Wednesday, May 11, 2011

The Rainshadow

This LandSat image is of the northern end of the big island of Hawaii. LandSat images are an awesome way for meteorologists to analyze the impacts of weather. So what is so cool about this picture? Take a look at the vegetation differences from the northeast side of the island to the west end. A stark difference between lush greens and drought-like conditions. What causes this phenomena to occur so close together? It's known as the "Rainshadow" effect.
The Rainshadow is caused by two things: topography and prevailing winds. In this photo, the dominant winds are coming from the northeast and moving directly across to the west of the island. The prevailing winds encounter the land and it's topography. On Hawaii, winds meet the mountains of Mauna Kea to the south and Kohala to the north. The winds are forced to ride up and over the mountains. That process is called orographic lift.

As the prevailing winds are forced UP the mountain, that air will begin to cool. This side of the mountain is called the "windward" side. As the air rises and cools, it will condense and begin to form a cloud. We even see some clouds forming in the image. If and when the clouds cool enough, they will begin to dump rain as it continues to rise over the mountains. With all the rain that occurs on the windward side of the mountain, that provides ample water to keep the vegetation nice and green!
The prevailing wind is now on its way down the back side of the mountains, or the "leeward" side. Here, the opposite of the "windward" process takes place. As air sinks, it warms up. The air running down the leeward side of the mountain has thus lost all of its precipitation and is beginning to warm up, resulting in drier conditions. This process explains why it is so dry on the back side of the mountains! Sometimes, this whole process can lead to severe droughts near and behind mountainous terrains.

The Rainshadow phenomena occurs all over the world, including right here in Oregon! Our dominant wind pattern is from the west off the Pacific and to the east. Add the coastal mountain range and the larger Cascade range and presto, we have a rainshadow!
The dominant rainshadow is in Central Oregon. This is what gives places like Bend such a desert-like climate. The rainshadow isn't as dominant between the coastal range and the Cascades, but it is evident when you look at rain totals around the metro area. In large rain events, higher rain totals normally are expected EAST of the Portland-metro area into the foothills of the Cascades because that air is beginning to ride up the Cascades.

Wednesday, May 4, 2011

The Super Outbreak Aftermath

The dust is settling from the late April tornado super-outbreak. It will go down as one of the worst outbreaks in U.S. history. In fact, the April 27-28th outbreak will set the bar for most tornadoes in a single outbreak. As National Weather Service offices across Mississippi, Alabama and Georgia conduct damage surveys of the deadly super-outbreak, official numbers are still coming in. Initial raw data from the two days reported some 269 tornadoes alone. After the NWS surveys, the official number of tornadoes that actually occurred reached 153. That tops a 37-year old record of 148 tornadoes in April of 1974. Deaths from this outbreak have reached a staggering 340 and we are lucky that, given the severity of the storms, it was only that much. The most deadly tornado outbreak came from the infamous 1925 Tri-State outbreak that ravaged Missouri, Illinois and Indiana and left 695 dead.

NOAA complied satellite images into a movie that tracts the outbreak. The video is awesome.

NWS Offices have distributed some great information in regards to several of the hardest hit areas. As of May 3rd, there have been two EF-5 tornadoes reported from April's super-outbreak. As I detailed in my last entry, the EF scale is not based on wind speeds like the Saffir-Simpson scale used to determine hurricane category strength. Rather, the tornadoes receive their ratings after the storm is over and is based on how much the tornado "eats" or destroys. One of those EF-5 tornadoes ripped through Northern Alabama and hit the town of Hackleburg. The NWS put wind speeds of that tornado at around 200 miles per hour. By comparison, for a hurricane to reach its strongest category strength of Cat-5, its wind speeds only need to be 156 miles per hour. So this tornado is doing some SERIOUS damage. At least 25 fatalities were reported from the Hackleburg EF-5 and surveys put the damage path 25.5 miles long and 3/4 mile wide! The EF-5 monster was on the ground for only 23 minuets but managed to toss cars 150-200 yards into the air!

