Despite the sunny start, low clouds and drizzle engulfed the Brazos Valley quickly this afternoon. A warm front off the Gulf of Mexico slowly lifted north and helped to create a process called "overrunning." Typically, these type of events occur with mid-latitude cyclones, but can occur on a much smaller scale as well. To simply put it, warm air is moving on top of colder air at the surface, hence, overrunning.
Early this morning, a push of air from the southeast began to advance behind a warm front off the Gulf of Mexico. At the same time, a shallow layer of colder air was in place ahead of the front. Air behind the front is warmer with additional moisture. The exact opposite is true ahead of the front, where the air at the surface is much cooler and drier. What you need to know here is that drier, cooler air has a higher density than air that is warm and moist. In other words, air that has a higher density is essentially "heavier." The "heavier" more dense air will move when it wants to and will not let less dense air push it around. Therefore, instead of pushing the colder air out of the way, the only place for warmer (less dense) air to go, is over the colder air, that stays put at the low levels.
Take a look at this phenomenon from the side. You can clearly see that the warmer air rises over the colder, more dense air. As air rises, it tends to cool, become saturated and generates cloud cover. In our situation, the depth of cold air is so shallow, that precipitation remained light in the form of drizzle and showers. This also answers why its been so chilly today because the cold air did not want to move. However, this type of event is quite tricky to track, especially on doppler radar. Satellite pictures are perfect, but not so much when the radar is superimposed on top. 
The picture on the right displays what Max Storm saw this afternoon. Plenty of clouds, but radar echos are not returning, illustrating any precipitation. As noted earlier in this blog entry, our event is shallow. Cloud cover is close to the ground, and therefore precipitation that forms and falls is close to the ground as well. When an event sets up like this, it is almost impossible for doppler radars to pick up precipitation. Two key reasons for this is due to how dopplers work. One, you need to factor in that when a beam from the doppler radar goes out to find targets (precipitation, bugs, birds....etc.) the radar beam shoots out at an angle. Plus, the curvature of the Earth comes into play as well. The farther an object is from the radar site, the lower chance that the radar will pick up on it, especially if the target is close to the ground.
The illustration on the left should help you understand the two issues with curvature and angle. Note how the radar beam is shot at an angle, but how the Earth's curvature comes into play. A thunderstorm close to the radar has no problem, but the storm farther away from the radar cannot be picked up. Therefore, the best way to know about any shallow precipitation are from first hand accounts, weather watchers and official weather reporting sites, such as airports. Nonetheless, the doppler radar is one of the best tools that a meteorologist has. Sometimes, we need to do an little extra digging to understand what exactly is going on.
