Category: Meteorology 101
The weather situation continues to develop over AL this evening. A persistent area of storms in Tennessee has produced an outflow boundary that is now moving into northern Alabama, and thunderstorms have fired on this boundary.
I was starting to wonder how we were going to get the high dewpoints and shear the models were predicting (due to winds out of the SW instead of SSW or S), but then, between 4 and 5 pm, the surface winds at many spots in central AL backed around out of the SSW. At TCL, for example, wind direction went from 220 degrees to 190 degrees in one hour. If this continues, it will bring Gulf moisture in here faster and produce larger wind shear (helicity has already increased to 300 m2/s2 over most of AL).
We at UAH have been examining the changes that occur in the atmosphere during the afternoon-to-evening transition (AET), when the sun goes down and things stabilize a little right at the surface. My thought was that, since the air stabilized just a tad at the surface between 4 and 5 pm since the sun is going down, the SW flow from aloft can’t get mixed down to the surface as easily, friction then plays a bigger role, and winds turn more toward the low pressure to the north near the surface. I ran this by my colleagues Dr. Kevin Knupp and Ryan Wade a few minutes ago, and they agreed. They also pointed out that the storms did not fire along the line in north Alabama until about 430 pm either…a similar AET mechanism.
Given this development, wind shear may increase to the model-predicted 500 m2/s2 by 7 or 8 pm. In addition, winds blowing from Biloxi as opposed to Natchez may bring in the higher dewpoints faster. That may still be the key as to whether we get severe weather tonight or not. With this type of wind shear and cold air aloft, we have to be wary of isolated tornadoes.
Another update before 800 pm.
…FOR JAMES’ FULL FORECAST SCROLL DOWN A LITTLE…
Radar shows the beginnings of a large mass of precipitation moving into western Alabama. The air very dry over Alabama, so most of this precip (especially east of Tuscaloosa) is not reaching the ground.
The temperature in BHM is currently 49 degrees. But, the dewpoint is 15 degrees. This means the air has almost no moisture in it at all (it is 0.2% water vapor), but with a temp of 49, the air can hold 0.7% water vapor). So in every pound of air, the amount of water can be multiplied by 3 before we are saturated. So, as precipitation falls into a dry air mass like this, the evaporation of water cools the air and yet adds water to the air, raising the dew point. Where the two meet at equilibrium is the wet bulb temperature. This is the temperature we should settle out at once it rains/sleets for while. In BHM that temp is about 37. Here is a map:
And, the air 3,000 feet above the surface is even colder. So, as some of the warm rain formed up at 5,000 to 10,o00 feet falls initially, it will freeze on the way down in this wet-bulbing process and form sleet. This has already occurred over parts of NW Alabama.
Fortunately, surface temperatures are expected to stay above freezing (since the wet bulb is above freezing, so no widespread travel problems are expected in central AL. NWS Huntsville has issued a winter weather advisory, as they have wet bulbs near 34-35, and some of the valleys in NE Alabama, where this morning’s cold air got trapped, or the hilltops, probably have wet bulbs near 32. So, there may be a few slick spots up in extreme north and northeast Alabama this evening.
Once it starts raining harder overnight and warmer air moves in aloft, it should go to all rain in TCL/BHM/ANB. But, in NE Alabama, there still could be some icy spots by morning.
I have updated this blog with pictures sent to me by Troy and Jen Gibbons, who have a friend serving in Siberia. The blog discusses how, until the last couple of weeks, the cold air had been bottled up in the Arctic. Things are changing now though, as the AO is negative and the NAO is headed down.
This blog will attempt to explain, in fairly plain language, some of the material on atmospheric dynamics that I have taught in graduate courses at UAH. However, I think weather enthusiasts, and even those who just want to know the forecast, will get something out of it.
It has clearly been a warm winter so far in Alabama, and actually over the eastern half of the US. By my calculations (using data from Alabama state climatologist John Christy’s website), looking back at winters (Nov-Mar) since 1897, we’ve had the 21st warmest December, the 15th warmest January, and overall, if temperatures stay on the current track, we’ll have the 12th warmest winter on record. Normally, for Nov 1 through Feb 6, the average temperature at BHM is 46.9. Since Nov 1 2011, it has been 52.2, or 5.3 degrees above normal. That’s a lot for a 3+ month period.
Normally in an average late fall/winter/early spring, we have 52 nights at or below freezing in BHM. So far this year, we have only had 25.
1. The general atmospheric circulation, maintaining balance, and how we get our cold air
To begin the discussion of all this, I will start with the basic, large-scale dynamics of the atmosphere over the Northern Hemisphere. There is a lot more sun energy warming the earth in the tropics than in the arctic, and the difference is much larger in winter (they get practically no sun energy at all in the Arctic in winter). If there were no weather systems to balance things out, temperatures would drop to -100 F or lower in the Arctic, and stay in the 70s or so in places like Alabama all winter.
But, we do have weather systems, in the form of areas of surface low pressure, associated cold fronts, and upper-level troughs and ridges. When a low pressure area forms and intensifies along a cold front (lows tend to stay near fronts most of the time), a counterclockwise flow develops around it. As shown in the example below (solid blue lines represent pressure, and dashed green lines are temperature), this pushes warm air north and brings cold air south.
These systems are part of the general circulation of the atmosphere, that redistributes heat northward from the tropics to the Arctic, and angular momentum and other stuff. The cold air is generally associated with upper-level troughs (since it is more dense and pressure changes rapidly with height in cold air), and warm air is associated with upper-level ridges. The atmosphere has to develop these low pressure areas and associated upper waves (called Rossby waves) to maintain balance.
