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SJU
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN JUAN PR
537 AM AST THU JUL 2 2009
.SYNOPSIS...AT UPPER LEVELS...A HIGH DOMINATES THE WESTERN
ATLANTIC BUT WEAK LOW PRESSURE IS IN THE WESTERN TROPICAL
ATLANTIC. THE LOW IS EXPECTED TO TRACK NORTHWEST JUST NORTHEAST OF
PUERTO RICO ON SATURDAY. DURING THE FOLLOWING WEEK THE HIGH WILL
ROTATE BACK AROUND THE LOW AND INTO THE CARIBBEAN AND THE WESTERN
ATLANTIC AGAIN.
AT MID LEVELS...HIGH PRESSURE CONTINUES FROM AFRICA TO FLORIDA
BETWEEN 20 AND 30 NORTH LATITUDE WITH LIGHT TO MODERATE EASTERLY
FLOW OVER THE LOCAL AREA.
AT LOWER LEVELS...TROPICAL WAVES AND AFRICAN DUST ARE QUEUED UP
FROM THE CARIBBEAN TO AFRICA. CURRENTLY ONE IS TRACKING SOUTH OF
PUERTO RICO...A SECOND WAVE IS NEAR 51 WEST...WHILE A THIRD IS
NEAR IS NEAR 39 WEST.
&&
.DISCUSSION...SHOWERS WERE ISOLATED ACROSS THE LOCAL WATERS AND
THE ENVIRONS OF THE U.S. VIRGIN ISLANDS AND ONLY A FEW STILL
PATCHES OF MOISTURE REMAIN OF THE SHOWERS THAT MOVED ONSHORE IN
SOUTHEAST AND SOUTHERN PUERTO RICO OVERNIGHT. SHOWERS WERE
GENERALLY MOVING FROM THE EAST SOUTHEAST AND EXPECT THIS FLOW TO
CONTINUE FOR AT LEAST ANOTHER 12 HOURS AND LONG ENOUGH TO BRING
NEAR RECORD TEMPERATURES TO THE SAN JUAN AREA AIDED BY THE DRIER
AIR STILL LINGERING OVER THE LOCAL AREA. THE RECORD FOR TODAY WAS SET
IN 1995 AND 1990 WITH 93 DEGREES. THE MIMIC PRODUCT SHOWS THE DRY
SLOT SOUTH OF PUERTO RICO BECOMING MORE AND MORE NEGATIVELY TILTED
WITH SOME MOISTURE INCREASING JUST EAST OF THE ISLAND AND OVER THE
LEEWARD ISLANDS. THIS MOISTURE WILL BRUSH BY SAINT CROIX AND
PUERTO RICO TODAY YIELDING SOME SCATTERED SHOWERS AND STRONG
THUNDERSTORMS BUT MOST AREAS WILL FIND THAT CONVECTION WAS IMPEDED
BY DRY AIR ABOVE 700 MB. THE NEXT WAVE...CURRENTLY NEAR 13 NORTH
AND 51 WEST IS EXPECTED TO CROSS THROUGH THE AREA WITH BETTER
MOISTURE SATURDAY NIGHT AND SUNDAY MORNING...FOLLOWED BY A MORE
EXTENDED DRY PERIOD AND SOME CONSIDERABLE AFRICAN DUST. FOR NOW
WAVES WERE MOVING AT AROUND 18 KNOTS.
THE FORECAST REQUIRED LITTLE CHANGE...BUT DID BOOST THE MAXIMUM
TEMPERATURE IN THE SAN JUAN METRO AREA AFTER YESTERDAYS 93. AM
HOLDING WITH THE BASIC IDEA OF A SUCCESSION OF WAVES WITH SOME
SHOWERS FOLLOWED BY AFRICAN DUST AT LEAST OUT THROUGH DAY 8. WINDS
AT 850 MB SHOW A MAXIMUM LATE SUNDAY AFTER AN UNSTEADY INCREASE
THAT PEAKS AT AROUND 38 KNOTS. THE GFS IS SHOWING WINDS AT 700 MB
INCREASING TO 40 KNOTS BY SATURDAY. SOME OF THESE WINDS WILL
SPREAD DOWN TO THE SURFACE RESULTING IN INCREASING
WINDS...ESPECIALLY OVER LOCAL WATERS SATURDAY EVENING.
&&
.AVIATION...MOSTLY VFR CONDITIONS WILL PREVAIL AT ALL TAF SITES
DURING THE MORNING HOURS. IN THE AFTERNOON...BETWEEN
17Z-22Z...SCATTERED SHOWERS AND AN ISOLATED THUNDERSTORM MAY DEVELOP
ACROSS PARTS OF THE WESTERN INTERIOR AND NORTHWESTERN SECTIONS OF
PUERTO RICO...WHICH WILL PRODUCE MOUNTAIN OBSCURATIONS WITH PERIODS
OF MVFR CONDITIONS ACROSS TJMZ...AND TJBQ. WINDS WILL BE MAINLY EAST
TO SOUTHEAST AT 10 TO 15 KTS...EXCEPT FOR SEA BREEZE VARIATIONS AT
COASTAL AREAS.
&&
.MARINE...WINDS AND SEAS WILL INCREASE ACROSS THE SOUTHERN
SECTIONS OF THE FORECAST AREA SUCH THAT SMALL CRAFT ADVISORIES
WILL HAVE TO BE HOISTED BY LATE TONIGHT ACROSS THOSE AREAS. SEAS
WILL PEAK JULY 4 AND 5.