234
FXCA62 TJSJ 100117 AAA
AFDSJU

Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
917 PM AST Mon Sep 9 2024

.UPDATE...Rainfall coverage over land and sea has greatly
diminished since 09/22Z, but isolated to sct SHRA cont over the
Caribbean and to a lesser extent Atlantic waters. The evening
sounding from San Juan, Puerto Rico at 10/00Z came in with a deep
and rich moisture profile, with precipitable water coming in at
2.34 inches and no truly dry layers. Therefore although the
atmosphere is not particularly unstable and does have some
negative buoyancy or convective inhibition in the very lowest
layer, thunderstorms tonight cannot be ruled out. Almost all of
these will be over the local waters. Currently most echo tops are
between 20 to 30 Kft, but a few just outside the forecast area
reach 38 kft. Total precipitable water over the area will decrease
to just below 1.9 inches by early Wednesday morning. This is still
just above the normal value for the month. So the expectation is
for continued above normal rainfall across the area.

&&

.AVIATION...
(00Z TAFs)

VFR conds will prevail except for isolated SHRA/TSRA in ern PR
where mtn obscurations and MVFR/IFR conds will occur. Sfc and llvl
flow is now E at 5-10 kt. Some land breeze influences are seen but
some SHRA were still moving onshore in NEern PR. Aft 10/14Z winds
will become E 10-14 kt with sea breeze influences. SHRA/TSRA will
incrs btwn 10-14-17Z, forming mainly in wrn and interior PR.

&&



.PREV DISCUSSION... /issued 548 PM AST Mon Sep 9 2024/

SYNOPSIS...

A wet and unstable pattern will persist for the rest of the short
term period increasing the flood threat risk across the islands.
For the long term period, the National Hurricane Center is
monitoring an area named Invest 92L with a 60% chance of formation
in the next 7 days. Residents and visitors should monitor the
development as the tropical wave moves in the central Atlantic.

SHORT TERM...Tonight through Wednesday

An active afternoon was observed across the islands today due to
the arrival of tropical moisture from a tropical wave moving
westward, with its axis over Hispaniola. The heaviest activity
began across northeastern Puerto Rico, followed by active
convection across the interior sections of the islands. According
to the latest Doppler radar, rainfall accumulation estimates
reported over 6 inches in the northern section of San German and
around 3 inches near the boundaries of Utuado, Adjuntas, and
Jayuya. Doppler radar estimates of 1 to 2 inches of rain were
observed across other sections. At around 3 PM another round of
showers started to affect the southeastern coastal areas of
Puerto Rico, leaving almost 2.5 inches over Patillas. Temperature-
wise, the islands experienced highs in the upper 80s to lower 90s
in most coastal areas and upper 70s to mid-80s across the
mountains. Despite persistent cloudiness across the eastern
sections in the morning, heat indices climbed to 108-112 degrees
in the northwest to north-central areas, the west coast, and the
southwestern coastal regions.

A Tropical Upper Tropospheric Trough (TUTT) will continue to
influence the region tomorrow, moving eastward into the central
Atlantic. This will result in colder temperatures at 500 mb,
making atmospheric conditions more conducive to widespread
thunderstorm activity. A favorable pattern will also be present at
the surface, as abundant tropical moisture with a PWAT of 2.3
inches, as shown by GOES-derived imagery, continues to reach the
local islands. These combined conditions will lead to an unstable
weather day, particularly in the late morning into the afternoon,
influenced by local effects and daily heating. The forecast so
far today, calls for widespread scattered to numerous showers
across the islands, with a chance of isolated to scattered
thunderstorms, as suggested by the Galvez-Davison Index (GDI). The
risk of excessive rainfall will remain elevated, especially for
Puerto Rico, where saturated soils could lead to localized flash
flooding and mudslides, particularly in small streams, creeks, and
areas of steep terrain. Residents and visitors should exercise
caution or avoid outdoor activities during the afternoon. In the
U.S. Virgin Islands, minor flooding may occur on roads and in poor
drainage areas as shower activity increases. The southeasterly
wind flow will continue to feed the islands with abundant moisture
Wednesday, producing a similar pattern of widespread shower
activity over areas with saturated soils. This could result in
urban and minor stream flooding, as well as mudslides in steep
terrain areas.

LONG TERM...Thursday through Monday...
..from previous discussion issued at 447 AM MON SEP 9 2024....

No major changes to the long term forecast. The upper trough that
persisted during the short term period, will gradually move
northward and away from the region early Thursday. A mid-level
ridge will then build and reduce instability aloft. Precipitable
water (PWAT) values will continue mainly above 2.1 inches during
most of the period, which is above normal values for this time of
the year. East to east southeast winds will aid in providing
moisture to the area. Expect variable weather conditions as local,
orographic and diurnal effects generate afternoon shower and
thunderstorm activity, increasing the risk of flooding during the
afternoon for sectors of interior and western Puerto Rico. Lines
of convection can also develop from the USVI, Vieques, Culebra,
and from El Yunque. Periods of passing overnight and early morning
showers are also forecast across windward sectors of the islands.
With available moisture and light to moderate east to east
southeast winds, heat indices in coastal, urban and lower
elevation areas can reach above 108 degrees Fahrenheit. The
Climate Prediction Center`s (CPC) Global Tropics Hazards Outlook
indicates a greater than 50% probability of above-average rainfall
across the northeastern Caribbean during the period.

The National Hurricane Center (NHC) continues to monitor an area
of low pressure (Invest 92L), currently in the central tropical
Atlantic, with a high cyclonic formation chance through the next
seven days. While it is too early to assess any potential impact,
if any, on our forecast area, please continue to monitor future
forecast updates from the NHC and from WFO San Juan.

AVIATION...
(18Z TAFs)

SHRA/TSRA cont to affect most of the interior sections resulting
in lower CIGs and reduction in VIS. That will result in MVFR
conds due to CIGs below FL020-030 across the mountains areas
until 09/23Z. VCSH will persist for all the TAF sites from 09/23Z
to 10/16Z. Winds will diminish to 10 kts or less at 09/23Z,
increasing once again from the E at 10/14Z. Another set of SHRA &
TSRA are forecast for tomorrow resulting in MVFR to IFR
conds across all the TAF sites.

MARINE...

A surface high pressure system across the Atlantic will promote
gentle to moderate easterly winds over the next few days. An
upper-level trough to our north, combined with abundant tropical
moisture moving across the islands, will increase shower and
thunderstorm activity during the next two days. Thunderstorms will
affect the coastal waters of western Puerto Rico each afternoon,
potentially resulting in localized hazardous marine conditions.

&&

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

&&

$$

SHORT TERM/AVIATION/MARINE...WS
UPPER AIR/PUBLIC....MMC