138
FXCA62 TJSJ 120847
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
447 AM AST Wed Jun 12 2024

.SYNOPSIS...

Excessive heat continues. Excessive Heat Warnings are in effect
from 9 AM to 6 PM AST for lower elevations and coastal areas of
Puerto Rico. Heat Advisories are in effect from 9 AM to 6 PM AST
for valleys and urban areas of the eastern and western interior
sections of Puerto Rico, Vieques, Culebra, and the US Virgin
Islands. Since dangerous hot conditions will likely continue, an
Excessive Heat Watch is now in place for Thursday. A tropical
wave south of the area, frontal remnants and an upper trough are
expected to increase moisture and the potential for showers and
isolated thunderstorms. Still, this activity will remain localized
and amplified by daytime heating, especially across western
sections of Puerto Rico today and Thursday. Southeasterly winds
and saharan dust will promote a hot and hazy weekend.

&&

.SHORT TERM...Today through Friday...

An overnight radar and satellite imagery analysis indicated mainly
clear to partly cloudy skies, with persistent mid-to-upper level
clouds moving over the eastern half of the forecast area. A few
light showers moved into eastern sections of Puerto Rico, leaving
rainfall totals up to a quarter of an inch. Reports again
highlighted warm overnight low temperatures above 80 degrees
Fahrenheit in coastal areas of the San Juan metropolitan area,
eastern and southern Puerto Rico, and the local islands. Coastal
observation sites and local buoys have also recorded warm low
temperatures, hovering around 82-84 degrees Fahrenheit. Although
slightly lower than yesterday`s lows, this situation offers little
overnight relief and will exacerbate the region`s ongoing heatwave
situation. Winds were mainly light to calm and variable.

Weather conditions throughout the forecast period should slightly
deviate from the persisting stable and shower-less pattern that
has dominated the region recently, primarily due to increased
instability yielded by an upper-level trough moving in from the
west and increasing moisture levels from the remnants of an old
front, steered by an approaching surface trough from the east.
Driven by a surface high pressure meandering to the north of the
local islands and the subsequent influence of the approaching
surface trough, the general steering flow will undergo some
changes. Easterly trade winds today, with peak daytime speeds
ranging from 15-20 mph, will gradually weaken to 10-15 mph and
shift from the east-northeast by tonight into early Thursday.
Winds will further diminish to 5-10 mph, gradually veering from
the east on Thursday and from the southeast on Friday.

Precipitable water values will stabilize throughout the forecast
period, rising from around 1.4 inches this morning to about 2.0
inches by Thursday afternoon and settling within typical seasonal
thresholds of around 1.8 inches on Friday. This trend and marginally
unstable conditions aloft suggest a higher probability of showers
with isolated thunderstorms. Still, this activity will remain
localized and amplified by daytime heating, especially across
western sections of Puerto Rico today and Thursday. Additionally,
expect increased passing showers with a few isolated thunderstorms
over the local waters tonight, with some moving into eastern Puerto
Rico and the local islands as remnants of an old front approach the
area. Regarding flooding impacts, ponding on roadways and poorly
drained areas, with localized urban and small stream flooding, is
likely with the heaviest and most frequent rains.

Dangerously hot conditions are likely again today, prompting
Excessive Heat Warnings for lower elevations and coastal areas of
Puerto Rico from 9 AM to 6 PM AST. These conditions pose a serious
risk to anyone without adequate cooling or hydration. Heat
Advisories are also in effect for valleys and urban areas of the
eastern and western interior sections of Puerto Rico, Vieques,
Culebra, and the US Virgin Islands, indicating a significant risk
for individuals sensitive to heat. When outdoors, take extra
precautions: wear lightweight, loose-fitting clothing and limit
strenuous activities to early morning or evening. Staying hydrated
and recognizing signs of heat exhaustion are crucial. Since
dangerous hot conditions will likely continue, an Excessive Heat
Watch is now in place for Thursday.

.LONG TERM...Saturday through Wednesday...

Current model guidance suggests precipitable water (PWAT) at
below normal to normal values this weekend, with only few patches
of more humid air moving through the islands, last remnants of a
past frontal boundary pushed towards the region by a high pressure
system over the Atlantic. This system will also promote
southeasterly surface windflow this weekend. This will steer
warmer tropical air towards our area and promote overall warm
conditions for the islands. Heat related products will likely be
issued for the islands later on. 925 mb temperatures are forecast
to be above normal this weekend. Saharan dust particles will
increase to start the period as well, promoting hazy skies during
the weekend at least. Most available moisture will remain below
850 mb. Although model guidance suggest some moisture over the
mid levels, a more notable increase is suggested by Tuesday and
into Friday. This increase in moisture, will lead to increasing
PWAT values Tuesday and Wednesday, in part due to the possible
passing of a tropical wave. High pressure aloft will also
strengthen over the Western Caribbean through at least the
weekend. Model guidance suggests more easterly winds to start the
workweek as the Atlantic surface high weakens, uncertainty
remains.

&&

.AVIATION...

(06Z TAFS)

VFR conditions will likely prevail across all terminals during the
next 24 hours. However, SHRA/TSRA may result in brief MVFR
conditions at TJSJ and TJBQ between 12/16-22Z and after 12/22Z at
TJSJ and USVI terminals. Light to calm and variable winds will
become more easterly and increase to 14-18 knots between 12/13-23Z,
accompanied by higher gusts and sea breeze variations.

&&

.MARINE...

Surface high pressure over the Atlantic will promote moderate to
locally fresh easterly winds. A tropical wave, frontal remnants and
an upper trough are expected to increase the potential for showers
and isolated thunderstorms. Moderate winds will back and become east
northeast tonight into early Thursday, veering to become east for
the rest of Thursday. Winds will then continue to diminish and veer,
becoming east-southeast to southeast throughout the weekend.


&&

.BEACH FORECAST...

There is a moderate risk of rip currents for St. Croix, NW to
north-central PR, and SW Puerto Rico today. Although the rip
current risk decreases to low for Puerto Rico tonight, a moderate
risk will persist for easternmost St. Croix. Low risk elsewhere
today and tonight.


&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Excessive Heat Warning from 9 AM this morning to 6 PM AST this
     evening for PRZ001>003-005-007-008-010-011.

     Excessive Heat Watch from Thursday morning through Thursday
     afternoon for PRZ001>003-005-007-008-010>013.

     Heat Advisory from 9 AM this morning to 6 PM AST this evening
     for PRZ004-009-012-013.

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

     Heat Advisory from 9 AM this morning to 6 PM AST this evening
     for VIZ001-002.

AM...None.

&&

$$

SHORT TERM...ICP
LONG TERM....MRR