625
FXCA62 TJSJ 182046
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
446 PM AST Wed Sep 18 2024

.SYNOPSIS...

Generally fair weather this evening, with shower activity
increasing later tonight. A wetter pattern is anticipated in the
coming days, driven by rising instability and moisture under weak
steering winds. Excessive heat will continue to be a concern for
vulnerable individuals, especially those outdoors without adequate
cooling or hydration. Additionally, a moderate risk of rip
currents will remain over the next few days.


&&

.SHORT TERM...Tonight through Friday...

Radar, satellite, and surface analyses indicate generally fair,
warm-to-hot conditions, with skies becoming progressively partly
to variably cloudy. Shower activity began this morning with
isolated showers over eastern Puerto Rico, followed by streamers
downwind of the Sierra de Luquillo and the local islands. The
strongest showers, accompanied by isolated thunderstorms,
developed over the island`s interior and western areas in the
afternoon. Radar estimates indicated the highest rainfall totals,
around 2 inches, between Lares, Las Marias, and Maricao in western
Puerto Rico, prompting the issuance of a Flood Advisory. Surface
observations report consistently hot temperatures up to 93-96F,
with heat indices rising into the mid-100s, while higher
elevations recorded temperatures in the lower 80s. Winds were
predominantly from the east to east-northeast at 10-15 mph, with
gusts up to 25 mph along coastal areas.

The short-term forecast highlights a period of significantly
below-normal precipitable water (PWAT) levels for tonight, with
values dropping to as low as 1.2-1.4 inches, which is about two
standard deviations below normal for September. PWAT will rapidly
increase to above-normal levels on Thursday, reaching 2.0-2.2
inches, before returning to typical values around 1.8-2.0 inches
by Friday. A deep layer high to the west-northwest will be
replaced by a tropical upper tropospheric trough (TUTT) moving in
from the east-northeast in the coming days. This shift will
increase instability and the potential for isolated to scattered
thunderstorms as the trade wind inversion weakens and 500 mb
temperatures drop to around -6C. At the surface, east-
northeasterly winds of 10-15 mph will become more northeasterly
and decrease to 5-10 mph on Thursday before shifting to
southeasterly on Friday, influenced by the remnants of Gordon
located far to the northeast over the central subtropical
Atlantic. Despite the expected moisture and wind variations,
intense heat will persist, with 925 mb temperatures exceeding two
standard deviations above normal thresholds for September, which
may require the issuance of Excessive Heat Advisories or Warnings.

Tonight`s forecast calls again for generally fair weather
conditions to shift towards increased shower activity over
northern and eastern sections as high moisture airmass reaches the
region behind a drier-than-normal airmass later tonight. However,
flooding impacts are not expected, with rainfall totals ranging
from trace to around a quarter of an inch. Expect gentle to light
and variable winds, with overnight lows ranging from the upper 60s
in higher elevations to the lower 80s in coastal areas of eastern
Puerto Rico and the local islands.

Further increase in moisture and favorable conditions will lead to
more showers and isolated thunderstorms on Thursday and Friday,
with a broader area likely experiencing limited to elevated
flooding and lightning risks. For details on excessive heat,
excessive rainfall, or other potential hazards in the coming
days, please refer to the Experimental Graphical Hazard Weather
Outlook at https://www.weather.gov/erh/ghwo?wfo=sju.

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

From previous discussion...issued at 500 AM AST Wed Sep 18 2024/

Latest satellite-derived precipitable water values indicate a
patch of drier air moving into the area around next Saturday, with
PWAT values expected to range from 1.4 to 1.6 inches. Heights will
begin to increase as the axis of an upper-level trough moves away,
leaving the subsiding portion over our region, which will help
limit rain development a little. While there wont be enough
moisture for widespread rainfall, local effects and diurnal
heating will still promote afternoon convection in some areas,
particularly in the interior, northwestern, and north-central
Puerto Rico. In contrast, the U.S. Virgin Islands are expected to
experience mostly isolated showers. Similar conditions are
anticipated to persist into Sunday.

By Monday and Tuesday, moisture will increase from the south,
resulting in better rain chances across the islands. During this
period, winds are expected to remain very light and somewhat
variable. As a result, urban and small stream flooding may occur
due to locally induced slow-moving showers and thunderstorms with
heavy rainfall, especially in the central and western sections of
Puerto Rico.

Hot conditions will likely dominate over the long-term period,
with potentially heat indices remaining elevated. Over the
weekend, guidance are suggesting that temperatures at 925 mb will
remain two standard deviations above normal. The combination of
high temperatures and humidity could create hazardous heat
conditions, particularly in urban and low-elevation areas of the
islands. Therefore, it is likely that Heat Advisories and Excessive
Heat Warnings will be issued. Residents and visitors are encouraged
to monitor the forecast, take precautions to stay hydrated by
drinking plenty of water, wearing light and breathable clothing,
and avoiding prolonged sun exposure, especially during peak heat
hours.


&&

.AVIATION...
(18Z TAFs)

Mainly VFR conditions are expected to prevail across all terminals
through the forecast period. However, locally induced SHRA/TSRA over
portions of the islands may cause tempo MVFR cigs thru 22z. Also,
SHRA is expected to increase aft 19/12z in and around TJSJ/TIST.
Winds will continue to gradually turn from the NE and become lighter
through Thursday.


&&

.MARINE...

East-northeast winds will dominate, gradually becoming variable
and weakening from Thursday throughout the weekend. Showers are
expected over the local Atlantic waters and Caribbean passages
later tonight, with a chance of isolated thunderstorms off the
western coast of Puerto Rico each afternoon. Increased showers
with isolated thunderstorms over the local waters are likely on
Thursday and Friday. Overall, seas will range from 3 to 4 feet,
occasionally reaching 5 to 6 feet in some areas, while southern
coastal regions of the islands will experience lower seas in the
coming days.


&&

.BEACH FORECAST...

The risk of rip currents will remain low at most local beaches,
except for northern Puerto Rico, where a moderate risk will
persist. By Friday and into the weekend, a moderate risk will
return for beaches in western Puerto Rico, Vieques, Culebra, and
the U.S. Virgin Islands. For a more detailed forecast, please
consult the Surf Zone Forecast for Puerto Rico and the U.S. Virgin
Islands (SRFSJU).


&&

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

$$

SHORT TERM...DSR/CVB
LONG TERM....ICP
AVIATION...DS/CVB
PUBLIC...MRR/MMC