quite a nice August (again, a lot like last summer) – perhaps even our coolest in 10 years.
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 possible. (Prior to last summer, we produced a very successful outlook but we can’t say the same for our more recent winter outlook. Seasonal forecasting is a work in progress.)
Our summer outlook by the numbers
We expect that this summer will finish with temperatures around normal, leaning slightly above normal
We predict that precipitation will finish right around normal.
June: 1 to 2 degrees above average
July: 1 degree above average
August: 1 to 2 degrees below average
Overall: Average to 1 degree above average
Number of 90-degree days for June/July/August: 30-35 (Normal is 31)
Number of 100-degree days: 0
Longest Streak of 90+: 7-9 days
The main methodology for creating the outlook was the use of analog years. Analog years are past years in which conditions leading up to summer most closely resemble conditions leading up to summer 2014. Analog years are far from a perfect predictor due to the complexities of weather, as no two years are exactly alike. However, they can be of considerable value in giving us a general idea of what to expect.
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 Niño/La Niña
We are currently experiencing the formative stages of an El Niño. We expect weak El Niño conditions to develop and maintain throughout the summer, with some strengthening possible, particularly toward the end of summer. (By themselves, El Ninos tend to favor cooler than normal summers in the Mid-Atlantic, but they are not the only factor.)
The second half of April was quite cool, but May has been mostly warm. D.C. metro is generally running around 2 degrees above normal and the cooler shots have been less anomalous (or different from normal). The area of upper level high pressure near Alaska that persisted through much of the winter is still there, but is no longer delivering formidable cold shots to the East. Lately, the cooler air masses have mostly been relegated to the center of the country, while it has been warm on the coasts.
Other factors considered
The PDO (Pacific Decadal Oscillation) is currently in a negative long term phase, but it has been positive the last few months, and while month to month variability is common, we can expect it to gravitate positive over the summer. The NAO (North Atlantic Oscillation) and AO (Arctic Oscillation) have mostly been positive for the past year, and we expect that to continue. The QBO (Quasi-Biennial Oscillation) which has been in a positive phase for over a year is currently weakening, and could turn negative over the summer.
While there have been some fleeting signs of relief, parts of Texas and the Southern Plains are in a severe long-term drought. That will tend to enhance hot air masses developing in that part of the country that may migrate east.
Persistence. 8 out of the last 9 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.
The analog years, or the years where some of the above factors were present to some extent, are: 1986 (primary) and 2013 (secondary).
The weather during these summers served as loose general guidance for our outlook.