*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:

icestorm

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.

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If you’re in Nashville you’re no doubt sitting around waiting for this massive ice-mageddon that is supposed to happen today. Make no mistake, ice storms can be extremely dangerous when they actually happen, but more often than not they end up being far less severe than initially forecast. This is because these types of storms are incredibly complex and the most difficult of all weather phenomena to forecast. Here’s how they work, and why I’m usually skeptical of forecasts involving them:

When an ice storm happens there is always a strong mass of cold air in place (just look at how cold it was yesterday and monday!). Then a low pressure system moves in from the south or southwest, creating a mid-level wind flow from the south or southwest. Mid-level in meteorology is loosely defined as 5,000-10,000 or so feet above sea level. This southerly wind flow brings both warm air and moisture with it, creating a layer of warmer air above the cold air at ground level. This means the precipitation starts out high up as snow or rain, then becomes all rain as it falls through the warmer layer, then refreezes when it gets to the stubborn layer of cold air at the surface. Depending on the thickness of this cold layer, the rain drops will freeze while falling (sleet), or will freeze upon contact with the ground (freezing rain). Usually in these situations, the surface temperature is right at or just below freezing. This freezing precipitation only lasts as long as the cold layer stays below freezing, and that surface layer always eventually warms up. Here’s a graphic I’ve posted on this blog before, to illustrate:

Based on the forecast models, and published forecasts I’ve seen, there’s no doubt Nashville will see some frozen precip today, but I have a feeling that it will change to all rain, and the surface temp will creep up to the mid 30’s by late afternoon or so. The current forecast calls for the ice to remain the longest in the northeastern counties of middle TN, so those folks may see enough accumulation to start downing trees and power lines. I doubt there will be enough accumulation for that to be a major problem in Nashville, and all areas south and west. I’m fairly certain that Nashville’s heat island effect will aid in getting the surface temps up sooner rather than later. Furthermore, most roads have been salted heavily, and even the secondary roads have some salt on them transplanted by car tires, and with the surface temp hovering right around freezing the salt will have no problem preventing ice on the roads. Since the temperature is forecast to continue rising as the night goes on, due to the warmer air in the mid-levels working its way down, there shouldn’t be any problems at all by midnight or so.

PLEASE DON’T THINK THAT I’M TELLING YOU TO DRIVE NORMALLY IN WINTER WEATHER. YOU SHOULD ALWAYS ERR ON THE SIDE OF CAUTION WHEN THERE’S A CHANCE OF WINTRY ROAD CONDITIONS. PLEASE BE CAREFUL, ESPECIALLY IF CROSSING A BRIDGE OR OVERPASS AS THOSE FREEZE MUCH EASIER.

What I’m saying here is that this will NOT be ice-mageddon. For Nashville at least.

Skiing/weekend stuff

February 12, 2010

Megan and I are headed for my hometown of Kingsport this weekend to take advantage of this last snowstorm that dumped about 10 inches in the southern Appalachians. We’re going skiing at either Beech or Sugar mountain. Skiing in the southern Apps can be tricky due to the fickle winter weather around here… yes the mountains are always colder than other areas, but they still have to make most of their snow with machines, and sometimes they’ll have a long period of daytime highs in the 40s and nighttime lows in 20s, which means some of the snow melts in the day, then re-freezes at night, causing icy spots that are a huge thorn in any skiers side. But with this last snowstorm, and the fact that temps there haven’t gotten above freezing for several days, and won’t until sometime next week, the slopes should be pretty nice & powdery this weekend.

That means I won’t be out & about seeing shows/partying/etc… this weekend, but that doesn’t mean you shouldn’t be.

The big show tonight is Tortoise with Disappears at Exit/In. Also: Neil O’Neil and Max & the Wild Things at the 5 Spot.

Saturday- Oblio, Shoot the Mountain, Deleted Scenes, Steve LaBate at The Basement.

Also on Saturday, two things will be coming together that I honestly thought I’d never EVER say in the same sentence: Infinity Cat and Vanderbilt. Yes, according to Nashville’s Dead, Infinity Cat bands Natural Child and Daniel Pujol will be playing at Vandy’s McGill Hall (that’s a dorm), along with a band called Colossi. I have no idea if Colossi is associated with Infinity Cat. Pretty crazy huh? Although, being a Vandy employee, I do know that McGill Hall is definitely the extreme exception to the typical Vandy kid stereotype. It’s basically where most of the students who don’t fit into that Greek/trust-fund-kid stereotype live. So while Vanderbilt and Infinity Cat definitely have a bit of an oil & water vibe, McGill and Infinity Cat seem much more like peanut butter & jelly.

