Washington has received 0.11 inches of rain this month, more than two inches less than the norm, the sixth driest September on record. No more than 0.05 inches of rain has fallen in a single day. By comparison, Washington was deluged by 9.73 inches of rain last September.
This month’s minuscule rain total follows both a drier-than-normal August and second half of July. Since mid-July, Washington has posted about three inches of rain, compared with the historical average of about seven inches.
Meanwhile, the forecast shows little promise for substantial rainfall in at least the next week. If the spotty showers forecast for Monday night do not materialize, we may go rainless through the end of September, extending our streak of five straight dry days to 16.
This dry forecast is supported by the National Weather Service’s six-to-10-day and eight-to-14-day outlooks, which favor below-normal precipitation.
The sudden stoppage in rainfall is probably related to the decline of El Niño, which tends to favor above-normal precipitation in the Southeast and Mid-Atlantic.
Capital Weather Gang’s Matt Rogers, who specializes in long-range forecasting, said we’ve seen high-altitude winds around the Northern Hemisphere slow way down in response. He said these winds are among “the lowest we’ve seen in September” and are similar to 2005 and 2007, when Washington also experienced abnormally dry weather. In 2007, there were a record 34 straight rainless days spanning September and October.
When these winds are weak, “you tend to have more expansive high pressure over the U.S.,” Rogers said. “Everything tends to slow down, and there is less energy in the jet stream."
Rogers said he sees this pattern persisting through the first half of October, before potentially breaking down in its second half when we might see more storminess return. The National Weather Service’s three-month outlook for October, November and December favors above-normal precipitation for the region.
Jason Elliott, a hydrologist at the National Weather Service office serving the Washington region, said fall dry spells such as this aren’t terribly unusual. “It’s the dry season, where it’s sort of feast or famine,” he said. “Either we get a tropical storm and rain, or we get absolutely nothing.”
Because of all the rain earlier in the year, the effects of this sudden dryness aren’t as acute. “This is a short-term flash drought,” Elliott said, noting that its arrival toward the end of the growing season was fortunate. “We’d definitely be in worse shape if we had worse issues going into this.”
The lack of rain has reduced streamflows to their lowest levels in a couple of years, Elliott said, meaning that some replenishment is needed in the fall and winter “to recharge the groundwater and the streams.” Otherwise, drought concerns could turn more serious in the spring.