2/11 - 5pm - A Wintry West Tennessee Forecast

A front that passed through West Tennessee last night brought rain showers during the overnight hours and snow showers this morning across the region. These have since moved out of the area, but the snow came down in some heavy bursts at times even though there was really no accumulation. High pressure will provide the setup for a very cold couple of days with highs in the 30's and overnight lows in the teens. Yikes! Tonight could be the coldest night of 2012 so far, which really isn't saying much since we've been pretty mild as of late.

Monday's forecast is flat-out complicated. Warm air overriding the retreating arctic air mass in place from this weekend will set the stage for a storm that could put down snow, sleet, and even freezing rain. Most indications are that this will start off as snow across most of West Tennessee on Monday afternoon as the lower layers of the atmosphere moisten up, but this snow could switch over to a period of wintry mix later on in the evening as the atmosphere warms up more (notice the warm "push" at 850 mb - 5000 ft on the left). Once we reach a certain threshold of warming on Monday night into the wee hours of Tuesday morning, all the snow, sleet, and freezing rain should transition to plain rain and the wintry weather headaches will be no more. Before everything switches over to rain an inch or so of snow could be on the ground around the I-40 corridor, including Jackson, according to the latest forecast models. Areas in Northwest Tennessee will switchover to rain a little later on than I-40, so there may be up to two inches on the ground. This is nowhere near set in stone yet and I'm willing to bet we'll see a few small changes in this forecast before the weekend is over.

The models agree that snow will fall across West Tennessee on Monday, but there are differences in when exactly this warm layer aloft will come in enough to switch the snow and wintry mix over to rain. The 18z NAM seems the most conservative on snow amounts at this point, with only a half inch in Jackson on Monday night. It also suggests that we might avoid a wintry mix and switch immediately over to rain. There's even a brief shot of rain just before the snow starts on that model. The GFS is bullish on snow amounts with over an inch in Jackson with the aforementioned wintry mix period (depicted as freezing rain on the BUFKIT output to the right) before the switchover to rain. The main issues seem are the timing of the warm air intrusion aloft and how strong it will be. So while this won't be a huge winter weather event by any stretch of the imagination, a few travel issues may crop up on Monday evening as the snow and wintry mix is falling.

Check out the video below from this morning's newscast on WBBJ for more details on your forecast!

12/26 - 12:30pm - Post-Christmas Snow Chance?

While many did not have a white Christmas yesterday, it certainly was a nice day across much of the East US with warm temperatures. We now turn our attention to a system that will be bringing much of the Southeast rain and possibly some snow as you head northward. This system is brewing in the Gulf Coast states right now and will bring West Tennessee rain this afternoon through tomorrow and the Louisville area rain tonight through tomorrow. While West Tennessee will likely stay all rain throughout the duration of this system due to warmer temperatures, Kentuckiana may have to deal with some snowfall during the afternoon tomorrow. The track of this storm from Tennessee to Eastern Kentucky is usually one that gives us our bigger snow events during the winter, but this time that won't be the case because of the lack of cold air we'll have to work with.

The forecast for snow is a tough one because we'll be dealing with colder air coming in on the backside of this system to give us our snow. This means that the atmosphere will cool from top to bottom and while snow will be falling it will likely not accumulate much, if any, because the air at the surface will still be warm while the upper atmosphere cools down. Nevertheless, a switchover to snow in the Louisville area will likely happen sometime tomorrow during the early to mid afternoon hours as cold air penetrates enough of the atmosphere to keep snow falling all the way to the surface. The NAM (left) seems to be the most aggressive on this switchover right now due to the surge of cold air it brings in early on in the afternoon in Louisville but the forecast sounding, or a vertical snapshot of the atmosphere at a given time, still indicates that we'll have temperatures just above freezing near the surface. This means that snow will be making it to the ground, but will likely melt on contact or accumulate lightly on the grass and elevated areas in heavier snow bands. Speaking of banding, that's another issue we could face. Snow banding is when you get a thin strips of heavy snow falling that look like bands due to winds coming together in these areas. These bands can produce snow accumulations even when temperatures are above freezing, just like in Jackson, Tennessee earlier this month when over an inch of snow accumulated with temperatures just above freezing.

Overall, I think areas in and close to Louisville can expect a dusting and maybe up to an inch of snow on grassy and elevated surfaces at some point tomorrow evening. If cooler air does not come in as quickly as expected this will remain a rain/snow mix or even just plain rain. Areas north and west of the Ohio River have the best shot for a light accumulating snow as they'll be slightly cooler and have an earlier switchover during the late morning hours. Most of the models are agreeing that we'll stay above freezing during most of this event, but we will be dipping down below that mark late Tuesday evening. This may create slick spots if any roads are wet from the snowfall during the afternoon. The 12z NAM (top left) is indeed the most aggressive on snowfall totals around here by looking at the snowfall output map, which you can see a larger view of by clicking. The 0z GFS (bottom left) is less aggressive, which is typical for it lately, and the Euro (which I cannot post here to copyright policies) keeps the snowfall along and north of the Ohio River. There are still a lot of small variables like exactly how fast the precipitation will move out of here and how much cold air will actually work into this process, so little details like these can drastically alter who gets snow and how much.

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!

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.

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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.

12/29 - 7pm - Snow Update, Newspaper Article Tomorrow

The warm air looks to be winning out on the computer models for the system coming through tomorrow and Thursday. Rain should start tomorrow evening and transition to snow showers by Thursday evening. I think we're in for a half inch on the ground by the time Friday morning rolls around, but we could see up to an inch if temperatures drop faster than expected. The precipitation graph on the left was generated from the 18z GFS using BUFKIT, and it shows well how the rain (green bars) switches over to snow (gray bars) on Thursday evening. We may have some road problems on New Year's Eve as the wet roads freeze, so be careful out there!

If you live in Southeast Louisville, particularly the suburbs of Jeffersontown, Middletown, Fern Creek, and Hikes Point, open up the Neighborhoods section of the Courier-Journal tomorrow. They're running a follow-up article on the one they published about my website back in January. This follow-up article may also run in an abbreviated format in other Neighborhoods sections around East Louisville, so be sure to check your paper even if you don't live in the suburbs I mentioned. I'll be sure to post a link to the online version once it becomes available on my website, blog, and Twitter/Facebook. Thanks again to the Courier-Journal for writing these articles!