Wednesday, December 15, 2010

12/15 - 1am - Late Night Winter Storm Update

The 0z suite of computer models (GFS/NAM) is in and the verdict is not good for our winter storm here in Louisville and surrounding areas. Both computer models have increased the amount of precipitation that will fall tomorrow night through Thursday, but it still appears the time line I laid out in the blog post below is valid. The GFS model is now putting out 0.35" of liquid equivalent precipitation and the NAM just output 0.75". This is up considerably on both models compared to this afternoon's runs. Check out the 18z NAM model's accumulated freezing rain estimate for this storm on the left... it's showing the potential for a half inch or more of accumulated ice around the region.

I still think Southern Indiana north of Scottsburg will see mostly snow, which could add up to four or five inches especially as you head eastward. Here in Louisville we're on track to see a brief burst of moderate snow on Wednesday night before we go to sleet and then to freezing rain by Thursday morning. Like I said earlier today, Louisville's morning commute on Thursday is going to be treacherous if all the data we're seeing right now comes to fruition. Surface temperatures should be below freezing in Louisville throughout Wednesday night and Thursday (Downtown Louisville could get just above freezing on Thursday afternoon), so ice will have no problem accumulating on cars, roadways, overpasses, and everything in between. The computer models aren't accounting for the snow we have on the ground already, which will cool our surface temperatures, so it's difficult to gauge just where exactly it will get above freezing just south of town. The good part in all of this is that we will be fairly close to the freezing mark, which will make the salt on the roads much more effective. If surface temperatures were to be in the teens or 20's during this event, we'd have much worse road conditions. The HPC (Hydrometeorological Prediction Center) has points just south of Louisville in a slight risk for 0.25" of freezing rain or more (see right image), but we'll see how that plays out. I think we have the potential for 0.25" or higher of ice accumulation here in Louisville on Thursday, but those numbers will be fine-tuned as we get closer to time.

I'll post tomorrow with the latest data. Again, this storm won't break any records, but it will have a significant impact on much of Kentuckiana. Remember that I-64 and south will be under a Winter Storm Warning from tomorrow afternoon through Thursday afternoon and points north of I-64 will be under a Winter Weather Advisory for the same time period. The difference between these two is the potential ice accumulations, which warrant a Winter Storm Warning in place of an advisory.

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