566
AXNT20 KNHC 171204
TWDAT
TROPICAL WEATHER DISCUSSION
NWS NATIONAL HURRICANE CENTER MIAMI FL
804 AM EDT MON SEP 17 2018
TROPICAL WEATHER DISCUSSION FOR NORTH AMERICA, CENTRAL AMERICA
GULF OF MEXICO, CARIBBEAN SEA, NORTHERN SECTIONS OF SOUTH
AMERICA, AND ATLANTIC OCEAN TO THE AFRICAN COAST FROM THE
EQUATOR TO 32N. THE FOLLOWING INFORMATION IS BASED ON SATELLITE
IMAGERY, WEATHER OBSERVATIONS, RADAR AND METEOROLOGICAL ANALYSIS.
BASED ON 0600 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH
1115 UTC.
...SPECIAL FEATURES
TROPICAL DEPRESSION JOYCE IS CENTERED NEAR 34.2N 329.0W AT 17/0900
UTC OR 250 NM SW OF THE AZORES MOVING E AT 15 KT. ESTIMATED MINIMUM
CENTRAL PRESSURE IS 1008 MB. MAXIMUM SUSTAINED WIND SPEED IS 30 KT
WITH GUSTS TO 40 KT. LATEST SATELLITE IMAGERY SHOWS THAT JOYCE IS A
SHEARED SYSTEM UNDER STRONG WEST TO NORTHWEST VERTICAL SHEAR AS
NOTED BY ITS DEEP CONVECTION BEING SHEARED OFF TO THE NORTHEAST OF
ITS CENTER. THE CONVECTION IS DEFINED AS THE MODERATE TYPE INTENSITY
WITHIN 30 NM EITHER SIDE OF A LINE FROM 35N25W TO 35N29W. OVERCAST
TO BROKEN LOW AND MID-LEVEL CLOUDS WITH EMBEDDED SCATTERED SHOWERS
AND ISOLATED THUNDERSTORMS ARE WELL TO THE EAST AND NORTHEAST OF
JOYCE FROM 34N TO 38N BETWEEN 20W AND 25W. STRONG SOUTHWEST TO WEST
WINDS ALOFT AND DRY AIR IN ITS SURROUNDING ENVIRONMENT WILL CONTINUE
TO HAMPER JOYCE OVER THE NEXT FEW DAYS CAUSING IT TO GRADUALLY
WEAKEN. JOYCE IS FORECAST BY THE LATEST NHC ADVISORY TO SLOW ITS
FORWARD MOTION WHILE TURNING TOWARD THE SOUTHEAST TODAY, SOUTH ON
TUE, AND SOUTHWEST ON WED. SEE THE LATEST NHC FORECAST/ADVISORY
UNDER AWIPS/WMO HEADERS MIATCPAT5/WTNT35 KNHC FOR MORE DETAILS.
..TROPICAL WAVES
A CENTRAL ATLANTIC TROPICAL WAVE HAS ITS AXIS EXTENDING ALONG 43W
FROM 04N AND 20N, MOVING WESTWARD AT 15-20 KT. MODEL GUIDANCE
INDICATES THAT THE WAVE IS MOVING THROUGH A PRETTY WELL PRONOUNCED
700 MB JET STREAM BRANCH ACCOMPANIED BY A JET CORE OF 30-45 KT
EAST WINDS TO THE EAST OF THE WAVE AXIS. THE WAVE MARKS THE
LEADING EDGE OF QUITE AN EXTENSIVE AREA OF SAHARAN AIR FOR THIS
TIME DURING THE SEASON. THE AFRICAN DUST EXTENDS FROM 10N TO 25N
EAST OF THE WAVE TO AFRICA. DUST THAT SURROUNDED THE WAVE HAS
THINNED OUT SINCE YESTERDAY AS A SURGE OF LOW-LEVEL MOISTURE IS
ON EITHER SIDE OF THE WAVE SOUTH OF 15N TO THE ITCZ. THIS
MOISTURE SURGE CONSISTS OF SCATTERED SHOWERS AND THUNDERSTORMS
MOVING QUICKLY WESTWARD FROM 10N TO 15N BETWEEN 38W AND 44W, AND
WITHIN 120 NM EITHER SIDE OF THE WAVE IN THE VICINITY OF THE ITCZ
REGION FROM 04N TO 09N.
..MONSOON TROUGH/ITCZ
THE MONSOON TROUGH AXIS EXTENDS FROM WELL INLAND AFRICA SOUTHWESTWARD
TO THE COASTLINE OF NORTHWEST SENEGAL NEAR 17N16W AND CONTINUES
TO 09N22W. THE ITCZ EXTENDS FROM 09N22W TO 06N30W TO JUST WEST OF
THE TROPICAL WAVE ALONG 43W. IT RESUMES AT 08N43W TO 08N58W.
