Flash Flooding and Lightning Strike in Florida, May 20, 2013

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MIAMI FL
912 AM EDT MON MAY 20 2013

.UPDATE...
HAVE UPDATED GRIDS TO REFLECT CURRENT SHOWER ACTIVITY OVER PALM
BEACH COUNTY, AS WELL AS TWEAKS TO OTHER ELEMENTS. THIS MORNINGS
SOUNDING DOES SHOW THE 500MB TEMP UP TO AROUND -8, WITH A WEAK
CAP. THIS SHOULD ERODE EARLY THIS AFTERNOON, ALLOWING BETTER
DEVELOPMENT OF CONVECTIVE CELLS. SO, MAINLY RAIN SHOWERS THIS
MORNING, WITH POSSIBLY A FEW STRIKES OF LIGHTNING, ALTHOUGH HAVE
YET TO PICK UP ANY THIS MORNING. BUT, THEY CAN NOT BE RULED OUT.
COVERAGE WILL INCREASE LATER TODAY AND SPREAD WESTWARD. THE HRRR IS
SHOWING THAT THE ACTIVITY MAY ACTUALLY BE IN THE WESTERN HALF OF
SOUTH FLORIDA, SO ANOTHER UPDATE MAY BE FORTH COMING LATER THIS
MORNING, IF THE MESOSCALE MODELS CONTINUE TO SHOW THIS TREND OF
CONVECTION OCCURRING TO THE WEST, RATHER THAN THE EAST COAST.

&&

.PREV DISCUSSION... /ISSUED 738 AM EDT MON MAY 20 2013/

AVIATION...
A SHORTWAVE TROUGH COMBINED WITH PLENTY OF MOISTURE...DAYTIME
HEATING AND LIGHT AND VARIABLE LOW-LEVEL FLOW WILL KEEP THE RAINFALL
CHANCES UP TODAY ACROSS SOUTH FLORIDA...ESPECIALLY THROUGH THE
AFTERNOON AND EVENING PERIODS. ADDITIONALLY...A FEW SHOWERS THAT ARE
CURRENTLY DEVELOPING OVER THE OFFSHORE ATLANTIC WATERS WILL CONTINUE
TO TRACK WEST AND MAY BEGIN TO SPREAD ASHORE THROUGH THE MORNING
HOURS. INCLUDED VCSH FOR THE EAST COAST TERMINALS THROUGH THE
MORNING PERIOD AND VCTS THROUGH THE AFTERNOON/EVENING PERIODS FOR
ALL SITES. PERIODS WITH SUB-VFR CONDITIONS WILL REMAIN POSSIBLE IN
AND AROUND THIS ACTIVITY TODAY.

PREV DISCUSSION... /ISSUED 357 AM EDT MON MAY 20 2013/

DISCUSSION...
MAIN FOCUS FOR THIS FORECAST PACKAGE IS THE POSSIBILITY FOR STRONG
TO SEVERE THUNDERSTORMS ACROSS THE AREA THIS AFTERNOON.

DEEPER MOISTURE OVER THE ATLANTIC WATERS CONTINUES TO PRODUCE
ISOLATED SHOWERS EARLY THIS MORNING. THESE SHOWERS SHOULD CONTINUE
TO SLOWLY PUSH TOWARD THE EAST COASTAL AREAS UNDER WEAK EASTERLY
FLOW. HEADING INTO THE PEAK HEATING HOURS OF THE DAY THE FOCUS WILL
SHIFT TO POSSIBLE STRONG TO SEVERE THUNDERSTORM DEVELOPMENT ACROSS
MUCH OF THE AREA. DEEP MOISTURE ADVECTION INTO THE SOUTH FLORIDA
PENINSULA WILL CONTINUE THROUGH THE LATE MORNING AND EARLY AFTERNOON
HOURS. AS A RESULT...MODEL SOUNDINGS DEPICT PWAT VALUES AS HIGH AS
1.75 INCHES. THIS COMBINED WITH WEAK OVERALL STEERING FLOW WILL
ALLOW FOR VERY HEAVY RAINFALL AND THE POSSIBILITY OF LOCALIZED
FLOODING ASSOCIATED WITH ANY STORMS THAT DEVELOP.

