Very interesting late fall weather awaits us. You may recall I’ve mentioned a few times over the last couple years the “upper level low pressure sytem”, or “cutoff low”. This is a low pressure system that gets cut off from the general upper air flow(jetstream) pattern and slowly migrates over a longer period of time that is typical for a low pressure system that is being driven by the general atmospheric flow.
We will have the development of one of these upper level cutoff low pressure systems over the next few days, and that will provide a deluge of rain for us(which will prime us for more winter precipitation than normal this winter), followed by a change to sleet and snow, for Monday through Wednesday morning, for many areas of the southern US. We could actually see accumulating snowfall for parts of Mississippi, Alabama, Georgia, Tennessee, South and North Carolina, by the time this system is captured by the upper level flow and moved to the northeast. At that time, it’ll provide snowfall for the NE US region.
All the major computer models are in agreement now on this system. Upper level low pressure areas can attach to a northern stream of energy and bring colder air down from the northern plains area, but can also bring down cold air from the upper levels of the system, dynamically cooling the lower levels(surface where we live), depending on the strength of the system. This one looks to develop into a strong system, that will slowly develop over Louisiana and Mississippi tonight and tomorrow, and then migrate to the east on Sunday night through Tuesday, and then to the northeast across North Carolina and Eastern Tennesseee Tuesday into Wednesday.
If we continue to get these upper level lows into December, which I suspect we will, we may see a couple chances for snow and sleet from these type systems for December, as well. Another thing this system appears to do on the model output, is help to reconfigure the atmospheric pattern. The PNA(roller coaster) sets up a ridge in the west and trough(low point) in the south, after this system leaves, and the NAO(North Atlantic Oscillation) appears to go negative, which would promote an intrusion of very cold air, very soon.
Bottom line: I anticipate 2-4 inches of rain for northern Georgia through central and western North Carolina and eastern Tennessee, followed by a transition to sleet and then snow, at some time on Monday, continuing into Tuesday, in a scattered fashion. Accumulations are possible for northern Georgia, into eastern Tennessee and west and central North Carolina, with significant accumulations for the northern portions of Mississippi and Alabama, as well as eastern Tennessee and western North Carolina. Even central areas of Mississippi, Alabama, and Georgia may see at least some snow flurry and snow shower activity overnight on Monday, into Tuesday morning. I am including a couple maps to show you the upper level low. These maps are credited to www.raleighwx.net. With these systems, surprises abound. As the saying goes, “upper level low causes weatherman’s woe”………………….Keith
Notice the aqua blue colors that are separated from the green and yellow orange colors. Also notice the general flow to the north is west to east. The upper level low separates from this flow, causing it to slowly migrate to the east north east. The blue colors represent the atmospheric thickness, which is directly related to temperature. Any thickness of 546 or below can support snowfall. At this point, northern Alabama and Georgia, most of Tennessee are in the 540-546 thickness area, and could see snowfall. This map is from Tuesday morning, at 1 am. The map below this one shows the freezing line at 850mb(5000′ above the surface), at 7 am on Tuesday, as well as the precipiation that has fallen for the prior 6 hours(in green). Notice that many areas of Mississippi, Alabama, northern Georgia, Tennessee, western North Carolina and South Carolina, have freezing(0 Celcius) or below 5000′ temps at this point, as well as precipitation. This would be in the form of sleet or snow, by this time. The surface temps and upper air temps support snow and/or sleet for even central areas of the southern states, by this time. This area moves to the east and northeast on Tuesday into Wednesday, putting central North Carolina into the snow/sleet area.