Archive for April 11th, 2012
Frost Advisory
**No afternoon Weather Xtreme video this afternoon due to travel**
HEADED FOR THE 30s: We note that we actually had one or two spots up in Northeast Alabama at freezing early this morning, so no doubt another cold night is ahead for Alabama tonight with a clear sky and diminishing wind.
The National Weather Service continues a frost advisory for tonight, and there is no real change in our thinking. Most places will see a low somewhere between 32 and 39 degrees. The coldest valleys across Northeast Alabama could reach the upper 20s, and the ridge tops over 1,000 feet will probably stay above 40. But, most communities will see frost, and you will need to cover up and protect any plants that will be harmed by frost tonight.
Tomorrow will be another beautiful day, with sunshine in full supply and a high in the upper 60s.
We note that Friday morning will be very cool and while most places will see a low in the low 40s, the colder valleys will have some risk of light frost again. Then, a warming trend begins Friday afternoon with a high in the mid 70s.
OUR WEEKEND: Dry is the word for the weekend. Partly to mostly sunny Saturday and Sunday… the high Saturday will be near 80, followed by low 80s Sunday as the upper ridge keeps the active weather west of Alabama.
NEXT WEEK: Still looks like the next round of showers and storms will come Tuesday afternoon, Tuesday night, and possibly into Wednesday morning. Still too early to determine if severe weather will be an issue; still doesn’t look like a classic setup at this point. See the morning Weather Xtreme video for details.
HEADED FOR NORTHPORT: We will be at the Publix on McFarland Blvd in Northport this afternoon from 3:30 until 7:00 programming weather radios…. come see us if you need help. I will be there doing the weather live on ABC 33/40 News at 4, 5, and 6:00.
WEATHER BRAINS: Don’t forget you can listen to our weekly 90 minute netcast anytime on the web, or on iTunes. This is the show all about weather featuring many familiar voices, including our meteorologists here at ABC 33/40.
CONNECT: You can find me on all of the major social networks…
I had a great time this morning visiting with the kindergarten students at Paine Primary School in Trussville… be looking for them on the Pepsi KIDCAM today at 5:00 on ABC 33/40 News! The next Weather Xtreme video will be posted early tomorrow morning by 7:00….
Wild images of hail (photos and dual-pol radar)
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).
Palm Sunday Tornado Outbreak, April 11, 1965
On the Palm Sunday morning of April 11, 1965, warm, humid air was flowing northeastward into the Midwest ahead of a fast moving low pressure center that was dropping southeast out of Minnesota toward Iowa. A warm front was moving northward across Illinois and Indiana. Strong warm air advection was occurring across the region in the wake of the warm front with temperatures in the lower 80s in some places. A strong cold front trailed southward from the low. In the upper levels of the atmosphere, a huge, positively tilted trough extended from an upper low center over the upper Midwest back to southern California. A strong jet stream was flowing over the Midwest with a 185 mph jetmax located on the morning sounding at Dodge City, Kansas. The Weather Bureau’s Severe Local Storms Unit (SELS) at Kansas City issued five tornado forecasts that day. Every tornado but two occurred in a forecast area.
The first tornado touched down in Iowa just before 1 p.m. The Weather Bureau in Waterloo had no radar. A radio station in Cedar Rapids spotted the developing storm on their own aviation radar and alerted the Weather Bureau forecasters. The next twelve hours would be horrible. When everything was over, there were 47 tornadoes across six states, including Iowa, Wisconsin, Illinois, Indiana, Michigan and Ohio. There were 21 killer tornadoes. Seventeen of the tornadoes were violent (F3 or greater.) A total of 271 people were killed and 3,442 injured. It was the most significant tornado outbreak since March 18, 1925 when the Tri-State Tornado occurred. It is the fourth deadliest tornado outbreak on record behind the Tri-State outbreak, the April 3, 1974 Superoutbreak, and the April 27, 2011 outbreak. In Illinois, Crystal Lake was hard hit, with five fatalities from an F4. A massive F4 killed ten people at Koontz Lake, Indiana. A terrifying series of pictures was snapped near Goshen, Indiana as a pair of massive funnels roared by. The towns of Russiaville and Alto were destroyed, with dozens of fatalities. At one point, blanket warnings had to be issued, covering up to nine counties at a time, as forecasters struggled to keep up with the fast breaking storms. Indiana was the hardest hit state, with 138 fatalities. As some of the powerful tornadoes moved from Indiana into Michigan, warnings could not be telephoned ahead because the phone networks were out. A total of 53 people died in Michigan. In Ohio, 60 people died from violet tornadoes during the late evening. Toledo was hard hit.
