National Weather Service Weather Forecast Office

Event
Very shallow, chilly, and moist air at the earth's surface will gradually become overtaken by much warmer and also moist air only a thousand feet above the ground (Figure 1, below) through the night and into the early morning of Thursday, December 18^th. As the warmer air arrives, areas of dense fog will develop, particularly after sunset on the 17^th, likely continuing overnight and into the daylight hours of Thursday, December 18^th.

Motorists across much of Deep South Texas can expect to encounter areas where visibility drops to near zero, particularly in preferred locations such as low lying areas. Those planning travels between sunset tonight and an hour or two after sunrise Thursday should remain alert to sharply varying visibility, and plan on using low beam headlights and/or fog lamps, slowing down, and maintaining a safe distance between vehicles.

Meteorology
A few times each winter, shallow cold air reaches into the Lower Rio Grande Valley and sometimes well into extreme northeastern Mexico. Often, the originating air mass comes from locations well north, anywhere from northeast Russia through Alaska and into western Canada. Initially, the air mass extends high into the atmosphere. As the cold air spreads south and east, the support high in the atmosphere dissipates.

Meteorology, continued
However, because cold air originating near or above the Arctic Circle is very dense, it is able to sink to the earth's surface and "ooze" deep into continental North America. Even as the air mass modifies while spreading far south, it still maintains enough arctic characteristics to allow surface temperatures to plunge well below average. Sometimes, if the originating air is cold enough, subfreezing surface temperatures can reach the Lower Rio Grande Valley!

Initially, a low ceiling of clouds will form at the top of the surface inversion; in this case, around or just below 1000 feet. However, as warmer air begins to erode the chilly air at the surface, both the temperature and dew point will slowly rise. When the inversion level drops close the surface, the cloud ceiling height lowers; at the point the dew point just off the surface rises higher than that at the surface, fog will form.

Note that fog can be as fleeting as the wind. In some cases of this type of fog, can be disrupted by increasing winds, generally above 5 mph, which can bring enough warm air to the surface to disrupt the temperature and dew point contrast, improving visibility. For the overnight and early morning of the 18^th, this is a possibility in some areas.