Caribbean - Central America Weather

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
451 AM AST Sat Jul 19 2025

.KEY MESSAGES...

* Another hot day for the US Virgin Islands and Puerto Rico
today, before the formation of intense thunderstorms during the
afternoon and evening of Saturday, which could lead to flooding
rains.
* The trailing moisture from today's surface trough will
bring another round of thunderstorms on Sunday afternoon.
* The excessive heat trend is expected to persist throughout
much of the forecast period, particularly between Tuesday and
Thursday.
* If model guidance follows the correct track, a robust tropical
wave could bring a wet and unstable weather pattern around next
Friday.

&&

.SHORT TERM...Today through Monday...

Satellite and radar imagery showed partly cloudy skies and isolated
showers during the overnight period across the local waters, with
some showers reaching the northern and eastern coasts of Puerto Rico
and the U.S. Virgin Islands (USVI). Radar estimates did not indicate
significant rainfall accumulations. Temperatures ranged from the
upper 60s in the mountains to the upper 70s and low 80s in coastal
urban areas. Winds were mainly from the northeast at 10 mph or less.

The short-term forecast remains on track. Today, a TUTT low moving
north of the region will induce a surface trough that will pass near
the islands, enhancing afternoon convection and increasing the
potential for thunderstorms. Model guidance suggests that
precipitable water (PWAT) values will remain between 1.7 and 1.9
inches. Winds will shift from the northeast to the east-southeast
throughout the day. As a result, expect showers across windward
locations during the morning, followed by an active and unstable
afternoon with showers and isolated to possibly scattered
thunderstorms over interior and northwestern Puerto Rico, as
indicated by the Galvez Davison Index (GDI). The USVI also has a
good chance of experiencing occasional periods of showery weather
throughout the day. There is an elevated risk of ponding of water on
roads and in poorly drained areas, as well as localized flooding in
urban zones, roadways, and small streams. In addition, a period of
warm to hot heat indices is expected to persist. Although showers
and thunderstorms may help reduce the heat risk, high humidity will
contribute to muggy conditions in urban and coastal areas of Puerto
Rico, especially before rain develops or in places that receive
limited rainfall. As a result, a Heat Advisory is in effect between
10 a.m. and 4 p.m. AST for coastal urban areas of Puerto Rico.

Looking ahead to Sunday and Monday, expect a more typical summer
pattern for the tropics, with morning showers across the eastern
half of Puerto Rico and the USVI, followed by afternoon convection
mainly confined to the northwestern quadrant of Puerto Rico.
Rainfall should be more localized, and the flood risk will be
limited. However, warm to hot temperatures will continue. Residents
and visitors are encouraged to stay hydrated, limit outdoor activity
during peak heat hours, wear light clothing, and check on vulnerable
individuals and pets.

.LONG TERM...Tuesday through Saturday...

A mid-level high pressure will build over the northeast Caribbean
from Tuesday into next weekend, promoting dry air aloft, at least
through Friday. While a surface high-pressure will persist across
the Atlantic Ocean, promoting an increase in local winds from
around Wednesday into the end of the workweek. A weak easterly
perturbation is expected to move across the islands around
Tuesday, enhancing the potential for rain and thunderstorm
activity, especially during the afternoon hours. After Tuesday's
easterly disturbance, the local weather will be dominated by the
arrival of patches of moisture embedded in the trades, which will
promote the climatological hydrological pattern of the US Virgin
Islands and Puerto Rico on Wednesday and Thursday. Based on the
latest model guidance during this period, the primary hazard could
be excessive heat resulting from the combination of available
moisture and maximum temperatures across the islands.

A tropical wave, which currently appears to be the primary rain
and thunderstorm producer, is expected to arrive around Friday,
and its trailing moisture could persist near the region through at
least next Saturday, potentially pausing the excessive heat
trend. We are monitoring this wave, as it could increase the
potential to observe flooding rain, especially around Friday, for
both PR and the USVI. Additionally, NHC is currently monitoring
this same wave in their Tropical Weather Outlook for the Atlantic
waters, and it has a low chance of formation over the next 7 days.