8:30 PM CST | January 1, 2019
I have been busy lately, but I wanted to write an update this evening. First off, there will be two rounds of winter weather. The first round will begin later tonight ahead of the system. The main winter precipitation types will be Freezing Rain and some Sleet across western North Texas. The Dallas-Fort Worth metroplex is caught in an all too familiar spot.
Temperatures across far northern and western sections of the metro could certainly fall to 30-32*F by late tonight/early tomorrow morning. Thus, Freezing Rain is possible for these areas. The urban heat island will help protect central, southern, and eastern sections from falling to freezing. It could be close, though.
What I am watching are the dew points as they could be the real factor in whether or not the immediate metro will see a period of freezing rain. DFW is currently 36*F with a dew point of 28*F. As moisture begins to fall, the temperature will come down some. This is because, as a certain amount of the moisture falling evaporates, it pulls heat from the environment - a process called evaporative cooling. The greater the temperature-dew point spread, the greater the potential for cooling. A quick way to estimate how much cooling can take place is by dividing the temp-dew point depression by 3 and subtracting from the temperature. This gives us the potential wet bulb temperature, or the temperature when it is cooled to saturation. A current depression of 8*F at DFW, divided by 3, would be 2.67*F. Subtracting 2-3*F from 36*F gives us a wet bulb temperature of 33-34*F which would be a very cold rain. Meanwhile, Denton and NW Fort Worth are 34*F with dew points of 27*F. Their wet bulb temperatures are about 32*F or 31*F. Rockwall, to the east of Dallas, currently has a wet bulb temperature of 36-37*F. Hence, the headache for forecasters right now.
The second round begins by tomorrow evening as the low moves out of New Mexico. Additional freezing rain, sleet, and SNOW is expected for western and northern parts of the state. This will continue into Thursday night as the low pivots eastward. Keep an eye on your local forecast, and plan accordingly.