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Posted at 10:30 AM ET, 08/12/2009

Checking the Forecast for a Rare August Day

By Capital Weather Gang

Weather Checker

* Staying Humid: Full Forecast | Climate Change & National Security *

Weather Checker is an outsider's analysis of CWG's forecast accuracy. See previous Weather Checker posts.

By Jamie Yesnowitz

After a showery and overcast morning, this satellite image shows clearing skies beginning to work their way into the D.C. area (from northwest to southeast) at 1:45 p.m. on Thursday Aug. 6, 2009. Courtesy UCAR.

Until the recent searing heat, this summer has been strangely mild, and probably the most tolerable of my seven years in the D.C. area. Still, it's a novelty when an August day passes with temperatures that don't surpass 80, as was the case this past Thursday (Aug. 6) for many areas downtown and to the south and east of D.C.

So, I thought it might be instructive to look back and see how CWG fared in its forecast for this rather rare August day.

Keep reading for what the Weather Checker has to say...

To review, Thursday's weather at Reagan National (DCA) consisted of 0.12 inches of light rain in the early morning, followed by consistent cloud cover with temperatures in the low 70s, until late afternoon when the clouds broke and the high made it to 79 just after 4 p.m. BWI and Andrews Air Force Base (ADW) only reached 78 and 75, respectively, while IAD spiked to 83 (the sun came quicker north and west of town as clearing skies moved in from northwest to southeast). IAD and BWI reported only .04" and .05" of rain, respectively, with .22" at ADW.

Jason Samenow's Monday forecast picked up on a cooling trend for Thursday (from low 90s to near 90 earlier in the week), and predicted with medium confidence that Thursday temperatures would moderate to the mid-80s with a mix of clouds and sun and the potential for an isolated storm in the afternoon. While Jason got the trend right, the temperatures were still overstated, and it turned out to be be a showery morning with no afternoon storms.

Tuesday's forecast by Matt Rogers correctly predicted, with low-medium confidence, the Thursday morning showers and significant cloud cover, but again overshot (for many parts of the area) on highs being "in the 80s."

Likewise, Dan Stillman's Wednesday forecast accurately noted with medium confidence the potential for morning showers and the eventual change to partly sunny skies in the afternoon. Predicted highs of mid-80s or low 80s, depending on the timing of clearing skies, verified for the north and west suburbs, but not for locations in D.C. and to the south and east. In addition, the forecast headline "Typical Summer Stuff, Then Real Heat?" failed to characterize Thursday's weather, which in fact was quite atypical for August.

Ian Livingston's Wednesday afternoon forecast finally raised the potential of Thursday temperatures remaining in the 70s all day and noted the challenge of predicting when the rain and clouds would depart.

Finally, Brian Jackson's same-day forecast (medium-high confidence) initially overestimated the intensity of rain as "moderate to heavy," before a mid-morning forecast change to "light to moderate." As for temperatures, it was a little less clear: The "Express Forecast" at the top of the post predicted highs of 75-79 degrees, correct for DCA and BWI, but too low for IAD and other spots north and west of town.

The quibble from my perspective was Jackson's more detailed forecast narrative, which said, "...temperatures will struggle and we'll spend the majority of the day in the mid-to-upper 70s. If our wave [of low pressure] gets some giddy-up and moves out sooner than expected, some afternoon and evening sun may be able to push the mercury above 80."

While I liked the use of the term "giddy-up," I'm not sure mid-to-upper 70s was an accurate characterization of the day, as many locations saw temperatures no higher than the low 70s through early-to-mid afternoon.

About the Weather Checker...

Jamie Yesnowitz has been interested in the weather since he rooted for school-closing snowstorms while growing up in Brooklyn and East Rockaway, N.Y. After graduating from Dartmouth College with a bachelor's degree in economics and government, his focus on the accuracy of weather predictions took hold when he moved to Coral Gables, Fla., to attend the University of Miami School of Law. Class was scheduled to begin on August 24, 1992. Hurricane Andrew had other ideas, however, shutting down the school for weeks. But what stuck in Jamie's mind was the final unpredicted swerve of the eye that saved those living in Miami and points north, and completely devastated areas about 20 miles south of Miami.