A more deadly EF-4 tornado hit Tuscaloosa, Alabama and stretched Northeastward into Birmingham. I was watching The Weather Channel as this monster moved through Birmingham and the images were awesome from a weather standpoint, horrific from a human standpoint. NWS reports have this EF-4 with winds up to 190 miles per hour. The death toll reached 65 from this one tornado, making it the deadliest single tornado since May of 1955 when 80 people were killed in Kansas. The Tuscaloosa tornado traveled about 80 miles in close to an hour and a half. It was believed to have a damage width of 1.5 miles! The storm that spawned this tornado began in Mississippi and was sustained all the way to North Carolina. The tornado itself did not last that long but the storm that spawned it dropped several different twisters.

Take a look at some imagery from these two tornadoes. This Google map shows the path's of all tornadoes confirmed so far by the NWS across the Gulf coast states. The pink trail in Northern Alabama is the EF-5 and it also gives a good idea of how far the Tuscaloosa tornado traveled. It is rare that a tornado is sustained for an hour and a half. Most tornadoes have a life span on the matter of a few minuets so this was one powerful twister.
Landsat images are helpful in locating tornado tracks. NASA released this image of the damage path left by the Tuscaloosa tornado. Note the other tracks from different tornadoes on the image as well.

The Super-outbreak also tormented Georgia. The NWS in Peachtree City, GA put out great information regarding the outbreak in their state. One EF-4 and four EF-3 tornadoes were reported by the NWS office.

The Catoosa County EF-4 tornado touched down just after 8 p.m. on the 27th and ultimately had a damage path 13 miles long and 1/3 wide. Here is an image of the track and intensity.

And another image of the damage path taken by the surveyors:
Seven deaths and more than three times as many injuries resulted from the Catoosa twister.

Another tornado, this one an EF-3 rating, hit Bartow, Cherokee and Picknes counties. The tornado touched down just before 9:30 p.m. and had winds that topped out at 150 miles per hour. No deaths were reported and only 3 injuries. Check out the 23 mile-long path of this storm:

A huge tool in the fight to forecast tornadoes is radar. We use reflectivity to determine where precipitation is falling and how heavy it is. Tornadoes have specific characteristics that show up on radar, the most dominant being what is called a "hook echo". Here is the radar image of the B-C-P tornado:
The intense reds, whites and some pink show the heaviest areas of rain. Note how the radar image has a "hook" to it in Northeast Bartow county. That is a textbook "hook echo" and is a sign that the environment may be tornadic because the winds are wrapping the rain around a center of rotation in a counterclockwise direction.

Also detectable on this radar image is a "V-notch". Note how the precipitation out in front of the hook echo takes on a V-like formation. This indicates strong upper-level winds that are running into the storm's updraft from the surface. The westerly winds hit this updraft and must move around it, thus fanning out precipitation to either side of the storm and creating a V-like echo on radar. Strong upper level winds are critical for tornado development.

Another product that meteorologists use on radar is called "storm relative velocity". It basically shows where winds are moving towards and away from a radar site. It allows us to see inside the storm and indicate areas were winds may be moving in opposite directions right next to each other, giving us a strong indication of rotation. On the image below from the same EF-3 event, areas of red are winds moving away from the radar site and areas of green are winds moving towards the radar site:
Notice the areas of green smack in the middle of all the red. That is a textbook indication that there is rotation in the area, possibly leading to a tornado. In this case, an EF-3 tornado resulted.

So those are some tools that can be used to identify severe weather. The creation of radar has increased severe weather safety over the years and has helped in the further understanding of how tornadoes work. All the above information has been provided by various National Weather Service offices including the photos. The statistics are staggering yet, it could have been so much worse if not for inventions such as radar and warning systems. This definitely puts April's Super-outbreak into perspective and displays the awesome power of Mother Nature.