869
FXCA62 TJSJ 132059
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
459 PM AST Thu Jun 13 2024

.SYNOPSIS...

A surface trough passing through the region tomorrow will continue
a pattern of passing showers and afternoon convection, mainly
across western Puerto Rico. Isolated areas may experience heavy
rainfall at times, however mid to upper-level ridging will limit
widespread rainfall. By Saturday, a plume of Saharan dust of
moderare to high concentration is forecast to spread across the
region, causing hot and hazy conditions that last through the
second half of the weekend into next week.

&&

.SHORT TERM...Tonight through Saturday...

A mid to upper ridge is forecast to build and then linger across
the region into this upcoming weekend. Surface high pressure
extending southwards to just north of the region and an induced
surface trough east of the Lesser Antilles will maintain a light
to moderate east to northeast wind flow tonight, then becoming
east- southeast to southeast throughout the weekend.

Low level moisture from a departed tropical wave and an old
frontal boundary will continue to allow showers to linger this
evening before dry air moves in, lasting through tomorrow morning.
The passage of the surface trough tomorrow afternoon will bring an
increase of moisture and instability across the region which will
assist afternoon convection mainly across western interior
sections of Puerto Rico. Isolated areas may experience heavy
rainfall at times, however the mid to upper-level ridging will
limit widespread rainfall.

By Saturday, a plume of Saharan dust of moderare to high
concentration is forecast to spread across the region while the
local low level wind flow becomes more southeasterly By then with
the southeasterly winds expect another round of hotter and hazy
conditions due to the combination of suspended dust particulates
and the mid to upper leve ridging, which will favor subsidence and
compression of the warm air resulting in limited convective
development across the islands. Daytime high temperatures will be
in the low 90s especially along the north and west coastal areas
with maximum heat indices expected to be in the low 100s once
again with excessive heat impacts probable particularly along the
coastal and urban areas.

&&

LONG TERM...Sunday through Wednesday...
/from previous discussion/
Hot and hazy conditions will continue to start the long term
period. Current model guidance suggests precipitable water (PWAT)
at below normal to normal values to start the period, with only
few patches of more humid air, from frontal remnants, moving
through the islands. Southeasterly surface windflow is expected to
persist on Sunday as a Surface high is present over the Atlantic.
This will steer warmer tropical air towards the islands,
promoting overall warm to hot conditions. Heat related products
will likely be issued for the islands later on. 925 mb
temperatures are forecast to be above normal to start the period.
Model guidance suggests more easterly winds to start the workweek
as the Atlantic surface high weakens. Saharan dust particles will
continue to start the period as well, promoting hazy skies at
least through Tuesday. Although model guidance suggest some
moisture over the mid levels, a more notable increase is suggested
by Wednesday. Most available moisture for the period will remain
below 850 mb, however. A decrease in PWAT values is now forecast
for Monday and most of Tuesday, with an increase by Tuesday night
and Wednesday. High pressure aloft will start to weaken as we head
into the workweek.


&&

.AVIATION...
(18Z TAFS)

VFR conditions will continue across the area except for SHRA/TSRA
dvlpg aft 13/17Z ovr wrn and interior PR till 13/23Z. Wind flow is
easterly 10-15 kt with gusts to 25 kt on the coasts in sea breezes.
Aft 14/01Z winds bcmg easterly 10 kt or less with land breezes. SHRA
will be mainly ovr the local waters and isold ovr the east coast of
PR. Aft 14/13Z winds will become sea breezes in east to southeast
flow.


&&

.MARINE...

Surface high pressure over the Atlantic and an induced trough east of
the area will promote moderate east to northeast winds through
tonight. Frontal remnants from the northeast and an upper level
trough are expected to increase the potential for showers and
isolated thunderstorms through tomorrow. Winds will continue to
diminish and veer, becoming east-southeast to southeast throughout
the weekend with increasing Saharan dust through Sunday.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Excessive Heat Warning until 6 PM AST this evening for PRZ001-
     005-008-010.

     Heat Advisory until 6 PM AST this evening for PRZ002-003-007-
     011>013.

VI...Excessive Heat Warning until 6 PM AST this evening for VIZ002.

     Heat Advisory until 6 PM AST this evening for VIZ001.

AM...None.

&&

$$

WS/RC/YZR