U.S. heavy precipitation events are increasing, but drought is not

By: Dr. Jeff Masters , 3:12 PM GMT on January 25, 2011

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Yesterday, I introduced the National Climatic Data Center's Climate Extremes Index, which uses temperature and precipitation records to see if the U.S. climate is getting more extreme. Today, I'll focus on how the drought and precipitation extremes that go into the Climate Extremes Index have changed over the past century. The three precipitation-related factors to go into the Climate Extremes Index are:

1) The sum of: (a) the monthly percentage of the United States in severe drought (equivalent to the lowest tenth percentile) based on the Palmer Drought Severity Index (PDSI) and (b) the percentage of the United States with severe moisture surplus (equivalent to the highest tenth percentile) based on the PDSI.

2) Twice the value of the percentage of the United States with a much greater than normal proportion of precipitation derived from extreme (equivalent to the highest tenth percentile) 1-day precipitation events.

3) The sum of (a) percentage of the United States with a much greater than normal number of days with precipitation and (b) percentage of the United States with a much greater than normal number of days without precipitation.

Items 1 and 3 have shown no change in annual average value over the past century, but there has been a marked increase in the number of heavy 1-day precipitation events in recent decades. Thus, the record and near-record values of the Climate Extremes Index in recent years have been due to a combination of the increase in heavy 1-day precipitation events and an increase in maximum and minimum temperatures.


Figure 1. The Annual Climate Extremes Index (CEI) for heavy 1-day precipitation events shows that these events, on average, have affected 10% of the U.S. over the past century (black line). However, heavy precipitation events have increased in recent decades. The seven most extreme years since 1910 have all occurred since 1995, with 2010 ranking as the 5th most extreme year in the past 100 years. Image credit: National Climatic Data Center.

Heavy precipitation events
Global warming theory predicts that global precipitation will increase, and that heavy precipitation events--the ones most likely to cause flash flooding--will also increase. This occurs because as the climate warms, evaporation of moisture from the oceans increases, resulting in more water vapor in the air. According to the 2007 Intergovernmental Panel on Climate Change (IPCC) report, water vapor in the global atmosphere has increased by about 5% over the 20th century, and 4% since 1970. The Climate Extremes Index plot for extreme 1-day precipitation events (Figure 1) does indeed show a sharp increase in heavy precipitation events in recent decades, with seven of the top ten years for these events occurring since 1995, and 2010 coming in 5th place in the past 100 years. The increases in heavy precipitation events have primarily come in the spring and summer, when the most damaging floods typically occur. This mirrors the results of Groisman et al. (2004), who found an increase in annual average U.S. precipitation of 7% over the past century, which has led to a 14% increase in heavy (top 5%) and 20% increase in very heavy (top 1%) precipitation events. Kunkel et al. (2003) also found an increase in heavy precipitation events over the U.S. in recent decades, but noted that heavy precipitation events were nearly as frequent at the end of the 19th century and beginning of the 20th century, though the data is not as reliable back then.

Drought and extreme wetness
Global warming theory predicts that although global precipitation should increase in a warmer climate, droughts will also increase in intensity, areal coverage, and frequency (Dai et al., 2004). This occurs because when the normal variability of weather patterns brings a period of dry weather to a region, the increased temperatures due to global warming will intensify drought conditions by causing more evaporation and drying up of vegetation. Increases in drought and flooding are my top two concerns regarding climate change for both the U.S. and the world in the coming century. Two of the three costliest U.S. weather disasters since 1980 have been droughts--the droughts of 1988 and 1980, which cost $71 billion and $55 billion, respectively. The heat waves associated with these droughts claimed over 17,000 lives, according to the National Climatic Data Center publication, Billion-Dollar Weather Disasters. Furthermore, the drought of the 1930s Dust Bowl, which left over 500,000 people homeless and devastated large areas of the Midwest, is regarded to be the third costliest U.S. weather disaster on record, behind Katrina and the 1988 drought. (Ricky Rood has an excellent book on the Dust Bowl that he recommends in a 2008 blog post).


Figure 2. The Annual Climate Extremes Index (CEI) for drought. The worst U.S. droughts on record occurred in the 1930s and 1950s. There has been no trend in the amount of the U.S. covered by drought conditions (blue bars) or by abnormally moist conditions (red bars) over the past century. About 10% of the U.S. is typically covered by abnormally dry or wet conditions (black lines). Image credit: National Climatic Data Center.

