Wednesday, December 15, 2010

12/15 - 5:30pm - Louisville Winter Storm CRUNCH TIME!

The onset of Louisville's first major mixed precipitation winter storm of the season is just hours away and the forecast is still very difficult. The National Weather Service has extended tonight's and tomorrow's Winter Storm Warning to counties in Southern Indiana that were previously only in an advisory. The reason for this is the increase in precipitation totals that both the NAM and GFS have been hinting at today. Those areas in Indiana will see all snow and sleet, with accumulations up to six inches in places.

Here in Louisville the snow should start from the southwest near the 10pm hour and possibly mix with sleet before 1am, with accumulations of around 1-2". Based on the model guidance I've seen, I think we'll switch over to a period of sleet with some freezing rain mixed in after 1am. Freezing rain will become dominant in Louisville just a couple hours after that and last until around 7am with ice accumulations ranging from 0.25" to 0.5". The 18z GFS model (pictured top left) and the NAM both keep Louisville just below freezing at the surface during the entirety of the winter storm, but we're going to have to watch that. The snow we have on the ground will influence the surface temperatures across the area, so the computer models may be running too warm at this point. As I've said before, Downtown Louisville could get a degree or two above freezing during the afternoon and that will help road conditions improve tremendously. The 18z NAM BUFKIT output (second image on left) gives Louisville 1.7" of snow/sleet with a half inch of ice accumulation.

Even with all of this data nothing is set in stone. The storm is forming to our west right now and we're already seeing light snow and sleet falling to our west and southwest. Watching the radar trends and surface observations overnight (called nowcasting) will be crucial to see who gets what kind of precipitation. Louisville is historically the battleground between precipitation types and we'll be in that situation for sure tonight. The HPC (Hydrometeorological Prediction Center) has Louisville and areas south in a Moderate Risk for 0.25" of ice accumulation. Louisville and points north have been put under a Moderate Risk for 4"+ of snow accumulation as well, so we may very well end up with quite a bit of both snow and ice here in the city.

Should this system shift south just a few miles, Louisville would end up with mostly snow and accumulations would pile up to nearly half a foot. I've posted the 12z NAM snow accumulation output on the left just so you can see how incredibly close the heavy snow will be to Louisville, at least according to that model run. A shift northward would severely limit snow accumulations and put us in a mostly ice situation. Either way, Kentuckiana is set to get walloped by winter weather tonight. This will not be as bad as the January 2009 ice storm due to lesser ice accumulations and the potential for slightly above freezing temperatures during the daytime hours in Downtown Louisville and areas south of town as well. We'll see how it all plays out!

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