Extreme Drought to Extreme Flood: Weather Whiplash Hits the Midwest

By: Dr. Jeff Masters , 2:42 PM GMT on April 19, 2013

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It seems like just a few months ago barges were scraping bottom on the Mississippi River, and the Army Corps of Engineers was blowing up rocks on the bottom of the river to allow shipping to continue. Wait, it was just a few months ago--less than four months ago! Water levels on the Mississippi River at St. Louis bottomed out at -4.57' on January 1 of 2013, the 9th lowest water level since record keeping began in 1861, and just 1.6' above the all-time low-water record set in 1940 (after the great Dust Bowl drought of the 1930s.) But according to National Weather Service, the exceptional April rains and snows over the Upper Mississippi River watershed will drive the river by Tuesday to a height 45 feet higher than on January 1. The latest forecast calls for the river to hit 39.4' on Tuesday, which would be the 8th greatest flood in history at St. Louis, where flood records date back to 1861. Damaging major flooding is expected along a 250-mile stretch of the Mississippi from Quincy, Illinois to Thebes, Illinois next week. At the Alton, Illinois gauge, upstream from St.Louis, a flood height of 34' is expected on Tuesday. This would be the 6th highest flood in Alton since 1844, and damages to commercial property in the town of Alton occur at this water level. In addition, record flooding is expected on at least five rivers in Illinois and Michigan over the next few days. A crest 1.5' above the all-time record has already occurred on the Des Plaines River in Chicago. This river has invasive Asian Carp that could make their way into Lake Michigan if a 13-mile barrier along the river fails during an extreme flood. Fortunately, NPR in Michigan is reporting today that U.S. Army Corps of Engineers crews stationed along the 13-mile Asian carp barrier have seen no evidence of the fish breaching the structure, and it would have taken a flood much larger than today's record flood to breach the structure. A crest on the Grand River in Grand Rapids, Michigan nearly 4' above the previous record (period of record: at least 113 years) is expected this weekend. At this flood level, major flooding of residential areas is expected, though the flood wall protecting downtown Grand Rapids will keep the commercial center of the city from flooding.


Figure 1. The rains that fell in a 24-hour period ending at 7 am EDT Thursday, April 18, 2013 over Northern Illinois were the type of rains one would expect see fall only once every 40 years (yellow colors), according to METSTAT, Inc. (http://www.metstat.com.) METSTAT computed the recurrence interval statistics based on gauge-adjusted radar precipitation and frequency estimates from NOAA Atlas 14 Volume 2, published in 2004 (http://dipper.nws.noaa.gov/hdsc/pfds/.) METSTAT does not supply their precipitation recurrence interval forecasts or premium analysis products for free, but anyone can monitor the real-time analysis (observed) at: http://metstat.com/solutions/extreme-precipitation-index-analysis/

Damages from the April 2013 Midwest U.S. flood in Illinois, Michigan, and Missouri are likely to run into the hundreds of millions of dollars. Some of the impacts at the flood levels predicted include:

St. Louis, MO:
Major flooding begins. At this level the Choteau Island Levee, protecting 2400 acres, is overtopped. Also, Lemay Park just south of Lemay Ferry Road will begin flood

Cape Girardeau, MO:
Many homes in the Cape Girardeau area are affected and evacuations may be required. Over 100,000 acres is flooded. Numerous roads are closed.

Hannibal, MO:
The Sny Island and South Quincy levees are overtopped between River Mile 315.4 and 264.3, flooding 110,000 acres. The South River levee is overtopped between River Mile 320.5 and 312.1, flooding 10,000 acres.

Quincy, IL:
Missouri Highway 168 east of Palmyra near the BASF plant closes; Quincy Waterworks inundated.

Note that sandbagging efforts may be able to prevent some of these flooding impacts from occurring. Wunderground's weather historian Christopher C. Burt discusses the latest rainfall and flooding records from this week's epic storm in his latest post. He plans on an update Friday afternoon.


Figure 2. Water levels on the Mississippi River at St. Louis are predicted to crest at 39.5', near major flood stage, on Tuesday. This would be the 8th greatest flood in history at St. Louis. Records at the St. Louis gauge to back to 1861. Image credit: National Weather Service.


