The Heat Begins to Falter as Autumn's Arrival Is a Mere Two Weeks Away
Good day, folks! After weeks of stubborn heat, the weather pattern is finally showing some signs of change, with cold fronts potentially beginning to appear more often across the Central Plains starting later this week.
The weather pattern across the western United States has been extremely favorable for cooler temperatures over the past couple of weeks. Persistent troughing over the Pacific Northwest has kept much of the West unusually cool for early September. These troughs have helped keep the heat dome largely confined to the central and eastern United States. At times, troughing farther east has pushed the core of the heat back towards the west, resulting in particularly hot conditions across for everyone in the Central and Southern Plains
However, for those in the Central Plains, there is hope.
Looking at model trends over the past week, there are growing indications that ridging will become more established across the western United States while increasingly frequent troughs move into the central and eastern parts of the country. This would open the door for a series of cold fronts to push southward through the Plains, potentially bringing the first meaningful taste of autumn-like temperatures to areas that have spent much of the past couple of weeks dealing with the oppressive heat.
The first cold front has already entered the United States. As I write this, the front is draped across portions of the Midwest and Central Plains, and it will bring some much-needed relief to the northern tier. Some locations that reached the low 100s today could struggle to get out of the mid-80s tomorrow, September 9th. For anyone tired of the heat, that kind of drop should feel like the atmosphere finally remembered what September is supposed to be.
The second cold front is expected to enter the United States early Friday morning. As it digs southward into the North-Central Plains, it could bring another round of severe weather—we'll talk more about that later.
The front will likely stall across the North-Central Plains, allowing areas to the south that had relief from the first cold front to warm back into the upper 80s and low 90s. However, as the front slowly begins pushing southeastward, those same areas could eventually find themselves with highs in the 70s and overnight lows in the 50s. There is one important catch: because this front is expected to move rather slowly, those cooler temperatures probably won't arrive everywhere at once. Depending on how far north and east you are, the most noticeable cooldown may not arrive until sometime between September 14th and 16th.
Now, switching over to those in the Southern Plains. I'm afraid the heat is not ready to let go just yet. This stubborn ridge is being so lazy, and it really does not want to move. It's like someone just dropped a boulder over Texas and Oklahoma and nothing can push it.
But there is some good news: the heat won't last forever.
There are some indications, albeit very uncertain, that some relief could begin to develop around, or shortly after, the two-week period. As we continue into September, the pattern should continue to evolve in a way that eventually lets one or two decent cooldowns into the Southern Plains, likely in the latter half of the month.
So while I wouldn't put away the summer clothes just yet, autumn is slowly making its way into the country and it's only a matter of time before the leaves start changing and the temperatures are consistently comfortable.
And with all of these cold fronts beginning to move across the country, there's another thing that we'll need to keep an eye on: severe weather.
A busy stretch of severe weather ahead
Graphic Showing the Slight Risk Area for Portions of Ohio, Pennsylvania, and New York
The first severe weather threat comes Wednesday, with thunderstorms possible from the Central Plains all the way into the Ohio Valley. Most areas will probably only see isolated strong to severe storms, but the threat becomes more organized farther northeast, particularly from eastern Ohio into western New York.
A mid-level trough will be moving from the Upper Midwest toward the Northeast on Wednesday, with stronger winds aloft developing along its southern side. At the surface, a strong low will move across northern Ontario while a cold front stretches southwestward through the Midwest.
Ahead of this front, increasingly moist air will spread northward. Dewpoints in the low 70s are expected across much of Ohio, Pennsylvania, and western New York, providing plenty of moisture for thunderstorms to work with. Although the atmosphere won't have particularly steep lapse rates, moderate instability combined with moderate wind shear should be enough to support an organized line of storms from central Ohio into western New York.
Graphic Showing the 15% Damaging Wind Threat Across Portions of Ohio, Pennsylvania, and New York
Damaging winds will likely be the primary concern with an isolated threat for large hail and tornadoes. The storms will likely form into a line which should keep the tornado threat relatively low.
Farther west and south, the severe threat becomes more conditional.
Moderate instability is expected to develop from the Central Plains into the Ohio Valley, but the environment will be quite different depending on where you are. Across the Plains, instability will be driven largely by the hot and deeply mixed atmosphere, while areas farther east will benefit from the very moist air mass and dewpoints in the 70s.
The problem is that winds higher up in the atmosphere will remain fairly weak across much of this region. That will make it more difficult for storms to organize and mature. If severe weather does occur, strong downbursts and damaging wind gusts will likely be the primary hazard. Drier air across the Plains could also promote strong microbursts.
There could also be an isolated hail threat from eastern Missouri into western Indiana if the stronger instability can develop beneath stronger winds aloft. However, morning thunderstorms and cloud cover could interfere with that setup, so there is still quite a bit of uncertainty regarding how favorable the environment will actually become.
Moving into Thursady, it looks considerably quieter.
Severe thunderstorm potential appears low as surface high pressure builds into the Midwest and Great Lakes behind Wednesday's system. A front will stretch from Virginia westward into the Central Plains, but widespread precipitation along the boundary should limit instability across much of the region.
Farther west, moderate instability could develop near the front from the Lower Ohio Valley into the Southern High Plains. However, winds aloft will remain weak, meaning there won't be much vertical wind shear to help storms organize. A marginal severe threat could eventually develop if instability becomes greater than currently expected or storms begin to cluster, particularly around the Ozarks or Southern High Plains, but the signal is weak right now.
There could also be a few fast-moving storms across north-central Montana Thursday night into early Friday morning as another upper-level trough moves across the Northwest. However, instability should be weak and the storms will be occurring during an unfavorable time of day, making significant severe weather unlikely.
By Friday, our attention turns back toward the Plains and Upper Midwest.
The Storm Prediction Center has already highlighted a 15% chance for severe weather.
As another cold front advances southeastward, moisture will spread northward ahead of it, allowing moderate to strong instability to develop. At the same time, a modest upper-level disturbance moving through the northern Plains and Upper Midwest will provide stronger winds aloft.
That combination of instability and wind shear could be enough to support organized severe thunderstorms from portions of the Plains into the Upper Midwest, particularly from Friday evening into early Saturday.
Saturday could keep some severe weather potential alive as the front stalls across the Central and Southern Plains while continuing eastward across the Upper Great Lakes. There should still be plenty of moisture near the boundary, but the strongest winds aloft may become displaced north of the best instability. That could prevent the severe threat from becoming particularly widespread.
As we move into next week, another upper-level trough is expected to move eastward from the Northwestern United States across the Plains and Great Lakes. This would bring yet another cold front through the region, potentially producing another round of thunderstorms from the Plains into the Upper Midwest and Great Lakes sometime Sunday through Tuesday.
There is still considerable uncertainty that far out, and the ingredients don't currently line up perfectly. Forecast guidance generally shows the strongest instability displaced from the strongest winds aloft, partly because of the earlier frontal passage and the strong high pressure expected to build into the Great Lakes afterward.
I will, however, be monitoring the situation and I will let you know if things change.
The Summary
And with that, we've reached the end of today's forecast.
To summarize. The big story over the next couple of weeks will be the gradual breakdown of the stubborn ridge that has dominated much of the country. The Southern Plains will have to wait a little longer, but increasingly frequent cold fronts across the Plains and Midwest will likely bring the cooler air down to the south sometime in the near future
At the same time, several of these fronts could bring rounds of severe weather, so there will be plenty to watch as we move through the coming days. Forecast details will undoubtedly change, especially once we get beyond the first few days, so I'll continue keeping an eye on the situation.
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Thanks for reading, and I'll see you in the next forecast!