It’s been quite a while since I posted much of anything weather-related, but now is as good a time as any! Our middle TN National Weather Service office started a Facebook page a while back, and I’ve been loving the hell out of it. Let’s face it, the weather service has always had a bit of an image problem. I’d be willing to bet that when most people think of the NWS, they think of a boring office with a bunch of crotchety old dudes staring at weather maps & computer screens. But in reality, the meteorologists are dedicated and passionate scientists who truly love what they do, and they enjoy talking about weather on a more conversational level, too! That’s exactly what their Facebook page is for, and it’s filled with tons of interesting factoids, tidbits, and more humanized/conversational information about the forecast. The forecasters are on there quite often, and are very good about responding to comments or questions posted. They also just closed submissions on a summer weather photo contest, and you can browse through the album and vote on your favorites. The photo with the most likes as of June 20th will be declared the winner. So check out their Facebook page and Like it. Even if you’re not as big of a weather nerd as I am, I promise you will learn something interesting if you Like the page and follow their posts!
Speaking of weather, I’m looking forward to the very favorable forecast for this year’s Bonnaroo! I will be there taking photos for the Scene as I have for the last 5 years, and thus I’m issuing my yearly Bonnaroo posting disclaimer: *This will be my last post before Bonnaroo, and posting will resume sometime early next week.* Be sure to follow our coverage and photos over at the Nashville Cream! Also, my girlfriend Lauren will be there with me blogging about the new & improved food options on her blog Old Red Boots, so follow her coverage to see the festival through the eyes of a foodie. But back to the weather- while it’d be nearly impossible to beat last year’s utterly euphoric Bonnaroo weather, this year’s forecast looks very good. Thursday is a little iffy as there’s currently a 30% chance of showers & storms, and the Storm Prediction Center has Manchester right on the edge of their ‘Slight Risk’ area for severe weather in their convective outlook. BUT, that activity should be pretty scattered/isolated so the risk is still pretty low. Friday & Saturday are damn-near perfect, however- sunny skies with mid to upper 80s for highs and low to mid 60s for lows- very similar to last year! That temperature span also suggests lower humidity! Sunday will be a little hotter with highs in the upper 80s and Sunday night brings back a 20% chance of showers. Overall, you really couldn’t ask for a better Bonnaroo weather forecast. See you here next week!
A rough overview of what’s behind our recent crazy weather
January 16, 2013
*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.
Science Friday: Coriolis Effect does NOT affect water drains/indirectly affects tornadoes
April 29, 2011
So I’m thinking that some Fridays I’m gonna steal NPR’s “Science Friday” idea and write a post debunking some popular myths. Not gonna happen every Friday, but I’m gonna make a pointed effort to do it somewhat often.
Today we’ll tackle the myth about water spinning in opposite directions down the drain in the northern and southern hemispheres. This is simply not true at all. While large-scale weather systems do indeed follow this pattern due to a phenomenon called the Coriolis Effect, water going down a drain does not. Minute things such as imperfections in the angle at which a basin was installed, inconsistencies in the surface or shape of the basin, and any residual motion in the water itself are what determine the direction the water rotates when drained. Once the water begins to drain, the conservation of angular momentum takes over and any hint of rotational motion in the water, whether clockwise or counter-clockwise, gets amplified as it moves toward the drain, creating a vortex. This is the same law of physics that causes tornadoes and dust devils to behave the way they do. The classic explanation for this concept is the spinning ice skater. As he/she moves her limbs closer to their body the conservation of angular momentum forces their rotational speed to increase. They can then move their limbs further away and they will slow down again. So no, no matter what you’ve heard, water does NOT always drain counter-clockwise in the northern hemisphere and clockwise in the southern. That being said, experiments have been done that show the Coriolis Effect can be seen in draining water IF AND ONLY IF all other forces are completely removed. A large and absolutely perfect cylinder-shaped container with one very small hole exactly in the center was filled with water and allowed to sit untouched for 24 hours to allow any residual motion in the water to die out. The container was perfectly balanced with extreme precision. The plug in the hole was then carefully removed and the water did eventually start rotating counter-clockwise, and continued to do so when the experiment was repeated. BUT clearly this only happens in extremely controlled conditions. In your sink or toilet, a myriad of other forces are orders of magnitude stronger and completely overwhelm the minute effect of the Earth’s rotation. Snopes has a decent debunking of this myth as well.
