As we get closer to the end of August and the beginning of September, many of us are starting to look toward autumn. Cooler temperatures, changing leaves, and the eventual end of the summer heat are probably sounding pretty good by now.
Unfortunately, summer has other ideas.
Much of the central and eastern U.S. will be dealing with another round of above-average temperatures as a persistent ridge that has established itself over the central U.S. for several days begins to shift eastward. Meanwhile, scattered thunderstorms are expected across parts of the northern tier, with a few storms capable of becoming severe. So, what can we expect over the next several days?
The Heat Dome
A ridge of high pressure has parked itself over the central U.S., bringing temperatures 20 to 25 degrees above average across parts of the Plains.
Looking ahead, the ridge is expected to gradually shift eastward, with its center moving from the South-Central U.S. toward somewhere over the Ohio Valley. As this happens, the hottest temperatures will shift with it, bringing temperatures around 20 degrees above average to parts of the Midwest.
Courtesy of www.pivotalweather.com
The ridge itself will likely remain firmly in place across much of the central and eastern U.S. over the next week or two. This is supported by the Climate Prediction Center's 6–10 and 8–14 day temperature outlooks, which place a large portion of the central and eastern U.S. in an area with a 50% or greater chance of above-normal temperatures.
Severe Weather
While ridging will keep the southern tier of the U.S. rather toasty, the northern tier will need to watch for strong to severe thunderstorms. A Slight Risk of severe thunderstorms is currently in place for Saturday, August 29, across parts of the Northern Great Basin, Northern Rockies, and Northern Plains.
Across the Northern Great Basin and Northern Rockies, widely scattered afternoon thunderstorms are expected. While low-level moisture will be limited, strong daytime heating, steep lapse rates, and 40–50 knot winds in the mid and upper levels of the atmosphere could allow a few storms to become organized. Damaging wind gusts should be the primary concern, with the strongest storms potentially producing gusts around 50–60 mph. Some marginally severe hail is also possible.
Farther east, storms are expected to develop around the Black Hills, eastern Wyoming, and southeastern Montana before moving toward the Dakotas. Increasing moisture and instability, along with around 40 knots of deep layer shear, could provide enough support for a few stronger storms, including the possibility of an isolated supercell or two. Large hail and damaging winds would again be the primary threats, with the strongest storms potentially producing wind gusts around 50–60 mph.
There is still some uncertainty regarding how organized storms will become across eastern Wyoming. However, recent guidance has suggested the potential for a stronger corridor of severe winds extending into northeastern Nebraska. Because of this, the Marginal Risk has also been expanded southward into parts of the region.
Looking onward to the Day 3 outlook for Sunday, August 30, the severe weather threat becomes a little more interesting. At least isolated severe thunderstorms are possible from the Central and Northern Plains into the Upper Mississippi Valley, with damaging winds and large hail expected to be the primary threats.
Across the Northern Plains into the Upper Mississippi Valley, multiple rounds of thunderstorms could develop throughout the day. Storms developing near the warm front could become strong to severe, with damaging winds and an isolated brief supercell or two possible. Later in the afternoon and evening, additional storms may develop from eastern Montana through the Dakotas. Some of these storms could eventually merge into a larger complex, which would increase the potential for a more widespread corridor of damaging winds.
Farther south across the Central Plains, stronger capping will make storm development more uncertain. However, increasing moisture and instability during the evening could eventually allow some storms to break through the cap. If they do, a few could become severe, with damaging winds and large hail possible into the late evening.
For now, the exact location of the greatest severe weather threat remains uncertain. However, I would not be surprised to see a Slight Risk added into parts of central or western North Dakota in later outlooks.
Looking ahead a few days, the weather pattern remains roughly the same, with multiple shortwaves expected to move along the northern periphery of the ridge. These disturbances could bring multiple rounds of strong to severe thunderstorms.
There is still uncertainty about which of these disturbances will produce severe weather and exactly where that threat will develop, though the strongest signal currently appears to be across the Ohio Valley, Mid-Atlantic, and southern New England, particularly Tuesday, September 1, and Wednesday, September 2. Model guidance is still having difficulty determining exactly when and where storms will develop on Tuesday. However, there is enough consistency to suggest that a severe weather threat could develop somewhere from the Great Lakes toward the East Coast.
Wednesday currently has a stronger signal. Models suggest that another disturbance will move from the eastern Great Lakes toward the Mid-Atlantic and southern New England. A fairly strong mid-level jet will move over a seasonably moist environment, providing enough instability and wind shear for organized severe weather. The Storm Prediction Center has opted to add a 15% probability of severe weather across portions of this region where the confidence is currently highest.
There could be additional opportunities for strong storms later in the week, particularly across the Northern Plains and Upper Mississippi Valley as instability builds back into the region. However, the details become much less certain at this range, so it is too early to say exactly when and where the greatest threat will be.
For now, Tuesday and Wednesday are the days I would be watching most closely. It is still too early to get a perfectly clear picture, but as we get closer to the event, we should see more confidence in what threats dominate and how intense they could be.
The Tropics
The Atlantic has been relatively quiet so far this hurricane season, and for now, that trend continues. The ongoing El Niño has been one of the factors contributing to the quieter Atlantic, with increased wind shear making it more difficult for tropical systems to organize. Over the past several hours, Tropical Storm Dolly has encountered this strong wind shear and weakened below tropical-storm strength, although there is still a small chance of redevelopment.
The remnants of Dolly are currently located several hundred miles east of the Leeward Islands and are moving west-northwest at around 25 mph. The National Hurricane Center has highlighted a 10% chance of redevelopment as the system moves toward the Caribbean, with the potential for redevelopment currently focused northeast of Cuba or just north of Hispaniola.
In other news, there is also a very weak area of low pressure over the northeastern Gulf of America, associated with disorganized showers. Development is expected to be slow, if it occurs at all, as the system remains nearly stationary over the next several days.
The National Hurricane Center currently gives this system only a 10% chance of developing into a tropical cyclone over the next 7 days. For now, there is little reason for concern, although areas along the Gulf Coast could see some rain from the disturbance over the coming days.
A Break From the Heat?
While much of the central and eastern U.S. prepares for another stretch of summer heat, not everyone will be dealing with the same story. Areas across the far Northeast and Pacific Northwest should enjoy some of the more pleasant weather over the next several days, with temperatures closer to or even below average at times.
But that brings us to the big question: is there a cooldown in sight for the rest of us?
Unfortunately, there does not appear to be a significant cooldown on the horizon for much of the central and eastern U.S. The ridge appears likely to hold for much of the extended forecast. Model guidance suggests that troughing could start bringing bursts of cooler air farther south around the two-week period, but we all know that forecasts that far out are pretty much just fairytales.
That does not necessarily mean every day will be brutally hot everywhere. Temperatures will still fluctuate, and there may still be occasional cold fronts pushing south into the northern edge of the heat dome. However, there is currently little indication of a pattern change capable of bringing a widespread, sustained cooldown.
So, if you were hoping for an early fall, unfortunately, you may have to wait a little longer. Summer doesn't seem quite ready to give up yet.