Most frequent question of the day on the Weather Journal blog and on Twitter: Where is the storm? Why is the radar so clean? Even now that darkness has fallen on this presumably pre-winter storm Saturday in Southwest Virginia, pulling out the radar on this page to the wide view only reveals what you see at left as evidence of the storm (bigger view here) — a few tiny streaks of precipitation arond the Arklatex region, moving northeast. Believe it or not, this storm based on Gulf of Mexico moisture overrunning cold air near the surface is pretty much on track at this point. The radar was supposed to look pretty clean at this time of day, between the departed storm that encased much of the central U.S. in ice and snow, even deep in the heart of Texas, and the newer storm just taking shape and heading our way. The North American Model has a not-perfect but somewhat-useful “simulated radar” function that tracks how radar is expected to look at various times. Based on its early afternoon run (18Z), it shows precipitation widespread across North Carolina and from central Tennessee southwestward by 4 a.m. Then, 6 hours later, at 10 a.m., a broad area of precipitation has overspread most of Virginia. Again, this may or may not be similar to how it unfolds, but it is what is projected, and shows how rapidly that wide-open radar can fill in.
The second critical piece of this is the cold-air damming. It’s pretty cold out there already, but since mild air will be streaming in aloft, it will need to be somewhat colder and drier in the layers nearer the surface for frozen/freezing precipitation to develop and last for hours. Many times in winter we see imperfect timing between the cold-air high pressure system and the advancing Gulf moisture — either the cold air suppresses the moisture, or the Gulf air overruns the cold air quickly as the high supplying it exits stage right. This time, the two appear to be running into each other’s arms, with high pressure (the multiple H’s in the map inset at left, based on the 18Z NAM projection for 4 a.m.) building in just to our north just as the moisture begins to overrun the cold air from the south. The cold air continues to build at the surface, according to forecast guidance, as the mild, moist air continues to overrun high overhead it. It gets colder at the surface while it gets warmer aloft — a textbook freezing rain situation, if it happens that way. This is why the winter storm warnings are out. It does not appear likely that the irresistible force of the warm, moist air aloft will be able to push out the immovable object of the dammed cold air for many hours, likely not til after midnight Sunday and possibly as late as after sunrise Monday. That could allow ice to continue to build in many locations through Sunday evening as temperatures only slowly dawdle back up to near the freezing mark.
This is weather, so yes, there are things that can still go awry in forecasts, especially if either of the two things I mentioned above is a little weaker or stronger than projected. The early thing to watch on Sunday will be the level of snow/sleet falling. The models have backed off quite a bit on this potential, limiting significant snow/sleet accumulations mostly to areas north of Roanoke. But early and deep cooling of layers between the clouds and ground could allow more of this to develop than the models show.
We’re almost in “nowcasting” mode with this storm — once something shows up on radar worth nowcasting about.