Tornadoes are different. I was asked about this yesterday, in the wake of the massive tornado outbreak on Wednesday. The Coriolis Effect does influence the direction tornadoes spin, but in a more indirect way. There have, in fact, been clockwise (anticyclonic) tornadoes documented in many cases in the U.S. There have even been a few storms that dropped multiple tornadoes, both cyclonic and anticyclonic, at the SAME TIME. As I said earlier, the Coriolis Effect is what causes large-scale weather systems such as hurricanes, low pressure systems, and high pressure systems to rotate the way they do. Low pressure systems and hurricanes always rotate counter-clockwise in the northern hemisphere and clockwise in the southern. The opposite is true for high pressure (clear weather) systems. The storms which produces tornadoes are always associated with a large-scale low pressure system. The Coriolis Effect determines the rotation of that large-scale system, which in turn has an indirect influence on the structure of the supercell thunderstorms which spawn tornadoes. Tornadoes are far, far more common in the U.S. than anywhere else in the world, and because the warm moist air which “feeds” these storms at the surface is moving in from the southeast, and the cooler, drier air aloft is moving in from the northwest, that setup naturally lends itself to counter-clockwise rotation, hence most but not all tornadoes in the U.S. spin counter-clockwise. This illustration from NOAA’s National Severe Storms Laboratory shows the inner workings of a tornadic storm quite well:
The U.S. Department of Energy’s Office of Science Education has a website called NEWTON, with a forum of sorts called “ask a scientist.” This very question regarding the rotation of tornadoes was asked, and I found this particular response quite helpful:
At least the great majority of tornadoes rotate counterclockwise (as do all low-pressure systems) in the northern hemisphere, and clockwise in the southern hemisphere, for the reason (coriolis force) given by Eric Peterson in response #1. But occasionally, it would seem, northern hemisphere tornadoes do rotate clockwise: S. Flora’s book “Tornadoes of the United States” cites an 1890 article in the American Meteorological Journal. Its author, a J.P. Finley, states that, of 550 American tornadoes he studied, 29 were deemed to have rotated clockwise. I have not been able to find any “modern” study of this question. But I believe it could be true. The region of swirling air that contracts to become the tornado is not itself large enough in extent to have its rotation dictated by the coriolis force; rather, it “inherits” this tendency from the great masses of air whose movement sets the stage for the storms and any associated tornadoes. If the study cited is correct and representative, on occasion the direction of rotation is set by some other factor, perhaps the topography in the area where the tornado forms, for example.
So there you have it. I won’t have as much time every week to write up a post this in-depth, so don’t expect this every Friday, but I’ll do my best.
Thundersnow/new rocket for NASA
February 9, 2011
Thundersnow is possibly the coolest-sounding meteorological term I can think of. It happens very rarely, but when it does it’s awesome. Convection strong enough to cause lighting during a snowstorm is simply amazing, as are most things that rare. Through a ridiculous stroke of luck, scientists in Huntsville, AL got a rare opportunity to study this bizarre phenomenon in-depth. As you may know, Huntsville is home to NASA’s Marshall Space Flight Center. There are meteorologists and other scientists there with a barrage of better-than-average instruments that monitored the inner-workings of the snowstorm that hit the area Jan. 9th. They got the most thorough data ever recorded for thundersnow, and observed one bolt of lightning that traveled a whopping 50 miles horizontally before hitting the ground. The fact that such a rare and interesting phenomenon happened right on top of a facility so well-equipped to study it is quite remarkable, and those scientists were very excited to be able to study the thundersnow in such great detail. I look forward to seeing what is learned from this experience. (Via Discovery News)
NASA has been presented with yet another viable commercial option for replacing the Space Shuttle’s role of ferrying astronauts to and from the space station, as well as carrying cargo. A U.S. company called Alliant Techsystems teamed up with Europe’s Astrium to draft the proposal for a new rocket called Liberty. This new rocket would combine research of the now-dead Constellation program with the proven components of the European Space Agency’s Ariane 5 launch system. Since this collaborative effort combines mostly well-proven technology, it would be both cheap and relatively quick to build, shortening the problematic gap between the last Space Shuttle flight and the first availability of commercial access to space. If it truly will shorten said gap, I’m all for it. I hope it’s really as good as it sounds, but things like this always run into unforseen problems/delays. Right now, SpaceX still has the edge simply because it’s already had 2 very successful tests of its launch system, the Falcon 9 rocket and Dragon capsule. But Liberty could give SpaceX a run for their money. Check out the promo video below. (Via Universe Today)
How ice storms work/today will not be ice-mageddon
December 15, 2010
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.
Humidity myths/other science-y tidbits
June 29, 2010
If you live in TN, or even the southeast at all, you know damn well that it’s been hot and humid as hell lately. The entire southeast has been a sauna for several weeks in a row. You always hear people say stuff like, “man, it’s 90 degrees and 90% humidity out there!” Anyone with half a brain knows that’s a VAST exaggeration, but honestly the concept of humidity is a rather confusing one, and even some meteorologists don’t explain it very well. I’m not going to attempt to fully explain it because it’s already been done quite well at this Cincinnati meteorologist’s website. Please click through that link if you want a very detailed, but still in layman’s terms, explanation. Read on if you want my extremely condensed version.
