Extreme heat, record-high temperature days start with morning overcasts

AUSTIN, Texas - Extreme heat and record-high temperature days have started with strange things, clouds.

Mornings have been almost completely overcast, but the afternoons are entirely sunny. This is all under a powerful heat dome that's working hard to keep all the clouds and rain chances away. So what gives?

It's all because of the geography of and how the weather systems are set up over the region. There is a constant battle between the very humid, tropical airmass that lives over east Texas and the Gulf of Mexico, and the very dry airmass that lives over the deserts of west Texas. In this case, the tropical airmass dominates in the morning, but then gets squeezed out by the afternoon.

Moisture at the low levels is sneaking in from the Gulf of Mexico under our heat dome. The clouds that cover our skies aren't much higher than about a thousand feet. This can happen because of a strong inversion around 3,500 feet above the ground. The tropical moisture worms its way under the heat dome in the middle of the night. By morning, the sun starts to heat up the ground and the drier air in the Hill Country forces the clouds and humidity out.

You can see this happening on satellite imagery. Low level clouds move in overnight but don't move far into the Hill Country. This is because they're getting blocked by the Balcones escarpment and the inversion. Once the sun rises, hotter conditions spread from west to east evaporating the clouds as they go.  

Most days the clouds will be gone by about 10 a.m. But they're a good indication of about how hot the day will get. If they hold on longer, the temperatures will end up slightly cooler, but about 1 to 3 degrees. The opposite will happen with clear blue skies from sunup to sundown. When that happens get ready to roast. Even more than we have been recently.