Undeterred by the hurricane, Jamie ultimately served as editor-in-chief of his law school newspaper, and earned both a juris doctorate and master's degree in taxation. Following law school, Jamie practiced corporate and securities law in New York before shifting to the state and local tax consulting world. Jamie moved from New York to the Washington area in 2003, and he is presently a state and local tax senior manager at a major accounting firm. Jamie lives in Potomac with his wife, Sandra, and their two daughters, Sarah and Carly.

By Capital Weather Gang  | August 12, 2009; 10:30 AM ET
Categories:  Weather Checker  
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A very fair and thorough assessment, Jamie. I would add that it's also important to judge the forecast within the context of how tricky it was. To that end, I would give ourselves decent marks (maybe a B or B-), as there's always a high bust potential when the area is near the edge of a storm system as it was in this case.

In his forecast 3 days out, while off on the timing of precipitation (not unusual that far ahead of time), Jason captured the cooling trend and his forecast of mid-80s was pretty close for the northern and western portions of the area, not as much so for downtown and south and east.

In his Tuesday forecast, Matt did a good job of identifying the Thursday morning potential for rain, and cast an appropriately broad range (considering the uncertainty of clouds and rain) with "highs in the 80s."

My Wednesday morning forecast was mostly on the mark for northern and western portions of the area and would've even verified for downtown had National's high been one degree higher (i.e., probably 30 more minutes of sun). In retrospect, based on the data available at the time, I should've gone "mid-80s or near 80" rather than "mid-or-low 80s."

Ian's Wednesday afternoon update emphasized the uncertainty of the forecast and did a nice job on temperatures.

And Brian's day-of forecast, while maybe trying to too closely pin down what part of the 70s temperatures would spend most of the day in, was otherwise generally accurate. The change to light-to-moderate rain, from moderate-to-heavy, was made as soon as radar and model changes made it clear that earlier model runs had overestimated the intensity of the rain in the metro area.

While as a team we could've done a little better here and there, I've seen far worse performances -- both by CWG and other forecast outlets -- given similarly tricky weather situations.

Posted by: Dan-CapitalWeatherGang | August 12, 2009 10:42 AM | Report abuse

"rare August day":
15% of all August days in the last 80 years have been below 80°. That's a little more than once a week.

Posted by: CapitalClimate | August 12, 2009 4:55 PM | Report abuse


Maybe Jamie should've been more specific and said a rare "early" August day. And then for your part, the more appropriate analysis for the sake of nitpicking would be to look at the frequency of early August (or specifically August 6) days below 80.

Posted by: CapitalWeatherGang | August 12, 2009 5:04 PM | Report abuse

And maybe you should do your own homework before you break your arm patting yourself on the back. The percentage for Aug. 6 is 14%. That's still 1 out of 7, and the most recent one was 5 years ago. Who's picking nits now?
Here are the 11 occurrences out of 79 years:

8	6	1993	74
8 6 2004 74
8 6 1956 75
8 6 1929 76
8 6 1951 76
8 6 1953 76
8 6 2000 76
8 6 1994 77
8 6 1959 78
8 6 1948 79
8 6 1981 79

Posted by: CapitalClimate | August 12, 2009 10:04 PM | Report abuse


Congratulations. You win.

Posted by: CapitalWeatherGang | August 12, 2009 11:17 PM | Report abuse

I leave for a one day business trip and look at what happpens.

First Dan, I appreciate your comments.

Second, the Merriam-Webster definition of "rare" is as follows:

Main Entry: rare
Pronunciation: \ˈrer\
Function: adjective
Inflected Form(s): rar·er; rar·est
Etymology: Middle English, from Latin rarus
Date: 14th century
1 : marked by wide separation of component particles : thin
2 a : marked by unusual quality, merit, or appeal : distinctive b : superlative or extreme of its kind
3 : seldom occurring or found : uncommon

synonyms see choice, infrequent

— rare·ness noun

I think that definition (3) certainly gives credence to the fact that a weather occurrence that happens 14-15% of the time is rare (it seldom occurs, and it is uncommon).

It will be interesting to see whether we go below 80 for highs at DCA the rest of the month.

-- Jamie Y (Potomac) aka Weather Checker

Posted by: JamieYPotomac | August 13, 2009 9:59 AM | Report abuse

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