*I was going to create diagrams to go along with this post but I just don’t have time today. You’ll have to envision it in your mind’s eye. 

*For more on winter weather in general, check out my reeeeeally old post about winter weather in the south

I will go out on a limb and say that weather-wise, the past 5 days or so have been just about the ugliest, most depressing string of days Nashville and middle TN have seen in quite a while. This is all due to a rather interesting weather scenario across the whole nation. I will attempt to concisely explain the factors involved, and hopefully you’ll also see why these types of storms are so incredibly hard to forecast.

First of all, last weekend (Jan. 11th & 12th) were unseasonably warm and also very rainy, especially Saturday. This was due to a large upper level “cutoff” low pressure system that parked itself right over the southwestern U.S. Because this system was in the upper levels of the atmosphere (about 18,000-30,000 ft) where the main branches of the jet stream exist, it controlled on a broad scale which airmasses were influencing which parts of the nation. The big trough of low pressure stubbornly sat spinning over the western 1/2 of the U.S. for several days, causing the jet stream to dig southward over the west, then shoot northeastward over the midwest and on into the northeast. The southern branch of the jet stream was allowed to surge northward and pump lots of warm moist air from the Gulf and northern Mexico up across the southeast and even into the northeast U.S., bringing the soaking rain and warm temps we had over the weekend. Meanwhile, the northern branch of the jet stream was doing just the opposite, pumping dry frigid air from Canada down over the western and southwestern U.S.

Finally on Sunday Jan. 13th, the huge upper-level low pressure pushed eastward across the nation, and thus the cold arctic air across the west pushed eastward along with it, creating a very sharp cold front at the surface that moved across Nashville that afternoon. You may remember that the temp was a balmy 65F or so Sun. morning, and around 3:30pm it plummeted 15-20 degrees over the course of about 2 hours.

That was part 1 of this crazy chain of events. Now for part 2:

After that initial surge eastward, the upper-level low pressure trough stalled out yet again, leaving that sharp cold front draped roughly along the spine of the southern Appalachians. The cold airmass wedged down at the surface (we all remember from basic science that cold air sinks and warm air rises, right?) but after it stalled out, the southern jet stream was still active. Monday & Tuesday of this week we had a battle between the southern flow of warm Gulf moisture and the stubborn cold airmass that surged eastward Sunday. The warm moist air has been sliding up and over the heavy cold air at the surface and thus we’ve had sleet and freezing rain across the whole middle TN area as the two forces have been at a stalemate. Temps in the mid-levels of the atmosphere have been steadily 6-10 degrees warmer than at the surface. This is an old diagram I made showing a simplified version of how that looks as a cross-section:


Judging exactly where that freezing line will be on the surface is nearly impossible. Thus, forecasts for this type of weather are rarely accurate on a local scale. Last night, the Nashville metro area got lucky in that the freezing line at the surface ended up being just to the west of us- the temp held at about 33 or 34 degrees. This large trough is finally going to push further eastward tomorrow, as yet another piece of Gulf moisture and energy combines with a surface-level low pressure system moving east out of Texas. That will bring the possibility of significant snow and ice to east TN, southern VA, and western NC. Nashville will be spared any precipitation from that system, though, and most of the southeast is forecast to remain firmly in the grips of that arctic airmass for at least another week or so, so keep your heavy coats handy.



January 7, 2010

This has happened so many times before, you’d think forecasters would get a clue. As a weather nerd I’ve followed the same models the forecasters use, they are easily found on the NOAA/NWS website. For some reason, the models often fail to take into account just how ridiculously DRY the air is with these Canadian arctic highs, and how stubborn it can be. We’ve had a very cold and VERY DRY last few days because we’ve been under the influence of a high pressure center that slid down the midwest into the southeast from Canada. The low pressure system and associated front that is currently passing through middle TN is ahead of yet ANOTHER Canadian DRY airmass. Even though there is some moisture associated with this front, there isn’t enough to overcome the insanely dry air at the surface. There’s plenty of snow flying around waaaay up there, but it’s evaporating as it falls through the lower layers of the atmosphere via a process called sublimation. As of about 10am, we are finally starting to see a few tiny grains of snow making it to ground in downtown Nashville. Other areas to the north and west have already seen about 1/2 inch, but I doubt Nashville will even get that much. It won’t be over till the front pushes on through later this afternoon, though. The only thing we can be 100% sure of with this system is that it will be FYAO cold this weekend.

UPDATE 3pm: Snowflakes finally started fleshing out around noon in downtown Nashville as moisture finally reached the lower levels, but it still has only amounted to a dusting.

This has happened before, and I just don’t see why conventional wisdom never makes its way into these forecasts sometimes. They rely too strictly on the computer models. To the meteorologists defense, however, winter weather has always been, and always will be VERY fickle and hard to forecast in the southeastern US. Also, the science of meteorology is always a work in progress, and any forecast past 1 day or so is at best an educated guess. No matter how sophisticated our computer models get, they will never be near as complex as the forces at work in our atmosphere, and until we devise some method of actually controlling the weather, a 100% accurate forecast past 12-24 hours is simply impossible. For more info on why snow is pretty hard to come by in TN, see my post on the subject from last year.

