Summer of Simmer: Hottest July (Tie), Hottest June-July, 2nd Hottest of Any 2 Months Hottest Summer on Tap?

July 30, 2011 Update: For new all-time hottest month record, see Washington's Hottest Month Ever, By Wide Margin

Aug. 1, PM Update: Today's minimally rain-cooled (a trace) high temperature of 82° is the coolest since July kicked off with 81°. That has already been factored into the August projection below.

Note on regression (see below): If the regression relationship shown in the July vs. August correlation plot were to hold exactly, the August average would be 78.9°, bringing the summer average to 80.9°.

Aug. 1, AM Update: Although there is no official record report as of yet from the National Weather Service, this morning's preliminary monthly climate report confirms that the Washington July average was indeed 83.1°, tying the record for hottest July (and hottest month of all time, since the hottest August was 82.8°).

Here's how much of a squeaker this record was, however. The monthly average is computed by averaging all of the daily high and low temperatures in a month. Since temperatures are reported in whole degrees Fahrenheit, the average is calculated to the nearest tenth of a degree. To 3 decimal places, however, the July 2010 average was 83.065°, which rounds up to 83.1° (thanks to Mr. Softee's Excel™). If any one of the daily highs or lows had been 1° cooler, the average would have been 83.048°, which rounds down to 83.0°, missing the record and tying for second place.

Now, consider yesterday's morning low of 71°. At midnight local time, the temperature had dropped only to 78°. Since daily records are kept on standard time, the month was not closed out until 1 am EDT, when the temperature was at 76°, still not a problem for the record. Showers were moving into the area, however, and by 2:16 am an updated report showed the temperature had already dropped to 70°. This means that the record held by as little as 1 hour and 16 minutes.

Since June set a monthly record, the June-July average of 81.8° is also a record by a wide margin over the 80.6° in 1994. As the summer seasonal average to date, it's also a huge excess over 1980's record pace. On the other hand, average temperatures decline in August, so it's unlikely that pace can be continued. Notice that despite 1994's record performance for the first 2 months, a cool August by about 2° below average knocked it completely out of the top 10 hottest summers. In 1980, however, a relatively cool June was followed by a very hot July and August.

What con be expected for the remainder of this summer? Even projecting an average August, the final summer average would still break the heat record. The upcoming week, however, is already likely to have temperatures in the 90s. Factoring in the forecast for the next 7 days and the climatological average for the rest of the month, this year's summer average would be 80.5°, a very impressive 0.5° above the current record from 1980.

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.

So, it's hardly a guarantee, but the odds are definitely in favor of a record. Stay tuned.

Original post:
Today's high in Washington of 86°, combined with a higher-than-forecast low of 71°, results in a July average of 83.1°. This unofficially squeaks out a tie by the narrowest of margins with 1993 for the hottest July in 140 years of temperature history.

Following a record for the hottest June, this June-July is by far the hottest such period and is the second hottest of any 2 consecutive months in Washington weather history.

More details and graphics to follow.

Video: Global Warming: Is it True?

Here's an excellent video summarizing the scientific data behind the global warming story, posted yesterday:



The author is a former professor of geology who has also held the following posts: Acting President of Oberlin, President of Franklin and Marshall College, President of Reed College, President of the Franklin Institute Science Museum in Philadelphia, and President and Director of the Los Angeles County Museum of Natural History.

Summer of Simmer: Party Like It's 1999

Midnight Update: Today's high is confirmed by the midnight report. Assuming the next 3 days go as forecast, July will end up with an average of 82.9°, the 3rd hottest in history. August can then average 0.75° below average or warmer and this will be the hottest summer in Washington's 140 years of temperature history. Normal for August is 77.4°. (That's right, August can average nearly 6° cooler than this month, and it will still be a record-breaking summer!)

9 PM Update: There was apparently a high of 93° after 5:00, as has happened several times recently.

Original post:
After 2 consecutive days below 90°, Washington's preliminary high temperature today was 92°. Combined with a morning low of 75°, the day was 5° above the long-term average, and the month-to-date July average is now a wannabe record of 83.3°.

