Bad signs for hurricane season/Severe weather outbreak this weekend

By: Levi32 , 3:16 AM GMT on February 22, 2007

Hints at what hurricane season might hold in store for us this year keep popping up everywhere lately. The ENSO has officially plunged into neutral, and I believe a coming La Nina is a strong possibility this spring. The latest ENSO report from Australia came out today. As always it's full of good information so I encourage you to read it. Also skyepony found an interesting link to an experimental product, the ESPI (ENSO Precipitation Index). It's just like the SOI (Southern Oscillation Index), but it measures precipitation instead of air pressure. The numbers are similar to most ENSO indices. The 30-day average ESPI has plunged from +0.61 to -0.14 in only two weeks, which supports the current trend towards La Nina.

Another thing that I'm concerned about is the wind shear values in the Atlantic. The SSD Tropical Genesis data page shows that the wind shear in the eastern Atlantic, and both the western and eastern Caribbean have all been decreasing rapidly over the last 3 weeks, and are now well below normal for this time of year. In fact the wind shear in these areas is so low that if it were summer they would be favorable for tropical development. This is a bad sign to me, as it is once again supporting a developing La Nina, and could mean an early start to the hurricane season due to more favorable conditions in the upper atmosphere. We'll have to see if the SST anomalies fall in line as well this spring, but right now it looks like we are going to have a more active season than last year at any rate.

On another note, a potentially significant severe weather outbreak could be in the works as early as Friday night for Texas, Oklahoma, and Kansas. A low coming down out of the pacific northwest will deepen over the plains with the support of a strong (110+ knot) jetstream aloft. A classic setup with good, moist, low-level inflow from the gulf of Mexico, strong upper level dynamics, and plenty of instability is what the models are forecasting at this time. The storms will begin to decrease in intensity as the low loses upper-level support by the time they get past the Mississippi River, but some potent storms could still threaten the areas of Tennessee, Mississippi, and Georgia on Saturday and the early part of Sunday. The SPC 3-day outlook already has a slight risk area posted for the plains.

However all this could change as the low which will spawn this outbreak is still in its formation stages off the California coast, and the models usually don't handle these storms well until they're inland and crossing the Rockies. Right now I think the SPC is on the right track putting a slight risk area down for the plains on Friday. I think the ingredients will be there for a bad outbreak, but how it actually turns out could be much different considering the last two outbreaks which looked like they might be bad but in reality turned out not to be so awful.

We shall see what happens!

What is El Nino?

El Nino is a reversal of the normal trade wind flow over the equatorial Pacific. When conditions are normal, trade winds flow from east to west. This usually sets up high pressure over northwestern South America, and low pressure in the western Pacific near Australia. A normal rainfall pattern with moist in Australia and dry in the eastern Pacific and South America is the result.

However when El Nino pops up, those trade winds can be reversed or greatly slowed down. The flow is then from west to east, which sets up the low pressure over NW South America and the high pressure over Australia. This is the total opposite of normal conditions. If El Nino sticks around for several months, Australia experiences severe droughts, and NW South America experiences heavy tropical rains. SSTs in the eastern equatorial Pacific are warmer than normal because no upwelling is occurring due to the reversed trade winds. Upwelling is brining up colder water from the deeper ocean to the surface. When the trade winds are from the east like normal, they "push" the ocean water westward from the coast of South America. As the warm surface water is pushed westward, the cold water from deep down moves up to replace it in response. But when the trade winds are reversed, upwelling is shut down, and the SSTs near the South American coast are warmed greatly. Warming of the SSTs near the equator is one of the first signs of an El nino, and is an easy signature to recognize on an SST anomaly map. El Nino is also known to cause global weather pattern changes which can be very severe. Where I live in Alaska, El Nino causes winters to be extremely mild and rainy, and summers to be very hot and dry. The affects are different for different parts of the world.

