Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 160151

Area Forecast Discussion
National Weather Service San Juan PR
951 PM AST Mon Jul 15 2019

Some showers are moving across the local waters, the north and east
coast of Puerto Rico, while the U.S. Virgin Islands, the west and
south coast of Puerto Rico have mostly clear skies with little or
no shower activity.

A drier air mass with another round of Saharan dust will continue
tomorrow across most of the islands, but with limited afternoon
convection across western Puerto Rico.


VFR conditions are forecast through the forecast period. Passing
showers should not be ruled out at times. Winds will continue
from the east to north-east at around 10 knots increasing at 10
to 20 knots with higher gusts after 16/13z.


Increasing winds will result in seas up to 7 feet across the
southwestern offshore Caribbean waters and Mona Passage.
Therefore, a small craft advisory is in effect through Tuesday
afternoon for these waters. Elsewhere, seas will range between 1
and 4 feet across protected waters and between 4 and 6 feet across
open waters.


.PREV DISCUSSION... /issued 414 PM AST Mon Jul 15 2019/


Drier air and very low precipitable water values are
moving in, with another round of Saharan dust. However, shower
activity will pick up somewhat on Wednesday as patches of moisture
ride in on the trade winds. Widespread showers are expected for
Thursday and Friday as another tropical wave moves in. Lingering
moisture lasting into the weekend will result in isolated showers
across the region.

SHORT TERM...Tonight through Wednesday...

Showers occurring across the higher terrain of Puerto Rico are
expected to dissipate into the nighttime hours as surface heating
slows down. Our pattern shifts to drier conditions over the next few
days. Precipitable water values, which are currently around
1.5-1.6 inches are expected to drop to around 1.2 by tomorrow
morning, which will be two standard deviations below the mean.
High pressure to our north will drive easterly trade winds,
bringing another bout of Saharan dust with subsequent hazy
conditions into the forecast area. Dust concentrations are
expected to peak tonight and into tomorrow morning.

Despite much drier air and dust moving in, we`ll see some isolated
showers tomorrow. For tomorrow morning, a few showers are
possible across the USVI and eastern Puerto Rico. However, the
best chance for showers will be in west central Puerto Rico
tomorrow afternoon where terrain effects including surface
convergence will be optimal. Given the drier air expected
tomorrow, minimal impacts are expected from these showers.

Patches of moisture embedded in the easterly trade winds will move in
for Wednesday. Thus, a higher likelihood for shower activity is
expected. A lack of upper level dynamic support, plus lingering dry
air will again limit the impacts of these showers. Again, the
highest likelihood for showers and scattered thunderstorms are
expected to be centered around Mayaguez for Wednesday afternoon.

LONG TERM...Thursday through Tuesday

PREV DISCUSSION... /issued 451 AM AST Mon Jul 15 2019/

Shower and thunderstorm activity may increase once again on
Thursday and Friday as another tropical wave moves in and
increases the available moisture. The moisture increase associated
with this tropical wave is forecast to encompass a larger
coverage area locally than the past few waves we have had
recently. The latest model guidance suggested once again a weak
upper low over the local area and to the west, which normally
suggests enhanced instability, but this time it is really weak,
and the model seems to be flip flopping between having no upper
level feature to enhance instability and having some weak trough
in the area. Therefore it looks like any upper level feature for
Thursday and Friday will be weak if there is any at all. That
said, the moisture will combine with diurnal heating and local
effects to cause more significant shower or thunderstorm
development across portions of Puerto Rico, but also the tropical
wave will bring shower activity to the local area overall,
affecting also the USVI. For next weekend, the moisture decreases
but some will still linger, mainly locally induced showers in the
afternoons across western PR would be expected, while the rest of
the area could observe isolated to scattered showers. Another
tropical wave would reach the local area on Monday into Tuesday
but the increase in moisture with this wave appears to be modest,
as suggested by the GFS model.


SHRA are expected to cont dvlpg from northeast PR to
btwn TJMZ and Cabo Rojo. Only a slgt chc of TSRA at this time, but
brief heavy SHRA had dvlpd by 15/1730Z. SHRA to diminish aft 15/21Z.
Winds 15-20 kt alg the coasts with gusts to 28 kt briefly psbl. Sea
breezes bcmg land breezes aft 15/22Z and winds diminish to 10-15 kt
less in the interior. Maximum winds E 18-23 kt btwn FL010-250 and
btwn FL525-550.


Fresh easterly trade winds are producing choppy seas across the
region. By this evening and into tomorrow morning, we`ll see a peak
in the winds of around 20 knots, with wave heights up to 7 feet
in the southwestern offshore Caribbean waters and Mona Passage. A
moderate risk of rip currents exists for most local beaches.
Improving conditions are expected by mid-week.


SJU  79  89  79  88 /  20  20  20  30
STT  80  90  78  89 /  20  40  40  30


AM...Small Craft Advisory until 2 PM AST Tuesday for Caribbean Waters
     of Puerto Rico from 10 NM to 17N-Mona Passage Southward to



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