Figure 3. Water levels on the Mississippi River at St. Louis bottomed out at -4.57' on 01/01/2013, the 9th lowest water level since record keeping began in 1861, and just 1.6' above the all-time low-water record set in 1940, after the great Dust Bowl drought of the 1930s. Image credit: National Weather Service.

Flood-Drought-Flood Weather Whiplash
Residents along the Mississippi River have experienced a severe case of flood-drought-flood weather whiplash over the past two years. The Mississippi reached its highest level on record at New Madrid, Missouri on May 6, 2011, when the river crested at 48.35'. Flooding on the Mississippi and Missouri rivers that year cost an estimated $5 billion. The next year, after the great drought of 2012, the river had fallen by over 53' to an all time record low of -5.32' on August 30, 2012. Damage from the great drought is conservatively estimated at $35 billion. Next Tuesday, the river is expected to be at flood stage again in New Madrid, 40' higher than the August 2012 record low. Now, that is some serious weather whiplash. I'm often asked about the seemingly contradictory predictions from climate models that the world will see both worse floods and worse droughts due to global warming. Well, we have seen a classic example in the Midwest U.S. over the past two years of just how this kind of weather whiplash is possible. A warmer atmosphere is capable of bringing heavier downpours, since warmer air can hold more water vapor. We saw an example of this on Thursday morning, when an upper air balloon sounding over Lincoln, Illinois revealed near-record amounts of moisture for this time of year. The precipitable water--how much rain could fall if one condensed all the water vapor in a column above the ground into rain--was 1.62", just barely short of the Illinois April record for precipitable water of 1.64" set on April 20, 2000 (upper air records go back to 1948.) Thursday's powerful low pressure system was able to lift that copious moisture, cool it, and condense it into record rains. So how can you have worse droughts with more moisture in the air? Well, you still need a low pressure system to come along and wring that moisture out of the air to get rain. When natural fluctuations in jet stream patterns take storms away from a region, creating a drought, the extra water vapor in the air won't do you any good. There will be no mechanism to lift the moisture, condense it, and generate drought-busting rains. The drought that ensues will be more intense, since temperatures will be hotter and the soil will dry out more.

The new normal in the coming decades is going to be more and more extreme flood-drought-flood cycles like we are seeing now in the Midwest, and this sort of weather whiplash is going to be an increasingly severe pain in the neck for society. We'd better prepare for it, by building a more flood-resistant infrastructure and developing more drought-resistant grains, for example. And if we continue to allow heat-trapping gases like carbon dioxide continue to build up in the atmosphere at the current near-record pace, no amount of adaptation can prevent increasingly more violent cases of weather whiplash from being a serious threat to the global economy and the well-being of billions of people.


Video 1. One person was hospitalized after a sinkhole swallowed three cars in the South Deering neighborhood on the Southeast Side of Chicago on Thursday, April 18, 2013. Witnesses said the hole opened up around 5 a.m. at 9600 South Houston Avenue, quickly growing from about 20 feet to about 40 feet. First two cars slid in, then a third as the hole widened, witnesses said. A fourth vehicle was towed from the edge as it was about to fall inside. The sinkhole was due to heavy flooding that broke a water main built in 1915.

Related Science
A 2010 study by Duke University scientists suggests that global warming is the main cause of a significant intensification in the Bermuda High that in recent decades has more than doubled the frequency of abnormally wet or dry summer weather in the Southeast United States. Thus, the Southeast U.S. may see greater than its share of "Weather Whiplash"--extreme droughts followed by extreme floods--in coming decades. Joe Romm at climateporgress.org has a post discussing the paper, with links to examples of how the Southeast U.S. has seen both extreme droughts and extreme floods since 2005.


Jeff Masters

Underpass flooding (aerojad)
Belmont St. at the Metra train station.
Underpass flooding

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Quoting TropicalAnalystwx13:

The ECMWF forecast should be completely disregarded. Its under the impression that a strong El Nino will develop prior to the peak of the season, which appears to be extremely unlikely.


The ensemble mean shows a warm-biased neutral ENSO or very weak nino, not a strong El Nino. That said, the ensemble spread is extremely large, ranging anywhere from moderate La Nina to strong El Nino.



