Once upon a time in a forecast model that was over a week out, there was the possibility of snow – for today. But as we know all too well, longer range models do not perform well into the ether world of voodoo land. But what is interesting tonight is that I still don’t know for certain if it’s going to snow or not on Wednesday night or Thursday morning in Birmingham. At this point in time, I’d have to say that it is a marginal situation – the kind that deserves a watchful eye.
When I did the Weather Xtreme Video this morning, the 06Z GFS was forecasting a weak low moving across the northern Gulf of Mexico somewhat offshore. The key feature seemed to be the strength of the surface low and the fact that it was fairly far south. It moved eastward across North Florida in the vicinity of Jacksonville before racing up the East Coast and presenting them with serious problems Thursday night and Friday.
The 12Z GFS model run didn’t seem to show a major difference. One difference was in the timing with the progress of the low a bit slower – on the order of 6 to 12 hours. The low was still relatively weak, but it was also positioned slightly closer to the coast traversing North Florida a bit north of Jacksonville. And a quick look at the 18z showed very little variation from the 12Z run.
Two elements that get a lot of attention are the location of the 5400 meter height contour (540 line) and the 850 millibar zero degree isotherm. In each of the runs mentioned above, the 540 line remained well north in Tennessee or Kentucky. The 850 zero line was near the Alabama-Tennessee line or even a bit further south to near Decatur. This solution, if true, did not present any serious winter problems for Birmingham and most of Central Alabama.
And now the North American Model (NAM) is in from the 00Z data. But instead of helping to make a strong case for or against snow, it seems to be continuing the trend of “not quite sure.” The 540 line on the NAM is along the Alabama-Tennessee border on Thursday morning (84-hour prog) with a surface low hugging the coast between Pensacola and Panama City. And the zero-degree 850 isotherm is projected to be in the vicinity of Birmingham. This points to a much closer ice situation for a good portion of Central Alabama.
As long as everything remains “close,” the prudent forecast is to mention both rain and snow. This also means the possibility of icing is there, too.
I have to say that the models have been fairly good in pointing the way toward the trend even as they bounce around a bit. But like most situations whether it is winter weather (ice and snow) or severe weather, the details are always in the small scale. And that small scale is extremely tough to nail down when we’re talking about a storm that has not even formed yet!! At least I can’t find signs of it in the data I have available to me.
Somewhere, someone will interpret this post as an excuse for a lousy forecast. This post is not that at all. It is, though, a brief discussion on the uncertainty meteorologists deal with as we try our best to pin down a forecast several days in advance – a forecast that is still tough to call as snow or ice or rain. Stay tuned!
-Brian-
