Very Unstable Air pushing up from the GOM, Tornado Watch for South FL
Posted by JAC on 12/18/2009, 7:43 am






URGENT - IMMEDIATE BROADCAST REQUESTED
  TORNADO WATCH NUMBER 804
  NWS STORM PREDICTION CENTER NORMAN OK
  820 AM EDT FRI DEC 18 2009
 
  THE NWS STORM PREDICTION CENTER HAS ISSUED A
  TORNADO WATCH FOR PORTIONS OF
 
         SOUTH FLORIDA AND THE FLORIDA KEYS
         COASTAL WATERS
 
  EFFECTIVE THIS FRIDAY MORNING AND AFTERNOON FROM 820 AM UNTIL 200
  PM EDT.
 
  TORNADOES...HAIL TO 0.5 INCH IN DIAMETER...THUNDERSTORM WIND
  GUSTS TO 70 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE
  AREAS.
 
  THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 80 STATUTE
  MILES NORTH AND SOUTH OF A LINE FROM 40 MILES SOUTH OF NAPLES
  FLORIDA TO 20 MILES SOUTHEAST OF WEST PALM BEACH FLORIDA.  FOR A
  COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE
  UPDATE (WOUS64 KWNS WOU4).

DISCUSSION...A BROKEN BAND OF THUNDERSTORMS...SOME OF WHICH ARE
  SUPERCELLS...WILL CONTINUE TO SPREAD NWD/NEWD ACROSS S FL AS A
  SURFACE LOW DEEPENS ACROSS THE NE GULF OF MEXICO.  A GRADUAL
  INCREASE IN LOW-MID LEVEL WIND SPEEDS AND VERTICAL SHEAR...ALONG
  WITH LOW-LEVEL MOISTENING AND SOME SURFACE HEATING IN SMALL CLOUD
  BREAKS...WILL CONTRIBUTE TO AN INCREASING THREAT FOR SUPERCELLS AND
  A COUPLE OF TORNADOES THROUGH THE MORNING.





MESOSCALE DISCUSSION 2255
  NWS STORM PREDICTION CENTER NORMAN OK
  0539 AM CST FRI DEC 18 2009
 
  AREAS AFFECTED...S FL...FL KEYS...AND NARROW PORTIONS ERN AND WRN
  COASTAL REGIONS NWD TO CENTRAL FL.
 
  CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE
 
  VALID 181139Z - 181345Z
 
  INLAND COVERAGE OF SFC-BASED TSTMS...SOME OF WHICH MAY BE
  SUPERCELLS...SHOULD INCREASE THROUGH AT LEAST 15Z...AS HIGHER-THETAE
  MARINE AIR OVERSPREADS GREATER PORTION OF FL...ESPECIALLY COASTAL
  AND INLAND AREAS NEAR AND S OF LAKE OKEECHOBEE.  
 
  SFC MESOANALYSIS SHOWS SYNOPTIC WARM FRONTAL ZONE ACROSS NRN FL WITH
  NOCTURNALLY STABILIZED AND SOMEWHAT DRIER AIR MASS LINGERING ACROSS
  MOST OF INTERIOR PENINSULA.  MLCAPE IS ESTIMATED AROUND 1000 J/KG
  INVOF KEYS...MIA AND SRN EVERGLADES...DIMINISHING NWD WITH RAPID
  INCREASE IN CINH W OF LAKE OKEECHOBEE.  AS LARGE-SCALE ASCENT
  INCREASES -- BOTH IN FORM OF LOW LEVEL WAA AND MIDLEVEL DPVA -- MORE
  FAVORABLE INSTABILITY AND GREATER TSTM COVERAGE SHOULD RESULT...
  INCLUDING POTENTIAL FOR SUPERCELLS SUCH AS THOSE ALREADY OBSERVED
  OVER GULF AND STRAITS SURROUNDING S FL AND KEYS.  LOW-LEVEL
  HODOGRAPHS...MID-UPPER FLOW...AND DEEP-LAYER SHEAR IN THIS AIR MASS
  EACH ARE FCST TO STRENGTHEN GRADUALLY THROUGH MID-LATE MORNING AS
  MID-UPPER TROUGH NOW OVER LOWER DELTA REGION AND CENTRAL GULF LIFTS
  NEWD.  
 
  FARTHER N UP E COAST...POTENTIAL WILL REMAIN MRGL/CONDITIONAL FOR A
  FEW MORE HOURS...WITH CONDITIONS REMAINING VERY NEARLY AS DISCUSSED
  IN PREVIOUS MCD 2254 BUT SLOWLY SHIFTING NWD.  STRONGEST-RIGHTWARD
  CELL DEVIANCE WILL OCCUR WITH MOST TORNADO-PRONE STORMS...BUT ALSO
  WILL TEND TO FAVOR OFFSHORE TRANSLATION GIVEN ORIENTATION OF
  COASTLINE.
 
  FARTHER N UP W COAST...AIR MASS WILL TAKE LONGER TO DESTABILIZE.
  COMBINATION OF LAND BREEZE AND AMBIENT/ISALLOBARIC FORCING CONTINUES
  ELY COMPONENT...WITH LATTER BEING DOMINANT PROCESS.  GRADUAL VEERING
  OF NEAR-COASTAL WINDS WILL PERMIT MORE FAVORABLE MARINE AIR MASS NOW
  BEING SHUNTED OFFSHORE TO APCH COASTAL AREAS AND RENDER RELATIVELY
  STABLE/LOW-DEWPOINT LAYER OF INLAND AIR MORE SHALLOW.  SVR THREAT
  FROM FMY VICINITY NWD ACCORDINGLY WILL TAKE LONGER TO DEVELOP...BUT
  MAY REQUIRE WW LATER THIS MORNING.
 
  ..EDWARDS.. 12/18/2009
82
In this thread:
Very Unstable Air pushing up from the GOM, Tornado Watch for South FL - JAC, 12/18/2009, 7:43 am
< Return to the front page of the: message board | monthly archive this page is in
Post A Reply
This thread has been archived and can no longer receive replies.