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July outlook: D.C.'s hottest month to play the part

We lean toward conditions being slightly hotter and wetter than average

Tubers try to stay cool floating down Seneca Creek on June 27 near Darnestown, Md. (Jeannie in D.C./Flickr)

As we set foot into what’s typically our hottest month of the year, we may be able to use the past 30 days as a guide as to what to expect. June has been slightly hotter and wetter than normal, which is where July appears to be headed as well.

The 30-year (1991 to 2020) average temperature in July is around 81 degrees, and we estimate an outcome between 81 and 83 degrees this year.

Average rainfall is 4.33 inches, and we’re forecasting 5 to 6 inches this July.

Models project a hot, humid and stormy first half

July will start out much the same way June performed, with variable temperatures that average out to the hotter side and intermittent periods of heavy rainfall. While the upcoming holiday weekend leans cool, the overall two-week forecast from the various model simulations, as shown below, features slightly hotter than normal temperatures and above normal precipitation.

The models differ somewhat over the position of the highest heat in North America, but they generally project it to focus between the Pacific Northwest and Northern Plains. A slight dip in the jet stream over the western Great Lakes would set up a pathway for occasional thunderstorm complexes to roll eastward.

Occasionally, some cool fronts are likely to pass by, potentially producing pleasant low humidity in their wake, not unlike what we experienced several times in June. Other times, however, it should deliver days of higher humidity, soupy 90s and high heat index values.

The National Weather Service forecasts heavy rainfall to kickoff the month and has even issued a flash flood watch between Thursday afternoon and Friday morning. Rainfall totals could reach one to three inches with isolated 4-inch amounts possible.

The month overall

The latest outlooks from the National Oceanic and Atmospheric Administration’s CFS model call for the month overall to bring slightly above average temperatures and rainfall totals about 1 to 2 inches above average.

We have found that amid severe drought conditions in the West, sometimes models underestimate the extent of the heat in that area, but overestimate temperatures in the East. If this holds true, the more durable, severe summer heat may avoid our region and lead to a tolerable July overall.

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