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.
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)
V-day/Exploding eggs
February 13, 2009
This is the cover art for the new Yeah Yeah Yeah’s album, It’s Blitz! which is coming out sometime in April. I think this is pure genius. This has to be one of the best album covers I’ve seen in a long time. I don’t normally get REALLY excited about cover art, but this is AWESOME. (And not just because it’s a photograph and I’m a photographer.) I’ve never been good at explaining in words why I really like or dislike anything…. but YES. Click the image to see a larger version on the Stereogum post about it.
Your daily dose of WTF?!?… Two German gay men who worked in a metal factory decided it would be fun for one of them to poke an unloaded air rifle up the other one’s ass and pull the trigger. The result? His intestines exploded. Via Vice Magazine.
I’m officially ashamed of my state after hearing that FOUR state representatives from TN have agreed to be plaintiffs in a future lawsuit by a California man who is challenging Obama’s citizenship. Come on, people. We understand, you’re pissed off that you lost the election AND lost majorities in the Congress and Senate, but for fuck’s sake this is PATHETIC.
Scientists have mapped 60% of the Neanderthal genome, which will hopefully lead to a better understanding of the common ancestor of them and us moden humans, and why Neanderthals died out about 28,000 years ago. Via Clusterflock.
Speaking of DNA, scientists have also mapped DNA of the common cold (aka the human rhinovirus). This could finally lead to medications that treat the actual cause of the cold rather than just the symptoms. But it’ll still be a while, because there are 99 known strains of the virus, and if a person is infected with 2 strains at once, parts of the virus’ genetic code can get swapped out, resulting a new strain. So there are potentially HUNDREDS of different versions of this thing floating around out there.
I rarely post on weekends, so here’s an early treat for you from XKCD- a Sierpinski Valentine.
Exomoons, ice, and an avian dance party
December 16, 2008
Score one for the weather forecasters. This morning on my way out the door I inspected my surroundings to find a thin glaze of ice on almost everything except the road (THANKFULLY!). We were under a freezing rain advisory and freezing rain is indeed what we had. It’s rare, but it’s kinda cool to see when it happens. When it hits you on the hand, it’s wet, yet when it hits anything else, it freezes! But I really don’t think any roads had problems as they were just a little too warm for anything to freeze on them. I also found this cool slideshow on the Tennessean’s website showing snow photos over the past several years.
Ok, enough nerdy weather science. Back to the usual links of interest:
Since I’ll be flying to NYC this Wed. with Megan, I thought it appropriate to post this video showing the astounding number of flights world wide over a 24 hour period.
Apparently astronomers are already tired of discovering exoplanets. Now they’re onto discovering exomoons around exoplanets! David Kipping at the University College London is working on a method of measuring the wobble of planets around other stars. So, we look at a distant star and observe its wobble caused by the tug of a large planet orbiting it. Then we directly image that planet with Hubble or even some ground-based telescope, and observe its own wobble, caused by it’s moon(s). Obviously this can only be done (at least for now) with very large planets (think Jupiter-sized or even bigger) that are close enough to be directly imaged, but still… AMAZING. Read the rest of the the story at Universe Today.
Finally, I give you Snowball, the Dancing Cockatoo:
steve cross loves music and science 