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NWS Tampa Bay ‏@NWSTampaBay 8m
Showers and thunderstorms have begun to expand across the area. A few severe storms will be possible in the area today. #FLWX
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nothing here yet..
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883. SLU
Quoting Tropicsweatherpr:
Here is the CFS MSLP forecast and is quite different from the ECMWF one as it has lower pressures while the Euro had higher pressures.



Do you have the link for the original website?
Member Since: July 13, 2006 Posts: 13 Comments: 5367
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warning after warning coming out now south florida.....SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE MIAMI FL
241 PM EDT SUN APR 21 2013

FLZ069-070-211915-
COASTAL COLLIER COUNTY FL INLAND COLLIER COUNTY FL
241 PM EDT SUN APR 21 2013

...A SIGNIFICANT WEATHER ADVISORY IS IN EFFECT FOR WESTERN COLLIER
COUNTY...FOR FREQUENT TO EXCESSIVE LIGHTNING...GUSTY WINDS FROM 45 TO
55 MPH...UP TO NICKEL-SIZED HAIL...

* UNTIL 315 PM EDT

* AT 236 PM EDT...NATIONAL WEATHER SERVICE METEOROLOGISTS DETECTED A
STRONG THUNDERSTORM NEAR BELLE MEADE...AND MOVING SOUTHEAST AT 5
MPH.

* THE STORM WILL AFFECT...
FIDDLERS CREEK...
MARCO ISLAND AIRPORT...
KEY MARCO...
AND SURROUNDING COMMUNITIES.

FREQUENT TO EXCESSIVE LIGHTNING...GUSTY WINDS FROM 45 TO 55 MPH...UP
TO NICKEL-SIZED HAIL...TORRENTIAL DOWNPOURS...OR A COMBINATION OF
THESE ARE POSSIBLE. LIGHTNING IS THE NUMBER ONE WEATHER RELATED
KILLER IN FLORIDA. TREES AND OPEN SHELTERS OFFER NO PROTECTION. THESE
WINDS CAN DOWN SMALL TREE LIMBS AND BRANCHES...AND BLOW AROUND
UNSECURED SMALL OBJECTS. SEEK SHELTER IN A SAFE BUILDING UNTIL THE
STORM PASSES.

TORRENTIAL RAINFALL CAN BE EXPECTED...CAUSING TEMPORARY PONDING ON
SOME ROADS AND MINOR FLOODING OF POOR DRAINAGE AREAS. MOTORISTS
SHOULD SLOW DOWN IN HEAVY RAINFALL TO REDUCE THE RISK OF HYDROPLANING
AND LEAVE A SAFE DISTANCE BETWEEN OTHER VEHICLES.
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Quoting indianrivguy:


you gave away your age there..... Cookie
LOL..i was going to add..only some of can appreciate the humor in that..gee lol
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Quoting LargoFl:
LMAO


you gave away your age there..... Cookie
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i figure i would get a smile outta ya
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FLOOD ADVISORY
NATIONAL WEATHER SERVICE MIAMI FL
247 PM EDT SUN APR 21 2013

FLC021-212045-
/O.NEW.KMFL.FA.Y.0014.130421T1847Z-130421T2045Z/
/00000.N.ER.000000T0000Z.000000T0000Z.000000T0000 Z.OO/
247 PM EDT SUN APR 21 2013

THE NATIONAL WEATHER SERVICE IN MIAMI HAS ISSUED AN

* URBAN FLOOD ADVISORY FOR...
WEST CENTRAL COLLIER COUNTY IN SOUTHWEST FLORIDA...
THIS INCLUDES THE CITY OF NAPLES...

* UNTIL 445 PM EDT

* AT 245 PM EDT NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED 1
TO 2 INCHES OF RAINFALL OVER SOUTHWEST COLLIER COUNTY OVER THE LAST HOUR.
AN ADDITIONAL 1 TO 2 INCHES CAN BE EXPECTED OVER THE NEXT HOUR OR SO.