The good news is that the intensity and areal coverage of U.S. droughts has not increased in recent decades (blue bars in Figure 2). The portion of the U.S. experiencing abnormal drought and exceptionally wet conditions has remained nearly constant at 10% over the past century. A recent paper by Andreadis et al., 2006, summed up 20th century drought in the U.S. like this: "Droughts have, for the most part, become shorter, less frequent, and cover a smaller portion of the country over the last century. The main exception is the Southwest and parts of the interior of the West, where, notwithstanding increased precipitation (and in some cases increased soil moisture and runoff), increased temperature has led to trends in drought characteristics that are mostly opposite to those for the rest of the country especially in the case of drought duration and severity, which have increased."

Other portions of the globe have not not been so fortunate. Globally, Dai and Trenberth (2004) showed that areas experiencing the three highest categories of drought--severe, extreme, and exceptional--more than doubled (from ~12 to 30%) since the 1970s, with a large jump in the early 1980s due to an El Niño-related precipitation decrease over land, and subsequent increases primarily due to warming temperatures. According to the Global Drought Monitor, 98 million people world-wide currently live in areas experiencing the highest level of drought (exceptional).

References
Andreadis, K. M. Lettenmaier, D. P., "Trends in 20th century drought over the continental United States", Geo. Res. Letters 33, 10, L10403, DOI 10.1029/2006GL025711

Dai A., K.E. Trenberth, and T. Qian, 2004: A global data set of Palmer Drought Severity Index for 18702002: Relationship with soil moisture and effects of surface warming", J. Hydrometeorol., 5, 11171130.

Gleason, K.L., J.H. Lawrimore, D.H. Levinson, T.R. Karl, and D.J. Karoly, 2008: "A Revised U.S. Climate Extremes Index", J. Climate, 21, 2124-2137.

Groisman, P.Y., R.W. Knight, T.R. Karl, D.R. Easterling, B. Sun, and J.H. Lawrimore, 2004, "Contemporary Changes of the Hydrological Cycle over the Contiguous United States: Trends Derived from In Situ Observations," J. Hydrometeor., 5, 64-85.

Kunkel, K. E., D. R. Easterling, K. Redmond, and K. Hubbard, 2003, "Temporal variations of extreme precipitation events in the United States: 1895-2000", Geophys. Res. Lett., 30(17), 1900, doi:10.1029/2003GL018052.

A new Nor'easter for New England
A low pressure system currently centered along the Gulf Coast near New Orleans is bringing heavy rain to much of the south. Rains in excess of 3 inches have fallen over central Mississippi, and the rain is expected to change to snow over northern Mississippi, northern Alabama, and much of Tennessee late tonight. A swath of 2 - 4" of snow is expected in these regions, with higher amounts in the mountains. The low will move off the coast of North Carolina on Wednesday morning, then northeastward out to sea, potentially bringing heavy snows of 4 - 8" to inland portions of New England and the mid-Atlantic. At this time, it appears that the storm will track far enough from the coast and there will be insufficient cold air in place for snowfall amounts of a foot or more to fall. A nasty mix of rain, sleet, and snow is likely for much of the coast, with the heaviest snows expected to miss New York City, Washington D.C., and Boston (Figure 3.) As the low drags its cold front over Florida this afternoon, a slight risk of severe thunderstorms exists, and Florida could see a few tornadoes.


Figure 3. Probability of more than 8 inches of snow falling, for the 24 hour period ending 7am EST Thursday January 27, 2011. Image credit: National Weather Service HPC.


Jeff Masters

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Quoting Jedkins01:
Good Lord it is hard to get Calculus homework done on a day like today, even more so when the homework is about derivatives and integrals, lol.
Get 'er done, dude. If you do what I do, you may just need that stuff...

L8R, y'all.
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It has turn really windy here and I just recorded a wind gust of 36 mph with my hand held. I can also feel the moisture increasing mostly because my skin feels it. I am going to say we will get a big storm today.
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Quoting Jedkins01:



Woah yeah, that would be awesome!
Of course, the resolution wouldn't be what you are used to seeing from radar...
(4km vs. less than 1km)
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Good Lord it is hard to get Calculus homework done on a day like today, even more so when the homework is about derivatives and integrals, lol.
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It is only rainfall rate, but an idea of convection in the gulf (about 75 minutes old):


(click for full size)
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Quoting atmoaggie:
Believe it or not, I am writing a proposal in response to a NOAA RFP to further apply their radar reflectivity to rain rate algorithm in order to generate a satellite derived reflectivity estimate for that very "hole".