ASIDE FROM THE SHOWER AND THUNDERSTORMS ACTIVITY ASSOCIATED WITH
THE TROPICAL WAVE, SCATTERED MODERATE ISOLATED STRONG CONVECTION
IS SEEN WITHIN 210 NM NORTH OF THE ITCZ BETWEEN 53W AND 58W,
WITHIN 180 NM SOUTH OF THE ITCZ BETWEEN 45W AND 49W, AND WITHIN 60
NM SOUTH OF THE ITCZ BETWEEN 51W AND 53W. SIMILAR CONVECTION IS
NORTHWEST OF THE ITCZ WITHIN 60 NM OF A LINE FROM 09N58W TO THE
COAST OF S AMERICA AT 08N60W. SCATTERED MODERATE CONVECTION IS
WITHIN 150 NM NORTH OF THE ITCZ BETWEEN 48W AND 51W.
..DISCUSSION
GULF OF MEXICO...
IN THE UPPER-LEVELS, AND ELONGATED UPPER LOW IS OVER THE WESTERN
GULF NEAR 25N93W, WITH A TROUGH EXTENDING NORTHEASTWARD TO
INLAND SOUTHEASTERN LOUISIANA AND CONTINUES WELL NORTHEASTWARD OF
THE AREA. ANOTHER TROUGH EXTENDS SOUTHEASTWARD FROM THE LOW TO
THE EASTERN BAY OF CAMPECHE AND THE SOUTHWEST YUCATAN PENINSULA.
UPPER-LEVEL CYCLONIC FLOW COVERS THE WESTERN GULF W OF 90W. AN
UPPER LEVEL HIGH IS CENTERED OVER THE EASTERN GULF NEAR 25N84W.
UPPER-LEVEL ANTICYCLONIC FLOW COVERS THE EASTERN GULF. AT THE
SURFACE, A TROUGH IS JUST INLAND THE COAST OF MEXICO FROM 20N TO
NEAR SAN FERNANDO. PLENTY OF ATMOSPHERIC INSTABILITY ALONG WITH
DEEP MOISTURE IN PLACE IS LEADING TO SCATTERED SHOWERS AND
ISOLATED THUNDERSTORMS SOUTH OF 25N AND WEST OF 94W TO INLAND
MEXICO. A 1015 MB HIGH IS CENTERED OVER THE NW GULF NEAR 27N93W.
RELATIVELY WEAK HIGH PRESSURE IS ACROSS THE BASIN. A SURFACE
TROUGH EXTENDS SOUTHWESTWARD FROM WHAT IS NOW TROPICAL DEPRESSION
FLORENCE LOCATED OVER THE BORDER OF EASTERN GEORGIA, SOUTHWESTWARD
TO THE FLORIDA PANHANDLE. SCATTEROMETER DATA HIGHLIGHTED LIGHT TO
GENTLE ANTICYCLONIC FLOW WEST OF 87W, AND LIGHT TO MODERATE SOUTH
TO SOUTHWEST FLOW EAST OF 87W. LITTLE CHANGE IN SURFACE GULF
WINDS IS EXPECTED THROUGH THE MIDDLE PART OF THIS WEEK.
CARIBBEAN SEA...
A SURFACE TROUGH EXTENDS ACROSS THE CENTRAL CARIBBEAN FROM JUST
WEST OF HAITI TO A 1009 MB LOW NEAR 17.5N76W, AND SOUTHEASTWARD
TO NEAR 13N74W. THESE FEATURES AND RELATED CONVECTION DENOTE A
BROAD AREA OF LOW PRESSURE ASSOCIATED WITH THE FORMER TROPICAL
CYCLONE ISAAC. ABUNDANT MOISTURE AND ATMOSPHERIC INSTABILITY, THAT
IS FURTHER ENHANCED BY THE TAIL-END OF A CENTRAL ATLANTIC DEEP
LAYER TROUGH THAT EXTENDS TO HAITI AND TO NEAR 17N85W, REMAINS
ENTRENCHED ACROSS THE CENTRAL CARIBBEAN AND MUCH OF THE EASTERN
CARIBBEAN. THIS IS RESULTING IN SCATTERED MODERATE TO STRONG
CONVECTION FROM 16N TO 18N BETWEEN 75W AND 76W, INCLUDING THE
EASTERN PORTION OF JAMAICA. SCATTERED SHOWERS AND ISOLATED
THUNDERSTORMS ARE ELSEWHERE BETWEEN 65W AND 80W, WHILE ISOLATED
SHOWERS AND THUNDERSTORMS ARE EAST OF 65W. THE BROAD AREA LOW
PRESSURE REMAINS DISORGANIZED. ONLY SLOW DEVELOPMENT WILL BE
POSSIBLE DURING THE NEXT DAY OR SO DUE TO PROXIMITY TO DRY AIR AND
LAND INTERACTION WITH JAMAICA. BY WED, ENVIRONMENTAL CONDITIONS
ARE FORECAST TO BECOME LESS CONDUCIVE FOR RE-DEVELOPMENT TO OCCUR.
