Special Weather Statement, NWS JSJ

Area Forecast Discussion
National Weather Service San Juan PR
335 PM AST Thu Aug 1 2024

.SYNOPSIS...

The unstable weather pattern will persist through at least Friday.
Weather conditions are expected to gradually improve by the
weekend with drier airmass entering along with some particles of
Saharan Dust.

&&

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

Mostly cloudy skies and occasional showers prevailed across
Puerto Rico and the U.S. Virgin Islands today as a well-defined
tropical wave continued to affect the area. This wave produced a
large area of disorganized showers and thunderstorms over the local
islands and waters. The Culebra and Vieques municipalities and
eastern Puerto Rico, as well as Saint Thomas, Saint John, and
Saint Croix, experienced the most frequent shower and thunderstorm
activity today. Temperature-wise, they were in the low 90s to 80s
along the coastal/urban areas and in the 70s in the mountains as
the cloudiness and rainfall activity dominated.

Overnight, we foresee periods of thunderstorms and rainfall activity
across Puerto Rico and the U.S. Virgin Islands from time to time
as weather conditions remain unstable, with precipitable water
values in the 75th percentile or above normal values for this time
of the year. Therefore, there is a moderate to high chance of
showers and thunderstorms through at least the beginning of the
weekend.

On Saturday, the trailing moisture could promote some rainfall,
particularly during the overnight and early morning hours across
the windward sections. The sea breeze variation, excessive
heating, and local effects will enhance the afternoon convection
across the mountains and downwind from them. However, heat indices
in the 100 degrees will return during the weekend, especially in
areas where it will not rain.

&&

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

Relatively calmer weather conditions will dominate the long-term
period compared with the past week. An easterly perturbation is
expected to move across the area by the weekend along with light
to moderate concentrations of Saharan Air Layer. Patches of
tradewind moisture will come along too, promoting the enhancement
of shower activity over portions of the islands. This activity
will be more focused over the western and northwestern portions of
Puerto Rico due to the combination of southeasterly winds,
diurnal heating and local effects. In addition, the San Juan
streamer could bring showers into the San Juan metro area and
adjacent municipalities. Across the U.S. Virgin Islands, fair
weather conditions will dominate as well, with little or none
precipitation over the islands.

A continuation of the dry weather pattern is expected by mid-week,
with the presence of a strong high pressure in the central
Atlantic and the approach of another pulse of Saharan Air Layer.
In response to the surface high pressure, the potential for heavy
showers and thunderstorms will remain limited. However, a strong
thunderstorm cannot be ruled out across the interior and western
Puerto Rico during the afternoon hours.

Warm temperatures will continue to prevail. With highs in the upper
80s to low 90s along the urban and coastal areas, and in the mid 80s
elsewhere. Heat indices could reach over the 108 degrees Fahrenheit
at times, leading to Heat Advisories or Excessive Heat Warning.
Moving into the tropics, at this moment the latest model guidances
(GFS & ECMWF) are not suggesting tropical development for the
local area during the period. Nonetheless, please continue to
monitor the Tropical Weather Outlooks (TWO) provided by the NWS
National Hurricane Center (NHC) as we move into the peak of the
Hurricane Season.

&&

.AVIATION...
(18Z TAFS)

SHRA/TSRA may result in brief MVFR to IFR conditions across the
local terminals throughout the fcst period through 01/22Z and after
01/22Z across TJSJ and USVI terminals. ESE-SE at 10-20 kt with
higher gusty winds due to sea breeze, becoming light to calm and
variable between 01/22Z and 02/13Z. Higher wind and gusts are
possible near SHRA/TSRA.

&&

.MARINE...

The departing tropical wave will continue to bring showers and
thunderstorms along with locally higher seas and squally weather
conditions through the end of the workweek. Moderate to fresh easterly
winds will generate choppy to occasionally rough seas late in the
workweek and into the weekend.

&&

.BEACH FORECAST...

The moderate risk of life-threatening rip currents along the
northern and eastern beaches of Puerto Rico, including Culebra and
St. Croix should prevail tonight.

&&

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

SHORT TERM...MMC/ERG
LONG TERM...MMC/ERG/RC
AVIATION...ICP
MARINE/BEACH FORECAST...MMC