Use the 15 minute GOES satellite rain rate, apply the algorithm in reverse, validate it over land (well, over water near land...satellite rainfall rate over land is suspect) where reflectivity from radar exists, make adjustments, and we'll have a 15-minute reflectivity in your "hole".

It's a little on the edge of doable...we'll see. But would be an awesome product!



Woah yeah, that would be awesome!
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Quoting Neapolitan:
Lightning. Lots and lots of lightning:

Click for large loop:

Appropriate tropical weather-related image.


this year is starting off giving Florida its lighting dues :)
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Oops sorry I meant to say thank you Aquak for the compliment about my bird obsession :)
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Lightning. Lots and lots of lightning:

Click for large loop:

Appropriate tropical weather-related image.
Member Since: November 8, 2009 Posts: 4 Comments: 13719
Quoting Levi32:


Dang, that GOES product is going to be awesome this hurricane season.
No way it will be there for this season...they take at least 3 month reviewing proposals. The season after, maybe.

(RE: My intentions to build a satellite-derived near-realtime reflectivity product)
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Hi Flood! I'm doing fine thanks! How about you and your lady?
Just sitting here with 3 snoozing dogs watching the weather. Doesn't get much better than that :)
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Quoting atmoaggie:
Believe it or not, I am writing a proposal in response to a NOAA RFP to further apply their radar reflectivity to rain rate algorithm in order to generate a satellite derived reflectivity estimate for that very "hole".

Use the 15 minute GOES satellite rain rate, apply the algorithm in reverse, validate it over land where reflectivity from radar exists, make adjustments, and we'll have a 15-minute reflectivity in your "hole".

It's a little on the edge of doable...we'll see. But would be an awesome product!


Dang, that GOES product is going to be awesome this hurricane season.
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Quoting Jedkins01:
Convection is absolutely exploding now in the gulf, those thunderstorms are getting really nasty.


It is currently 79 in Fort Myers with a dew point of 70. Our winds are out of the SE at 8. Our local mets are telling us to expect possible squally weather after the evening commute and more likely very late tonight or very early tomorrow morning. Possible rain fall totals of 2 inches, and we are under a lake wind advisory
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oh man, Florida is gonna get soaked today!
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Quoting icmoore:
Hi Aquak, Flood, and all you good people just here to see what my favorite weather people are saying about the weather coming into FL out of the GOM and Aquak you knew I'd be here lurking I'm sure :) Anytime there is a potential you know I'm here.


ICMoore, how are you doin', darlin?
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and my local barometer keeps dropping as my eyes try to bulge outta my skull...oh Lordy my aching head.
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117. Jax82
now that is rain.

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On the radar I am noticing a nasty-looking little bow echo showing up about 130-150 miles west of Tampa. There is also an extraordinary amount of lighting in that cell. My guess is that the the warm frontal boundary is located just above that bow echo, as indicated by the broad and more diffuse shield of moderate to heavy rain to the north of there.
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Convection is absolutely exploding now in the gulf, those thunderstorms are getting really nasty.
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Iceeee!!! I was thinking of you last night, thinking of your gorgeous bird pics, wondering if you were far south enough to be involved in this mess later on tonite.

still i think the worst will be S, SW of you, but you're liable to see some good winds and sideways rain for a bit.
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Quoting Surfcropper:
I once survived a water spout that went over me on a bridge in the keys...it was such a rush...even an F-zero will try to take the shirt off your back


Outran one in a boat once. Too much for this non adrenalin junky!!!
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Quoting Jedkins01:


I completely agree, I think a lot of it does have to do with the unknown "hole" in the gulf.
Believe it or not, I am writing a proposal in response to a NOAA RFP to further apply their radar reflectivity to rain rate algorithm in order to generate a satellite derived reflectivity estimate for that very "hole".

Use the 15 minute GOES satellite rain rate, apply the algorithm in reverse, validate it over land (well, over water near land...satellite rainfall rate over land is suspect) where reflectivity from radar exists, make adjustments, and we'll have a 15-minute reflectivity in your "hole".

It's a little on the edge of doable...we'll see. But would be an awesome product!
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The weather channel local on the 8's says severe thunderstorms with rainfall greater than 2 inches, you know when TWC has a forecast like that for us we are gonna get whacked...
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Still waiting on initiation in front of the cluster squall... lots of cloud cover but the soup is getting warmer.
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Shows activity further north in the gulf
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Sorry
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http://www.aprsfl.net/weather/wasp2.png?1295976878211

ALOT of lightning!!!!
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Quoting beell:
Maybe gotta give a little bit of a skew to FL tornadoes associated with tropical systems?


