Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 170130

Area Forecast Discussion
National Weather Service San Juan PR
930 PM AST Mon Jul 16 2018

.UPDATE...Although the tropical wave will continue to move away
overnight, lingering moisture is expected to prevail across the
local islands through at least early Tuesday morning. Therefore,
still expect passing showers across the USVI and e PR overnight.
As a drier air mass moves across the eastern Caribbean Tuesday
afternoon into Wednesday, a fair weather pattern is then expected
to prevail with hazy skies. PoPs were adjusted to match current
weather conditions.


.AVIATION...VFR conds expected to prevail at all TAF sites
overnight with VCSH possible in and around the Leeward and USVI
terminals as well as JSJ. Aft 17/16z, TSRA/SHRA across west
Puerto Rico may result in MVFR conds at JMZ/JBQ through 17/22z.
ESE winds at 10 knots or less increasing to 15 knots aft 17/13z.


.MARINE...Choppy marine conditions to continue across most of
the local waters due to seas near 6 feet and winds 15 to 20


.PREV DISCUSSION... /issued 434 PM AST Mon Jul 16 2018/

A tropical wave will continue to move away from the local area
tonight. Weather conditions will improve as a drier air mass and
Saharan dust moves into the region from the east. Although drier
conditions are expected, low level moisture should enough to
combine with the local and diurnal effects to aid in shower and
thunderstorm development on Tuesday and Wednesday afternoon. The
GFS indicates the passage of a broader tropical wave on Thursday
with higher precipitable water values of around 2 inches over the
majority of the forecast area. Weather conditions will improve on
Friday through the weekend. Patches of low level moisture will move
across the area on occasion, which should bring passing showers
over the windward areas of the islands in the mornings and aid
shower and thunderstorm development over interior and western PR
in the afternoons.

SHORT TERM...Rest of today through Wednesday...

Showers and thunderstorms affecting the interior, north and
northwest portions of PR this afternoon will dissipate gradually
tonight. Although weather conditions are expected to improve as
drier air moves into the forecast area, passing showers are still
expected particularly over the eastern and northern portions of
PR and USVI overnight and Tuesday morning. Based on the latest
NAAPS Aerosol guidance, a surge in Saharan dust will affect the
local area early Tuesday morning, producing hazy skies once again.
Low level moisture should be sufficient to combine with the local
and diurnal effects to produce another round of showers and
thunderstorms Tuesday afternoon, particularly across the interior,
north and northwest sections of PR. Also, east southeast surface
winds will allow for streamers to affect some portions of east and
northeast PR.

Based on latest GFS model guidance, relative humidity values in
the mid to upper levels of the atmosphere will prevail low
through Wednesday, along with Saharan dust. The 3 KM and 9 KM WRF
model do not show impressive rainfall accumulations for Tuesday
and Wednesday, and the HiRes Precipitation guidance only shows
accumulations between 0.25-0.50 with isolated amounts up 1.25 -
particularly over the western and northwestern portions of PR in
the afternoon hours.

East to east southeast winds will prevail with daytime maximum
temperatures ranging between upper 80s to lower 90s across the
coastal areas of the local islands during the next few days.

LONG TERM...Thursday through Monday...

Another tropical wave is forecast to move across the forecast
area on Thursday. The GFS continues to indicate precipitable
water values of around 2 inches with the wave. The wave is also
much broader than the previous one with these precipitation
values extending across the majority of the forecast area. Good
atmospheric dynamics is also expected with the wave for strong
shower and thunderstorm development over portions of the regional
water and local islands.

Weather conditions will improve gradually on Friday as drier air
and another surge in Saharan dust moves into the local area.
Patches of low level moisture will move across the region on
occasion. This will help maintain enough moisture to aid in
passing showers across the windward area of the islands in the
mornings followed by showers and possible isolated thunderstorms
in the afternoons across the interior and western sections of PR
through the rest of the forecast period.

Tropical wave is bringing TSRA/SHRA to the local
terminal sites. Brief periods of MVFR conds are possible as
TSRA/SHRA briefly reduce visibility and also there may be occasional
brief MVFR ceilings. Winds will be mainly from the east at 15 TO 20
kts with higher gusts. TSRA/SHRA will dissipate around 17/01Z
across all terminal sites.

Choppy seas of 4 to 6 feet and winds between 15 and 20 knots are
expected to continue during the next few days. Small craft should
exercise caution across most of the local waters. Shower and
thunderstorm activity is expected to increase on Thursday as
another tropical wave moves across the region.


SJU  80  90  80  88 /  40  20  40  40
STT  79  89  80  89 /  40  20  40  40




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