San Antonio temperatures will be hitting the upper 90s all week as if it’s midsummer already — and there might be no looking back: The city’s first 100-degree day of the year could arrive this weekend.

No, this kind of heat this time of year in South Texas is not normal — at least not according to National Weather Service records.

Let’s take Tuesday’s forecast high of 97 degrees: Not only is that almost 10 degrees warmer than the normal high for May 21 (88 degrees), it’s also within 4 degrees of the record high for the date, 101 degrees, set in 2022, according to the weather service data.

To establish what is  “normal,”  the weather service compiles averages from 30 years of data. Forecasters these days are using observations collected from 1991 to 2020.

But what if we include all San Antonio weather records for May 21 going back to 1885? We then find that the forecast high of 97 is now about 11 degrees warmer than the average high of 86.4 degrees and that daily highs on May 21 are warmer than 90 degrees only about 24% of the time.

High pressure, high humidity

What’s driving this heat is a classic combination of cloud-clearing high atmospheric pressure that’s opening the skies to unimpeded sunshine and a low-level inflow of humid air from the southeast and the Texas Gulf Coast.

“Upper-level ridging (of high pressure) will continue to dominate our weather through the long-term period, resulting in an early season heat wave,” the weather service said in a forecast bulletin Monday.

While the high pressure will squelch any rain chances, the additional tropical moisture in the air will help keep morning low temperatures above 76 degrees, forecasters said. The humidity also will make those afternoon temps in the upper 90s feel more like they’re 104 or 105 degrees.

Humidity, or the level of water vapor in the air, can make our perception of heat seem much hotter than the actual temperature because the body cools itself through the evaporation of sweat. But evaporation isn’t as effective when there’s too much moisture in the air.

The heat index takes that into account, which is why the actual air temperature might be 96 degrees but a high humidity level (say, a dew point of 71 degrees) can make it feel like it’s 104.

Weather service forecasters noted that a cold front entering the Texas Panhandle late Tuesday would creep southward, “but don’t get your hopes up, as it’s unlikely to have much of any impact on our sensible weather in the short term.”

“As far as storm chances go, we may see a stray shower or storm develop Tuesday afternoon/evening over the Edwards Plateau or Rio Grande Plains,” the weather service said, but the chances of severe storms would be less than 10%.

Wednesday and beyond

In San Antonio on Wednesday, expect partly sunny skies with an afternoon high temperature of 96. South-southeast winds of 10 to 15 mph, including gusts as strong as 20 mph, will continue to bring in muggy coastal air. Overnight and morning low temperatures will stay above 75 degrees, forecasters said.

Thursday looks to be sunnier but just as unseasonably hot with a high near 96. The south-southeast winds will persist, as will balmy overnight low temperatures.

“Another chance for isolated storms seems possible Friday afternoon/evening across northern portions of the Hill Country before the (high pressure) ridge expands slightly northward over the weekend and into the beginning of next week,” the weather service said. “This will lead to the warmest temperatures of the year so far with widespread upper 90s to upper 100s expected for high temperatures and even warmer heat indices.”

Heat safety alerts likely will be necessary, forecasters said.