What Is Going to Happen Thursday
THE PROBLEM
Thursday will be a marginal day. By that I mean we won't see feet of snow (so there's no reason to panic), but we will see moisture (rain, mix, snow). Below are the three different models (European, NAM, and GFS) and what type of precipitation each is forecasting at 5 PM Thursday as some start to show a transition from rain to snow. Notice how much they vary in type, placement, and amount of precipitation.
THE EXPLANATION
With so much variety between models, we're going to help you understand the most likely outcomes Thursday. First thing to note here is the arrival. Models have been suggesting this initially sets up as two separate lines that merge together as they arrive in Kentuckiana. Those are circled below.
Notice also the timing above. This is right around daybreak Thursday when temperatures are the lowest. Lows will be in the lower 40s Thursday morning, so I think most of this falls as rain at the start. As precipitation begins, though, that can lower temperatures slightly. A few ice pellets mixing in as this beings are not out of the question. As temperatures climb into the upper 40s through the morning and early afternoon, that will quickly switch back to all rain. There won't be enough sleet/ice early Thursday to cause travel problems. The precipitation will stay all rain until temperatures start to fall, then the impacts come late Thursday and early Friday.
THE FORECAST
The precipitation forms as snow up in the atmosphere, but with temperatures above freezing in the lowest 1 km most of the day, those snowflakes will melt before they reach us. The forecast hinges on when that lowest layer of the atmosphere becomes cold enough for the snowflakes to fall through without melting.
The inconsistencies between models boil down to temperatures. The map above is showing 4 PM Thursday. Notice the warm "nose" I've circled. Even though this system is bringing a cold front, the cold does not hit right away. This map is showing warm air advection right along that front. That means warmer air is blowing into the area which will keep temperatures in the 40s until closer to sunset. This warm "nose" is not a permanent feature. We lose that warmer air after an hour or two, but it will delay the onset of ice/snow.
The exact timing of the transition is going to be fluid. The day starts with strong wind coming out of the southwest. That will bring warmer air (see above note about the warm "nose"), so I don't expect the transition from rain to snow until the wind shifts. Even after the wind does shift, that change over will not be instantaneous. The wind from the northwest has to bring in cooler air that will drop temperatures allowing the snow to fall un-melted. I see the wind shift happening around 7 PM, then an hour or two later we could start to see the transition from rain to wet snow.
Impacts to travel will come very late Thursday (after the evening commute) and Friday morning. Any snow accumulation will be minor. Remember, the high temperature the day before this happens will be in the 50s. Ground and pavement temperatures will still be well above freezing, so it will be tough to get much snow to stick. Marc will talk you through this forecast during WDRB News tonight, and we will all provide you updates on TV and online as this system draws near.
-Hannah Strong
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