Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 200139

Area Forecast Discussion
National Weather Service San Juan PR
939 PM AST Wed Sep 19 2018

A surge of moisture is producing shower across the surrounding
waters of the U.S. Virgin Islands and eastern Puerto Rico. This
activity will continue overnight and some of them could moved
inland producing periods of moderate rainfall at times. However,
Saharan dust particulate was detected making its way through the
region and hazy conditions were reported later this afternoon.
Afternoon showers and one or two thunderstorms should not be ruled
out across the northwest quadrant and downwind from El Yunque
into the San Juan Metro by Thursday. Heat index in the low to mid
100s are possible along the coastal areas of Puerto Rico and the
U.S. Virgin Islands the next few days.


VFR conds will prevail across the islands through much of the
forecast period. However a Saharan Air Layer will reduce
visibilities between 5 and 10 miles through much of the forecast
period. In addition, showers and thunderstorms could form across
the northwest quadrant of Puerto Rico and in the VCTY of TJBQ/TJMZ
between 20/16-22z. Easterly winds at 7 to 12 knots expected
overnight becoming more ESE at 10 to 15 kt and gusty aft 20/13z
with sea breeze variations.


Tranquil marine conditions with E-ESE winds at 10 to 15 knots and
seas at 4 feet or less. Winds and seas are forecast to increase
across the Atlantic and Caribbean Offshore waters on Thursday.


.PREV DISCUSSION... /issued 526 PM AST Wed Sep 19 2018/

SYNOPSIS...Saharan dust has invaded the U.S. Virgin Islands and
eastern Puerto Rico and will continue over the area through
Friday. Little shower activity will occur after tonight except in
northwest Puerto Rico Thursday and southwest Puerto Rico on
Friday. Winds become more northeasterly on Friday. After Friday
moisture increases through mid week next week when another
tropical wave is expected.

SHORT TERM...Tonight through Saturday...
Drier air has invaded the area and is drastically reducing the
shower activity. Nevertheless, the best activity of the tropical
wave is yet to follow, though it will be sapped by the very dry
air accompanying the dust. As evidence of this activity and the
dryness of the air around it many elliptical outflow boundaries
have been seen southeast of the forecast area headed toward the
area. Most of these will be gone by tonight. Current expectations
are that some showers overnight will make it through, impacting
southeast and eastern Puerto Rico through the morning on Thursday,
then the convection will disappear over eastern Puerto Rico and
what is left of the moisture in northwestern Puerto Rico will
spawn some thunderstorms Thursday afternoon. These should be
brief and will likely not produce significant heavy rains. Friday
morning is expected to have the lowest precipitable water of the
next 10 days over the area. Then lower level flow begins to turn
to the northeast and some moisture is brought into the area Friday
night after the dust has passed. This should yield some showers
Friday afternoon in the southwest and then on the northeast coast
of Puerto Rico and around the U.S. Virgin Islands Friday night,
but amounts will be light. On Saturday afternoon there is a good
chance for showers in southwest Puerto Rico with isolated showers

LONG TERM...Sunday through Wednesday...
Low pressure migrating across the western Atlantic will double
back and grow closer to the area while remaining north of 30
degrees north. Nevertheless the trough that forms to its southwest
will weaken the trade wind flow and it will begin to shift back to
the east. Limited moisture above 850 mb but especially at mid
levels will allow only limited shower activity with some
thunderstorms Sunday and through Tuesday. A tropical wave will
move through the area on Wednesday depicted by the GFS, but most
of its moisture and energy will move through the Caribbean to the

At upper levels...on Sunday morning a trough extends out of the
TUTT in the western tropical Atlantic to the west northwest. It
slowly moves south until it reaches the area on Wednesday, this
will help some of the activity on Wednesday as will the peak of
the columnar moisture on Wednesday and Thursday next week.

AVIATION...Mainly VFR conditions are expected to prevail across all
terminal through the forecast period. SHRA/TSRA over west PR
expected to end around 21z. HZ due to suspended Saharan dust will
continue  across the local terminals. Low level winds will
continue mainly from the ESE at 15-25 kt. Lighter land breeze
variations expected across the PR terminals overnight. Maximum
winds blo FL540 next 48 hrs ESE 20-25 kt on 20/12Z btwn

MARINE...Marine conditions are currently responding to moderate
to locally fresh east southeast trade wind flow of 10 to 20 kt.
This will bring areas of 6 foot seas in the Caribbean and along
the north coast of Puerto Rico. Strongest winds will be on
Thursday. A north to northeast swell will enter the area on
Saturday night from the remnants of Florence. Seas then are
expected to remain below 7 feet.


SJU  78  91  79  90 /  40  30  10  60
STT  80  89  81  90 /  40  40  30  30




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