HEAVY RAINFALL WILL CAUSE PONDING OF WATER IN URBAN AREAS, HIGHWAYS,
STREETS AND UNDERPASSES AS WELL AS OTHER POOR DRAINAGE AREAS AND LOW
LYING SPOTS. RUNOFF WILL ALSO CAUSE ELEVATED WATERS LEVELS IN CANALS
AND DITCHES.
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Quoting indianrivguy:


77 Sunset Strip
LMAO
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Quoting Grothar:


Very funny, KEEPER. Guess which one? :)





77 Sunset Strip
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solid overcast here now and clouds building..yesssss
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Quoting Tropicsweatherpr:
Here is the CFS MSLP forecast and is quite different from the ECMWF one as it has lower pressures while the Euro had higher pressures.


The ECMWF forecast should be completely disregarded. Its under the impression that a strong El Nino will develop prior to the peak of the season, which appears to be extremely unlikely.
Member Since: July 6, 2010 Posts: 113 Comments: 32855
Quoting Grothar:


Very funny, KEEPER. Guess which one? :)




That one.
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Miami, FL WFO - Visible Loop
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Quoting KEEPEROFTHEGATE:
here is another angle with grother house in the view

zoomed in a little bit


XX/XX/XX


Very funny, KEEPER. Guess which one? :)



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Here is the CFS MSLP forecast and is quite different from the ECMWF one as it has lower pressures while the Euro had higher pressures.

Member Since: April 29, 2009 Posts: 75 Comments: 14906
Quoting LargoFl:
wow GFS for after midnight tonight.............

That is not a model forecast from the GFS, but rather a human output from the forecasters at the Weather Prediction Center (WPC, formerly the HPC).
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SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE MIAMI FL
234 PM EDT SUN APR 21 2013

FLZ071-073-211900-
INLAND BROWARD COUNTY FL INLAND MIAMI-DADE COUNTY FL
234 PM EDT SUN APR 21 2013

...A SIGNIFICANT WEATHER ADVISORY IS IN EFFECT FOR SOUTHWESTERN
BROWARD COUNTY...NORTHERN MIAMI-DADE COUNTY...FOR FREQUENT TO
EXCESSIVE LIGHTNING...GUSTY WINDS FROM 45 TO 55 MPH...UP TO
NICKEL-SIZED HAIL...

* UNTIL 300 PM EDT

* AT 232 PM EDT...NATIONAL WEATHER SERVICE METEOROLOGISTS DETECTED A
STRONG THUNDERSTORM 11 MILES SOUTHWEST OF INTERSECTION ALLIGATOR
ALLEY AND MIAMI CANAL...AND MOVING SOUTHEAST AT 5 MPH.

* THE STORM WILL AFFECT...
RURAL SOUTHWESTERN BROWARD COUNTY
AND SURROUNDING COMMUNITIES.

FREQUENT TO EXCESSIVE LIGHTNING...GUSTY WINDS FROM 45 TO 55 MPH...UP
TO NICKEL-SIZED HAIL...TORRENTIAL DOWNPOURS...OR A COMBINATION OF
THESE ARE POSSIBLE. LIGHTNING IS THE NUMBER ONE WEATHER RELATED
KILLER IN FLORIDA. TREES AND OPEN SHELTERS OFFER NO PROTECTION. THESE
WINDS CAN DOWN SMALL TREE LIMBS AND BRANCHES...AND BLOW AROUND
UNSECURED SMALL OBJECTS. SEEK SHELTER IN A SAFE BUILDING UNTIL THE
STORM PASSES.
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wow GFS for after midnight tonight.............
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ORANGE COLOR IS MODERATE SEVERE POSSIBILITY...
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here is another angle with grother house in the view

zoomed in a little bit


XX/XX/XX
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Quoting JrWeathermanFL:
A bunch of clouds broke from the front that was in the GOM yesterday and now (as in comment 803) another might be forming. Then there's that cool swirl in the CATL



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** ITS STARTING UP SOUTH FLORIDA,STAY ALERT TONIGHT**..................SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE MIAMI FL
219 PM EDT SUN APR 21 2013

FLZ073-075-174-211900-
FAR SOUTH MIAMI-DADE COUNTY FL INLAND MIAMI-DADE COUNTY FL MAINLAND
MONROE FL
219 PM EDT SUN APR 21 2013

...A SIGNIFICANT WEATHER ADVISORY IS IN EFFECT FOR WESTERN MIAMI-DADE
COUNTY...EASTERN MAINLAND MONROE COUNTY...FOR FREQUENT TO EXCESSIVE
LIGHTNING...GUSTY WINDS FROM 45 TO 55 MPH...UP TO NICKEL-SIZED
HAIL...

