By the midpoint of August, fall weather fans (myself included) might be impatiently scanning the extended forecast for the first hints of a crisp low in the 50s and a weekend with flawless blue skies.
Autumn weather isn’t on the horizon yet — probably more of this warm, muggy and sporadically stormy theme over the next week or two — but it’s not too soon to take a look at how the next season could shape up.
This is also the time of year when forecasters at the National Oceanic and Atmospheric Administration release a seasonal outlook for the months of climatological fall: September, October and November.
On Thursday, NOAA’s fall outlook revealed that Virginia’s odds are tipped toward a season that will be warmer and wetter than average.
If that sounds familiar, it’s because that has generally been fall’s flavor here over the past several years. And over the past century, there has been a notable rise in both mean temperature and rainfall across Virginia from September to November, likely influenced by climate change.
That’s not to say it won’t get chilly in October — it inevitably will — but those brisk, dry days might take their time getting here once again, and we might get less than our fair share overall.
Warmer autumns mean more than just wearing short sleeves long after Labor Day. Extended warmth is also detrimental to an on-time display of brilliant fall colors. For the energy-conscious, the fall of 2018 racked up a higher cooling degree day sum in Richmond (478) than ever before, double the typical 241.
We can see later bug activity, too. Since 2015, Richmond’s official first freeze date has landed between Nov. 10 and Nov. 15, about a week to 10 days behind the 30-year average.
On the monthly scale, September is favored to be warmer than normal for Virginia, along with the Eastern Seaboard.
As we saw last year, an above-normal September can feel more like an extension of August: several afternoons in the 90s, but only two nights below 60.
Last fall, as a whole, brought an unprecedented 33 days with a low at or above 65 degrees in Richmond. In past decades, we averaged about 11 such days in fall months. Those balmier nights came about because the first half of last fall was dominated by a humid flow of air from the warmer-than-usual tropics, which also helped make it the wettest autumn on record statewide. Ocean temperatures are still running above normal off to our south and east.
Over each of the past four years, the autumn months have averaged out to be significantly warmer when compared with a 20th century baseline.
Though warmer and wetter are the favored outcomes this time around, the odds aren’t prohibitive. A stubborn flip of the weather pattern might keep a cooler trend in play, or a robust dip in the jet stream could give us a killing freeze in mid-October for the first time in a generation.
Therein lies the distinction between weather and climate.
In the probabilistic terms of NOAA’s seasonal outlook, the mean temperature from September to November has a 48% chance of being above normal, a 33% chance of being near normal, and a 19% chance of being below normal.
For precipitation, the Richmond area has a 46% chance of a wetter-than-normal fall, 33% for near-normal rainfall, and there’s only a 21% chance that we end up drier than normal.
A healthy amount of rain in the short term would help stave off the development of a drought.
For the first time since May 2018, when we flipped to a prolonged and extreme wet pattern, the U.S. Drought Monitor shows “abnormally dry” conditions across much of central Virginia, where hit-or-miss, late-summer rain has been more on the “miss” side.
As of Aug. 13, that pre-drought category covers areas from Chesterfield County to Farmville and north through the Piedmont to Washington. No area in the state is currently seeing drought conditions, however.
The storms we have seen since Monday could even erase some of those dry spots in time for next week’s report.
But there’s also the matter of when and how we get our autumn rain. September is often the time when tropical influences can come into play.
This NOAA outlook bases our odds of a wetter fall on patterns and trends that have emerged over the decades, rather than any particular system.
There’s no way to discern a hurricane landfall in September this far out, or a soaking remnant low in October. But we shouldn’t let our guard down, either.
The updated NOAA hurricane season outlook, released last week, boosted the odds of above-normal activity in the Atlantic because of the fading El Niño pattern in the Pacific Ocean.
Although the prediction of above-normal fall precipitation along the Eastern Seaboard is consistent with that hurricane outlook, according to NOAA forecaster Brad Pugh, it “by no means implies landfalling tropical systems in the Mid-Atlantic.”
As always, all it takes is one.
Elsewhere in the U.S., the fall outlook calls for elevated chances of above-normal precipitation from Florida to the Mid-Atlantic, from the Rockies to the northern Great Plains, and also throughout Alaska. No region is explicitly favored for drier-than-normal conditions, but the odds are evenly split across much of the South, Northeast and West Coast.
The entire United States is favored for above-normal autumn mean temperatures, with the highest probabilities across Alaska, the Four Corners states and the Northeast.