139
FXCA62 TJSJ 261545
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1145 AM AST Thu Sep 26 2024

.UPDATE...

Showers and thunderstorms related to the leading edge of a
tropical wave continue to move across the eastern and southern
waters, affecting the USVI and the eastern third of PR. Therefore,
rain activity and cloudiness will increase across most of the
islands, reducing the risk to reach critical heat thresholds.
Therefore, the Heat Advisory was cancel.

&&

.PREV DISCUSSION... /issued 455 AM AST Thu Sep 26 2024/

SYNOPSIS...

Another day with intense heating across the islands. Therefore, a
Heat Advisory was issued for the coastal areas of Puerto Rico.
Showers and isolated thunderstorms are expected this afternoon
across portions of the interior and western Puerto Rico due to the
combination of local effects and the available moisture. Ponding of
water in poor drainage areas and urban and small stream flooding
is expected with this activity. A tropical wave is expected to
reach the islands on Friday, increasing once again the potential
for showers across the region.

SHORT TERM...Today through Saturday...

Scattered showers and isolated thunderstorms were observed overnight
across the local waters and over portions of the islands. This
activity was associated to an upper-level low that has moved south-
southwest and away of the region. Although warming of the 500 mb
temperature is expected today, shower and thunderstorm development
is still expected due to the proximity of the low and the higher
than normal moisture content across the eastern and southern
Caribbean. Ponding of water in poor drainage areas and urban and
small stream flooding is expected with this activity across PR and
the USVI. Strong gusty winds are possible with this afternoon
thunderstorms over western PR.

Furthermore, a broad tropical wave will increase moisture content
across the islands on Friday, and showers and thunderstorms will
increase in coverage and intensity. Latest model guidance suggest
precipitable water values between 2.00-2.30 inches during the peak
of afternoon convection on Friday. Therefore, urban and river
flooding can be expected across PR, as well as mudslides in areas of
steep terrain. For Saturday, a drying trend is expected as a broad
ridge develops from the western Atlantic and another upper-level low
develops over the Leeward Islands. However, typical afternoon
showers and isolated thunderstorms will develop over the western
sections of PR due to the combination of daytime heating and local
effects.

Even though showers and cloudiness are expected over land areas,
intense heating during the late morning and early afternoon hours
combined with high humidity will promote excessive heat conditions.
Therefore, a Heat Advisory(NPWSJU) was issued for the coastal areas
of PR.

LONG TERM...Sunday through Wednesday...

After the passage of a weak tropical wave across the islands on
Saturday, a drier airmass is expected to quickly move into the area.
Therefore, mainly calm and stable weather conditions are anticipated
from Sunday onward. A migrating surface high pressure located over
the Bahamas and moving into the central Atlantic will promote
easterly winds until Tuesday. On Wednesday, this surface high is
expected to be located north or over the local islands promoting
winds from the northeast. Shower activity will be limited during
this period. However, passing showers dragged by the trade winds
cannot be ruled out across the eastern municipalities of Puerto Rico
and the USVI. In the afternoon hours, some localized showers are
possible due to the combination of moisture, warm temperatures and
local effects. Hot temperatures are very likely during most of the
long-term period with highs reaching the mid 90s along the coastal
and urban areas. Precipitable water will be mostly between 1.55 to
1.80 inches from Sunday through at least late Tuesday.

AVIATION...

(06Z TAFS)

SHRA and -TSRA en route from the Leeward Islands may cause brief
MVFR conds at times across TIST/TISX/TJSJ thru 26/15z. Thereafter,
SHRA/TSRA expected to spread across the interior and western
sections of PR, causing possible TEMPO MVFR conds at TJPS btw 26/18z-
22z. Elsewhere, VCTS expected. East winds expected at 10-15 kt with
locally higher gusts and sea breeze variations aft 26/14z.

MARINE...

A weak high pressure ridging over the region will maintain gentle to
moderate easterly winds over the next few days. A northerly swell
will maintain seas to around 4 to 5 feet through the weekend.
Shower and thunderstorm activity will increase from today into
Friday as a tropical wave approaches our region.

BEACH FORECAST...

There is a moderate risk of rip currents along the northern side
of most of the islands. The risk may become high for the north-
facing beaches of Puerto Rico and Culebra on Friday morning.

&&

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

$$

CAM/MRR/YZR