As an upper-level low pressure system digs southeastward towards the Great Lakes, a potent shortwave is going to continue digging southeastward across the Great Plains and Mississippi Valley tonight. An associated surface low is currently located in the Great Lakes region, and an Arctic cold front will continue sweeping southeastward. The positive-tilt (southwest to northeast orientation) of the trough has resulted in westerly/southwesterly flow across a deep layer of the atmosphere extending from the Southern Plains to the Mississippi Valley/Mid-south, Southeast, Tennessee/Ohio Valley, Mid-Atlantic, and Northeast. Despite a shortwave feature this past weekend ushering in colder and much drier air, the current pattern configuration has allowed/is allowing for some moistening of the atmosphere across most regions ahead of the Arctic front. Across most regions that are expecting to get snowfall tonight into tomorrow, this should allow for snow to begin reaching the ground sooner, although a decent amount of evaporation will still occur initially.
The unusual aspect of this setup, especially for southern locations that will be getting snow and/or ice, is the fact that there will be no generation of a surface low that will trek from southwest to northeast across the South. This is a scenario where enough lift will be generated along the Arctic front that snow will be able to fall at a decent clip, which is already occurring along a zone from southern Missouri/northern Arkansas to the Ohio Valley/Great Lakes region. That southwest to northeastward band of snow will continue moving/developing southeastward as favorable dynamics/lift sets up across additional regions (map included below). This upward motion will be further enhanced by a jet streak (a region of fast-moving air) that will be located to the right of the trough axis. When more air in the upper-levels of the atmosphere leaves than what is coming in, the atmosphere tries to compensate for that; thus, air begins to rise across that region. If there is enough moisture across that zone, it begins to condense and can produce precipitation.
The big concern with the air mass behind the front is how severely cold it is. As the drier and much colder air mass begins to intrude southeastward, any snowfall that initially melts on the roadways due to warmer temperatures today will result in major travel issues. Snow ratios will increase as the colder air seeps southward, so it will take less moisture to cause hazardous conditions. I’ve noticed that some meteorologists are drawing some comparisons to the event that unfolded in Birmingham and Atlanta in 2014, which resulted in people being stranded along roadways for hours, and honestly, it is a fair comparison.
We made some tweaks to our forecast since this morning. We upped the totals in our heaviest (purple) accumulation zones. Using a 10:1 snow to liquid equivalent ratio that is often shown on forecast model pages will not suffice with this setup; thus, this is why these totals are a bit higher than what you might be seeing on the model guidance. After much discussion, we went ahead and widened the 1-2 inch and 2-4 inch zones across the Carolinas and parts of eastern Georgia and included a zone of accumulations in southeastern Alabama and southwestern Georgia. We also specified ice accumulations across the zones that we foresee getting ice. Out of an over-abundance of caution, I suggest residents in regions, such as Atlanta, prepare for this event accordingly, despite our actual accumulations being on the low-side. Given that temperatures will rapidly drop behind the Arctic front, just a dusting to light accumulations could result in extremely hazardous traveling conditions. While a dusting/light accumulations may not be a concern in most cases, it actually will be in this scenario if they do occur.
Please be on our Facebook page tonight at 9:30 pm ET (8:30 pm CT) for a live Facebook discussion with Chris. You will be able to ask him any questions that you might have. In the meantime, we will continue to monitor this situation and will mention any necessary tweaks to the forecast in that video.