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 NiƱo-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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You can see how the radar colors dim as if weakening as it goes over the Dopplar site and now it's starting to brighten back up to more intense storms as that torrential rain moves outta the way of the site
Member Since: July 14, 2008 Posts: 1 Comments: 9645
119
WFUS52 KMLB 252255
TORMLB
FLC069-095-117-127-260000-
/O.NEW.KMLB.TO.W.0004.110125T2255Z-110126T0000Z/

BULLETIN - EAS ACTIVATION REQUESTED
TORNADO WARNING
NATIONAL WEATHER SERVICE MELBOURNE FL
555 PM EST TUE JAN 25 2011

THE NATIONAL WEATHER SERVICE IN MELBOURNE HAS ISSUED A

* TORNADO WARNING FOR...
EASTERN LAKE COUNTY IN FLORIDA...
THIS INCLUDES THE CITIES OF...MOUNT PLYMOUTH...CASSIA...
WESTERN ORANGE COUNTY IN FLORIDA...
THIS INCLUDES THE CITIES OF...WINDERMERE
WESTERN SEMINOLE COUNTY IN FLORIDA...
THIS INCLUDES THE CITIES OF...WEKIVA SPRINGS...SANFORD...
LONGWOOD...LAKE MARY...FOREST CITY...CASSELBERRY...ALTAMONTE
SPRINGS...
WESTERN VOLUSIA COUNTY IN FLORIDA...

* UNTIL 700 PM EST.

* AT 551 PM EST...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A
SEVERE THUNDERSTORM CAPABLE OF PRODUCING A TORNADO NEAR
WINDERMERE...MOVING NORTH NORTHEAST AT 45 MPH.

* OTHER LOCATIONS IN THE WARNING INCLUDE BUT ARE NOT LIMITED TO
WINDERMERE...PINE HILLS...ORLOVISTA...FERN PARK...AND WEKIWA SPRINGS
STATE PARK.

THE IMMEDIATE DANGER OF A TORNADO IS HIGH. PEOPLE IN THE PATH OF THIS
DEVELOPING TORNADO ARE URGED TO QUICKLY PREPARE FOR ITS APPROACH.
THIS IS A DANGEROUS SITUATION! ACT NOW AS SECONDS CAN SAVE LIVES!
TAKE COVER NOW! TAKE COVER NOW!

PRECAUTIONARY/PREPAREDNESS ACTIONS...

THE SAFEST PLACE TO BE DURING A TORNADO IS IN A STRONG BUILDING ON
THE LOWEST FLOOR...IN AN INTERIOR ROOM SUCH AS A BATHROOM OR CLOSET.
KEEP AWAY FROM WINDOWS. GET UNDER A WORKBENCH OR OTHER PIECE OF
STURDY FURNITURE. USE BLANKETS OR PILLOWS TO COVER YOUR BODY.



LAT...LON 2941 8113 2834 8135 2835 8173 2938 8142
2927 8141 2927 8117 2933 8115 2941 8115
TIME...MOT...LOC 2253Z 202DEG 54KT 2853 8155



15


CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab FL Page Main Text Page
Member Since: July 3, 2005 Posts: 426 Comments: 129088
Quoting Ossqss:


Most tubs are cast iron and can provide some protection. :)
? Yeah, if they're better than 35 years old. Mostly plastic, otherwise. (Unless it's a special)
Member Since: August 16, 2007 Posts: 6 Comments: 12463
I'm in Northeast St. Pete and it was getting pretty bad here. Pieces of tree where flying down the road. Rain was going completely sideways, power keep flickering on and off. I think the possible tornado was near my house, because everything was blowing in every direction. Be intresting to see some of the damage photos.
Member Since: May 28, 2007 Posts: 0 Comments: 288


Radar in Ft. Myers (Little further away from the storms)
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Quoting NRAamy:
Expect be moving into the bathroom with my computer in a few minutes.

does your bathroom have a shower curtain with fish on it?


LOL...a good one ;)
Member Since: September 16, 2010 Posts: 0 Comments: 1496
Hail and wind
Member Since: July 14, 2008 Posts: 1 Comments: 9645
They just issued a Tornado warning for Central Hillsborough, came over the AM radio as a NWS broadcast
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Quoting NRAamy:
Expect be moving into the bathroom with my computer in a few minutes.

does your bathroom have a shower curtain with fish on it?

hehehe
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I really can't remember the last time I've seen so many tornado warnings issued across Florida.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
Radar beam cant penetrate the heavy rains as good when its on top of the site,
Member Since: July 14, 2008 Posts: 1 Comments: 9645
Quoting RitaEvac:
Check skies tonight and see if it turns aqua blue/green, long time ago Houston Astros were in spring training down there and the players were interviewed about what they saw. That was the 93' storm I think


Rita, I had those aqua blue/green skies on my way home a little bit ago... what does that indicate?
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weakening flag on
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Expect be moving into the bathroom with my computer in a few minutes.

does your bathroom have a shower curtain with fish on it?
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Oh, it's because the storms are right on top of the NWS site, it does that. Same thing happened at Houston/Galveston radar site the other day
Member Since: July 14, 2008 Posts: 1 Comments: 9645
Ossqss... I never thought of that... good idea!

:)

thanks press....

:)
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They've adjusted the radar scheme or something
Member Since: July 14, 2008 Posts: 1 Comments: 9645
868

WOUS20 KWNS 252227

WWASPC

SPC WW-A 252230

FLZ000-CWZ000-252340-



STATUS REPORT ON WW 7



SEVERE WEATHER THREAT CONTINUES RIGHT OF A LINE FROM 20 WSW PIE

TO 30 S OCF TO 20 WNW SGJ.



..GRAMS..01/25/11



ATTN...WFO...JAX...MLB...TBW...



&&



STATUS REPORT FOR WT 7



SEVERE WEATHER THREAT CONTINUES FOR THE FOLLOWING AREAS



FLC009-027-035-049-055-057-061-069-081-083-085-093-095-097-101-

103-105-107-109-111-115-117-119-127-252340-



FL

. FLORIDA COUNTIES INCLUDED ARE



BREVARD DESOTO FLAGLER

HARDEE HIGHLANDS HILLSBOROUGH

INDIAN RIVER LAKE MANATEE

MARION MARTIN OKEECHOBEE

ORANGE OSCEOLA PASCO

PINELLAS POLK PUTNAM

ST. JOHNS ST. LUCIE SARASOTA

SEMINOLE SUMTER VOLUSIA

$$





AMZ454-550-552-555-GMZ830-853-252340-



CW



. ADJACENT COASTAL WATERS INCLUDED ARE



COASTAL WATERS FROM ST. AUGUSTINE TO FLAGLER BEACH FL OUT 20 NM



FLAGLER BEACH TO VOLUSIA-BREVARD COUNTY LINE 0-20 NM



VOLUSIA-BREVARD COUNTY LINE TO SEBASTIAN INLET 0-20 NM



SEBASTIAN INLET TO JUPITER INLET 0-20 NM



TAMPA BAY WATERS



COASTAL WATERS FROM ENGLEWOOD TO TARPON SPRINGS FL OUT 20 NM



$$

THE WATCH STATUS MESSAGE IS FOR GUIDANCE PURPOSES ONLY. PLEASE

REFER TO WATCH COUNTY NOTIFICATION STATEMENTS FOR OFFICIAL

INFORMATION ON COUNTIES...INDEPENDENT CITIES AND MARINE ZONES

CLEARED FROM SEVERE THUNDERSTORM AND TORNADO WATCHES.

$$
Member Since: Posts: Comments:
Quoting charlottefl:
2210 75 2 N SAINT PETERSBURG PINELLAS FL 2779 8267 ESTIMATED WIND GUSTS OF 70 TO 75 MPH. BRANCHES BREAKING OFF TREES. (TBW)

Person in stable condition, was trapped in car. Gas station hit.
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Member Since: July 14, 2008 Posts: 1 Comments: 9645
SPC TORNADO AND SEVERE THUNDERSTORM REPORTS

UNOFFICIAL - FOR OFFICIAL REPORTS, SEE PUBLICATION 'STORM DATA'

FOR 06CST TUE JAN 25 2011 THRU 16CST TUE JAN 25 2011



EVENT LOCATION REMARKS (CST)TIME



.....TORNADO REPORTS..........TORNADO REPORTS..........TORNADO REPORTS.....