* UNTIL 300 PM EDT

* AT 214 PM EDT...NATIONAL WEATHER SERVICE METEOROLOGISTS DETECTED A
STRONG THUNDERSTORM NEAR PA-HAY OKEE OVERLOOK...AND MOVING WEST AT
10 MPH.

* THE STORM WILL AFFECT...
RURAL WESTERN MIAMI-DADE COUNTY
RURAL EASTERN MAINLAND MONROE COUNTY
AND SURROUNDING COMMUNITIES.

FREQUENT TO EXCESSIVE LIGHTNING...GUSTY WINDS FROM 45 TO 55 MPH...UP
TO NICKEL-SIZED HAIL...TORRENTIAL DOWNPOURS...OR A COMBINATION OF
THESE ARE POSSIBLE. LIGHTNING IS THE NUMBER ONE WEATHER RELATED
KILLER IN FLORIDA. TREES AND OPEN SHELTERS OFFER NO PROTECTION. THESE
WINDS CAN DOWN SMALL TREE LIMBS AND BRANCHES...AND BLOW AROUND
UNSECURED SMALL OBJECTS. SEEK SHELTER IN A SAFE BUILDING UNTIL THE
STORM PASSES.

LAT...LON 2538 8094 2562 8089 2549 8058 2531 8067
TIME...MOT...LOC 1819Z 107DEG 7KT 2546 8081

$$

10
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THE 12Z MIAMI SOUNDING SHOWS THAT NOTABLE COOLING HAS OCCURRED ABOVE
600 MB SINCE 00Z...WHICH WHEN COUPLED WITH A COMPARABLY MORE MOIST
BOUNDARY LAYER...WILL RESULT IN A MORE UNSTABLE AIR MASS THAN
YESTERDAY WITH MLCAPE APPROACHING 1500 J/KG. MOREOVER...AN ELY
COMPONENT TO THE LOW-LEVEL WIND FIELD WILL AUGMENT MODEST /20-30 KT/
WLY WINDS IN THE MID LEVELS TO YIELD SUFFICIENT VERTICAL SHEAR FOR
SOME STORM ORGANIZATION.

INCREASED LARGE-SCALE FORCING/DCVA ASSOCIATED WITH A WEAK MIDLEVEL
VORTICITY MAXIMUM CURRENTLY OVER THE CNTRL G.O.M. WILL ENHANCE
LOW-LEVEL CONVERGENCE ALONG DEVELOPING SEA BREEZE FRONTS AND
CONVECTIVE OUTFLOW BOUNDARIES /OBSERVED OVER THE E-CNTRL/SERN
PENINSULA/ TO YIELD SCATTERED TO NUMEROUS STORMS THIS AFTERNOON.
GIVEN THE MODERATE INSTABILITY AND RELATIVELY COOL MIDLEVEL
TEMPERATURES...A RISK FOR ISOLATED DAMAGING WIND GUSTS AND HAIL WILL
EXIST THROUGH THIS EVENING.
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I would be parking the cars undercover,nws said possibility of Quarter sized hailstones..that could do a number on your car huh....
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XX/XX/XX
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Quoting indianrivguy:


is this a pre blob blob alert ?



LOL. Don't worry, IRG. I just categorize them as "splats" I do, however, think it may cause some more problems over the Florida peninsula later today. Splats can do that.
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storms around orlando moving slowly eastward.....
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Nam midweek.....................
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Nam takes that blob(LOW) in the gulf up to nyc as a nor'easter..midweek
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851. Skyepony (Mod)
Midwest sees record floods, road closures, runaway barges, and evacuations

After a week of torrential rains, six Midwestern states are struggling with massive flooding. Two Mississippi River bridges and part of the river have been closed as the waters continue to rise.
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some nice rains inland..............
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Quoting Grothar:


is this a pre blob blob alert ?
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Quoting CybrTeddy:


The Saturn was sure a beauty.



