Category: Meteorology 101
As a Weather-Ready Nation Ambassador, AlabamaWX is committed to keeping people informed of any initiative which will better prepare them to “Be a Force of Nature.” Starting today, NOAA and FEMA begin their annual National Flood Safety Awareness Week. This a week dedicated to providing life-saving information on being prepared for the threat of flooding.
On average, flooding causes more property damage in the United States than any other weather related event. It is a threat to life and property that can occur in any of the fifty states or U.S. territories at any time of year. In 2013, the nation watched as deadly floods from severe storms impacted the Great Plains during May and record rainfall caused devastating flooding in Colorado in September. Last year, 85 people lost their lives to freshwater flooding. More than half of those fatalities were a result of people driving into floodwaters. On average, there are 89 fatalities and $8.3 billion in damages annually.
During National Flood Safety Awareness Week , March 16-22, 2014, NOAA and FEMA will highlight the importance of preparing for a flood before it strikes and teach the actions you should take when faced with a flooding situation.
The National Weather Service has created an online marketing toolkit to help you get the word out about National Flood Safety Awareness Week. The toolkit contains daily web stories (including Spanish versions), a poster, a social media plan and flood safety information. All of this content is in the public domain and can be used for your web site, email newsletter, social networks or elsewhere.
In addition, the National Weather Service has a redesigned flood safety web site, with information on what to do before, during and after a flood. The site includes an interactive flood map, where you can look up flood dangers in your state. Flooding is a nationwide threat. But with your help, we can highlight the importance of preparing for a flood before it strikes.
“Be a Force of Nature”
Farewell to a valuable set of eyes 22,300 miles above us!
GOES-12, the weather satellite launched in July, 2001 by the National Oceanic and Atmospheric Administration, is being retired, its operators have announced.
Intended to operate for about five years, GOES-12 turned out to be a warrior, instead, wildly surpassing expectations.
Going into service April 1, 2003, the satellite sat above the Equator, monitoring developing weather systems across the eastern United States and part of the Atlantic ocean for about ten years.
Under its watch, the satellite—which became known as GOES-East—saw a lot, including Hurricane Katrina’s devastating strike along the Gulf coast in 2005, the Christmas blizzard that pounded the central U.S. in 2009, and countless severe weather episodes.
There’s no telling how many lives have been saved with the help of this marvel of technology.
GOES-12 even did a bit of traveling, itself. In May, 2010, the satellite was shifted south of the Equator and provided coverage over South America, where it monitored wildfires, drought and volcanic ash clouds.
In the place of GOES-12 is GOES-13, serving as the current GOES-East, which works in partnership with GOES-15. Together, these two satellites provide coverage from the Atlantic ocean west across the entire continental U.S., into the Pacific ocean.
If you’re curious about the break in numbers (what happened to GOES-14?), there’s an explanation.
GOES-14 is up there, too, but is not in service. Instead, it remains available as a stand-by/back-up, should a problem develop with either GOES-13 or GOES-15.
GOES, by the way, stands for Geostationary Operational Environmental Satellite.
What’s next for GOES-12?
NOAA will use up the satellite’s remaining fuel to boost it to a higher orbit (so it will be less of a collision threat to operational spacecraft), its batteries will be disabled, and transmitters will be turned off (to reduce any potentially interfering signals).
Just how much did GOES-12 see during its service time? NOAA has taken ten years worth of GOES-12 observations and packed them into 3 minutes:
This was the radar loop around 700 pm on March 31, when a hailstorm came across my location at Bluff Creek on the Warrior River. The radar shows the purple shades (near 70 dBZ, or 100 times as much return as 50 dBZ that as is the lower edge of red on radar in its logarithmic pattern.)
I have never personally seen hail that large. What was perhaps more fascinating was the speed and force the hailstones fell with (we’ll discuss that below), and the noise they made. I was fortunate to be outside on a screened-in porch, and with a tin-roof boathouse near me, it sounded like a machine gun for a while. There was a strong smell of pine and other trees in the air, as the hail knocked branches off trees.
1. Photos and video
First of all, some photos and video. The hailstorm came through Bluff Creek mainly after dark, so you couldn’t see much while it was happening. That was not the case over Smith Lake earlier. Watch the lake and listen to the noise of the intense hailstorm.
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Hail is ice that falls from a thunderstorm. It forms when strong thunderstorm updrafts hold ice crystals up in a storm long enough, for them to get big enough, to fall to the ground without melting. Some hailstones make several trips through the updraft and downdraft of a storm (see diagram from NC State).
2. Radar images (including dual pol)
First of all, the large hail over Walker County that night produced a huge “hail spike” (see below). The radar determines the distance a raindrop or piece of ice reflects energy from by timing how long it takes for the emitted energy to get back to the radar. In cases of large hail, enough of the radar beam may bounce off a hailstone down to the ground, back up to other hailstones, then back to the radar, making the radar think the reflector was farther away than it was. This creates a streak of false echo beyond the storm (farther from the radar) in large hail.
You may have heard that the National Weather Service is upgrading its radar network to “dual polarization”, or dual-pol. This means the radar sends out the traditional horizontally-oriented radio waves, and also vertically-oriented ones, to detect precipitation. Since raindrops are actually flat like hamburgers when they fall, the horizontal waves will bounce back to the radar more efficiently than vertical ones. In hail, the shapes are more spherical or irregular and tumble as they fall, so the horizontal and vertical waves bounce back to the radar more equally. Using these measurements, the radar computes differential reflectivity (Zdr), or a ratio of returned horizontal waves divided by vertical waves. In rain, the Zdr is high, since most returned energy is horizontal. In hail or snow, the Zdr is often low, since you get an equal amount of horizontal and vertical.