El Nino also has a large impact on the Atlantic and eastern Pacific Hurricane seasons. Warming of the SSTs in the eastern equatorial Pacific tends to be counter-acted by a cooling of the SSTs in the Gulf of Mexico, the Caribbean, and the western Atlantic. It also tends to increase wind shear. This lowers the average intensity and number of hurricanes. A classic example is last year, when even a weak El Nino greatly reduced the number and strength of hurricanes. The affects of El Nino can reduce the number of U.S. hurricane landfalls as well. When low pressure sets up over NW South America, high pressure sets up over the Caribbean, which directs tropical waves south and west over South America. These waves then pop out on the other side in the eastern Pacific, where they have a much better chance to develop. This results in a much more active eastern Pacific hurricane season. Therefore El Nino decreases hurricane activity in the Atlantic, and increases activity in the eastern Pacific.

La Nina is the opposite of El Nino, and is simply an intensification of normal conditions. Easterly trade winds are stronger, which causes more upwelling of colder deeper water in the eastern equatorial Pacific. SSTs are colder than normal, which is the signature of a La Nina on an SST anomaly map. Rainfall in Australia is increased, and NW South America experiences very dry conditions. La Nina, like El Nino, also has major impacts on weather patterns across the globe, though usually the opposite of El Nino. Likewise the affects on the eastern Pacific and Atlantic hurricane seasons are opposite. Strong high pressure builds in the eastern equatorial Pacific, which results in low pressure over the Caribbean. Tropical waves in the Atlantic are steered northwest towards the U.S. and Mexican coasts, increasing the number of storm landfalls. The cooling of the SSTs in the equatorial Pacific also tends to warm the SSTs in the western Atlantic. Wind shear is also decreased. This results in a more active Atlantic hurricane season. On the other end the eastern Pacific sees very little tropical activity, as SSTs are lowered and few tropical waves make it across into the pacific basin.

El Nino and La Nina are some of the toughest pieces of the weather to forecast. Either one could pop up any time without us foreseeing it. However lots of research is being put into these phenomenon, and hopefully some day we will be able to accurately predict these climate-changing events.

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Reader Comments

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32. Tazmanian
11:34 PM GMT on February 22, 2007
Member Since: December 31, 1969 Posts: Comments:
31. Levi32
11:33 PM GMT on February 22, 2007
Sleet is not quite like hail. Sleet is clumps of ice that are formed when rain falling high up freezes before it hits the ground. Don't confuse that with freezing rain either, they're not the same thing.
Member Since: December 31, 1969 Posts: Comments:
30. Tazmanian
11:26 PM GMT on February 22, 2007
4" of snow now geting a little sleet is sleet like vary small haill?
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29. Levi32
11:20 PM GMT on February 22, 2007
Cool Kris! They must have added that to the site recently I've never seen it before. Too bad it only goes out to 48 hours.
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28. weatherboykris
10:46 PM GMT on February 22, 2007
Check this out.You can get model averages and differences.LinkOnly place I've seen that.
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27. Levi32
10:07 PM GMT on February 22, 2007
Hey, while we're all sharing links, have any of you checked out the models on the University of Wyoming site? In my opinion they're really sweet. Hint: the North America view covers a nice area from the central Pacific to central Atlantic, as well as tropical areas.
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26. weatherboykris
9:39 PM GMT on February 22, 2007
They're for June,July,and August.
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25. weatherboykris
9:39 PM GMT on February 22, 2007
H23,have you seen these 500mb heights forecast models.

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24. Levi32
9:23 PM GMT on February 22, 2007
Thanks Michael! For some reason I never saw that link on their page. They have some good analysis. Really interesting how the whole MJO thing works.
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22. Levi32
3:25 PM GMT on February 22, 2007
Thanks {{Red, Rays}}!

Hey Adrian! Thanks for stopping by. I agree as I said I think the wind shear will eventually bounce back up. It's pretty crazy to think it can stay at 10 knots during February and March. Now keep in mind that I have little experience with past seasons, so I'm not real savvy in terms of what past seasons were like in relation to climatological factors. However I've been doing some research, and my discoveries have shown that La Nina usually results in a strong ridge near the east coast of the U.S., not the other way around. One of the ways I got this was from this climate data site. Look at the temperature anomalies for La Nina and El Nino years (found here) and compare. I found that most La Nina years had warmer than normal temps in the eastern US, and El Nino years generally saw cold temps in the east. Also if you look at a a hurricane track chart for all El Nino years and then compare it with a chart from all La Nina years, there are clearly more recurving storms during El Nino than during La Nina.

