Wednesday, April 27, 2011

Severe Weather

We here in the Pacific Northwest are moaning and groaning about below average temperatures and wet weather. Meanwhile, the Midwest, Southeast and Ohio Valley are being pounded with round after round of severe weather. It is just one wave after another of tornadoes, hail and damaging winds for these states. By the way, many of these states endured record snowfall totals during this past winter. Breaks in the weather have come at days at a time, but no real relief has been offered since the beginning of April. The numbers from the Storm Prediction Center really tell it all.

Here are preliminary tornado reports over the last 5 days. Keep in mind these are preliminary, and will take a while to officially be confirmed:Tuesday, April 26: 54 tornado reports Monday, 25: 44- one tornado may receive a EF-5 rating in Arkansas. Sunday, 24: 13 Saturday, 23: 9 Friday, 22: 28
That is an ugly stretch of storms. They were spread out across all of the Midwest and South. But that stretch of 5 days does not compare to reports a few days earlier. On April 19th, the SPC had
77 reports of tornadoes mainly in Missouri, Indiana, Illinois and Ohio. That was the third-highest total for the month so far. A few days earlier on the 16th, 139 unconfirmed tornado reports across North Carolina and Virginia. And the most reports of tornadoes for April came on the 15th when 146 reports came in from Mississippi and Alabama. In all, 654 reports have been made for the month of April, according to the Storm Prediction Center. Again, they are preliminary and it is possible that not all will be confirmed as tornadoes.Take a look at this map that shows the distribution of tornadoes so far this year (notice one in Oregon! It received an EF-0 rating). Often times, severe wind damage can be confused as tornado damage. It is the job of the NWS to analyze storm damage in person. This process is what gives us tornado ratings. Often people think that the Enhanced Fujita ranking system is based upon wind speeds when it actually is based upon damage.
How does this April's reports compare Aprils' past? In 2009, 226 tornadoes were confirmed. The 3 year average for April is 185 tornadoes. So, should all 654 reports be confirmed, that would be more than twice the 3-year average for the month of April! Incredible.
What's even more incredible is that the peak tornado season hasn't even started yet! That
distinction belongs to May with a 3-year average of 322 tornado reports. May also holds the all-time record for tornado reports of 543 in 2003.
What is the possibility of all 654 April reports this year being confirmed? Given the large number of reports, it would be hard to imagine that all of them were tornadoes. But all we have to do is look at January 2011 for evidence to the contrary. There were 10 preliminary reports of tornadoes this January. The actual report of tornadoes by NOAA was 16 (that also stands as the total number of official tornadoes so far this year, but February doesn't have official totals yet). So it is possible that these reports will be confirmed.
Along with severe weather comes loss of life. Over the decades of weather reporting and forecasting, deaths resulting from severe weather has improved greatly. More information at the fingertips of forecasters means improved warning systems. But the shear amount of storms this Spring season is starting to take its toll. April 2011 has 43 confirmed deaths from tornadoes. The 3-year average is only 6.
So WHY is all this happening? Let's take a look at the charts and find some similarities between April 15, 16, and 19.
Here are the charts for the 15th around 5 a.m.

The maps we want to analyze are the two upper maps. The one on the left is the 850mb map. Look towards the Gulf Coast. Notice how warm, moist air is pouring in off the gulf into Louisiana and Mississippi indicated by the warmer colors. That causes instability and uplift in the atmosphere. The map to the right is the 300mb map. This shows the position of the jet stream. A strong, westerly stream is preferred for severe weather. That is exactly what we have. Lots of warm air, plus strong westerly jet is a great recipe for storms. Watch this as it moves east on the 16th. Here is the setup for April 16th around 5 a.m.

Now all the action has swung to the east. Cold stable air is now pouring into Louisiana and Mississippi a day after the severe storms and the warm moist air is now being forced up the eastern Atlantic. This was an interesting situation where the warm Gulf moisture was being sucked up and mixed in with the warm Atlantic moisture off the east coast. The warm air, plus a strong jet stream on the 300mb map that is evident over the Carolinas spawned a tornado outbreak of record proportions in North Carolina. It definitely helped that the storms really were initiated the day before over the south and continued as they migrated towards the east. But another thing that aided these storms the Appalachian Mountains. As air descends down the mountains, it is forced to stretch. Imagine an ice skater that is spinning. As the skater pulls their arms in, they spin faster and faster. This is conservation of momentum. That is what the air is doing as it travels down the back side of the Appalachians. The air is warming, compressing and spins a bit faster in order to conserve energy. A perfect setup for tornadoes. A few days later, it was the Ohio Valley's turn.
Here is the setup for April 19th
Overall, this setup isn't much different than the initial outbreak on the 15th. There is a tighter low over the central part of the country again that is pulling the warm air from the south. The broad cool air stretches east-west along Ohio, Indiana, Illinois. That is our target. A strong jet stream just to the north of those states helps enhance the storms. The main factor though is the tight gradient between the warm and cold air. In the lower left hand map, you see surface pressure plotted. The main feature is that low. The bulge in the isobars protruding eastward is the warm front. It shows up nicely on the 850mb map as well, huh? This system resulted in those 77 tornado reports!
So knowing that April could set a record for confirmed tornadoes, what does that mean for May when historically we see the most tornadoes? We don't know. The broad scale weather pattern is always changing. We can only see 3-5 days out in regards to issuing credible severe weather threats. So in that sense, there is no use in forecasting out any further. In my eyes, it is smart to only look ahead one day at a time. As of today, looking ahead shows a slight break for tomorrow. A break which will be much needed for many as clean up efforts have been hampered by more storms. The actual number of tornadoes will take a while to be confirmed but given what is going on, take solace in the rain and mid-50's.

Yesterday's storms that ripped through the South added to what will no doubt become a historic and infamous April. Various reports have the death toll reaching 194 as of 6 a.m. this morning. No doubt that number will rise as the clean up effort begins in earnest this morning. I would assume that there would be potential for an EF-4 or even EF-5 rating out of this outbreak given the reports and videos.
There have been 162 reports of tornadoes from yesterday, making it the most so far this year. As I mentioned yesterday, most likely not all will be confirmed as tornadoes and some reports may even be of the same tornado. While watching coverage yesterday, news stations in Birmingham, Alabama caught a twister that may have been on the ground for 45 minuets! It is tough to put this event into words, so I'll let the pictures do the talking.

Tornado caught in Tuscaloosa, Alabama- one of the hardest hit areas- via Crimson Tide Productions:

4-27-11 Tornado Tuscaloosa, Al from Crimson Tide Productions on Vimeo.

A days worth of action caught by the guys from TornadoVideosdotnet-

Photo snapped by a reporter for WBRC as people take shelter in Cullman, AL

Another video from Tuscaloosa. Not sure who to give credit to, but it isn't me...

An image from Twitter of the damage from Tuscaloosa, completely demolished- via @danamlewis

This monster skirts just past Bryant-Denny Stadium, home of the Crimson Tide-via Clay Hasenfuss

Tuscaloosa Tornado (4/26/2011) by sportsxbrooks

No comments:

Post a Comment