Take a look at the Zdr picture below. The area near Bluff Creek shows low Zdr, indicating hail, while areas around it show higher Zdr, indicating rain. There are other random areas of low Zdr, but if they are not coupled with overall high reflectivity, they typically do not indicate hail.
These measurements will also be helpful in finding the rain/snow line in the winter.
There are other dual-polarization measurements that we can discuss in later blogs, including the correlation coefficient (that has been shown to indicate tornado debris in some cases) and the specific differential phase (to better quantify rain rates).
The above map shows temperature anomalies (degrees above or below normal) over the USA and Canada yesterday. Notice the huge area of above normal temperatures covering the eastern half of the country…as much as 20 C (35 F) above normal in the Great Lakes area, and about 11 C (20 F) above normal from Alabama all the way to Hudson Bay. The temperatures over the eastern half of the US since last Monday have been more like late May or even early June than March. However, notice the western half of the country. Temperatures are way below normal. As much as 12 C (22 F) below normal in Arizona and New Mexico.
Since the upper air winds tend to mirror the temperatures (troughs in cold air and ridges in warm air), there is a very intense trough in the west, and a huge ridge in the east. Check out the 500 mb chart from yesterday.
Instead of the usual west to east flow at upper levels, winds were diving south along the Pacific Coast, then turning north and going from Texas straight to Hudson Bay. So, the primary reason it is so warm here is because of the amplified flow, with very cold air in the western US/western Canada/Alaska and very warm air in the eastern US/eastern Canada. This system is finally cutting off over the Plains, producing heavy rain that is slowly moving our way.
I have discussed before (see number 1. in http://www.alabamawx.com/?p=56954) that weather systems like the big one over the US now are the way the atmosphere is balanced. A lot more sunshine comes in to the tropics than it does at the North Pole. If we didn’t have low pressure systems with cold fronts and warm fronts, moving cold air south and warm air north, it would eventually heat up to 150 F in some places in the tropics, while the Arctic would drop to -100F. But, these weather systems redistribute the heat so that doesn’t happen. The weather system over the country right now has been doing this. Cold air has moved from the Arctic all the way to Mexico, and warm air has moved from the Gulf and Caribbean north into Canada (via Alabama).
But why is this one so extreme? Record high temperatures have been broken by the hundreds over the past few days in the eastern US. I don’t have the answer for sure, but I have an idea of why it may be.
It is NOT global warming. Over the entire globe right now, the temperature is only 0.4 degrees above normal, and these oscillations are typical, and the global temperature will be below normal by next week according to the models.
The extreme weather over the country right now may very well be because the winter in most of the US was so warm. We had a positive NAO, preventing Arctic air from making big moves into the US. The AO, a measure of how much cold air gets flushed out of the Arctic by weather systems over the whole Northern Hemisphere, was also mainly positive.
Because of the positive NAO and mainly positive AO, the cold air stayed locked up in the Arctic for most of the winter, and when it did move south, it mainly went into Europe. So, we had a warmer than normal winter, and Arctic areas like Alaska and Canada had colder than normal winters. One of the lowest temperatures ever recorded in the US occurred this winter in Alaska, and temperatures between -30 and -60 were widespread. (This is just the opposite of last winter and the winter of 2010, when the AO was negative. Cold air moved south often, and we had temperatures in the teens many nights in 2009-2010, and 4 snow events in 2010-2011, including a White Christmas. At the same time, the Arctic was warmer than normal).
The summary is that the atmosphere was very well-balanced in the past two winters, with cold air moving south and warm air moving north, keeping a really strong gradient from developing. This year, that has not happened as much. Just a week ago, temperatures were still near -50 in parts of the Arctic. I wonder if the imbalance created by the big temperature gradient between the Arctic and the midlatitudes (especially the US) this year is finally, naturally, being corrected.
Does this mean that there will be more big weather systems in late March and April? I don’t know. But, I would keep in mind that the average date of the last freeze in BHM is around March 25-30, and we have seen 20s well into April before. Hopefully, we’ll get some good rain out of this system as it finally presses toward Alabama.
A long time friend and blog reader, Wally Coker, has been keeping 1-minute resolution observations at his house in Clay, just off I-59/Deerfoot Parkway, for several years. His station ID on the mesonet was KC4ANB. Since getting 1-min data is tough in the short-term, I have sometimes asked Wally for his data, and he always obliges. I have used his data in blog posts, conference talks, etc.
Wally’s home was hit by the Center Point-Clay-Trussville tornado of Jan 23. His house is pictured below. Fortunately, his family was safe.
His Davis instruments weather system was wireless, and continued taking wind observations until his roof came off the house around 414 am. Apparently, the barometer is inside the weather station inside his basement, so it took readings for a few more minutes (it and computer must have been on battery back up and kept running until rain water came in). His highest observed 1-minute average wind speed was 53 mph. His highest recorded gust was 79 mph according to an internet site he was linked in to, but given the damage I would say it got a lot higher than that when the roof and the anemometer left.
Here is a trace of pressure and 1-minute average wind speed, then a trace of temperature, both at Wally’s house.
The rapid drop in pressure began even before the tornado arrived, but got worse when it did. This drop in pressure is due to the rotation in the mesocyclone and then the tornado…centrifugal force literally throws air out away from the center of the rotation, just like you get pushed to the outside of the curve in a car. The wind averaged 53 mph for the minute ending at 4:13 am, then dropped and went away.
Interestingly, the temperature suddenly rose 2 or 3 degrees as the tornado passed by. This could be an electronic problem with the sensor due to debris, etc. But, some have suggested that there are occasionally downdrafts in the middle of a tornado that can actually warm the air right in the middle…so maybe it was real.
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?