Tuesday, December 14, 2010

12/14 - 1pm - Winter Storm WATCH for Louisville

A Winter Storm WATCH has been issued by the National Weather Service for Louisville and surrounding areas until Thursday afternoon. An area of low pressure passing to our south will give us a quick-hitting winter storm, meaning we'll only see its effects in Kentuckiana from late Wednesday into midday Thursday. There's talk at the NWS of issuing an Ice Storm Warning for portions of Central and North Central Kentucky if the freezing rain accumulations appear that they'll get near a quarter of an inch.

Storm Timing

Wednesday morning/midday: All Clear

Wednesday afternoon/evening: Light snow/sleet begins in the west and spreads toward Louisville as snow. Under an inch of accumulation expected.

Early Thursday morning: Snow switches over to sleet and freezing rain, becoming mostly freezing rain as the morning wears on. The morning commute will be a royal mess in Louisville. Based on the forecast models and guidance I'm seeing right now, most schools should be closed or delayed on Thursday due to the treacherous driving conditions on Thursday morning. Areas north of Scottsburg, IN should have snow throughout the entire event and I would not rule out a couple inches of accumulation up that way.

Thursday afternoon: Near and south of the Ohio River should see freezing rain end from west to east in the early afternoon hours. There could be a few snowflakes as temperatures aloft cool down on the west side of the low. Again, Southern Indiana from Scottsburg northward should stay with snow and possibly a little sleet as this system pulls out of the area.

The devil is in the details with this storm. How much freezing rain will accumulate in North Central Kentucky? Right now I'm thinking Louisville will see just under a quarter inch of ice accumulation whereas areas near Elizabethtown, Fort Knox, and Bardstown could pick up about a quarter of an inch. The GFS model is being conservative with precipitation amounts (0.27" liquid) whereas the NAM is pumping more into the area (0.56" liquid). I'm willing to bet we'll see something in between those numbers here in Louisville, some of that liquid falling as snow/sleet while the rest falls as freezing rain. Check out the annotated BUFKIT sounding from the NAM model at 4am on Thursday above (click to make it larger). You'll notice that temperatures between 4000 and 8000ft will be above freezing, meaning the snow will melt to rain while it's falling through that layer of air. The lower 4000ft or so of the atmosphere will be below freezing and that already thawed liquid rain will likely freeze when it hits the surface. There will likely be a period of sleet in Louisville as we transition from snow to freezing rain on Thursday morning. Like I said yesterday, only a few miles will make a big difference. We still have a decent snow pack in Louisville, so I do expect temperatures to stay below freezing here at the surface throughout Thursday except for some areas downtown where they may reach 33 degrees. Even though we're only about 36 hours away from this event, there's still wiggle room for all of this. If temperatures aloft aren't as warm as forecast, we'll see mostly snow in Louisville and the freezing rain risk would shift southward. On the flip side, temperatures at the surface could also nudge a couple degrees higher than expected (unlikely due to snow cover in my opinion) and decrease the ice problems on Thursday morning.

We'll see how this one plays out. The NWS in Louisville will upgrade the Winter Storm WATCH to either a warning or Ice Storm Warning at some point later today and I'll post the details of that on Twitter and Facebook immediately. Should any significant changes occur in the forecast, I'll post those on the blog.

No comments: