[Tropical] Tropical Weather Discussion

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AXNT20 KNHC 201750
TWDAT

TROPICAL WEATHER DISCUSSION
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
205 PM EDT THU AUG 20 2009

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...METEOROLOGICAL ANALYSIS...WEATHER
OBSERVATIONS...AND RADAR.

BASED ON 1200 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH
1715 UTC.

...SPECIAL FEATURES...
LARGE HURRICANE BILL IS CENTERED NEAR 22.6N 61.7W OR ABOUT 330
NM...610 KM...NORTH OF THE LEEWARD ISLANDS MOVING NW OR 305 DEG
AT 16 KT. THE LATEST MINIMUM CENTRAL PRESSURE REPORTED BY A NOAA
P-3 HURRICANE RECONNAISSANCE AIRCRAFT WAS 951 MB. MAXIMUM
SUSTAINED WIND SPEED IS 105 KT WITH GUSTS TO 130 KT. SEE THE
LATEST FORECAST/ADVISORY UNDER AWIPS/WMO HEADERS TCMAT3/WTNT23
KNHC FOR MORE DETAILS. ALTHOUGH BILL WEAKENED SLIGHTLY TO A
CATEGORY 3 STORM TODAY...REINTENSIFICATION IS FORECAST DUE TO
ITS PASSING OVER THE WARMER WATERS AND LIGHTER WIND SHEAR.
HURRICANE BILL CONTINUES TO DISPLAY A WELL DEFINED EYE WITH
NUMEROUS STRONG CONVECTION WITHIN 60 NM NE AND 80 NM SW
SEMICIRCLES. SCATTERED TO ISOLATED STRONG CONVECTION IS OBSERVED
IN OUTER SPIRAL RAINBANDS. LARGE SWELLS GENERATED BY THIS
HURRICANE ARE AFFECTING THE NORTHERN LEEWARD ISLANDS AND SHOULD
BEGIN TO AFFECT THE BAHAMAS...BERMUDA...MOST OF THE EASTERN U.S.
COAST... AND THE ATLANTIC MARITIMES OF CANADA DURING THE NEXT
FEW DAYS. THESE SWELLS WILL LIKELY CAUSE EXTREMELY DANGEROUS
SURF AND LIFE-THREATENING RIP CURRENTS.

...TROPICAL WAVES...
A TROPICAL WAVE IS ALONG 23W S OF 18N MOVING W AROUND 10 KT. THE
WAVE IS EMBEDDED WITHIN AN AREA OF DEEP LAYERED MOISTURE MAXIMUM
OBSERVED IN THE TOTAL PRECIPITABLE WATER IMAGERY. AN INVERTED-V
LOW LEVEL SIGNATURE IS NOTED ON VIS SATELLITE PICTURES WITH SOME
CYCLONIC TURNING NEAR THE INTERSECTION OF THE WAVE WITH THE
ITCZ. SCATTERED SHOWERS AND ISOLATED TSTMS ARE WITHIN 160-200 NM
EITHER SIDE OF THE WAVE AXIS. THE WAVE WILL CONTINUE TO MOVE
ACROSS THE CAPE VERDE ISLANDS THIS AFTERNOON.

A TROPICAL WAVE IS ALONG 49W/50W S OF 15N MOVING W 15-20 KT. THE
WAVE SHOWS UP WELL IN THE UW-CIMSS TPW WHERE A BULGE OF MOISTURE
IS NOTED. ONLY FEW SHOWERS ARE ASSOCIATED WITH THIS FEATURE DUE
TO THE PRESENCE OF DRY AIR. THIS WAVE WILL REACH THE TROPICAL N
ATLC LATE TONIGHT INTO FRI MORNING.

A TROPICAL WAVE IS CURRENTLY MOVING ACROSS THE YUCATAN PENINSULA
AND NORTHERN CENTRAL AMERICA. ITS AXIS IS ALONG 88W/89W S OF 21N
MOVING W 10-15 KT. THE WAVE IS WELL DEPICTED IN THE MOISTURE
PRODUCT...TPW...AND IS EXPECTED TO MOVE ACROSS THE BAY OF
CAMPECHE THIS AFTERNOON THROUGH TONIGHT HELPING TO INDUCE SOME
SHOWER ACTIVITY.

...THE ITCZ...
THE ITCZ AXIS IS CENTERED ALONG 12N16W 9N30W 10N44W 8N56W
10N62W. CLUSTERS OF SCATTERED MODERATE TO ISOLATED STRONG
CONVECTION IS FROM 10N TO 13N E OF 20W TO THE COAST OF AFRICA.
A BAND OF SCATTERED MODERATE CONVECTION IS ROUGHLY WITHIN 120 NM
N OF AXIS BETWEEN 28W AND 45W.

...DISCUSSION...

GULF OF MEXICO...
BROAD UPPER RIDGE COVERS MOST OF THE GULF REGION EXTENDING FROM
CENTRAL MEXICO NEAR 20N100W NNE TO OVER THE LOWER MISSISSIPPI
VALLEY TO THE CAROLINAS WHILE A UPPER-LEVEL LOW CONTINUES TO
SPIN OVER THE GULF OF HONDURAS. A SHORT-WAVE TROUGH IS OVER
SOUTHERN TEXAS. AT THE SURFACE...A RIDGE FROM THE W ATLC
CONTINUES TO BUILD ACROSS THE GULF PRODUCING MAINLY LIGHT TO
MODERATE SE TO S WINDS. ISOLATED SHOWERS CONTINUE TO DOT THE
AREA PARTICULARLY THE E GULF. ALL THESE SHOWERS ARE MOVING NWD
UNDER THE PREVAILING WIND FLOW.

CARIBBEAN SEA...
A QSCAT PASS FROM THIS MORNING SHOWED MODERATE TO STRONG WINDS
OF 20 TO 25 KT JUST OFF THE COAST OF COLOMBIA COAST. THE TRADES
WILL DECREASE FRI AND INTO THE WEEKEND AS THE ATLC HIGH WEAKENS
WITH HURRICANE BILL MOVING N OF THE FORECAST AREA. TYPICAL
SHOWERS EMBEDDED IN THE TRADE WIND FLOW ARE SEEN ACROSS THE
BASIN. NLY WINDS ON THE WEST SIDE OF HURRICANE BILL ARE
PUSHING SOME NARROW LINES OF SHOWERS SWD INTO THE FAR NE
CARIBBEAN AS SEEN ON THE SAN JUAN DOPPLER RADAR. THESE SHOWERS
ARE IN ASSOCIATION WITH THE CYCLONIC FLOW AROUND BILL.
ALOFT...AN UPPER-LEVEL LOW IS OVER THE GULF OF HONDURAS COVERING
THE NW CARIBBEAN. ANOTHER UPPER-LEVEL LOW IS OVER HISPANIOLA. IN
BETWEEN...AN UPPER RIDGE EXTENDS FROM PANAMA TO THE CAYMAN
ISLANDS. UPPER DIFFLUENCE ASSOCIATED WITH THIS FEATURE IS
HELPING TO INDUCE SCATTRED SHOWERS AND ISOLATED TSTMS OVER THE
SW CARIBBEAN S OF 12N.

ATLANTIC OCEAN...
ATTENTION REMAINS FOCUSED ON BILL...THE FIRST MAYOR HURRICANE OF
THE 2009 ATLANTIC HURRICANE SEASON. SEE SPECIAL FEATURES FOR
DETAILS. THE AZORES-BERMUDA HIGH EXTENDS A RIDGE OVER THE
CENTRAL AND EASTERN ATLC. BILL IS MOVING AROUND THE PERIPHERY OF
THIS HIGH. A WEAK SFC RIDGE ALSO DOMINATES THE SW ATLC AND THE
STATE OF FLORIDA. PRES GRADIENT BETWEEN HIGH PRES OVER THE
AZORES AND HURRICANE BILL IS PRODUCING A SWATH OF MODERATE TRADE
WINDS FROM 12N TO 24N E OF 50W. ALOFT...NARROW UPPER TROUGH
ENTERS THE AREA NEAR 32N68W THEN CONTINUES SW TO NEAR THE SE
BAHAMAS. AN UPPER LOW IS ALONG THE TROUGH NEAR 27.5N71W
GENERATING SOME SHOWER ACTIVITY. THE FLOW EAST OF THE UPPER-LOW
IS HIGHLY AMPLIFIED ANTICYCLONICALLY DUE TO THE VERY PRONOUNCED
OUTFLOW OF LARGE HURRICANE BILL. AN UPPER-LEVEL RIDGE FROM THE
GULF OF MEXICO EXTENDS INTO THE W ATLC W OF 73W.

$$
GR







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