THERMODYNAMIC PARAMETERS ARE ALSO EXPECTED TO BE FAVORABLE WITH CAPE
VALUES APPROACHING 3000 J/KG AND LIFTED INDICES RANGING FROM -6C TO
-8C WITH LITTLE IF ANY CAPPING. NORMALLY THIS WOULD BE ENOUGH TO
SPARK A FEW STRONG STORMS...BUT A FAIRLY BROAD H5 SHORTWAVE IS ALSO
EXPECTED TO SLIDE ACROSS THE FLORIDA PENINSULA THIS AFTERNOON WHICH
WILL ACT TO ENHANCE THUNDERSTORM DEVELOPMENT. THIS SHORTWAVE WILL
PROVIDE EXTRA COOLING ALOFT NEEDED FOR STRONG TO SEVERE
THUNDERSTORMS. POINT SOUNDINGS ACROSS THE AREA DEPICT 500 MB
TEMPERATURES AROUND -12C WHICH IS SEVERAL DEGREES COLDER THAN THE
AVERAGE 500 MB TEMPERATURE FOR THIS DATE. ALL OF THESE PARAMETERS
COMBINED WILL AIDE IN THE DEVELOPMENT OF ISOLATED SEVERE
THUNDERSTORMS WITH THE MAIN IMPACTS BEING GUSTY WINDS...LARGE
HAIL...AND LOCALIZED FLOODING. CURRENT THINKING IS THAT
THUNDERSTORMS WILL INITIATE ACROSS THE NORTHERN INTERIOR AND WITH
SUCH WEAK STEERING FLOW...SHOULD BE MAINLY OUTFLOW DRIVEN AND DRIFT
SLOWLY TOWARD THE EAST COAST METRO AREAS.

KEPT CHANCES FOR THUNDERSTORMS OVERNIGHT TONIGHT AND INTO TUESDAY
MORNING AS MID LEVEL DYNAMICS SHOULD PERSIST WITH THE LINGERING
SHORTWAVE. ANOTHER ROUND OF SHOWERS AND THUNDERSTORMS WILL BE LIKELY
TUESDAY AFTERNOON BUT 500 MB TEMPERATURES ARE EXPECTED TO BE A BIT
WARMER WITH SLIGHTLY BETTER LOW LEVEL CAPPING. STILL COULD NOT RULE
OUT AN ISOLATED STRONG STORM WITH DIURNAL HEATING AND DEEP MOISTURE
STILL IN PLACE.

AFTERNOON SHOWER AND THUNDERSTORM CHANCES WILL PERSIST THROUGH THE
REMAINDER OF THE WEEK AND INTO THE WEEKEND AS THE H5 SHORTWAVE
LINGERS ACROSS THE FLORIDA PENINSULA. IT IS EXPECTED TO SLOWLY
RETREAT EACH DAY ALLOWING HIGHER H5 HEIGHTS TO SLOWLY BUILD INTO THE
AREA WHILE STILL REMAINING IN A FAIRLY WET PATTERN.

MARINE...
NO MARINE ISSUES ARE EXPECTED OUTSIDE OF ANY THUNDERSTORMS THAT
AFFECT THE LOCAL WATERS. LIGHT TO MODERATE EAST AND SOUTHEAST
WINDS WILL CONTINUE THROUGH AT LEAST THE EARLY HALF OF THE WEEK
ACROSS THE ATLANTIC WATERS WHILE REMAINING MORE LIGHT AND VARIABLE
ACROSS THE GULF WATERS. THROUGH THE LATTER HALF OF THE WEEK WINDS
ARE EXPECTED TO BECOME LIGHT ACROSS ALL LOCAL WATERS.

FIRE WEATHER...
DEEP MOISTURE WILL REMAIN IN PLACE ACROSS THE SOUTH FLORIDA
PENINSULA THROUGH THE PERIOD KEEPING MINIMUM RELATIVE HUMIDITY
LEVELS WELL ABOVE CRITICAL THRESHOLDS.

&&

.PRELIMINARY POINT TEMPS/POPS...
WEST PALM BEACH  86  73  86  73 / 60 40 60 40
FORT LAUDERDALE  85  75  86  76 / 60 40 50 40
MIAMI            86  74  87  75 / 50 40 50 30
NAPLES           88  71  88  71 / 30 30 40 20