REGARDLESS OF DEVELOPMENT, LOCALLY HEAVY RAINS AND GUSTY WINDS
WILL BE POSSIBLE OVER PORTIONS OF HISPANIOLA, JAMAICA, AND EASTERN
CUBA DURING THE NEXT COUPLE OF DAYS WHILE THE SYSTEM MOVES WEST-
NORTHWESTWARD TO WESTWARD TOWARD THE NORTHWESTERN CARIBBEAN SEA.
GENTLE TRADES ARE WEST OF THE TROUGH AND LOW, WHILE, GENTLE TO
MODERATE TRADES ARE ELSEWHERE EAST OF THE TROUGH. THE CENTRAL
ATLANTIC TROPICAL WAVE WILL MOVE ACROSS THE TROPICAL WATERS EAST
OF THE WINDWARD ISLANDS TUE AND TUE NIGHT, AND THROUGH THE EAST
CARIBBEAN ON WED AND THU ACCOMPANIED BY FRESH TO STRONG TRADES AND
BUILDING SEAS.
ATLANTIC OCEAN...
PLEASE REFER TO THE SPECIAL FEATURES SECTION ABOVE, FOR DETAILS
ON TROPICAL DEPRESSION JOYCE.
A RATHER STRONG AND BROAD DEEP-LAYER TROUGH EXTENDS ACROSS THE
CENTRAL ATLANTIC FROM A LARGE UPPER-LEVEL LOW, CENTERED NEAR
27N65W. PLENTY OF INSTABILITY EXISTS WITH THIS TROUGH AS IT ACTS
ON A VERY MOIST ATMOSPHERE. THE UPPER-LEVEL LOW IS FORECAST TO
GRADUALLY MOVE WESTWARD TODAY, THEN MOVE IN A NORTHWARD DIRECTION
TONIGHT THROUGH TUE. AT THE SURFACE, A NEARLY STATIONARY 1013 MB
LOW IS NEAR 24N65W, WITH A TROUGH EXTENDING NORTHEASTWARD TO NEAR
30N59W, AND SOUTHWESTWARD TO NEAR 20N70W. TO THE EAST OF THIS
SYSTEM, ANOTHER SURFACE TROUGH EXTENDS FROM 29N50W TO NEAR 24N61W.
ALL OF THESE FEATURES ACTING ON THE PRESENT VERY MOIST AND
UNSTABLE ATMOSPHERE ARE LEADING TO THE DEVELOPMENT OF NUMEROUS
SHOWERS AND THUNDERSTORMS OVER THE WATERS FROM 19N TO 26N BETWEEN
59W AND 65W, AND FROM 18N TO 23N BETWEEN 65W AND 74W, INCLUDING
THE WINDWARD PASSAGE AND VICINITY WATERS. SCATTERED SHOWERS AND
THUNDERSTORMS ARE ALSO EVIDENT FROM 18N TO 23N BETWEEN 54W AND
59W, AND FROM 26N TO 32N BETWEEN 50W AND 56W. ANY OF THIS ACTIVITY
IS CAPABLE OF PRODUCING VERY HEAVY RAINFALL AS WELL AS FREQUENT
LIGHTNING STRIKES. DRIER AIR IS PRESENT TO THE WEST OF THE UPPER-
LEVEL TROUGH AND LOW, WHERE ONLY ISOLATED SHOWERS AND
THUNDERSTORMS ARE NOTED.
A 1022 MB HIGH IS CENTER OVER THE CENTRAL ATLANTIC NEAR 28N39W,
WITH A RIDGE EXTENDING SOUTHWESTWARD TO NEAR 20N60W. RATHER DRY
AND STABLE CONDITIONS ARE PRESENT EAST OF 50W, EXCEPT NEAR THE
TROPICAL WAVE ALONG 43W AS DESCRIBED ABOVE. ALSO AS STATED ABOVE,
A RATHER EXTENSIVE BATCH OF SAHARAN AIR IS OBSERVED OVER THE
EASTERN ATLANTIC FROM 10N TO 25N BETWEEN 15W AND 45W. THESE
CONDITIONS ARE CONTRIBUTING TO THE ALREADY DRY AND STABLE
CONDITIONS SUPPORTED BY THE HIGH PRESSURE THAT IN PLACE OVER THAT
PART OF THE ATLANTIC. THE DUST WILL CONTINUE TO MIGRATE WESTWARD
DURING THE NEXT FEW DAYS.
FOR ADDITIONAL INFORMATION PLEASE VISIT
HTTP://WWW.HURRICANES.GOV/MARINE
AGUIRRE/FORMOSA
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