Category: Meteorology 101
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.