Another round of winter weather follows the 15th cold front to invade Southeast Texas this fall. Officially, winter doesn't begin until this Sunday, but we've already seen snowflakes and by Tuesday morning, there could be ice on bridges and overpasses north and west of Houston. Officially, the National Weather Service has issued a Freezing Rain Advisory for several counties north and west of Houston:

Freezing rain is one of the hardest things to explain because unlike snow, you can't really "see" it. Yes, it's cold outside, but the cold air is only at the ground. If you could ascend in a balloon up a mere 2000 feet, the temperature would be an astonishing 50° - 30 degrees warmer than the ground! So the clouds at that level contain rain. As the rain falls, and hits the ground, it freezes where the ground temperature reaches 32° or below. How much ice forms depends heavily on how much rain falls; there's a lot of uncertainty about how much we'll see.

On elevated bridges and overpasses, it'll be a little colder than 32° and rain freezing there could cause dangerous driving conditions. I see the threat as mainly affecting Highway 290 west of Prairie View, I-45 north of the Woodlands, Highway 59 north of Shepherd and I-10 west of Sealy. If, however, temperatures in Harris County dip below 32°, then the threat for the morning commute becomes greater.

So how does this all happen that the ground is cold, but the air in the upper atmosphere is warm? Well, we had a very strong cold front blast through the city this morning just after 8 am when the temperature plummeted from 64° to 42° in the span of a few hours. That was the leading edge of a surge of cold air that originated in western Canada last week:

However, the jet stream winds are oriented from southwest to northeast, bringing much milder air from the central Mexico flowing on top of the cold air. This conditions, sometimes called "overunning", leads to steady clouds and periods of light rain as impulses ride the jet stream

So unlike last week's picturesque snow, today's scenario won't look like a holiday postcard if the worst happens. Stay alert and most of all stay warm because we won't see the cold retreat until sometime late Wednesday.

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