Another +5° or so are likely tomorrow, but a cooling trend going into the weekend could produce a below-average day on Saturday for the first time since July 3. Depending on how cool it can actually get Saturday morning, the monthly average will be in tight contention with 1999 for second place on the July temperature hit parade. The current forecast is for 63°, which would equal the low for this month. The last time the temperature was below 72° was July 4.

Regardless of the final standings for July, the combination with the record warm June is almost a mortal lock to be the hottest June-July by far. The 2-month average to date is a hefty 1.4° above the current record in 1994 and behind only July-August 1980 as the second hottest of any 2 consecutive months in Washington. All of the top 9 previous hottest pairs of consecutive months have been July-August, rather than June-July.

Washington Setting Double Daily Heat Record; Monthly and Seasonal Records Also Possible

The preliminary daily climate report as of 5 pm today shows a high temperature of 101° and a low of 81°. The high has easily broken the old record of 96° for July 24 (set in 1987, 1968, and 1884). The low, assuming it holds through midnight standard time (1 am EDT), will also break the previous record high minimum of 78°, also set in 1987.

Today's record temperatures bring the July month-to-date average up to 83.4°. If it were to hold through the end of the month, this would be a new record hottest July. Given the somewhat less hot temperatures forecast for next week, at least a tie for the record is likely.

The summer season-to-date average of 81.8° is also running well above the pace of the previous hottest summer of 1980.

Image (click to enlarge): Washington, DC average summer temperatures to date for 2010, 1980, 1991, 2002, and climatological period 1971-2000; CapitalClimate chart from National Weather Service data, background photo from Kevin Ambrose

Sinkhole Swallows SUV in Milwaukee Monsoon

Record-breaking rainfall caused widespread flooding across southern Wisconsin on Thursday. In Milwaukee, many roads and the city's airport were closed by high water. A sinkhole 40 feet wide and 20 feet deep swallowed a Cadillac SUV with the driver inside, along with a traffic light. The driver was rescued, although the fate of the vehicle was less certain.

The 5.79" of rain at Milwaukee was over 4 times the old record for the date, and it was the second highest daily precipitation of all time, second only to the 6.81" on August 6, 1986.

The 3.62" at Madison broke the old record for the date of 2.21" in 1885.

The National Weather Service reports:

A large area of moderate to heavy rain moved through southern Wisconsin Thursday morning, dumping 1.5 to 2 inches of rain across areas west of Madison, and .50 to 1.5 inches of rain east of Madison, including the Milwaukee metropolitan area. Milwaukee's Mitchell International Airport received 1.44 inches of rain during that morning round. Madison's Dane County Regional Airport received 1.47 inches. This helped set the stage for the major flooding that ensued later in the day.

During the afternoon, a persistent band of strong to severe thunderstorms developed and moved very slowly over south central and southeast Wisconsin through the evening hours. The individual storms were moving quite fast, about 40 to 50 mph, but the line containing these storms didn't, resulting in storms repeatedly training, or moving, over the same area. WIdespread 3 to 4 inch amounts were reported along and either side of the I-94 corridor, with locally higher amounts of 5 to 8 inches. Widespread reports of flooding and flash flooding associated with the heavy rain were received across southern Wisconsin, sending rivers and streams above flood level and water flowing over roads and stalling cars. Traffic was stopped on various stretches of the interstate system throughout downtown Milwaukee at the height of rush hour. Many motorists were stranded in their cars until the waters receded enough for traffic to move again.

Mitchell Field recorded 5.79 inches for the day...breaking a record for the date. The previous record was 1.26 set in 1948. This also is the second highest daily rainfall total on record for Milwaukee. The greatest one-day precipitation record was the 6.81 inches which fell on August 6, 1986.

Madison also set a record for precipitation for the date at 3.62 inches. This beat the previous mark of 2.21 inches set in 1885. The 3.62 inches ranks 13th for the most precipitation in one day. The record is the 4.96 inches which fell on August 8, 1906.

