12/11 - 10:30pm - Be Careful When Wishing for Snow... You Might Get it!

With all the warmth much of the eastern portion of the nation has been experiencing lately some folks are beginning to get antsy over whether we'll be able to fall into a more winter-like pattern in time for the holidays. The cold front that rolled through earlier this week has helped to assuage some of those fears as it brought much cooler temperatures and even some light snow/sleet to parts of Kentucky and Indiana.

So... What's next? First up is a system that will move through the region this weekend as a warm one. A low will travel from the Plains to Lake Michigan and keep the eastern half of the nation warm as it spreads rain from the Great Lakes to the South. This makes sense because the North Atlantic Oscillation (NAO) will be going neutral during this time, which generally means that cold air won't be able to spill down into the Eastern US from Canada. The Arctic Oscillation will be negative to support cold air coming out of the north, but that doesn't matter too much for us when the NAO isn't negative as well.

The system behind this weekend's is the one that is drawing some concern. A low will be moving across the South during the day on Tuesday and potentially spread some snow to areas that are north of it. At this point anywhere from West Tennessee to the Louisville area is in a potential corridor for snow. This all depends on the exact track of the low and how much cold air it will be able to bring down from the north. This scenario is supported by the NAO and AO both going negative during this time. The GFS model takes the storm along the southern border of Tennessee right to the Carolinas. The ECMWF (European) model starts out with the low a little further south along the Mississippi River but has it exit off the coast in about the same location as the GFS. The low would then become a storm that would ride up to the Northeast just off the coast. With the previous system pulling up toward the Chicago area, this low will likely stay suppressed to the south. So... the broad, general nature of this low's track isn't in too much question. My preliminary (read: subject to change!) thinking is that this will be a rain to light snow situation for portions of Tennessee and Kentucky since cold air would plunge in as the system is exiting to the east. Accumulations would be very light to none.

But here's an issue. The snow cover over the Northern Plains and Upper Midwest has expanded tremendously over the past week as a storm pummeled those areas with quite a bit of snow. We're now at 31.4% snow cover over the United States. Air moving from the north over that snow won't warm as quickly as it would over bare land and the computer models generally have a difficult time resolving that. With this snow cover, the air in place over us and the air coming down from the north may be a little cooler than the computer models think it will be at the moment. That could mean more snow from this system. We're still a week away from this event so while details are sketchy, it's now obvious that this is a system to watch. This likely won't be the only storm we'll have to contend with between now and the new year since a pattern is setting up that will allow cold air to sit close by to the Ohio and Tennessee Valleys while lows track through and just to the south of the region. There's hope for a white Christmas in the region but it will depend on whether we can get enough cold air and a storm to move through just before the 25th. It'll be fun to watch!

12/17 - 11:50pm - A Not So White Christmas

Positive NAO - http://www.ldeo.columbia.edu/NAO

Christmas is just a week away and snow chances for many in the eastern half of the nation look pretty bleak. The next major system to pull through on Tuesday and Wednesday will produce snow in the Plains, but to the east it's expected to be all rain. A second low moving through the Deep South on Friday will bring more rain, but most of that will stay south of Kentucky and the heaviest south of Tennessee. After that point it looks pretty dry right through Christmas. Why so rainy and relatively warm lately? It has to do with the North Atlantic Oscillation, or NAO for short. When the NAO is negative, storms tend to ride up the East Coast or just west of the Appalachian Mountains. This is a recipe for snow for many across the East since these storms have cold air streaming in just to the north of their track. When the NAO is positive, storms begin deflecting northward to Canada before they reach the Mississippi River. This means that the cold air and snow with these storms stays out in the Plains. The NAO is in this positive phase right now and isn't forecast to go neutral or even negative until around the new year (see forecast and history chart to the right). Until we can get the NAO negative, I wouldn't bet on a winter storm for Kentucky, Tennessee, or much of anywhere else in the East.

23% of the United States is covered by snow at the moment, which is quite a bit lower than the 47.3% covered by snow at this time last year. Aside from Lake Effect snowcover in Pennsylvania and New York and snowcover in northern Maine, the East is snow-less.

A big winter storm is forecast to hit New Mexico, the Texas/Oklahoma Panhandles, and Kansas on Monday and Tuesday as an area of low pressure cuts northeastward (because of the positive NAO as explained above) through the Plains. A Blizzard Warning (green color on right image) has been issued for Northeast New Mexico while the other areas are under a Winter Storm Watch. Areas in the core of the snowfall north of Amarillo, TX up through Dodge City, KS could get well over a foot of snow.

12/19 - 7pm - You've Got to be Kidding Me!

So yesterday I was on here describing how bleak our outlook was for snow this Christmas. Today the models pulled a 180 and are now showing a possible storm for this part of the country. Do we get snow from this storm? Well, not yet folks. A few things have to get tidied up before we get the big one.

First off we got to make sure this low goes SOUTH of here, not ON TOP OF US, but SOUTH of us (as in the TN mtns). This factor absolutely blasted our last storm to bits.

The second thing we got to get in line is the cold air, which the GFS isn't doing real well with. Other models such as the EURO are taking the cold air and putting it here for that, so the GFS is just being stubborn like the NAM-WRF was the other day.

The third and final factor in this is the NAO value, which is the North Atlantic Oscillation (just the tilt of the airmass in the N. Atlantic). That is now forecast to be a POSITIVE tilt, which means this low can swiftly exit the US to the Atlantic via the Carolinas. This helps because it keeps the low from cutting northeast in a hurry, which again killed our last storm.

So, I'm going to keep my eyes peeled on this one, especially since it is a Christmas storm. So here's what I'm going to do; I'm going to put a Christmas snow chance at the bottom of each blog post from now until Christmas to let you see exactly what I'm thinking at the moment. Until Christmas though, we'll see highs soaring into the 60's! Looks like I'll be flying my model airplane on Friday!

Christmas Snow Chance: 30%

10/22 - 9pm - Snow on the GFS!!!

Alright, I went to school this morning and took a look at the GFS for grins:



Was I dreaming? Disregarding the model, I came home this evening and looked at some later runs:



Yeah, that's snow on November 3rd. I still can't believe it. The GFS has had its weirdness over the past couple weeks predicting some "far out flurries" in October, but I think this one might be valid given the general consistency of the model (they all call for some sort of snow, although amounts vary greatly). Then I took a look at the NAO (a measurement of the North Atlantic Oscillation, which determines if cold storms can pass through the East US based on pressure levels in air over the Atlantic). The NAO could go negative at the beginning of November, which opens up the flap for any winter-like storms to come through our neck of the woods:



So we just need to sit tight. There is another system Nov 6th-7th that is similar, but that's way too far out to conjecture. We're still about 12 days out on this Nov 3rd system, whether it be rain or snow. I think I'll have a decent guess by this weekend, but this still could be a model game right now by the GFS taking the low too far south. In the meantime, enjoy the rain because we will get a couple inches by tomorrow, with temps dipping into the 60's!