Thats part of ncdc thoughts

"In the United States, there are two regions with a disproportionately high frequency of tornadoes. Florida is one and "Tornado Alley" in the south-central U.S. is the other. Florida has numerous tornadoes simply due to the high frequency of almost daily thunderstorms. In addition, several tropical storms or hurricanes often impact the Florida peninsula each year. When these tropical systems move ashore, the embedded convective storms in the rain bands often produce tornadoes. However, despite the violent nature of a tropical storm or hurricane, the tornadoes they spawn (some as water spouts) tend to be weaker than those produced by non-tropical thunderstorms."
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Hi Aquak, Flood, and all you good people just here to see what my favorite weather people are saying about the weather coming into FL out of the GOM and Aquak you knew I'd be here lurking I'm sure :) Anytime there is a potential you know I'm here.
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Quoting reedzone:
You know when TWC puts red on the map, it's a serious situation..



LOL true, they never do for Florida!

What I'm most excited about is gusty winds, lots of lightning, and tons of beneficial rains.

All the local METs models are putting a 3 to 5 inch bullseye on the bay area here. Heavy rain is always welcome in Florida!
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You know when TWC puts red on the map, it's a serious situation..

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How much longer are these 80 degree then 40 degree then 80 degree then 40 degree days and nights going to last in Florida!! My sinuses need a break!! I'll take 80-90 everyday please.
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Quoting reedzone:
Jedkins, scratch that, it is not THAT cloudy here, I see a few areas of blue sky. Instability getting unstable here with wind gusts.


Yeah its been completely sunny here except for low and fast moving cumulus clouds racing by, its now 79! Wonder if it will reach 80! the forecast is only 75, lol.
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Quoting beell:
Maybe gotta give a little bit of a skew to FL tornadoes associated with tropical systems?
Aha! True. But, still have that observational issue with those over the last 50 years...

It's still guesswork today to classify every tornado in a landfalling hurricane, also, though.

Dual pole radar should help with that soon.
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Quoting FLWaterFront:


That giant pool of water to the west of the FL peninsula still serves as a blind spot for forecasters, in my opinion. Without all of the ground stations that are available elsewhere to measure distinct and fluid changes in the atmospheric profile, there is no way to get as firm a grip on developing weather situations as in the Upper Midwest, for one example.

It is different with tropical weather of course. That is why they have hurricane hunter planes. This allows them to go where no ground instrumentation or ground stations can exist and still get up-to-the-minute observations. As a result, I think the SPC just takes the middle ground and plays it conservative when an atmospheric setup just like the one we are seeing today develops. Issue warnings, make a conservative assessment of risk but just in case some unforeseen circumstance develops which lowers the potential for extreme weather, don't go overboard with the outlook, at least not until very late in the game.

You mentioned the issue of colder near-shore waters in one post last night, Jedkins. Indeed, that is something that puts another "unknown" into the forecasting mix. However, in Florida when a storm such as this one is approaching, there is a strong potential for discreet supercell development over the land areas, especially when an adequate heating up of the lower levels is occurring, such as what appears to be happening today. And that one detail may be where the risk for strong and rapid rotational development is greatest on the FL peninsula today and tonight. Is my guesstimate of this setup at all accurate, in your opinion?


I completely agree, I think a lot of it does have to do with the unknown "hole" in the gulf. Us weather lovers have it easy, so when we see things, we just say it. As a real forecaster, it isn't that simple. They can"t go with what their "gut" tells them based on the current weather setup, they have to go by data to be wise practicing scientist.


That being said, its always better to err on the side of going conservative than being the "forecasters who cried wolf". Then nobody will believe a forecast.


I agree though, I just recently posted on how the one thing forecasters weren't sure about was surface instability. However, it is getting extremely warm, approaching the 80 degree mark, something we haven't seen since early November at my house! Not too mention dew point are rapidly rising, personally I think dew points may approach the dangerous 70 mark.


Its amazing it is getting so warm, considering shelf water temp is 54 off the coast here!
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I once survived a water spout that went over me on a bridge in the keys...it was such a rush...even an F-zero will try to take the shirt off your back
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Jedkins, scratch that, it is not THAT cloudy here, I see a few areas of blue sky. Instability getting unstable here with wind gusts.
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Quoting Jedkins01:



I agree man! Temps and dewpoints have already reached as high as they were supposed to at maximum. Currently its 78 here with a dewpoint of 64, the dewpoint when up 5 degrees from 59 last hour...