That being said, here’s my conspiracy theory: Kroger is in cahoots with the NWS and all other forecasters in the southeast. They promise them a cut of the massive profits from all the bread, milk, and eggs sales in exchange for inflating the snow amounts.


Winter weather in the south

December 12, 2008

So I’m sure if you’re from the Nashville area you know how ridiculous people around here can be when even a flurry of snow is mentioned in the weather forecast. If you’ve been paying much attention in the past couple of days, you’ll also know that Mississippi (yes, Mississippi!) got a pretty significant snowfall recently…. even 2-4 inches in some areas across central Miss. That’s absolutely unheard of in the south! If you know me, you’ll also know that I’m a weather nerd- I follow the forecasts and even chase storms when the opportunity arises. Often times I hear people talking about how crazy winter weather is in Nashville, and how we never get any snow (some people like it that way, some don’t), and how the forecast is never right. I thought this would be a good time to do a little post explaining a few things about why winter weather can be such a roller-coaster sometimes, and why it’s so difficult to accurately forecast winter weather in the south.

Basically, the location of TN, especially its latitude, causes it to always be influenced by air masses created in other areas. During the winter, the jet stream is essentially split into two branches, a southern branch that resides over northern Mexico and the Gulf, and the northern branch that resides over the northern US states and Canada. (Sometimes it can actually split into 3 branches, but for simplicity’s sake let’s leave it at 2.) TN’s latitude causes it to be right in the zone where those two branches can sometimes come together, and with them comes air masses created in their respective areas. The southern branch can sometimes bring northward a warm, moist airmass from the Gulf, and the northern branch can sometimes bring southward cold, dry airmasses created over Cananda. In order to have snow you have to have A) cold air- freezing or below- and B) moisture/clouds. Because of our location relative to the track of winter storms, we rarely ever see moisture and cold air at the same time. Most often we see the moisture in the form of rain as the system/front approaches, and then the cold air comes in after the system has passed, and taken the moisture away with it. I’ve made some diagrams to help illustrate what I’m talking about. To understand these you need to know a few basic things about weather- high pressure (blue H)=calm, clear weather and has clockwise circulation around it, and low pressure (red L)=cloudy, rainy/snowy weather and counter-clockwise circulation. The blue line with teeth is a cold front, which means that to the west (left) of it is a cold airmass advancing eastward. The red line with round humps is a warm front, which means that to the south of it is warm air advancing northward. In this first diagram I’ve made, you will see the typical scenario for a winter storm in the south. You can see that the rain is in the moist sector to the south and east of the low pressure system. The cold doesn’t make it to those areas because it’s being pulled down from the west of the system, behind the cold front. Because that cold air mass originated over land (Canada), it’s fairly dry, and if you’re to the south of the low, once the cold front passes the moisture is gone. It’s only in that northwest quadrant of the system that moisture gets pulled around and mixed with the cold air, creating snow. Most of these storm systems track a little too far north or south (this diagram has it going too north) and there either isn’t enough cold air in place over the northern plains, or the warm moist air simply rides up over the cold air (we’ll talk about that in a minute). Click on the image to show the full size diagram.

So what happens when the warm air is pulled up and then rides over the cold air? Ice. Normally this happens right along the warm front, in the northeast quadrant of the storm. The warm air is lighter and rises (we all learned that in elementary school) and creates a wedge. The precipitation starts as snow way up in the clouds, then melts when it gets to that layer of warmer air, then the rain drops re-freeze when they get closer to the ground where the colder air sits. This is called sleet. If the layer of colder air is really shallow, the rain may not freeze until it touches a surface. It will then freeze and form a glaze of ice on everything. This is called freezing rain. Another diagram to illustrate: (click to enlarge)

Finally, this last diagram will show you what the “ideal” scenario for a heavy snowfall event in Nashville/middle TN would look like. Unfortunately if you’re a snow-lover like me, this doesn’t happen very often because the low has to track in JUST the right place for it to happen. There also has to be a very strong, very cold airmass parked over the northern plains, waiting to plunge down to the southeast. The southern branch of the jetstream also must be very active and the low pressure must be very strong- strong enough that you have full 360 degree circulation (called a “closed low”). The exact track of the low makes all the difference. It has to track just to the southeast of us so that we are in that “sweet spot” of heavy snow just to the north and west of the low. The perfect Nashville snowstorm: (click to enlarge)

Essentially this is the great “Blizzard of ’93,” except its track is shifted slightly to the west of that storm. Ok, now that we’ve all had a full nerd-gasm, hopefully you now have a better understanding of why Nashville’s winter weather can be rather fickle and difficult to forecast. Back to the normal posting tomorrow.