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Posted at 09:00 AM ET, 07/29/2011

July 2011 to become the hottest month ever in Washington, D.C.


Top-10 July records, including 2011 to-date and a projected range to finish.
Bolstered by the Sweat Ceiling heat wave that we are now enduring part II of, this July is a sure bet to finish as the hottest month on record for Washington, D.C. As of yesterday, July 28, the average temperature for the month was 83.9F. That’s 0.8F above the monthly record, and 4.1F above the full-month average of 79.8F.

While we don’t yet know exactly where this July will finish on the temperature scale, with three days (including today) left we can come to relative conclusions about its range and how much it stands out compared to other hot months. We’ll follow up with more once the month is done with a broader examination, but let’s take a quick look at what we do know now.

Keep reading for more on July 2011’s record heat…

D.C. started the month in the 90s and that’s most likely where we’ll finish it. Amazingly, only six days have featured temperatures below that number, with just one dipping into the low 80s for a high. Even if an icy asteroid exploded overhead and froze us to finish the month, we’re already in elite company for all months on record.


Top 90-degree day months in terms of numbers of such days (y axis). *2011 is through the assumed July 29 90F+. Red is projected.

90-degree days are an easy metric to visualize, but of course temperature records are kept in averages (daily highs + lows / # of each). The hottest July (and any calendar month) we’ve ever seen was the 83.1F average obtained both last year and in 1993. I’m sure some were at least hoping after getting there in 2010, we’d get a bit of a break in coming years. Nope, sorry.

Where do we finish the month? To make a rough estimate, let’s use the last two (prior to this morning) primary runs of the Global Forecast System (GFS) and last night’s North American Mesoscale Model (NAM). The 12z July 28 GFS produced Model Output Statistics (MOS) values through Sunday that would give us an average of 84.1F, or one full degree above the current record. The 0z GFS MOS output remained quite similar, and would help us achieve an average of 84.0F. The 0z NAM, which tends to run cooler, spit out values that would achieve an 83.8 on the month.

MOS is not gospel, of course, so we should not assume it is right even in the short term. However, GFS MOS does particularly well with warmth, and an examination of its output seems to produce temperatures meshing with the current pattern. If anything, it may be too cool on lows as they tend to run high next to the super-heated river. While the NAM output is also possible, it seems less likely that we would see the lower values.

For those hoping against hope Washington doesn’t set this ugly record, let’s visualize a scenario that would miss the record by a tenth of a degree: briefly imagine 85 degree highs and 67 degree lows today and tomorrow, and then a cool down to 83F for a high 65F for a low on Sunday… Wow! That’s nice. Ok, back to sweaty reality...

By  |  09:00 AM ET, 07/29/2011

Categories:  Extreme Heat, Local Climate, Latest

 
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