Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 270759

Area Forecast Discussion
National Weather Service San Juan PR
359 AM AST Tue Jul 27 2021

.SYNOPSIS...Mainly fair weather conditions are expected for today,
with limited afternoon convection over southwestern Puerto Rico.
An increase in shower and thunderstorm activity is expected
Wednesday and Thursday as a Tropical Upper Tropospheric Trough
(TUTT) moves over the area. This will be followed by a drier and
more stable weather pattern on Friday through the upcoming weekend
as a mid and upper-level ridge builds overhead.


.SHORT TERM...Today through Thursday...

The available moisture will decrease today, causing the precipitable
water values to be close to 2 standard deviations below normal.
There is also an ENE wind flow today, which will cause sea breeze
convergence across the southwestern quadrant of Puerto Rico this
afternoon. There is overall not a significant amount of instability
to expect widespread or very heavy rain across the local islands.
That said, isolated to scattered brief showers are expected across
the local islands today, mainly in the form of streamers. Also,
scattered to numerous showers are expected this afternoon across the
southwestern sections of PR, with a slight chance of thunderstorms.
The overall impact is expected to be that of ponding of water in
areas of poor drainage, but some urban flooding will not be ruled
out for small areas of southwest PR.

The available moisture will increase on Wednesday, closer to normal
levels for the time of year, while a TUTT passes over the local
islands. This will cause an increase in shower activity across the
local area in both coverage and intensity. Scattered showers are
forecast for most of the day on Wednesday, with numerous showers and
isolated thunderstorms across the western to northwestern PR on
Wednesday afternoon. The general wind flow on Wednesday will be more
easterly, but that will change for Thursday. The general wind flow
will become southeasterly on Thursday. Also, the TUTT will be to our
west and in a better position to support thunderstorm development,
especially across the NW sections of PR where there will be sea
breeze convergence in the afternoon. The available moisture will be
near to below normal, and there is some indication that small
concentrations of Saharan dust could be moving in, so shower
activity may be modest. Therefore, isolated to scattered showers are
forecast across most of the local area, but numerous showers and
isolated thunderstorms are expected across the NW quadrant of PR on
Thursday afternoon.

The overall temperatures will be near normal, with highs in the
upper 80s to low 90s across the lower elevations, and in the upper
70s to mid 80s across the higher elevations. Low temps will be in
the mid 70s to near 80 across the lower elevations, and in the upper
60s to low 70s across the higher elevations.

.LONG TERM...Friday through Wednesday...

As the TUTT moves further away towards the west, a mid and upper-
level ridge will build over the area on Friday and prevail
through the upcoming weekend. This will result in drier and
more stable conditions aloft. A tropical wave is expected to move
across the eastern Caribbean on Friday, however, model guidance
continues to indicate that the most active part of the wave will
remain south of the area. Although some moisture associated with
the wave may reach the area and produce isolated to locally
scattered shower activity, no significant rainfall is expected due
to unfavorable dynamics aloft. Moisture then is expected to be
mostly below normal during the weekend. Therefore, mainly fair
weather conditions can be expected with rainfall activity limited
to the typical afternoon convection across western Puerto Rico.
The upper-air pattern then changes during the early to middle
portion of next week as another deep-layered TUTT retrogrades
towards the area. This feature in combination with increasing
moisture will result in more favorable conditions for better
convective activity across the local area, especially during the
afternoon time period. Therefore, if model guidance is correct,
a wetter weather pattern will be possible during the early to
middle portion of next week.


.AVIATION...VFR conds are expected across the local terminals through the
forecast period. Brief VCSH is possible in ISOL SHRA this morning,
but is not expected to pose a significant hazard to the local
terminals. Winds will be mainly from the ENE at 10KT or less through
27/13Z, but increasing to 15KT and gusty thereafter. Sea breeze
variations expected, causing SE winds at TJPS after 27/13Z.


.MARINE...More tranquil marine conditions with seas of up to 5
feet will prevail across the regional waters for today. Seas are
expected to remain generally below 6 feet through the rest of the
work week and into the upcoming weekend. For beachgoers, there is
a moderate risk of rip currents for the northern and southeastern
beaches of Puerto Rico as well as the eastern beaches of Culebra,
Vieques and Saint Croix.


SJU  88  77  86  76 /  20  40  40  40
STT  89  77  87  78 /  20  30  30  40





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