The one thing they said we would lack in would be surface instability, looks like that lacking has already ended quickly.

The local METS just noted there is ridiculous lightning activity occurring way out there.


My current conditions and temp so far..
Mostly Cloudy

72 °F
(22 °C)

Forecast for this afternoon...
This
Afternoon

Severe
Tstms
Hi 71 °F

We are already warmer then forecasted. The IR continues to look more impressive as the hours fly by and while it is cloudy outside, it's extremely bright, which means the clouds are not thick.
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Maybe gotta give a little bit of a skew to FL tornadoes associated with tropical systems?
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Quoting reedzone:
SPC continues to downplay the potentially dangerous situation for Northern/Central Florida.



I agree man! Temps and dewpoints have already reached as high as they were supposed to at maximum. Currently its 78 here with a dewpoint of 64, the dewpoint when up 5 degrees from 59 last hour...

The one thing they said we would lack in would be surface instability, looks like that lacking has already ended quickly.

The local METS just noted there is ridiculous lightning activity occurring way out there.
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Quoting hurricane23:


Our main threat in South Florida is winds as the storms will become more linear late tonight. You can't rule out a supercell or two, but more likely it would be wind gusts. There is an outside chance of late afternoon storms west of Lake Okeechobee with those having greater potential for tornadoes.

Adrian


Hello Adrian. Nice to see you! Don't let the lawn furniture blow away tonight
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Quoting MichaelSTL:
I see westerly wind anomalies in the western Pacific:



Also, heat content is very similar to two years ago; the West Pacific is also warmer:



Not saying that it means anything, but that is the first time since last April that there were westerly anomalies in the western Pacific:



what's it like up there in Missouri?
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I see westerly wind anomalies in the western Pacific:



Also, heat content is very similar to two years ago; the West Pacific is also warmer:



Not saying that it means anything, but that is the first time since last April that there were westerly anomalies in the western Pacific:

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Elements coming together nicely, you can see it in the water vapor image..

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Quoting atmoaggie:
Well, yeah, it does, by that. More than TX and NE.

Seems as though NCDC and SPC need to have a talk...

By annual average FL has a lot. By visual of all historical nado records, looks like very little.

*shrug*



That's what I'm saying, very confusing eh?

Maybe the visual map is based on record of visual sighting combined with recorded event as well. For example, very few tornadoes in Florida are visible, and there is often debate over the ones that are weaker then F-2, whether they are actually tornadoes, or just very strong winds from micro bursts.
in 2009 we had micro burst that produced wind gusts of 80 mph here, there was a severe thunderstorm warning, but after the the local METS said the NOAA observation team was debating whether or not it was a tornado or not. That's just one example.

Personally though, we get at least one tornado touch down in pinellas county a year that comes pretty close to me, and I personally have seen one tornado, based on the amount of warnings and tornado damage surveys I can remember since Ive lived here.

I will say though, Ive never heard of a tornado greater than F2 in the bay area since Ive lived here (1995). and most Ive known of are F1 and F0. I can remember only one F2 report since living here. And that covers all of the west central Florida counties I'm talking about. I'm not sure if it was F2 either, I just know there was a particularly bad tornado in this county years ago.
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Quoting Jedkins01:
However, Florida doesn't compare to tornado alley when it comes to strong tornado frequency:


Interesting that Indiana would have so much more (in stronger nadoes per area) than all surrounding states. Not coming up with a meteorological reason...
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Quoting Jedkins01:



Atmoaggie, this is also from NOAA, and so is the image below, that's why I said the map you posted is deceiving. I don't post facts unless I know they are facts.




However, Florida doesn't compare to tornado alley when it comes to strong tornado frequency:


Well, yeah, it does, by that. More than TX and NE.

Seems as though NCDC and SPC need to have a talk...

By annual average FL has a lot. By visual of all historical nado records, looks like very little.

*shrug*
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Atmoaggie, this is also from NOAA, and so is the image below, that's why I said the map you posted is deceiving. I don't post facts unless I know they are facts.




However, Florida doesn't compare to tornado alley when it comes to strong tornado frequency:


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Quoting Jedkins01:


I don't know, that seems like a rather deceptive map, the local NWS has released products saying tornado density per square mile is higher than any other state. Graphs can be kinda deceiving.
That has to include some function of population density in it...right?

Link?
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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