That's just what I've found, but as I said I havn't been around long enough to justify anything climatologically by experience. You might be right about the trough, we'll see. If you think the Cape Verde storms can make it all the way to the Carolinas you're actually suggesting a fairly strong ridge not a trough in the western Atlantic. We'll see. Obviously La Nina isn't going to replicate the same behavior every time it comes around. We could have another "really weird" season this year lol.
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17. Raysfan70
10:42 AM GMT on February 22, 2007
Good Morning {{Levi}}!
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16. hurricane23
5:11 AM GMT on February 22, 2007
Good evening levi32...

My thoughts on this drop in wind shear is its just an anomaly and it should pick up in the next 2 months.Iam also watching how the SAL is behaving across the basin cause if you remember played a big factor in surpressing activity in 2006.With neutral or a moderate la nina developing this season i think things look to be active number wise but there is alot to be determined in the next 3-5 months but it also tends to result in a pattern in which we have a strong Atlantic ridge that is located further to the east as the Azores High, resulting in a year like 1999 (not 2004/2005) pattern. This tends to mean a strong trough located right off the southeast/eastern U.S. coast, meaning that most Cape Verde-type storms that form far east in a La Nina pattern will likely curve away from Florida and miss the U.S. eastern coast or, if they make landfall, hit the Carolinas.Its all about timeing with tropical systems.Lets see how all this evolves in the coming months.Adrian
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15. Redhead
4:37 AM GMT on February 22, 2007

Words Images @
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14. Levi32
4:17 AM GMT on February 22, 2007
LOL Bob! We'll see how like Joe I am when he comes out with HIS analysis of the upcoming H season lol. Thanks!

Michael, yes this is by far the largest and longest drop in wind shear we've had in recent years. The fact that it covers a very large area is one of the reasons I think that it is a climatological thing, not just a spike that comes back. Of course it probably will come back eventually, but I think the trend is clear, when you consider all the other factors I talked about.

A season like 1908? Hmmm...that would be huge to have a storm of any intensity in March, but it's not out of the question. The only thing I'd like to see is that dry air to get out of the Caribbean. If it doesn't moisten up there during March, no TCs are going to form no matter how low the shear is.
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13. weatherboykris
4:12 AM GMT on February 22, 2007
Yeah,things are looking bad for te hurricane season.
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11. Tazmanian
3:42 AM GMT on February 22, 2007
what is the forcast for wind shear for the next few weeks?????

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10. Tazmanian
3:41 AM GMT on February 22, 2007
and this is what FEB going in to march her soon
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8. weatherguy03
3:34 AM GMT on February 22, 2007
NOOO!! Ya sound like Joe already!!..LOL Just kidding Levi. Thanks for the analysis. Good job. Yes, I am very concerned about the Severe weather this weekend!
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6. Levi32
3:29 AM GMT on February 22, 2007
I would David but I was just told that I have to start a fire to warm up the house. It's nearing zero degrees here and I must keep us warm. See you guys later.
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5. Tazmanian
3:28 AM GMT on February 22, 2007
ok i see now come to my blog
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4. Levi32
3:27 AM GMT on February 22, 2007
Yeah I noticed that on the GFS, but I expect it will bounce back tomorrow.
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3. Levi32
3:26 AM GMT on February 22, 2007
David, a La Nina could mean a further west-positioned Bermuda High, but I didn't mention it because there are no more model forecasts or anything out for it until next month.
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1. Tazmanian
3:24 AM GMT on February 22, 2007
what about the high any thing new on that you for got to add that part
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About Levi32

Masters student in tropical meteorology at FSU. Raised in Alaskan blizzards, but drawn toward tropical cyclones by their superior PGF.

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