Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 301459

Area Forecast Discussion
National Weather Service San Juan PR
1059 AM AST Sat May 30 2020


Despite the amount of African dust particles, limited afternoon
convection still possible over the northwest quadrant of Puerto
Rico and downwind from the U.S. Virgin Islands. Expect, warm
temperatures with highs in the low 90s and heat indices between
the upper 90s and low 100s. There is a slight chance to observe
isolated thunderstorms over the northwest quadrant and
surrounding waters.



VFR conditions expected today. African dust particles are creating
hazy conditions and reducing visibilities, but airports observers
are reporting visibilities P6SM. JSJ is reporting 9 miles VIS.
Limited SHRA could develop over the NW-quadrant between 30/18-23z.
If it develops, it could move over/near JMZ/JBQ. Winds will
continue from the ESE at 10 knots or less with sea breeze



Tranquil marine conditions will continue with seas below 5 feet
and winds below 15 knots.


.PREV DISCUSSION... /issued 522 AM AST Sat May 30 2020/

SYNOPSIS...Mostly sunny, hot and hazy weather conditions are
expected today and generally through the weekend across much of
the local area as a drier air mass and a Saharan Air Layer will
spread across the region. This should limit shower activity and
afternoon convection over the islands. However, showers and isolated
thunderstorms are still expected to develop across portions of western
and northwestern Puerto Rico during the afternoon hours due to local
effects and intense daytime heating. As a result of the Saharan Air
Layer, hazy skies can be expected at least until Monday.

SHORT TERM...Today through Monday... Hazy skies are expected to
prevail through the forecast area as a drier air mass along with
Saharan dust filtered into the islands. A surface high pressure over
the eastern Caribbean is generating an east-southeast wind flow.
Under this pattern, above normal temperatures are expected to
continue. In the central Atlantic, a mid to upper level trough will
hold north of the area, while maintaining 500 mb temperatures at -7
to -8 degrees. However, precipitable water values are expected to
fluctuate between 1.3 to 1.4 inches, which is below normal. In fact,
moisture is expected to remain confined to the lower levels of the
atmosphere and will mostly depends on fragmented group of clouds
carried by the trade winds. As a result, a few passing showers
cannot be ruled out across eastern Puerto Rico and the U.S. Virgin
Islands during the overnight and early morning hours. Then, in the
afternoon, the northwestern quadrant of Puerto Rico should
experience shower and isolated thunderstorm activity, aided by the
local effects and diurnal heating. Except for isolated areas, the
activity is not expected to be as widespread and intense due to the
drier air mass over the region. A gradual increase in low level
moisture is however forecast by Monday resulting a better chance
for afternoon convection.

LONG TERM...Tuesday through Saturday...

Recent model guidance continued to suggest a broad polar trough will
move across the western Atlantic and become amplified while sinking
southwards across the the region Tuesday through the end of the
work week. This will result cooler temps aloft with the 500 mb
temperatures to range between -8 to -10 degree Celsius. In addition
to this increasing instability aloft, a tropical wave will move across
the region Tuesday through Wednesday and therefore increase low level
moisture pooling and convergence across the forecast area. Therefore
there is increased potential for early morning and afternoon showers
and thunderstorm activity across the islands and coastal waters
at least during the early part of the period with enhanced afternoon
convection and rainfall possible over portions of the islands.

By late Wednesday through the end of the work week, a surface
high pressure ridge will build across the central and northeast
Atlantic to increase the easterly trade winds. This will tighten
the local pressure gradient over the eastern Caribbean resulting
in increasing trade winds and somewhat breezy conditions over the
islands. Consequently, expect more frequent passing overnight and
early morning shower activity to affect the east coastal areas of
the islands. The aforementioned upper trough (TUTT) is expected
to lift northeastward across the islands by Friday, accompanied by
a jet segment which will again destabilize the upper levels during
the afternoon hours. This feature along with available moisture
across the islands will increase the chance for enhanced afternoon
thunderstorms activity in and around the islands with periods of
locally heavy rains. A gradual improvement is forecast by the
following weekend as the upper trough pulls away and the pressure
gradient relaxes while local winds become more east northeast.

AVIATION...Mainly VFR conditions are expected to prevail across all
terminals. HZ due to Saharan air will reduce VIS through the
forecast period, although remaining at P6SM. After 17Z, VCTS will
affect TJBQ/TJMZ with MVFR TEMPO conditions possible. Rain should
taper off by 23Z. Winds remain out of the east southeast at around
10 knots with stronger gusts and sea breeze variation.

MARINE...Fairly tranquil marine conditions with seas ranging
between 2 and 5 feet across the regional waters through the
weekend. A moderate risk of rip currents is expected across the
northern coast of Puerto Rico as well as the northeast coast of
Culebra and the eastern coast of Saint Croix.


SJU  92  79  90  79 /  20  10  10  30
STT  88  79  88  78 /  10  10  30  40




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