Below is a computer model from Naval Research Lab showing a simplified Northerm Hemisphere, with the north pole in the middle, that is coldest in the middle (temperature on the right), and what happens when that unstable situation is present. Lows and highs (left) form and redistribute the heat north and cold south.
Some winters, the atmosphere is more efficient in moving cold air south and movjng warm air north. One way to explain the overall northern hemispheric weather pattern is through the Arctic Oscillation (AO). When the extremely cold air stays bottled up in the Arctic regions (Alaska, northern Canada, Siberia), and does not get flushed out very often into midlatitudes (US, Europe, Asia) by cold fronts and low pressure, it causes the surface pressures to be high in the Arctic relative to the midlatitudes (since colder air is heavier and often causes higher pressure). This is a positive AO. When the cold Arctic air frequently gets pushed southward into the US/Europe, pressures are higher here and lower in the Arctic…this is a negative AO.
The past two winters (2009-10 when it dropped into the teens so many times and the edges of the Warrior River froze, and 2010-11 when we had several snow storms) were very negative AO…the cold air got moved out of the Arctic often, making us cold but the Arctic warmer than normal.
2. Why has it been so mild this winter?
A similar indicator (and one James points out a lot on the blog) is the North Atlantic Oscillation (NAO). It measures a pressure difference, but only over the North Atlantic (comparing Iceland to Portugal/The Azores). A positive NAO is generally connected with a strong jet stream over the north Atlantic, and southwesterly upper flow over the eastern US that keeps Arctic air from being able to come south.
This year the NAO has been positive so far, so cold air can’t come south as easy over the eastern US and it doesn’t stay as long. The AO, representing the northern hemisphere, was also positive until late January. This means the cold air has stayed in the Arctic (it has been -20 to -60 degrees F over a large area this winter in the Arctic, abnormal for them, and there is debate about a possible record US low temperature, see http://www.alabamawx.com/?p=56957. But, the AO has gone negative. This is associated with bitterly cold air over Europe.
The map below shows the temperature anomaly (difference from normal) for December 1 through January 31. Note the abnormally cold air in Alaska and Siberia, and the abnormally warm air from western Canada down into the eastern US.
3. How about the rest of the winter?
This is hard to say. As James and I discussed early this week, with cold air moving into Europe the hemispheric pattern is changing, and perhaps this could kick the NAO to negative later in Feb or Mar, etc. The large scale upper-level waves have a lot to do with how all this plays out, but at least things are shifting now. What happens in the Pacific (El Nino/La Nina), what happens in Europe, etc. all can have an effect on our weather here over a longer period of time.
We are already back to near normal temperatures for early February now, and the models do indicate several pushes of cold air coming south over the next 2 weeks. But, as we get later into the winter, the sun is getting stronger over Alabama, so it makes long-term cold like we had the past two years much more unlikely. This is not to say it won’t get bitterly cold before the winter is over. A sudden pattern change in late February or early March could still get us down into the teens, and remember the Blizzard of 1993 was on March 13.
OK, here are the pictures from Siberia. Temperatures there -30 to -50 have been common this winter. That road must have buckled or something due to extreme cold/ice expansion?
Light snow showers continue over north Alabama this evening, mainly north of TCL-BHM-ANB. However, a few reports of snow in NE Jefferson County (Clay/Pinson/Trussville areas)! The snow has been a little heavier to the north, with slick spots developing, mainly on bridges, in HSV metro and over near Florence. I’m glad I’m not driving home over the I-65 bridge over the TV River today! Be careful tonight and tomorrow morning and check road conditions before heading out in case we get a little more snow than originally thought, mainly north US-278, but even a few elevated roads in the BHM and ANB areas could get icy.
As the big upper-level low and associated divergence moves through tonight, more light snow can be expected. The snow is a little like summer showers, as the air is unstable. It is 29 at the ground at UAH right now, but the MPR indicates it is 3 degrees at 4,000 ft., so the air is unstable!
It won’t be surprising to see temperatures drop more tonight, as it is in the teens and 20s upstream in MS, TN, AR, and MO. Wind chill in the morning may be near 15 at times as far south as BHM, so bundle the kids up! At least we are in a progressive pattern, and after a cold day tomorrow (highs in the 30s) and a cold night tomorrow night (lows 20-25 mainly), it will start to warm up by Saturday.
With the wet ground from the rain earlier today, and then skies clearing, dense fog has formed over Alabama tonight.
KBHM 260453Z 00000KT 1/4SM R06/1000V1400FT FG VV001 06/06 A3036 RMK AO2 SLP282 T00610061=
The visibility at BHM airport is 1/4 mile, and in some places it is lower than that. The vertical visibility (VV001) is 100 feet, so landing a plane is nearly impossible. With calm winds and radiative cooling (the skies are clear just above the shallow fog layer at the ground), visibilities will stay low through early morning, and may drop to 1/8 mile or lower in some locations, especially valleys. Avoid driving if possible, and if you have to, use common sense.
A weak gravity wave moved over BHM over the past couple of hours too. Check out the pressure trace at Helena.
It’s pretty clear from the strong temperature and dewpoint gradient in the analysis above where the front is…from near Demopolis to BHM area to Gadsden. Research has shown that on the near and just south of a front like this, atmospheric shear is enhanced. This explains the tornado warnings and rotating storms generally concentrated from south of Tuscaloosa earlier to Shelby county now. Given the location of the front, expect the tornado threat to stay south of I-59 for the most part, so people in Jefferson, Walker, Tuscaloosa, and Cullman counties can probably breathe easier. Lots of heavy rain coming.
South of the front, in places like Anniston, Springville, Alexander City, Jacksonville, and Roanoke, the threat for damaging winds and tornadoes will continue. However, the threat is not nearly what it was during the major outbreaks in the spring. That day, energy-helicity index was near 10, today it is only 1, and that is south of the front.