397
FXCA62 TJSJ 162022
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
422 PM AST Sun Jun 16 2024

.SYNOPSIS...

Warm days with heat indices near 108 to 112F for most coastal
areas and urban sections will continue for the beginning of the
week. Afternoon convection is expected across the western and
central interior to northern sections of Puerto Rico. Given the
expected conditions, an excessive heat watch is in effect for
Monday. Marine conditions remain calm with seas around 1 to 3
feet.

&&

.SHORT TERM...Tonight through Tuesday...

A broad surface trough with axis extending northward across
Hispaniola and the Mona passage and strong High pressure over the
central and northeast Atlantic will maintain a light east southeast
wind flow across the region. A moist airmass situated across the
region will allow showers and isolated thunderstorms to linger
into the evening hours, before clearing out overnight. The mid to
upper ridge will begin to erode tomorrow as an upper trough is
forecast to cross the west and southwest Atlantic and establish just
west of the region by Tuesday afternoon.

It appears that another surface trough will pass though the
region tomorrow, continuing a wet pattern, especially during the
afternoon hours. A pattern of clouds and plenty of sunshine will
be seen tomorrow, allowing afternoon showers and isolated
thunderstorm development over portions of the central interior and
west sections of Puerto Rico, lingering around and moving to the
northern coastlines meanwhile providing beneficial rain and
possibly urban flooding. More Saharan dust is projected to arrive
on late Tuesday after the passage of this wave, however will be
short lived as a thick tropical airmass moves into the region
afterwards. Expect an increase of low level moisture convergence
and lower level instability enhancing early morning and afternoon
convection across the islands during the latter part of the
period.

Daytime high temperatures are forecast to be in the upper 80s to low
90s especially along the north and west coastal areas. This along
with the humid conditions and southeasterly winds will again
favor maximum heat indices to exceed 100 degrees tomorrow. Please
refer to the latest Non- Precipitable Weather Message(NPWSJU)
issued by WFO SJU PR for additional information on the expected
heat impacts.

&&

LONG TERM...Wednesday through Sunday...
/from previous discussion/

More unstable weather conditions are expected at the beginning of
the long term due to an upper-level trough just west of the
region, with the axis over Hispaniola, leaving the CWA on the
divergent side of the TUTT. Although there are some discrepancies
in the intensity of the unstable conditions between global model
guidance, the islands should expect a deteriorating weather trend
from Wednesday into Thursday. At the surface, moisture from a
tropical wave will combine with this instability, increasing the
potential for shower and thunderstorm development. Isolated
thunderstorms are possible in the afternoon across the mountains
and over the local waters due to colder temperatures at 500 MB,
ranging from -7 to -8 degrees. According to the Glvez-Davison
Index (GDI), there is a potential for isolated to scattered
thunderstorms from Wednesday through Friday. Therefore, the
forecast reflects afternoon showers with a limited flood threat.
Hazy skies might be possible as a big pulse of Saharan Dust Air
Layer invades the local islands.

From Saturday into Sunday, a broad surface high pressure over the
Central Atlantic extending to the Eastern Atlantic will result in
variable weather conditions. Under the east-southeasterly wind
flow, plenty of surface moisture will continue to stream across
the local islands, keeping the potential for convection in the
afternoon, especially across the mountains and the western
interior. Although plenty of moisture will remain in place, more
stable conditions will be present at the upper level, provided by
an upper-level ridge.

A similar temperature pattern is expected across the islands by
the first part of the long term. According to model guidance for
925 MB temperature, the trend calls for warmer conditions,
resulting in heat indices around 108 to 112F. Residents and
visitors are urged to stay informed of possible advisories or
warnings due to excessive heat.

&&

.AVIATION...
(18Z TAFS)

sOME SHRA/TSRA dvlpd just bfr 16/16Z and are lingering just east of
TSJU, +SHRA with isold TSRA were also seen just NW of TISX.
SHRA/TSRA are still expected to dvlp wrn PR and interior aft 16/18Z
with lcl MVFR/IFR conds across TJMZ aft 16/1730Z and TJBQ aft
16/1830Z. Conds should return to VFR by 16/23Z. Winds SE less than
10 kt with strong sea breeze influences. Gusts up to 22 kt in sea
breezes and nr TSRA. Winds bcmg calm or land breezes aft 16/23Z.
Higher cloud coverage north of 19.5 degrees north with SHRA/TSRA
activity. Similar prognosis on Monday.

&&

.MARINE...
Surface high pressure over the Atlantic and an induced trough north
of the area will promote light to moderate east to southeast winds.
Moist air will allow showers and isolated thunderstorms to develop
over portions of the local waters resulting in localized hazardous
marine conditions. Winds will continue to diminish and veer,
remaining southeasterly through the beginning of the week.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Excessive Heat Watch from Monday morning through Monday
     afternoon for PRZ001-002-005-008-010-012-013.

VI...Excessive Heat Watch from Monday morning through Monday
     afternoon for VIZ001-002.

AM...None.

&&

$$

ERG/RC/YZR