Good morning! Please bear with me since I'm on the road right now but I still wanted to make a quick post to mention the major storm system making its way across the Country right now. This "double-barrel" low-pressure system will bring nasty impacts to the region as we start the weekend. I expect to do another update tonight once I'm set up and stationary and we'll discuss specific local impacts then. The models are tripping up because there will be multiple precip types (rain, freezing rain, sleet, snow) within a relatively short time and they haven't been reliable at all regarding accumulation yet. I expected they would start coming around today anyway so we'll see what they morning runs look like shortly.
What do we know? I'm glad you asked, that is a great question. I actually feel very confident about the large-scale details. That stuff is essentially locked in as I see it. We've still got a split-flow pattern aloft and the southern branch has brought a "bowling ball" upper-low (breaking out the book of weather terms today) into the West/Central CONUS. The northern branch has also brought a shortwave trough across the northern tier which has interacted with a thermal boundary at the surface to develop a surface-low. A second surface-low is developing over the Central Plains as a reflection of the southern upper-low as well. As our shortwave rounds the southern Rockies and ejects eastward it should take a very sharp negative tilt. It will also interact with a noticeable thermal boundary where two distinct air masses are clashing and the "frontogenetical" forcing (where the air masses clash) coupled with a strong upper-level divergence/low-level convergence relationship should result in rapid deepening of our surface-low. (The negative tilt, which means the low-pressure system tilts with height, also results in the surface-low being in a favorable location within the left-front exit-region of the upper-jet where vertical motion may occur, in turn leading to mass being removed from the column, decreasing the pressure).
A surface-low associated with the northern piece of energy will track toward the Great Lakes while the southern low will track toward the Appalachians. At that time, Thursday into Friday, I believe we will see an energy transfer between the northern-southern energy and our system should tighten up again as it passes through the Tennessee region.
I think that is one of the keys to how our transition from rain, to freezing rain/ice, to snow will play out. A quicker energy transfer with a faster pull of deep, cold air into the region would result in a faster transition to snow. We will see a transition with rain/ice/snow regardless but the storm track and duration greatly affect how much of a certain precip type we see. Those are the main questions from my perspective and once we can really answer them we will be able to pinpoint specific impacts more accurately.
Like I said, I really think today's guidance gets a better handle on things with the surface-lows both coming together so that's what's next, watching development and monitoring data trends.
Either way, we'll be under a warm advection regime as southerly flow takes over in response to the system off to our West so for the next 36hrs or so we will experience much warmer temps than we have in awhile. We'll actually get up into the mid 60's Thursday. Winds will occasionally be gusty as the pressure-gradient tightens as well, ~20mph gusts are possible. Rain should begin on Thursday with a transition to a wintry mix of freezing rain & sleet possible some time Friday. Friday night into Saturday we should see a changeover to snow. It is fairly likely we will see accumulating snow with ice being a realistic hazard as well. That's where we stand for now. Now it's back to patiently watching everything as it unfolds. I'll be back later tonight to make an update after I've had the chance to reassess everything.
DKK
