Friday, December 21, 2012

12/21 - 2pm - I'm Dreaming of a White (Day After) Christmas

How about that snow last night around Kentuckiana? Most places saw a dusting but areas in Southern Indiana near Orange County picked up about 3 inches since a snow band set up there for a few hours during the overnight. Here at my house in Southeast Louisville we picked up a dusting of snow that blew around a bit in the wind. Temperatures today aren't going to make it out of the mid 30's in Louisville today thanks to the cold air behind the front that passed through yesterday. Seems appropriate given that it's the first day of winter!

Temperatures will moderate for the weekend with southerly flow returning but an area of low pressure moving through will bring rain and even warmer temperatures for Christmas Eve. Booooo! Luckily Christmas Day isn't looking like a washout with just a few clouds and highs in the lower 40's. That's a different story further South in Mississippi, Alabama, and Georgia where rain will be setting up.

Now... onto the fun part. Over the past couple of days the forecast models have been jumping onto the idea of an area of low pressure moving from the Gulf states to the western side of the Appalachian Mountains the day after Christmas. This is concerning because this track favors a snow event for Kentuckiana and even some of West Tennessee. You need to be on the north and west side of these low pressure systems if you want to see some snow since that's where the cold air is confined to. The past few systems we've been on the south and east sides of the lows moving though so that has meant thunderstorms and warm weather lately.

The latest 3 runs of the GFS for this particular post-Christmas storm have been fairly consistent with the low track, which is good! The problem has been that there's been trouble determining how much cold air there will be to work with and how strong/large the low moving through will be. These issues have affected how much snow the Louisville area gets. The graphic on the left is a comparison of the last 3 runs of the GFS for December 26th. For entertainment purposes only, the 0z run had just shy of 8" of snow for Louisville, the 6z had about 3", and the 12z had around 2.5". It is still way, way too far out for specific totals and to worry about the cold air issues yet since things will change likely on the models between now and then. I will say though that the 6 and 12z runs seemed to have difficulty keeping the low as a single, compact system once it matured and instead wanted to go with an elongated large low that tries to interact with another low that develops east of the Appalachians. I've seen this kind of thing happen on the GFS earlier this year with the Nor'easters that we had. Eventually, based on what happened with these Nor'easters, these elongated lows should give way to more concentrated, compact lows on the GFS as we get closer to the actual event. This should help to clear up some of the murkiness with the cold air issues too.

The Euro has been fairly consistent with the track of the low as the GFS has been, but has also experienced a few issues of its own like the GFS. Yesterday's 12z run of the Euro had a blockbuster snow event in the Louisville area while the 0z last night backed off on that prospect thanks to, you guessed it, a more elongated low and trouble with cold air. I can't post the precipitation output from the Euro model since it's behind a paywall and copyrighted, but I can at least show you what the 0z run did with the low after it moved through. Lots and lots of cold air will be behind this system if you couldn't tell by the deep blues and purples on the map!

UPDATE 2:20pm - Today's 12z Euro model run is in and it has a blockbuster storm track for snow for us here in Kentuckiana and West Tennessee. That is quite a strong low in East Tennessee on the left-hand map compared to previous runs. Sheesh.

The CMC and the JMA, which are the Canadian and Japanese models, aren't worth showing here right now. The CMC has the storm running off the east coast and the JMA has a nice low track for some snow in Louisville. It's worth noting that this system might disappear all together on the models or make a radical change. It's still too far out for much certainty.

Also, West Tennessee may see snow from this storm should it take the track that we're seeing. It wouldn't be much snow but certainly more than an inch or so isn't out of the realm of possibility, especially in Northwest Tennessee. Once we get closer to time this will become a bit more clear.

The NAO, which is the North Atlantic Oscillation, frankly isn't agreeing that there will be support for this storm. A negative NAO means blocking near Greenland allows for cold air to flow into the Eastern US and stay locked in place. The current forecast is for a positive NAO during this time, but it has been flipping between negative and positive over the past couple of days. This storm doesn't have to have a negative NAO to happen, but it would help to have it for cold air support and for a storm track that rolls up to the northeast. This will, like the aforementioned models, sort itself out over the next few days.


Finally, NWS Louisville has been on board with this storm since yesterday. They're being cautious like they should be but mentioning at length the potential for this at 5 days out is something that they don't do very often. Here's their latest statement:

A much more significant weather system will arrive on the 26th. Both the 00z GFS/ECM and ensembles track a strong low pressure system just to the west of the Appalachian spine which may set us up for our first significant snow. If the low pressure system takes its current forecast track, precipitation would enter the area Wed morning with the bulk of precip occurring Wed during the day and then exiting late Wed night or Thurs morning.

Although there is still much uncertainty in the track of this weather system 6 days out, the possibility is there for significant snow based on the low track and anticipated thermal profiles. BUFKIT GFS showed 8 inches at SDF with a little more than 0.5 inch of qpf. The soundings denote a classic heavy snow sounding with subzero and deep isothermal layer super saturated. The synoptic features of interest are a 110 kt jet max entering a the base of a deepening trough.

For now will have 60% pops on the 26th with rain in the morning and then snow in the afternoon and Wed night. Bing Crosby`s I`m dreaming of a White Christmas may happen, just 24 hours later.

Stay tuned for upcoming forecast updates as this weather system may put a kink in holiday travel plans.


This will be a fun system to watch over the weekend. Hopefully there's some snow in our future!


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