The ongoing stretch of dry, comfortable weather in the Eastern half of the US may have many residents convinced that the dog days of summer have passed. But what does this oft-used phrase even mean? Let’s take a brief look into the history of the phrase to gain a better understanding.
Sirius, the “Dog Star,” is the brightest star in the “Greater Dog” constellation Canis Major. The dog days of summer are named for the coincident rising of Sirius with the Sun between July 3 and August 11. Ancient Egyptian, Greek, and Roman astronomers noticed that summer’s most intense heat, humidity, and drought coincided with the rising of Sirius alongside the sun. Traditionally, the Dog Days ended when Sirius first rose above the horizon before sunrise. But the Dog Days are just an approximation for the summer’s worst period of heat tailored to the Mediterranean coast. Sirius may now rise before the sun, but extreme heat and humidity will nonetheless spread from the Gulf of Mexico into eastern North America next week, possibly lasting through the first day of meteorological fall (September 1).
Upper-level high pressure will gradually intensify and build northward over the weekend. A few weak passing shortwaves in the Midwest and Northeast will help to somewhat slow the northeastward track of heat and humidity, keeping temperatures around normal this weekend. By Monday, a ridge of high pressure centered over the Deep South will build adjacent to the Bermuda High situated over the western Atlantic Ocean. The former high will pump heat and humidity directly from the Gulf of Mexico, around the high’s center, into the Midwest. The Bermuda High will pump Caribbean heat and humidity into the Mid-Atlantic and New England.
Intensification of the ridge over the South will continue through midweek, by which point it will have expanded over the entire eastern half of the US, kicking the Bermuda High further out to sea. This is when the heat and humidity will be most intense. High temperatures in the mid to upper 90s will be possible up and down the coast, from Savannah, GA to Portland, ME.
Long-term global model guidance is in substantial agreement about the intensity of the upcoming heatwave. Upper 80s to low 90s will dominate throughout much of the Eastern US next week. Temperatures may occasionally climb higher, especially midweek. These temperatures will be up to 15 degrees above normal, as depicted in the GFS 2 meter temperature anomaly figure below for next Wednesday afternoon. Alongside the heat the air will feel sticky and soupy, with dew point temperatures climbing into the mid-70s. This will make it feel like 95-105°F at times, mainly along the East Coast.
Unlike previous hot spells, rain chances look to be suppressed. This is good news for farmers, as dry weather alongside the heat will accelerate the drying of soils for harvesting fall crops. Beach-goers can also rejoice. Afternoon thunderstorms have frequently spoiled what would have otherwise been ideal beach weather this summer.
The timing of the heat’s retreat is less certain. A barrage of shortwaves may cross the Rocky Mountains and crash into the ridge baking the Eastern US late next week. These shortwaves would act to shift the ridge eastward. There are substantial differences between the models as to how these impact the ridge. Since this is more than one week away, specific details depicted in the global models are irrelevant. Rather, it is the overall pattern that is important. Recent runs of the ECMWF have suppressed the ridge to the southeast long enough for cool air to refresh the Midwest and Northeast by late next week. The GFS has been inconsistent with the timing and extent of the ridge’s suppression, but has generally maintained the heat through September 1.
Whether or not the heat is suppressed late next week says little about the long-term forecast into September. Cool spells will intermittently relieve the East of heat, but summer weather is expected dominate the region as North America descends further into the first few weeks of meteorological fall.