Thursday, July 28, 2011

Washington's Hottest Month Ever, By Wide Margin

See here for updated July data.

Aug. 1 Update: Chart updated with July 2011 record-breaking average of 84.5°.

July 31, 6 PM Update: Today's high of 99° and preliminary low of 80° each exceeded expectations by about 6°. If the low holds, the monthly average will increase to 84.5°, which is a remarkable 1.4° above the old all-time monthly record tied last year.

July 30, 5:30 PM Update: The daily high temperature at Washington National is 97°.

July 30, 2 PM Update: The high so far is 96°. The month-to-date average is up to 84.3°.

July 30, Noon Update: The morning low temperature of 81°, if it were to hold through midnight, would break the high minimum record by 3°.

July 30, 1:30 AM Update: The Washington daily low temperature of 80° is confirmed, tying the record high minimum set in 1949. The number of daily high minimums set or tied this month is now 6, and the record of 78° for the 30th is looking vulnerable with a current temperature of 85° (although the dewpoint is significantly lower than last night).

July 29, 5:30 PM Update: The preliminary daily climate report is in with a high of 104°. That smashes the previous Washington daily record by 5° and is just 2° below the all-time high. It's also enough to squeak out another tenth of a degree on the monthly average to date, raising it to 84.2°.

The high of 103° broke the Dulles record by an amazing 6°, and the 101° at Baltimore/BWI also set a record. Baltimore downtown (Inner Harbor) was 104°.

July 29, 5 PM Update: And, in fact, the 5:00 temperature is 103°, raising the record still further. Washington Dulles is at 102°, a daily record there by a margin of 5°.

July 29, 4 PM Update: Today's high/low temperatures so far of 102°/80° raise the month-to-date by 0.2° to 84.1° and the projected final average to 84.3°. Since the reading has been 102° for 3 consecutive hours, it's likely that the final high will be at least 103°.

Original post:
For the second year in a row, July will be the hottest month ever observed in Washington since official climate records began 140 years ago. Unlike 2010, however, when the July record accomplished a tie with 1993 by the narrowest possible margin, this July is literally burning up the record books. With today's high/low of 95°/78;deg;, the monthly average to date is 83.9°, well above the previous record of 83.1°.

Factoring in the forecast of a record-tying 99° tomorrow and more 90s for the last 2 days of the month, the average is likely to reach 84.2°, over 1° above the old record. This would be a bigger difference from the previous record than the difference between the current hottest July and the 5th place holder.

As for the prospects for the rest of the summer, here's what we said last year at this time:
Is there any clue to August's behavior from the fact that it's been so hot in July? Although there is little month-to-month correlation in Washington temperatures, the July-August correlation is one of the strongest in the year. The correlation coefficient of 0.227 says that about 23% of the variance in August temperature is explained by July's temperature. (The June-July correlation is only 0.133.) If you look at the right hand side of the chart of July vs. August temperatures, you can see that all of the hottest Julys are at or above the trend.

Furthermore, 10 of the top 11 hottest Julys have had August averages above the current August normal of 77.4°. Looking at all 28 of the past Julys with averages above 80°, the average for August was 78.1°, which is above average. In recent years, with the notable exceptions of 2004 and 2008, there has also been a tendency for August temperatures to be near or hotter than July on average, even though the long-term average is reversed.
Images (click to enlarge): July 2011 projected and observed average temperatures to date, plus 10 previous hottest Julys and long-term (1971-2000) average; 1871-2009 July vs. August average temperatures and regression line

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