The Palm Sunday outbreak led to significant improvements in the warning system. But why? There were good forecasts. The dangerous storms were identified in plenty of time, but the warning system had failed. People interviewed after the storms did not understand what a tornado “forecast” was. They didn’t understand the difference between “forecasts” and “warnings.” The next year, tornado forecasts would be named “watches.” Tornado forecast areas were defined as circles, triangles, and other kinds of shapes. The outbreak would lead a trend toward parallelogram tornado watches. They would begin in 1970. Another problem was that radar coverage was woefully inadequate across the outbreak area and there was poor communication between the Weather Bureau and media sources. After the outbreak, the Weather Bureau started holding preparedness meetings as well as spotter talks and training. The goal became to saturate the public with information on tornadoes. NOAA Weatheradio would be another legacy of the outbreak.
Low Temperature Roundup–Updated 11:30 am
ALABAMA
32 Broomtown
33 Valleyhead
34 Black Creek
35 Scottsboro
36 Russellville, Meridianville
37 Fort Payne Airport, Crossville
39 Decatur, Muscle Shoals, Courtland
40 Pinson
42 Huntsville
44 Cottondale, Birmingham Airport
45 Coker, Albertville
47 Jemison
48 Guntersville, Anniston Airport
49 Calera, Troy
50 Tuscaloosa Airport
51 Selma
52 Auburn
53 Dothan, Evergreen
56 Montgomery/Dannelly Field
REGIONAL
33 Crossville, Tenn.
35 Briston, Tenn.
35 Knoxville
36 Nashville
39 Corinth, Miss.
40 Chattanooga
43 Oxford, Miss.
44 Tupelo
47 Atlanta
49 Memphis
31Gatlinburg and Cades Cove
18 Mt. Leconte
24 Grandfather Mountain, N.C.
24 Beech Mountain, N.C
19 Mt. Mitchell, N.C.
USA EXTREMES
98 yesterday in Deth Valley
13 this morning at Park Rapids, Minn.
6 below zero at Wainwright, Alaska
68 inches of snow, Valdez, Alaska
Frost Advisory Issued
An all new edition of the ABC 33/40 Weather Xtreme video is available in the player on the right sidebar of the blog. You can subscribe to the Weather Xtreme video on iTunes by clicking here.
APRIL COLD SNAP: Blog readers won’t be surprised at this; the long awaited April cold snap is here, the one some call “blackberry winter”. We won’t get out of the 60s today with a cool breeze; the sky will be partly to mostly sunny.
Our Skywatcher at Black Creek (in Etowah County northeast of Gadsden), Vic Bell, reports on our chat this morning:
“34.6 with frost…..you’ll know when the colder air arrives…I’ve dropped 10 degrees since midnight”
That, of course, is one of the colder spots in the state, but a sign of things to come for the rest of us.
TONIGHT: The NWS has issued a frost advisory for late tonight and early tomorrow; we still project lows in the 32 to 39 degree range. The valleys of Northeast Alabama have the greatest chance of seeing a light freeze, but frost will be pretty common, and you will need to cover any plants tonight that could be harmed.
Tomorrow will be sunny with a high in the upper 60s. We should note that the colder valleys of Northeast Alabama could see a touch of frost early Friday morning as well, but most places will be in the low 40s. A nice warm-up kicks in during the day Friday with a high in the mid 70s.
WEEKEND FORECAST: A warm upper ridge should keep Alabama dry over the weekend with a high around 80 degrees both days with a partly sunny sky. Humidity levels will slowly rise, but the warm air aloft associated with the ridge should prevent showers from forming. All of the active weather remains west of Alabama.
NEXT WEEK: The 00Z GFS keeps us dry Monday, but a band of showers and storms should move in here late Tuesday and Tuesday night. The severe weather potential for now doesn’t look overwhelming with the main surface low and support passing to the north, but this is mid-April in Alabama so we will need to keep a close eye on it.
WEATHER BRAINS: Don’t forget you can listen to our weekly 90 minute netcast anytime on the web, or on iTunes. This is the show all about weather featuring many familiar voices, including our meteorologists here at ABC 33/40.
CONNECT: You can find me on all of the major social networks…
I will be doing a weather program this morning at Paine Elementary School in Trussville… then later today I will be at the Publix in Northport from 3:30 until 7:00 this evening programming weather radios. If you need help; come see us. I should be able to produce an afternoon Weather Xtreme video that will be posted by 3:00. enjoy the day!
