The 80s Won’t Last: September Begins With a Surge of Summer Heat

If you're enjoying the relatively comfortable temperatures across much of North Carolina this weekend, don't get too accustomed to them.

As August comes to an end and September begins, a significant surge of summer heat is expected to build across the region.

Instead of temperatures trending downward as we enter meteorological fall, much of the Piedmont and surrounding areas will climb well into the 90s during the first several days of September.

And once humidity is factored in, some locations could experience dangerous triple-digit heat index values.

A Rapid Warmup Begins Monday

Sunday will be the coolest day of the next several for much of the region.

High temperatures generally remain in the mid to upper 80s across the Piedmont, with some locations benefiting from morning clouds and fog.

That changes quickly Monday.

Temperatures are expected to reach around 90 to 93 degrees across much of the Piedmont and Charlotte region. By Tuesday, many locations will be in the lower to middle 90s.

Wednesday through Friday currently appears to be the hottest portion of the week.

During that period, widespread middle 90s are expected across much of the lower terrain, with some of the warmer locations potentially reaching the upper 90s.

That would make for an exceptionally hot beginning to September, even though summerlike temperatures are certainly not unusual this time of year.

The Temperature Isn't the Biggest Concern

A forecast high of 94, 95 or 97 degrees is hot enough by itself.

Add high humidity, however, and conditions can become considerably more dangerous.

The heat index estimates how hot conditions feel to the human body when relative humidity is combined with the actual air temperature.

That's going to become increasingly important this week.

Some locations, particularly south of Interstate 85, could already experience heat index values around or above 100 degrees Monday.

By Tuesday, triple-digit heat index values should become more widespread.

Across portions of central North Carolina, heat index values could approach 105 degrees Tuesday, with peak values potentially reaching 105 to 110 degrees Wednesday through Friday.

Exactly where the highest heat indices develop remains uncertain and will depend heavily on afternoon temperatures and dewpoints.

But the overall signal for dangerous heat is becoming increasingly difficult to ignore.

Why Is the Heat Building?

A strong ridge of high pressure is expected to strengthen across the Southeast during the first half of the week.

Think of a strong upper-level ridge as something of a lid over the atmosphere.

Air underneath the ridge sinks. As that air descends, it warms, while the sinking motion also suppresses the development of clouds, showers and thunderstorms.

With fewer clouds and storms, sunshine can efficiently heat the ground throughout the afternoon.

The ridge is expected to become firmly established across the region and show relatively little movement during the middle and latter portions of the week.

That will allow the heat to build day after day.

Very Few Thunderstorms to Provide Relief

One thing that can occasionally interrupt a hot summer afternoon across North Carolina is a thunderstorm.

That source of relief may be difficult to find this week.

Isolated showers and thunderstorms remain possible across the North Carolina mountains Sunday and Monday.

Across the Piedmont and much of central North Carolina, however, rain chances will be extremely limited.

As high pressure strengthens, the atmosphere becomes increasingly unfavorable for thunderstorm development.

By the middle of the week, much of the region could experience mostly sunny skies with little to no afternoon convection.

That means areas experiencing temperatures in the middle or upper 90s may receive very little rain-cooled relief.

The Mountains Will Be Hot Too

The most dangerous heat index values are expected across the lower elevations, but western North Carolina won't completely escape the warming trend.

Temperatures will increase across the Foothills and mountain valleys as the ridge strengthens.

Higher elevations will naturally remain cooler than the Piedmont, but above-normal warmth could still become noticeable.

The mountains will also have the best opportunity for isolated afternoon showers or thunderstorms early in the week before the strengthening ridge makes convection increasingly difficult.

Charlotte and the Southern Piedmont Could Be Particularly Hot

The Charlotte metropolitan area and southern Piedmont will be an area to watch closely.

Temperatures could climb into the middle and upper 90s during the middle of the week, while higher humidity pushes apparent temperatures even higher.

Locations south of Interstate 85 may be among the first to experience triple-digit heat index values.

If current trends continue, outdoor conditions could become increasingly hazardous during the hottest portion of the afternoon.

Central North Carolina Could See Heat Indices of 105 to 110

Farther east across central North Carolina, the heat could become even more significant.

Current forecasts indicate heat index values could reach around 105 degrees in some areas by Tuesday.

Wednesday through Friday could bring peak heat index values between 105 and 110 degrees in the hottest locations.

Those numbers remain several days away and will be refined as the event gets closer.

However, if those values continue appearing in the forecast, heat-related advisories will become increasingly likely.

Heat Advisories May Be Needed

Heat Advisories could eventually be issued for portions of the region during the middle and latter portions of the week.

Exactly where those advisories may be needed remains uncertain.

Small changes in temperature and especially dewpoint can have a significant effect on heat index values.

That's why simply looking at the forecast high doesn't tell the entire story.

A 95-degree afternoon with relatively lower humidity is very different from a 95-degree afternoon with tropical levels of moisture in the atmosphere.

I'll be watching both temperature and dewpoint trends closely as we get closer.

Warm Nights Will Add to the Heat

Afternoon temperatures will receive most of the attention, but overnight conditions are also important.

Many lower-elevation locations will struggle to cool much below the lower to middle 70s during the middle of the week.

Warm nights reduce the opportunity for buildings, pavement and people to recover from the previous day's heat.

When high daytime temperatures are followed by warm nights for several consecutive days, heat stress can accumulate.

That's particularly important for people without reliable air conditioning and anyone spending extended periods outdoors.

Meteorological Fall Begins Tuesday

There's some irony to the timing.

September 1 marks the beginning of meteorological fall.

Meteorologists divide the year into four three-month seasons, making September, October and November meteorological fall.

But changing the month on the calendar doesn't instantly change the atmosphere.

September can still produce significant heat across North Carolina, particularly during its opening weeks.

This year may provide a particularly good example.

While the calendar changes from August to September Tuesday, temperatures across much of the region will be climbing through the 90s.

By Wednesday and Thursday, some locations could be approaching the upper 90s with triple-digit heat index values.

Meteorological fall may be beginning.

The weather will still feel very much like summer.

What I'm Watching This Week

The first thing I'll be watching is the magnitude of the heat.

How widespread will temperatures of 95 degrees or higher become, and will any locations make a run toward the upper 90s?

The second factor will be humidity.

Dewpoints will determine whether this becomes simply a stretch of very hot weather or a more significant period of dangerous heat indices.

The third will be the potential for heat-related advisories.

Confidence in exactly where those become necessary should increase considerably over the next couple of days.

Finally, I'll be watching for any signs that the ridge could weaken or shift enough to eventually bring thunderstorms or cooler temperatures back into the forecast.

The Bottom Line

Enjoy Sunday's temperatures in the 80s because they won't last.

Heat begins building Monday before becoming increasingly significant Tuesday through the remainder of the workweek.

Much of the Piedmont could experience highs in the middle 90s, while some locations approach the upper 90s.

Once humidity is included, triple-digit heat index values are expected to become increasingly common, with parts of central North Carolina potentially experiencing heat indices between 105 and 110 degrees during the hottest portion of the week.

Rain chances will remain very limited across much of the region, providing little relief from the heat.

September begins Tuesday.

But across North Carolina, the first several days of meteorological fall are going to feel a lot more like the middle of summer.

I'll continue tracking the developing heat and provide updates as confidence increases in temperatures, heat index values and the potential for heat-related advisories.