Capital Weather Gang 2013 summer outlook: Less stifling than last 3 summers

Visitors take refuge from withering heat in the shade of the Washington Monument on Independence Day, 2010 (Bill O'Leary)

Visitors take refuge from withering heat in the shade of the Washington Monument on Independence Day, 2010 (Bill O’Leary)

This has been one of the nicest springs I can remember.  Lots of crisp, sunny days and beautiful weekends.  We will likely finish the March – May period with near normal temperatures, and it turns out normal in spring here can be quite nice.  Those who detest the quick jump from winter to summer have been rewarded this year.

While our periods of heat have been quite hot, they have been infrequent and brief.  But summer can only be delayed for so long.  Last summer, we were lulled into complacency in early June with beautiful spring-like weather before a significant late June heat sneak attack, commencing our third consecutive brutally hot summer.

This year the trend ends.  We expect this summer to be quite pleasant in the scheme of things.  All summers in D.C. have periods of uncomfortable heat and humidity.  That is unavoidable.  But given that context, we should mostly avoid the incredible heat that plagued the last 3 summers. We favor  lots of days in the 86 – 93 range with heat waves short-lived.  Thus, we anticipate our coolest summer since 2009, and a 50-50 chance at our 2nd coolest since 2005.

The task of doing a summer outlook is no easy one.

First, with few exceptions, summer in the D.C. area is mainly hot. Do we really care if it’s 88 and humid or 93 and humid?

Also, the weather varies less during the summer than it does the the rest of the year (especially compared to winter), with the vast majority of summer months finishing within a few degrees of normal. So, it follows that 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 large errors are possible. Nevertheless, we’ll do our best to convey what we expect for summer 2013 based on factors we’ve analyzed…

OUTLOOK SUMMARY

Temperatures

We expect that this summer will finish with temperatures closer to normal.  Given that the last 3 summers averaged over 3 degrees above normal, normal will feel quite reasonable.

Precipitation

This Spring has been a tad dry here.  We expect this trend to end and allow us to have a wet summer in contrast to last year’s quite dry one.

OUTLOOK DETAILS

Temperatures

June: Average

July: Average

August: 1 to 2 degrees above average

Overall: Average to 1 degree above average

Number of 90-degree days for June/July/August: 25-30 (Normal is 31)

Number of 100-degree days: 0

Longest Streak of 90+: 6-8 days

Precipitation

Somewhat above normal (125% of normal)

METHODOLOGY

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 2013. 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 neutral phase of this equatorial Pacific weather pattern.  While this past winter was also technically neutral,  it leaned much closer to a La Nina episode, when ocean temperatures are cooler than normal, than an El Nino, when they are warmer.  It has been 3 years since we experienced official El Nino conditions.  We expect neutral conditions to persist throughout the summer and perhaps beyond that as we head toward next winter.

Spring conditions

This Spring has been quite unlike last year’s record warm one, as the vestiges of winter hung around throughout much of March.  While we were somewhat warmer in April thanks to a brief and anomalous heat wave early in the month, this was not true for much of the U.S.  Very cold air masses continued to penetrate the northern Plains and spill into the heart of the country.  The core of the cold has moved into Dixie this May where it has been markedly cooler than normal. We have had a variable May here, but cooler air masses have been legitimate when they have made their way into the mid-Atlantic.

Other factors considered

Both the PDO (Pacific Decadal Oscillation) and NAO (North Atlantic Oscillation)  have varied between predominantly and somewhat negative the last few years, respectively. However, both are showing signs of more variability and less of a strong signal one way or the other for any extended period of time.

It has been wet in the Great Lakes and Tennessee Valley and the drought in much of the Plains is improving.  This moisture will help to modify air masses a bit, and should eventually spill east to make us wetter than we have been this year to date.

Persistence.  We can’t ignore or pretend that we haven’t had a slew of hot summers here for almost the last decade.  Other than 2009, every summer since 2005 has been hotter than normal, and 19 of the 24 summer months have been above normal.  Summer has momentum, and in weather, momentum is often very hard to reverse.

Analog years

The analog years, or the years where the above factors were present to some extent, are: 1947 and 1951

The weather during these summers served as general guidance for our outlook.

Conclusion

After the last 3 summers – the hottest 3 on record,  we should finally get a tolerable one. While we can’t avoid the heat in DC, we will likely get through summer mostly unscathed.  Once we acclimate, those upper 80s to low 90s aren’t going to feel that bad, and many of the nights might be quite comfortable compared to what we have endured in recent summers.  Enjoy, since any given summer here is always vulnerable to being brutally oppressive.  You never know when the next tolerable or intolerable one will come around.

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)

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