Green Bay, Wisconsin has already set a new July monthly rainfall record with 7.84". The normal rainfall amount through July 22 is 2.43".

Rainfall amounts of 3.5" to over 4" were reported in northern Illinois. Through 1 pm today, Rockford Illinois received 4.27", smashing the 1973 record of 1.73", and additional rainfall is possible through the rest of the day.

Flooding also occurred across Iowa, where the 2.24" broke the old daily record of 1.55" set in 1924 at Waterloo.

Images (click to enlarge): Southern Wisconsin precipitation for the 2 days ending July 23, 2010 from National Weather Service; Milwaukee sinkhole from Christian Science Monitor

Heat Streaking: Second 10-Day Stretch of 2010

Midnight Update: After dropping to 90°, the temperature is back up to 91° at midnight; the temperature has now been at 90° or higher for 13 consecutive hours.

8 PM Update: The high has actually popped up to 98° after 5:00.

6 PM Update: The preliminary high/low of 97°/77° came in 1° below forecast, but the predicted 102° tomorrow and continued 90s into next week still give a chance of at least tying the July temperature record. (As of 6 pm, it's been 97° for 3 consecutive hours; hard to believe it couldn't make it to 98° at some point.)

Original post:
With a high so far today of 97°, this is now the 10th consecutive day of 90° temperatures or higher in Washington, the second time this has occurred in the summer of 2010. The even hotter readings expected tomorrow will undoubtedly tie the 11 day stretch of 90°+ in June.

How do these current year heat waves compare with those of the past? Since temperature records began at or near the current location of National Airport in 1929, there have been 12 periods of 12 or more consecutive days with 90° temperatures or higher. The record, by far, was set by the 21 days ending August 14, 1980. That heat wave began with only a 2-day break following the 9-day streak ending on July 22. Ten days following the record streak, another 14-day streak extended from the last week of August through September 6. In all, there were a record 67 days of 90°+ temperatures in 1980, which also holds the current record for the hottest summer. The earliest 90° reading that year was May 13, and the latest was September 23.

The second and third place holders for heat wave length in the modern era are 1999 (18 days) and 1988 (17 days).

Temperatures are expected to moderate somewhat by early next week, but forecast highs of 90° or warmer through at least next Thursday could bring the current streak to 16, which would tie the 4th place record set on July 18, 1993.

Image (click to enlarge): Washington, DC longest streak of consecutive 90° days per year; CapitalClimate chart from National Weather Service data, background photo from Kevin Ambrose

Summer of Simmer, Mid Week Update: July Going For Bronze Silver

July 23, 1 AM Update: The midnight (standard time) temperature of 82° is in, so the low of 79° holds, just 2° below the record high minimum for the date. A couple of high minimum records are vulnerable over the weekend with 78° on Saturday and 79° on Sunday and Monday.

Summer-to-date average: 81.5°.

NOAA's Climate Prediction Center data shows that the 31-day running mean of temperature departures has not been negative since the end of February.

July 22 Update: The preliminary high/low of 96° and 79° today push the month to date (2010 TD) to 82.8°, firmly in position as the 3rd hottest July. The projection (2010 Proj) moves to 83.0°, tied for 2nd and within striking distance of the record for hottest July.

PM Update: Today's preliminary high comes in at 93° with a low of 75°.

Original post:
High temperatures generally in the low 90s (including a high as of 2 pm today of 91°) and lows in the mid 70s so far this week have continued to inch the Washington cumulative summer average higher, to 81.4° through yesterday. Forecast temperatures in the upper 90s to near 100° over the weekend will likely push it even further, to 81.8° by Sunday. Even with normal temperatures from the remainder of July through August, the final summer average would be 80.2°, a new record high.

Meanwhile, the July monthly average continues to increase as well, standing now at 82.6° (2010TD on the chart), tied with 1987 for third hottest July. Projections based on the next 7 days of forecasts and climatology would keep the final monthly average at the same value (within rounding error).