266
FXCA62 TJSJ 182116
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
516 PM AST Tue Jun 18 2024

.SYNOPSIS...

Unstable weather conditions will continue through at least late
tomorrow across the islands. An upper trough and area of low
pressure continue to become amplified west of the region through
tomorrow. This along with a tropical wave will maintain moist and
unstable conditions through mid week resulting in periods of
developing showers and isolated thunderstorms each day. Winds will
remain from the southeast before gradually backing to become
easterly tomorrow and further increasing on Thursday. Current
model guidance has the entire region with PWAT values above 2
inches tomorrow, with remnant moisture patches reaching the area
on Thursday. A slight to moderate concentration of Saharan dust is
forecast to spread across the area tomorrow into Thursday.

&&

.SHORT TERM...Tonight through Thursday...

Afternoon convection started over western and northwestern quadrant
Puerto Rico as strong thunderstorms started to develop. As these
thunderstorms gradually moved offshore, another cluster developed
over interior to north-central Puerto Rico moving northeast and
offshore before reaching Guaynabo. Pea-size hail was reported
between 2:45 and 255 PM in Corozal due to these thunderstorms.
Thunderstorm wind damage was also reported in Morovis as gusts
damaged plantain trees and displaced some trash collecting drones.
The highest rainfall amounts (around 2.6 inches) were reported in
the border of Mayaguez, Las Marias and Maricao. High heat indices
above 106 degrees were reported in several coastal areas. Unstable
weather conditions will continue through at least late tomorrow
across the islands. A surface high pressure system over the Central
Atlantic and an induced surface trough northwest of the region are
promoting light southeast surface winds across the islands. Winds
will remain from the southeast before gradually backing to become
easterly tomorrow and further increasing on Thursday. Current
satellite derived precipitable water (PWAT) values show normal
(mainly over southern PR and the Caribbean Waters) to above normal
values over most of the region. PWAT values will continue above
normal tomorrow as a tropical wave and abundant moisture behind it
is also near the area. Current model guidance has the entire region
with PWAT values above 2 inches tomorrow, with remnant moisture
patches reaching the area on Thursday. An upper trough and  low
pressure continue to become amplified west of the islands through
tomorrow. For tonight, remnant afternoon showers will continue
lingering and gradually dissipate while showers and possible
thunderstorms move towards the eastern region and coastal southern
PR. Shower and thunderstorm activity tomorrow can concentrate
over eastern PR, the metro area, and interior to western and
northwestern Puerto Rico, mainly during the afternoon hours. 500
mb temperatures will be normal to above normal (colder
temperatures) and increasing mid level moisture and relative
humidity will be present tomorrow. Convective activity on Thursday
will focus more on interior to western PR under easterly winds.
due to the easterly winds across the area. A slight to moderate
saharan air layer is also forecast to reach the area during the
period.

.LONG TERM...Friday through Tuesday...
./FROM PREV DISCUSSION...Issued at 445 AM AST Tue Jun 18 2024/

Conditions are so far forecast to improve by Friday with increasing
stability aloft, as the previously mentioned upper trough west of
the area weakens and high pressure builds across the area from the
central and eastern Caribbean. Low level moisture transport will
also erode as the easterly winds increase resulting in less frequent
passing late evening and early morning shower activity. The
potential for afternoon shower development and isolated
thunderstorms will continue on Friday but mainly due to local
effects and good daytime heating. Shower activity should be more
localized and focused over the interior and west section of Puerto
Rico steered by the moderate trade winds. Overall improving
condtions is expected elsewhere on Friday and into Saturday with
plenty of sunshine and near seasonal temperatures expected.

For the rest of the following weekend prevailing easterly winds are
forecast with periods of passing early morning showers affecting the
windward side of the islands. This will be followed by afternoon
convection limited to mainly parts of the central interior and west
sections of Puerto Rico. Plenty of sunshine and mostly pleasant
summertime weather is expected elsewhere although a weakly induced
easterly perturbation will cross the region late Saturday into
Sunday followed by the next topical wave so far forecast to enter
the eastern Caribbean late Sunday then cross the region Monday
through Tuesday. That said expect lesser showers and thunderstorm
activity across during the early part of the period with the best
potential for increasing instability and convection so far
forecast for the latter part of the period on Monday and Tuesday
due to the approaching tropical wave and associated moisture
field.

&&

.AVIATION...

(18Z TAFS)

Brief periods of MVFR conditions will remain possible at JSJ and JBQ
in TSRA/SHRA through at least 18/22z. Aft 18/22z VFR conditions will
prevail at all TAF sites with VCSH/VCTS. Winds are 10-15 kts from
the E-SE, and more VRB after 18/22z. The chance for SHRA/TSRA will
increase tomorrow with a tropical wave passage.

&&

.MARINE...

Surface high pressure over the Atlantic and a tropical wave crossing
the region will promote light east to southeast winds today, then
becoming more easterly and increasing to moderate to locally fresh by
Wednesday and through the rest of the week. Passing showers and
isolated thunderstorms accompanying and trailing the tropical wave
will continue to affect the coastal waters and local passages
resulting in localized hazardous marine conditions.

&&

.BEACH FORECAST...

There is a low risk of rip currents for all coastal areas tonight.
A moderate risk of rip currents will be present by tomorrow
morning for St. Croix and then by tomorrow night for Vieques and
sectors of eastern Puerto Rico. The moderate risk of rip
currents will then likely spread to Culebra, SW Puerto Rico, SE
Puerto Rico and northern Puerto Rico on Thursday.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.

&&

$$

SHORT TERM....CAM
LONG TERM.....MRR
PUBLIC DESK...YZR