This weekend ought to be an uneventful one. A weak cold front stretching from the Tennessee Valley to Eastern Texas is forecast to reach South Georgia this afternoon and early evening before washing out. Some stratocumulus clouds may be observed this morning, but we should end up with a partly cloudy day, on average. The cold airmass that was in place during the middle of this week continues to steadily modify. High temperatures should have no problem reaching the mid and upper 70s both today and Sunday, as a weak area of high pressure slides eastward from the Midwest to the Eastern Seaboard. A weak pressure gradient and dry air through a deep layer of the atmosphere will permit good radiational cooling tonight, so lows could dip below the 40 degree mark in the normally coldest neighborhoods for a couple of hours early Sunday morning.
The big weather-maker for the new week is a powerful upper trough moving from the Pacific Ocean into British Columbia and the Pacific Northwest this morning. The numerical models forecast the trough to fracture into two pieces, a southern part and a northern part. The southern part will merge (also known as "phase") with an upper low near the southern coast of California. As the system translates eastward, it will intensify rapidly Monday and Monday night in the Southern Plains and Lower Mississippi Valley. Surface pressure falls will begin to accelerate along the Texas Gulf coast, owing to a combination of strong low-level warm advection and the approach of the upper disturbance in an increasingly diffluent upper tropospheric wind pattern. The area of low pressure is then forecast to strengthen on its way to the Tennessee Valley and Central Appalachians on Tuesday. The global and now the regional models and their respective ensembles continue to be in stellar agreement on this scenario.
This leaves us only to nail down the always-important details. Unfortunately, the present state of the science only gives us limited predictability in this regard -- especially 3 or more days in advance. At this moment, it appears highly probable that strong or extreme vertical wind shear will be in place by Tuesday morning in the presence of impressive large-scale lift provided by the approaching upper trough. These two factors should be supportive of an eastward-moving squall line along the Gulf coast states Monday night into Tuesday. As is so often the case, the amount of destabilization is in question. Even with cloud cover, dewpoints in the mid to upper 60s on the heels of strong southerly low-level winds should lead to at least a weakly unstable airmass. Recent research clearly indicates that a strongly sheared environment can be more than adequate for severe weather in a weakly unstable environment. Just ask the folks in Gadsden, Decatur and Grady counties from the event on February 17 and 18. In any case, the atmosphere will be supportive of widespread wind damage from intense cells within the squall line. Furthermore, robust horizontal rotation could support tornadoes, especially if discrete supercells develop ahead of the squall line. However, tornadoes will be possible in the line, too, and some of them could be strong.
Another concern is the potential for minor to perhaps moderate coastal flooding in Apalachee Bay. Strengthening southerly flow from Monday through Tuesday morning will act push water toward the Franklin and Wakulla county coast. Once the front passes, the winds will veer to the west, which would increase the coastal flood threat to Taylor and Dixie counties Tuesday. Residents in flood-prone areas of Apalachee Bay are urged to monitor forecasts early next week to see if coastal flood watches and/or warnings are issued.
The one bit of good news is that the storm should move fast enough to prevent widespread flooding rains. That said, at least one-half inch seems likely and if a heavier thunderstorm develops over one of the swollen area rivers, it could create localized problems. Right now, we believe a repeat performance of the excessive rains on February 21 and 22 is unlikely.
We will update our blogs as the severe weather events draws near and the trends become clearer.