Don't worry, that was just an attention getting headline. It's the worst kind for forecasters. If you did not read my previous post you should do so now. The forecast page and on-air will be updated even when the blog is not. If you did not get a chance to try the "learn if you dare" tutorial post from Jan 7th now would be a good time to read it, as this is that type system at least in part.
Bad news from the NWS HQ, they've sent an administrative message indicating there are a lot of missing obs from the upper air balloon release today throwing a real wrench in forecasting models. Two of the sites AWOL are Peachtree City, GA and Birmingham, AL. This means if today's model cycles show big changes we won't know if its significant or a result of bad data ingest. Thankfully, there is time for them to fix it before we get to Wednesday.
Models agree only on the general setup, but not the fine (and all
important) thermal details. The exact track varies on each model some
inland, some South and East of the Gulf/Atlantic Coast. Plus the models
known weakness is just this type of pattern unfortunately. A wiggle of
just 50 miles on the track or 2-4 degrees in temp anywhere in the
vertical (all within the NORMAL margin of error) and its surprise city,
or some kind of different outcome scenario.
The key is, that with cold air aloft, and heavy precipitation falling out of a very cold upper atmosphere, the models might not be handling low-level temperatures very well. We just can not know for sure. The snowflakes will be very cold as they fall, cooling the atmosphere, and when they melt falling into warmer air, that will cool the atmosphere more. The simultaneous melting, partial evaporation, and refreezing absorbs heat and releases heat at the microscale. Some of the models are also showing strong WAA warm air advection at 850mb about 5,000 feet. That can be a snow killer. Bottom line, if the models are not handling these complex fluxes well, we could wind up with snow Tuesday afternoon or evening. But, if they are handling it, it will stay 40 or so most of the day Tuesday, with rain, and then a changeover to light snow late early Wed. morning, but no big deal around I-20 or South. The mountains toward TN could be looking at a big deal one way or another.
However, the 500mb low closes off and the vorticity max with it is impressive to create strong 700mb omega. For that reason, while it looks like more rain than anything else for Metro Atlanta, I CAN NOT just say its gonna be rain case closed game over that's that.
To put it another way, verbatim the model predicted thermodynamic diagrams (vertical sounding profiles) show too warm for snow, just rain! The million dollar question is are those projected thermals gonna prove right? Since its a known model weakness we just have to wait and see.
With the upper-level low spinning overhead, convective snow flurry squalls could form Wednesday mixed with sun.
Bottom line...this is a very complicated system and our confidence in the forecast is unusually low. Closed upper level low look out for surprises below.
It has some likeness to an even last March or the March before, rain changed to snow just about everywhere in Atlanta from the West Southwest to the East Northeast as the storm was leaving, MOST places got just a coating on the grass and briefly on roads that soon melted. BUT a band about 50 miles long and just 20 miles wide got 3 inches with isolated 6 inches and near Athens 9 inches with thundersnow. This band included central Fulton, Dekalb and part of Gwinnett. Those people felt we had miss-forecast a blizzard: Not realizing the forecast was spot-on for 80% of the metro and their winter wonderland was not representative of the area. And because of above-freezing surface temps at the time it was all falling, even radar did not show what was happening with the rain V snow line-- as it was happening.
There are limits to science and technology whether we like it or not.
The fact is the state of the science simply does not allow for the resolution of such small scale (microscale and mesoscale) atmospheric details.
I was not here at the time, but I am told that in 1983 there was a cold rain forecast up to the last minute for Atlanta and Athens. It snowed 6 inches in parts of Atlanta and 8 in Athens. It could happen this time. THERE JUST IS NO WAY TO TELL. We can only make the best forecast given what our data and model tools tell us. But knowing that this type animal (strong dynamical cooling aloft and evaporation cooling and latent heat release physics) are subject to boon or bust. I wish I could change that reality but I am not that powerful. In the end, God/mother nature always has the upper hand.
The good news is, that if the models are right, IF and where it snows, it will be a wet slushy snow and the cold air that follows is not as extreme as last time so we should get more melting on the roads even as its falling, and once the sun returns more melting than we did last time, and the sun return should be faster than after the last system. In fact both Tue and Wed when precip stops the temp will bounce up above freezing almost immediately. And yes, there are more winter threats showing up in the medium to long-range but lets deal one at a time.