Washington’s summer game is set to step up in July with intense heat building in the weeks ahead. While June’s hottest temperature was only 93 degrees, we could edge into the mid-90s by Friday and several more times next week. Drier-than-average conditions could also start to amplify the heat beyond that.
We predict July 2020 to run about 2 to 4 degrees hotter than the 30-year average (79.8 degrees) and for rainfall to be about 0.5 to 1.5 inches below average (3.73 inches). If our forecast is correct, July 2020 would be similar to July 2019, which was 2.3 degrees warmer than normal and ranked as the ninth hottest on record. And, if the high end of our forecast is right, the month would rank among the top five hottest Julys on record.
Rainfall is always very tricky to predict as it only takes one slow-moving thunderstorm to blow out that average but, like June, we suspect July would lean to the drier side.
The first half of July is forecast by the various weather models to run anywhere from 2 to 5 degrees hotter than normal in Washington. The following features the two-week forecast from the American (NOAA), European (ECMWF) and Canadian (CMC) weather models:
Some cooler shots might be possible later in the month, possibly easing the overall degree of the month’s heat.
The same models also predict drier-than-average conditions for the next two weeks:
While June was slightly drier than average, 2020 overall is still running about an inch wetter than normal since Jan. 1, and the federal government’s drought assessment has not placed our region in drought concern yet.
Move toward La Niña conditions may increase heat
The main driver of this hot and dry July outlook is the gradually developing La Niña in the tropical Pacific. That emerging feature is correlated with a reduction in high-altitude winds that tend to slow the jet stream down. This favors the development of persistent and hot high pressure zones over the mid-latitudes. Occasionally, rain breaks through these high pressure systems taking an edge off the heat but, usually, it struggles.
NOAA’s long range forecasting model, the CFS, which covers all of July, predicts hot and slightly dry conditions in Washington. The maps, shown below, present an average of the 20 most recent model runs:
One interesting aspect of the NOAA CFS forecast is that, like for May and June, it forecasts the Southeast to be wetter than average for July. If the wet boundary edges closer to the Washington area, it might not be as hot and dry.