Basically, in terms of actually knowing how much water vapor is in the air, relative humidity sucks. In order to really know how humid it is, and how uncomfortable you will be, look at the dew point. The dew point is simply the temperature at which the water vapor in the air will begin to condense. The higher the dew point, the more H2O is in the air. According to most charts that I’ve found, dew points in the 40-50 degree range feel very dry, like you would feel in a desert… Dew points between 50-60 degrees generally feel comfortable, dew points between 60-70 degrees are generally uncomfortable, and dew points 70+ degrees are utterly oppressive. Yesterday afternoon our dew point in Nashville was hovering around 70-72 degrees. Relative humidity takes into account the air temperature as well as the dew point, and the relationship between temperature and RH is inverse. That means that as the air temp goes up, the RH goes down. Of course the relationship between dew point and RH is converse. Again, if you want a really good, albeit long explanation then visit this website. Fortunately, TN is in for a bit of a relief from the oppressive conditions we’ve been enduring. Cold fronts during the summertime aren’t exactly “cold” though they do normally bring slightly cooler temps, but the main thing they usually bring is a drier airmass. The typical summer weather pattern in the southeast US often involves hot and humid air from the Gulf of Mexico being blown northward across the southeastern states, creating the conditions we’ve been experiencing the past few weeks. Sometimes, however, an airmass that originates over the northern US and Canada will make its way southward. That’s exactly what’s happening today, and the dew point is already falling (this morning it was 68, and right now at lunchtime it’s all the way down to 61!). This airmass is drier because it originated over a large area of land, rather than water. All forecast I’ve seen are in agreement that the dry weather will persist for at least a week, if not more, though the temps will creep back up into the 90s by this weekend. But 90 degree temps with a dew point of 60 is hella better than 90/70!
I must mention a couple of science news tidbits that pinged my radar today and yesterday…
The Obama Administration has announced a new national policy for aerospace that supports and guides the plans for NASA that were announced back in February. This is more of an over-arching “this is where we’re headed” type of policy, and it needed to be implemented to be in line with Obama’s NASA plans. Again, I fully agree with his desire to cancel the Constellation program, rely on the private space industry for low-earth orbit, and focus NASA on exploring beyond the moon. With this new policy, NASA basically has no choice but to use the plan unveiled in February. Hopefully this will get some of the opponents of Obama’s plan in Congress to STFU. But that’s probably a pipe dream. (Via Space.com)
The Large Hadron Collider in Geneva continues to creep closer and closer to its final goal of having the most intense proton collisions ever. Right now, Fermilab still holds the record for highest beam intensity, but the LHC just set a new record for overall number of proton collisions. It will be several more years before they have the LHC running at full capacity, but I have no doubt it will pay off. (Via Discovery News)
Snowpocalypse: Part Duex/NASA rumors
January 28, 2010
Here we go again. I recommend investing in Kroger stock this year, because they’re gonna get a big boost in sales in TN from all these “snowstorms” wherein a meteorologist utters the word “snow” and 75% of the population immediately clears the milk, bread, and egg isles. (Apparently people only eat french toast during snowstorms?) As usual, I’ve been monitoring the progress of the forecast and find it interesting that the NWS hasn’t issued the winter storm warning yet, only a watch. I’m sure the warning will come, but it’s kinda funny that they’re hesitating, no doubt because of the giant snow fail from a few weeks ago. (To be fair, some areas around the midstate did get something close to the forecasted amounts, though no one really got the full 2-3 inches that was initially forecast…) This system is a little different than the last one, however. This one will most definitely have enough moisture to generate the 3-5 inches, unlike the last system which had moisture “issues.” The big limiting factor with this system will instead be temperatures. Nashville will literally be right on the dividing line between having an ice/rain mix and having an all snow event. If this system decides to track just 50 to 100 miles further north than the models think it will, that will cause more warm air to advect farther north, and we’ll end up having mostly rain friday changing to a little snow on the backside fri. night into sat. morning. If it decides to track slightly further south, we’ll have all snow, but much less of it, and areas to the south of us could actually see more snow than Nashville does. This system has a little better chance of “success” in giving us a good ol’ fashioned snowfall than the last one, but I wouldn’t place any bets yet.
Big rumors abound in the blogosphere about Obama’s budget proposal due to land in Congress on Monday. The biggest rumor is that it will completely cutout NASA’s Constellation program, which is the rocket system currently under development to not only replace the Space Shuttle, but also put men back on the Moon. I reported many times on the progress of the Augustine Commission and its recommendations for how NASA should proceed given that its current “trajectory” was financially unsustainable. One of the options they proposed was to eliminate the Constellation program and let commercial spaceflight companies like SpaceX takeover the duties of getting astronauts to the International Space Station and other low-earth orbit missions. I have a feeling that if the budget really does cut the Constellation funds, that’s where we’ll be headed next. Honestly I think it may not be a bad idea, because it would allow NASA to focus more on getting man further out into the solar system, and eventually to Mars. I tend to agree with Dr. Phil Plait’s (the Bad Astronomer) sentiments on the issue (as usual) but I’m not in total agreement with him that we should still go back to the moon. But then again, he’s the astronomer with a Ph.D and I’m not. For even more info, check out Universe Today. Check those blogs again on Monday afternoon, as I’d say they’ll be able to update waaaay sooner than I will once the actual budget info is released.
SNOW FAIL
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.
KIDDING!!!
steve cross loves music and science 