Have a great weekend!

At a party in Murfreesboro several weeks ago I ran into an old acquaintance from college- Alice Buchanan. She’s from Memphis and had been in a band called Scandaliz Vandalistz which may or may not still exist… but when I talked to her she told me of this new band she’s involved with called Magic Kids that had just formed and were already gearing up for a tour with Girls. I was astounded at the speed of their ascent to label-signed status. As gorilla vs bear, Nashville’s Dead, and the Matador Records blog all report, they just signed to True Panther. Their quick emergence is not unfounded, though, as it’s literally impossible to deny the warm-fuzzy-feel-goodness of “Hey Boy.” It’s very summery, fun, and has the ability to turn even the dreariest winter day into a breezy summer afternoon. The first two bands that come to mind when I hear these songs are the Beach Boys and I’m From Barcelona. Grab two mp3s at Nashville’s Dead. Then just try to keep from putting them on repeat. I dare you. They’ll be playing at the Exit/In on Feb. 6th with Girls, and also hitting up SXSW in March. I’m sure they’ll be on my “must-see” list.

Monotonix singer Ami Shalev’s luck finally ran out at a show in Florida Wed. night. Though Brooklyn Vegan’s headline says he broke his leg, the statement from the band doesn’t quite make it that clear. Seriously dude, you should’ve expected this to happen sooner or later. You just can’t do crazy shit like that at shows without getting hurt eventually. Hopefully they’ll be able to resume melting faces soon enough.

As for this weekend, I’ll be attempting to get to the super-exclusive Clipse show at Phatkaps, but the snow/ice-gasm 2k10 may prevent it. The roads should be clear by tomorrow night, though, for the Wax Fang/How I Became the Bomb/Non-Commissioned Officers show at Exit/In.

And, given my post yesterday about the Mars rover Spirit, I must share this adorable comic from xkcd:

NMDinosaurs01The theory that the dinosaurs were wiped out from an asteroid impact near the modern-day Yucatan Peninsula is beginning to face major challenges. There’s no doubt that a huge impact caused the Chicxculub Crater, but some recent findings suggest that the impact may have occurred some 300,000 years earlier than originally thought. This comes as a second blow to the impact theory, with the first being the discovery/dating of the Deccan Traps in India. This gigantic volcanic feature is the result of a huge eruption that is believed to have lasted around 30,000 years. Can you imagine a massive volcano erupting for 30,000 years and covering an area equivalent to 1/2 of modern India with lava? Trust me, something on that scale is hard for the human mind to comprehend, but that much volcanic ash and gas in the atmosphere would’ve had a devastating effect on the ecosystem, and almost certainly played a major role in the extinction of the dinosaurs. Get ready for science textbooks to be re-written, because the asteroid impact theory is about to see the same fate as the dinosaurs themselves. (Via Daily Galaxy)

NASA recently teamed up with the Air Force Office of Scientific Research to test a new, Earth-friendly type of solid rocket fuel. I’m not exactly sure of the details, but somehow they’ve managed to make rocket fuel out of aluminum powder and ice. Yes ice… as in frozen water. The secret apparently is that the aluminum powder is so finely ground that it’s considered “nanoscale.” The nanoscale aluminum has so much surface area in contact with the water ice that the exothermic reaction when it burns is more efficient than normal solid rocket fuel, which is usually powdered aluminum (not nanoscale) mixed with an oxidizer such as ammonium perchlorate and a binding agent. Seriously though, who would’ve ever thought you could make rocket fuel out of ice and aluminum? It just sounds crazy, but it’s true. (Via EurekAlert)

When you think of the type of person who becomes an astronaut, you don’t typically think of race car drivers or musicians, but two of the astronauts about to launch on Space Shuttle Discovery tomorrow morning are just that- a former off-road truck racer and a drummer. Check out this Space.com article to find out more about Commander Rick Struckrow, formerly a Baja off-road race driver, Pilot Kevin Ford who is also a drummer, and several other astronauts who come from surprising backgrounds.

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.
winterweathermap-usualstorm

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)
icestorm

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)
winterweathermap-idealstorm

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.