&&

.MFL WATCHES/WARNINGS/ADVISORIES...
FL...NONE.
AM...NONE.
GM...NONE.
&&

DISCUSSION/MARINE/FIRE...13/SI
AVIATION/RADAR...84/AK

HAZARDOUS WEATHER OUTLOOK FOR SOUTH FLORIDA
NATIONAL WEATHER SERVICE MIAMI FL
1045 AM EDT MON MAY 20 2013

AMZ610-630-650-651-670-671-FLZ063-066>075-168-172>174-GMZ656-657-676-
210545-
LAKE OKEECHOBEE-BISCAYNE BAY-
COASTAL WATERS FROM JUPITER INLET TO DEERFIELD BEACH, FL OUT 20 NM-
COASTAL WATERS FROM DEERFIELD BEACH TO OCEAN REEF, FL OUT 20 NM-
WATERS FROM JUPITER INLET TO DEERFIELD BEACH, FL EXTENDING FROM
20 NM TO 60 NM-
WATERS FROM DEERFIELD BEACH TO OCEAN REEF, FL EXTENDING FROM 20 NM
TO THE TERRITORIAL WATERS OF THE BAHAMAS-GLADES-HENDRY-
INLAND PALM BEACH-METRO PALM BEACH-COASTAL COLLIER-INLAND COLLIER-
INLAND BROWARD-METRO BROWARD-INLAND MIAMI DADE-METRO MIAMI DADE-
MAINLAND MONROE-COASTAL PALM BEACH-COASTAL BROWARD-
COASTAL MIAMI DADE-FAR SOUTH MIAMI DADE-
COASTAL WATERS FROM CHOKOLOSKEE TO BONITA BEACH, FL OUT 20 NM-
COASTAL WATERS FROM EAST CAPE SABLE TO CHOKOLOSKEE, FL OUT 20 NM-
GULF WATERS FROM CHOKOLOSKEE TO BONITA BEACH, FL EXTENDING FROM
20 TO 60 NM-
1045 AM EDT MON MAY 20 2013

...STRONG TO SEVERE STORMS POSSIBLE TODAY...
...LOCALLY HEAVY RAINFALL AND FLOODING POSSIBLE...

THIS HAZARDOUS WEATHER OUTLOOK IS FOR SOUTH FLORIDA.

.DAY ONE...TODAY AND TONIGHT

THUNDERSTORMS: SCATTERED TO NUMEROUS THUNDERSTORMS ARE EXPECTED
TODAY ACROSS SOUTH FLORIDA. THE MAIN THREATS WILL BE LOCALLY HEAVY
RAINFALL...FREQUENT CLOUD TO GROUND LIGHTNING...GUSTY WINDS AND
HAIL. A FEW STORMS MAY BECOME STRONG TO SEVERE THIS AFTERNOON.

FLOODING: LOCALLY HEAVY RAINFALL AND PONDING OF WATER IN POORLY
DRAINED LOCATIONS IS POSSIBLE.

WIND: WINDS OF 40 TO 60 MPH ARE POSSIBLE WITHIN THE STRONGEST STORMS
THIS AFTERNOON.

HAIL: UP TO THE SIZE OF QUARTERS IS POSSIBLE WITHIN STRONGEST
STORMS THIS AFTERNOON.

RIP CURRENTS: THERE IS A SLIGHT RISK OF RIP CURRENTS ALONG THE
ATLANTIC BEACHES TODAY.

.DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SUNDAY

THERE IS THE POTENTIAL FOR ISOLATED STRONG TO SEVERE THUNDERSTORMS
ACROSS THE INTERIOR AND EAST COAST OF SOUTH FLORIDA ONCE AGAIN ON
TUESDAY. THE MAIN IMPACTS WILL BE STRONG WINDS AND HAIL ALONG WITH
FREQUENT LIGHTNING STRIKES. THE STORMS WILL BE SLOW MOVERS...WHICH
MAY RESULT IN LOCALIZED FLOODING WHERE THE HEAVIER ACTIVITY BECOMES
CONCENTRATED.

THUNDERSTORMS WILL REMAIN POSSIBLE EACH DAY THROUGH THE REMAINDER
OF THE WEEK LIGHTNING...GUSTY WINDS AND LOCALIZED HEAVY RAINS
BEING THE MAIN THREATS.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION IS NOT ANTICIPATED, HOWEVER INDIVIDUAL
SPOTTERS ARE ENCOURAGED TO REPORT HIGH WIND, HAIL AND FLOODING TO
THE NATIONAL WEATHER SERVICE FORECAST OFFICE IN MIAMI.

FOR MORE INFORMATION...VISIT THE NATIONAL WEATHER SERVICE IN
MIAMI WEBSITE AT WWW.WEATHER.GOV/MIAMI.