When it comes to weather, I-35 may be the great divide

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(KWTX) No matter if we’re in the heat of summer or in the cold grasp of winter, the Weather Authority Forecast Team frequently forecasts different weather depending on what side of I-35 you’re on.

I-35 doesn’t create or change Central Texas’ weather, but as luck would have it, it’s almost the perfect divider for the weather we see. (MGN/NOAA)

I-35 doesn’t create or change Central Texas’ weather, but as luck would have it, it’s almost the perfect divider for the weather we see.

First, the lay of the land

Channel 10’s forecasts cover what’s referred to as a “designated market area”, or DMA for short.

Designated Market Areas separate media markets in the United States on a county-by-county basis.

If you live in Bell County, KWTX is your CBS station but if you travel to a friend’s house in Georgetown to watch CBS, you’ll see the broadcast of KEYE in Austin.

DMAs divide the United States so your local news is local to your area and not focused on some larger city hundreds of miles away.

KWTX serves what’s known as the Waco-Killeen-Bryan market, which stretches from San Saba County all the way through Bryan-College Station.

Our sister station in Bryan, KBTX, covers cities and towns in the Brazos Valley, cutting the DMA in half so we can provide you with news and weather from your community.

Channel 10’s viewing area is roughly 205 miles east-to-west but only 95 miles north-to-south.

And now the interstate

By random chance, I-35 cuts the viewing area almost directly in half.

Since weather in the northern hemisphere moves west-to-east, the weather can vary greatly across Channel 10's market.

Heat is always the big story in the summer months, however there are some subtle differences in the atmosphere that cause one half of the area to be slightly different than the other.

Cities and towns along and especially east of I-35 frequently see more humid conditions since they’re closer to the Gulf of Mexico and the South Texas Coastal Plains.

Whenever we see southerly winds, which is a near-daily occurrence in the summer months, the humid air moves in.

Temperatures on an average summer day east of I-35 will be in the mid-90s, but heat index values, thanks to the extra humidity, range from 99 degrees to 105 degrees.

Humidity is also present in the atmosphere west of I-35 too, but since it’s farther away from the Gulf of Mexico, humidity isn’t quite as high as cities as towns just 50 to 100 miles to the east.

Areas west of I-35 are more likely to tap into the drier, more arid air mass of West Texas.

The difference between west and east is subtle but noteworthy.

Cities and towns west of I-35 tend to be a few degrees hotter, typically in the upper 90s and triple-digits, but since there’s less moisture in the atmosphere, heat index values also usually range from 99 degrees to 105 degrees.

Even in non-summer months, westerly winds bring dry air from West Texas and warm, sinking air off the mountain of Mexico to us.

The hotter and drier air usually only makes it as far east as I-35 so areas west of I-35 will be warmer but drier.

The overall difference in weather from west-to-east stays mostly negligible during the summer months and even in the winter months too, but the changes are much more pronounced whenever cold fronts move through.

Because of the west-to-east nature of the atmosphere, cities and towns west of I-35 naturally see colder weather before counties to the east.

On the first day and night post-cold front, colder air has more time to entrench itself and temperatures usually run three to six degrees below areas east of I-35.

Cold air typically doesn’t stay around for long either so as cold air moves out to the east a day or two later, cities and towns east of I-35 may be a degree or two colder than to the west.

On occasion, especially during the early fall and spring, cold fronts stall out across our area.

Coincidentally enough, frontal boundaries tend to stall near I-35 too, further aiding in the east-west split.

Cold fronts most frequently stall in Texas because they’re too far removed from the jet stream, a fast stream of air in the atmosphere that moves weather systems, or because the temperatures aren’t too different on one side of the front to the other.

Stalling or stalled fronts play havoc with the weather since the weather sometimes varies greatly over the course of just a few miles.

In Central Texas, we’ll sometimes see warmer and sunnier weather on the east side of a front while colder air and cloudy skies move in on the western side of a stationary front.

This isn’t a rule of thumb, though, since sometimes drier air on the west side of a front may keep the weather cooler but sunnier with cloudy skies and humid conditions a few dozen miles to the east.

Some Central Texans drive on I-35 daily, others rarely do.

Regardless of whether I-35 is a part of your day-to-day life, I-35 does drive Central Texas’ weather to an extent, not by creating or changing the weather, but because it does happen to be in just the perfect spot to frequently divide our weather from west to east.