Something about feeling 7.6 million pounds of thrust assaulting the atmosphere in person.... the shuttles were no slouch either..
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Animated

Link
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Quoting weatherbro:
The Southern Oscillation Index(SOI) has gone negative again.

Current SOI Values...

Average for last 30 days 6.2
Average for last 90 days 2.3
Daily contribution to SOI calculation -8.0


It continues to be positive in the 30 day index.

Member Since: April 29, 2009 Posts: 75 Comments: 14906
The Southern Oscillation Index(SOI) has gone negative again.

Current SOI Values...

Average for last 30 days 6.2
Average for last 90 days 2.3
Daily contribution to SOI calculation -8.0
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Re: #783 -- what an awesome shot - look at that gigantic low swirling in the broadest part of the Atlantic Ocean (takes up almost the whole thing)!!

Maaaaajorly psyched here in Cape Coral about the rain we had last night and the thought that more could be on its way (in this, the supposed peak of our annual dry season)! Headed out to play with a batch of palm trees I just got - have a great day!
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We're coming up on the three-year anniversary of the Yazoo City, Mississippi tornado. A 170 mph EF4, it killed 10 and injured at least 146. It was the state's ninth deadliest tornado since records began in 1900, and the state's worst natural disaster since Hurricane Katrina.

The 2011 Super Outbreak, which we know is also coming up on its anniversary, totaled smashed that.

I spent 15 minutes looking for a site that would upload my 3 mb radar/velocity animation of the tornado, but none decided they would do so without reducing the quality of the frames. So, I just provided a still image of the tornado as it moved through Yazoo City. Note the higher reflectivities (colors for a simple word) on the southern part of the city...this is what is referred to as a debris ball. It is indicative of particles...whether that is parts of trees, cars, or buildings...suspended in the air.

The gate-to-gate velocity in this scan is 193.2 knots, or 222.3 mph. This would classify the tornado as an EF5. However, the radar is measuring winds above the surface. As aforementioned, maximum sustained winds at ground level topped out at 170 mph, a midgrade EF4.

Member Since: July 6, 2010 Posts: 113 Comments: 32855
A bunch of clouds broke from the front that was in the GOM yesterday and now (as in comment 803) another might be forming. Then there's that cool swirl in the CATL
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837. beell
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1052 AM CDT SUN APR 21 2013

...FL THIS AFTERNOON AND EVENING...

THE 12Z MIAMI SOUNDING SHOWS THAT NOTABLE COOLING HAS OCCURRED ABOVE
600 MB SINCE 00Z...WHICH WHEN COUPLED WITH A COMPARABLY MORE MOIST
BOUNDARY LAYER...WILL RESULT IN A MORE UNSTABLE AIR MASS THAN
YESTERDAY WITH MLCAPE APPROACHING 1500 J/KG. MOREOVER...AN ELY
COMPONENT TO THE LOW-LEVEL WIND FIELD WILL AUGMENT MODEST /20-30 KT/
WLY WINDS IN THE MID LEVELS TO YIELD SUFFICIENT VERTICAL SHEAR FOR
SOME STORM ORGANIZATION.

INCREASED LARGE-SCALE FORCING/DCVA ASSOCIATED WITH A WEAK MIDLEVEL
VORTICITY MAXIMUM CURRENTLY OVER THE CNTRL G.O.M. WILL ENHANCE
LOW-LEVEL CONVERGENCE ALONG DEVELOPING SEA BREEZE FRONTS AND
CONVECTIVE OUTFLOW BOUNDARIES /OBSERVED OVER THE E-CNTRL/SERN
PENINSULA/ TO YIELD SCATTERED TO NUMEROUS STORMS THIS AFTERNOON.
GIVEN THE MODERATE INSTABILITY AND RELATIVELY COOL MIDLEVEL
TEMPERATURES...A RISK FOR ISOLATED DAMAGING WIND GUSTS AND HAIL WILL
EXIST THROUGH THIS EVENING.

Clouds/T-storms associated with the central GOM mid-level impulse showing up on wv.





MESOSCALE DISCUSSION 0547
NWS STORM PREDICTION CENTER NORMAN OK
1154 AM CDT SUN APR 21 2013

AREAS AFFECTED...PORTIONS CENTRAL/SRN FL.

CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY

VALID 211654Z - 211930Z

PROBABILITY OF WATCH ISSUANCE...20 PERCENT

SUMMARY...SEA BREEZE CONVECTION FCST TO INCREASE IN
COVERAGE/INTENSITY...BECOMING NUMEROUS THIS AFTN PER SPC ENHANCED
TSTM OUTLOOK. MAIN THREAT WILL BE ISOLATED DAMAGING
DOWNDRAFTS...WITH MRGL SVR HAIL ALSO POSSIBLE. FOCI FOR LOCALLY
ENHANCED RISK SHOULD BE INVOF OUTFLOW-OUTFLOW AND OUTFLOW-SEA BREEZE
BOUNDARY INTERSECTIONS. ATTM...POTENTIAL ORGANIZATION/COVERAGE
APPEARS TOO LOW FOR WW.

DISCUSSION...16Z SFC MESOANALYSIS INDICATES WEAKENING/ORIGINAL
SYNOPTIC FRONT IS QUASISTATIONARY OVER SRN FL...ACROSS PORTIONS PALM
BEACH/HENDRY/COLLIER COUNTIES...AND MAINLY IS MANIFEST AS WIND SHIFT
LINE AWAY FROM SEA BREEZE/OUTFLOW BOUNDARIES. SECOND/STRONGER
BAROCLINIC ZONE...WHICH ALSO WAS EVIDENT IN 12Z 925-MB
ANALYSIS...HAS BECOME BETTER EVIDENT AT SFC AMIDST DIFFERENTIAL
DIABATIC HEATING AND DEEP ELYS TO ITS S. LATTER FRONTAL ZONE WAS
LOCATED NEAR I-4 CORRIDOR DAB-TBW AND MAY DRIFT NWWD. VIS IMAGE
LOOPS AND SFC DATA INDICATE OUTFLOW BOUNDARIES FROM EARLIER
CONVECTION OVER GULF STREAM HAS EFFECTIVELY BOWED SEA BREEZE
BOUNDARY INLAND FROM NRN ST LUCIE INTO ERN OSCEOLA COUNTIES...AS
WELL AS OVER SRN PALM BEACH...CENTRAL BROWARD AND NRN MIAMI-DADE
COUNTIES. CONVECTION ALREADY HAS FORMED ALONG THAT BOUNDARY OVER
INTERIOR BROWARD/MIAMI-DADE COUNTY LINE...WITH ADDITIONAL TSTMS
EXPECTED FARTHER N UP E COAST SEA BREEZE IN NEXT 2-3 HOURS.

PW S OF NRN FRONT AND INLAND FROM SEA BREEZES COMMONLY IS IN
1.5-1.75 INCH RANGE AND LOCALLY GREATER...BASED ON GPS-PW
READINGS...MODIFIED RAOBS AND FCST SOUNDINGS. CONTINUED DIABATIC
HEATING IN MOISTURE-RICH/PRECONVECTIVE/INLAND AIR MASS ALREADY IS
RAISING 0-3 KM CAPE SUBSTANTIALLY AND REMOVING CINH...AND SHOULD
BOOST MLCAPE INTO 1500-2000 J/KG RANGE AWAY FROM TSTMS/OUTFLOW.
THIS...ALONG WITH DEPTH OF BUOYANT LAYER IN SOUNDINGS...INDICATES
POTENTIAL FOR LOCALLY STG PRECIP LOADING AND RELATED RISK FOR
DOWNBURSTS NEAR SVR LEVELS AND CAPABLE OF CAUSING DAMAGE.
KINEMATICALLY...WEAK/CHAOTIC 0-3 KM AGL FLOW YIELDS WEAK LOW-LEVEL
SHEAR FROM VWP. HOWEVER...STG UPPER/ANVIL-LEVEL WINDS IN MORNING
RAOBS AND FCST SOUNDINGS SUGGESTS ENHANCED CONVECTIVE VENTILATION
ALOFT AND RELATED CONTRIBUTION TO STORM MAINTENANCE/ORGANIZATION.

..EDWARDS/MEAD.. 04/21/2013
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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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