NONE REPORTED







.......LRG HAIL/STRONG WIND RPTS.............LRG HAIL/STRONG WIND RPTS......





NONE REPORTED







.........OTHER SEVERE REPORTS..................OTHER SEVERE REPORTS.........



1 A100 6 W BROOKSVILLE FL (44 NNE PIE) 25/1540

QUARTER SIZE HAIL REPORTED TBW/LSR 2855 8249

2 G 50 2 NNE MASARYKTOWN FL (40 NNE PIE) 25/1544

50 KNOT WIND GUST REPORTED AT BROOKSVILLE TBW/LSR 2847 8245

HERNANDO COUNTY AIRPOR




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Intense red on the line have just dissapeared at the same time, is that a radar screw up or is that actual...
Member Since: July 14, 2008 Posts: 1 Comments: 9645
2210 75 2 N SAINT PETERSBURG PINELLAS FL 2779 8267 ESTIMATED WIND GUSTS OF 70 TO 75 MPH. BRANCHES BREAKING OFF TREES. (TBW)
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Amy...the serious answer is: it just boils down to luck...
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Want to take this moment to thank all of you that make this the best weather blog during these types of situations.I like the verbal sparring but this blog shows it's true colors on weather days like this. Keep up the good work I'll bet the doc. appreciates you'll on days like this.
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409. flsky
Nothing yet in DBS, but my weather radio has sounded 4 times. Expect be moving into the bathroom with my computer in a few minutes.
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Quoting NRAamy:
Jerry....no...actually, I was asking a serious weather-related question....big shock, huh?


;)


Most tubs are cast iron and can provide some protection. :)
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1 hr rate rainfall

Member Since: July 14, 2008 Posts: 1 Comments: 9645
PBW.... thank you.... I'm in Southern Calif, but, I have friends in Boca Raton....that's why I'm anxious....
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NRAamy....My suggestion (what I am going to do) is stay up later than usual. This may be a midnight or later event for us...I am in Lake Worth on the Intracoastal
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The local mets at lunch time said that we would see 2 line of storms with the main squall line coming into Tampa tonight, around midnight. 4.5 hours later the squall line moves in about, 6-8hours early.
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Check skies tonight and see if it turns aqua blue/green, long time ago Houston Astros were in spring training down there and the players were interviewed about what they saw. That was the 93' storm I think
Member Since: July 14, 2008 Posts: 1 Comments: 9645
NRAamy.........I don't look for Palm Beach county until midnight or later
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weakening flag flag
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Gonna get worse for south Florida then
Member Since: July 14, 2008 Posts: 1 Comments: 9645
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are the storms near Boca Raton?
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Abundant energy available further south across South Florida and especially near the Keys for a significant severe weather event across the region as shown in the latest CAP analysis via the NWS in Miami.


Member Since: April 14, 2007 Posts: 8 Comments: 5169
The line of storms that is moving through Florida is the strongest I have seen in years..It is not weaking right now, but may weaken as it gets further south...But please keep aware of what is happening.
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Line bearing down on NWS
Member Since: July 14, 2008 Posts: 1 Comments: 9645
Member Since: July 14, 2008 Posts: 1 Comments: 9645
Jerry....no...actually, I was asking a serious weather-related question....big shock, huh?


;)
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Tornado into gas station in St Pete, Channel 10. Rescue in progress.
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Quoting EYEStoSEA:
A hallway or inner area...but too much glass in a bathroom....stay away from rooms, windows..ect...where there is glass or mirrows

EYES.The reason the bathroom is one of the top choices is because of the plumbing pipes..The pipes have an uncanny way of things falling on your head
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Quoting presslord:


Get out of the trailer...


Hmmm...I thought Amy lived in a Winnebago...LOL
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Something very concerning to note is that while the squall line has been producing some tornadic storms and dangerous weather across Central and Northern Florida, satellite imagery shows the squall line extending and intensifying further south. Definitely becoming the most impressive squall line to come through Florida since 1998 in my opinion.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
IKE...I would say just the opposite.I would say getting stronger
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A hallway or inner area...but too much glass in a bathroom....stay away from rooms, windows..ect...where there is glass or mirrows
Member Since: September 16, 2010 Posts: 0 Comments: 1496

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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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