But such warmth is not expected to endure, and temperatures are likely to jump around quite a bit in the coming month.
Overall, we don’t think this May will be as toasty as last year, but — assisted by this warm start to the month — still plenty warm. It should also be a wetter-than-normal month, but not as soggy as last May when 8.73 inches came down.
Confidence in this outlook is only moderate due to some competing influences on the prevailing weather pattern.
Early taste of summer. May can sometimes start to feel somewhat summerlike in Washington but usually not until the second half of the month when we more routinely break into the 80s or even 90s. This May, like the one before it, we’ll see some of these summerlike conditions right from the get-go. The early heat will put the month on warm footing.
But last year’s early May heat wave carried more staying power, generating three straight days at or above 90. This year, if we even manage to hit 90 on Thursday, it will be followed by slowly decreasing temperatures into the weekend. In other words, the early signal for warmth — while present — isn’t as strong this year.
El Niño slightly favors a warm month. Like many recent months, the presence of a weak-to-moderate El Niño event (warmer than normal waters in the tropical Pacific Ocean) should nudge temperatures slightly warmer than normal. This situation looks likely to persist through the summer, and even toward next winter.
Previous Mays in which weak El Niño conditions prevailed have averaged somewhat warmer than normal in the Mid-Atlantic:
But models are more mixed on the prospects for warmth. The temperature forecast from the National Oceanic and Atmospheric Administration’s CFS model is close to average for the eastern United States:
The main reason it’s not warmer is because it favors a cooler period in the second half of May. This is a reasonable risk showing up on other models, as well, and could be associated with wetter weather later in the month that could lower high temperatures some.
The El Niño pattern has a mixed signal for rainfall. Forecasting May rainfall, our wettest month of the year on average, is challenging.
El Niño generally boosts rainfall in our region, and that’s been the case in five of the past six months which have averaged wetter than normal. But April, which has been drier than normal, departed from that pattern. Further complicating the picture, Mays have historically leaned slightly drier than normal when weak El Niño conditions have prevailed:
However, notice that El Niño May periods are quite wet across the South and that sometimes that moisture can feed into the Mid-Atlantic. The CFS model (below) is suggesting that may happen this month, too. Here is the average of the most recent 20 cycles of the CFS model, with green and blue areas signaling wetter than normal conditions:
The model favors a wet South that bleeds north.
For now, we lean toward a warm and intermittently wet pattern carrying us through May. But expect springtime volatility, with some occasional cool spells in the mix.