038
FXCA62 TJSJ 172000
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
400 PM AST Tue Sep 17 2024

.SYNOPSIS...

Fair weather conditions will continue this evening, followed by
increased shower activity later tonight. A typical shower pattern
is expected over the next few days, heightened by the end of the
week due to increased instability and available moisture.
Excessive heat levels in the coming days will pose a risk to
vulnerable individuals, especially those outdoors without proper
cooling or hydration. A moderate rip current risk will also
persist for the next few days.


&&

.SHORT TERM...Today through Thursday...

Radar, satellite, and surface analyses indicate generally fair
and warm-to-hot conditions under mostly sunny to partly cloudy
skies throughout the day. Afternoon showers began developing
around 2 PM AST over parts of the western interior and western
Puerto Rico. So far, the highest radar-estimated rainfall totals,
around half an inch, have been recorded in Las Marias, Aguada, and
Moca in western Puerto Rico. Surface observations show
consistently hot temperatures of 90-95 degrees Fahrenheit across
most lower elevations of Puerto Rico and the local islands, with
heat indices climbing into the mid 100s. In contrast, higher
elevations have reported temperatures in the upper 70s to lower
80s. Winds were predominantly from the east to east-northeast at
15-20 mph, with gusts reaching up to 25 mph along coastal areas.
Radar imagery also indicated an intense wildfire, reported by
emergency managers within Camp Santiago`s facilities in Salinas.

The short-term forecast highlights two periods of significantly
below-normal precipitable water (PWAT) for September: tonight and
Wednesday night, with values dropping as low as 1.2-1.4 inches.
PWAT levels will likely rise to typical or above-normal ranges on
Wednesday and Thursday, reaching 2.0-2.2 inches. Additionally,
intense heat is anticipated, with 925 mb temperatures exceeding
two standard deviations above normal, potentially necessitating
Excessive Heat Advisories or Warnings. A meandering deep layer
high to the west-northwest will be gradually replaced by a
deepening tropical upper tropospheric trough (TUTT) moving in
from the east-northeast. This transition will increase
instability and the potential for isolated to scattered
thunderstorms as the trade wind cap inversion weakens and 500 mb
temperatures decrease to around -6C. At the surface, easterly
winds will gradually shift from the northeast and weaken on
Wednesday, with lighter and more variable steering flow expected
on Thursday.

Tonight`s forecast calls for fair weather conditions to shift
towards increased shower activity over northern and eastern
sections as a patch of low-level moisture moves through the area.
Flooding impacts are not expected, with localized rainfall totals
potentially reaching around a quarter of an inch. For those
planning to view tonight`s Super Harvest Moon partial Lunar
Eclipse, skies should remain sufficiently clear until
approximately 2 AM AST. 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.

While the increase in available moisture on Wednesday will likely
enhance afternoon convection, the risk of flooding remains
localized and limited. If any areas are at risk, they would be in
the western-southwestern quadrant of Puerto Rico, influenced by
northeasterly winds. Fair weather conditions will likely return on
Wednesday night with the arrival of a drier air mass. However, an
additional increase in moisture will lead to increased and slow-
moving showers with isolated thunderstorms on Thursday, with a
broader area likely experiencing limited flooding 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...Friday through Monday...

From previous discussion issued at 500 AM AST Tue Sep 17 2024

Recent model guidance indicates that an upper-level trough will
deepen and move southward between Puerto Rico and the northern
Leeward Islands by Friday. This will help destabilize the upper
levels and increase the likelihood of early morning and afternoon
convection across the interior and western sectors of Puerto Rico.
Additionally, the latest satellite-derived precipitable water
values show an increase from mostly below normal to normal levels.
Therefore, while there wont be enough moisture for widespread
rain, there will be enough to develop showers. Throughout 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 and diurnally induced slow-moving showers and
thunderstorms with locally heavy rainfall, especially in the
central and western sections of Puerto Rico.

By the weekend, surface winds are expected to shift to a more south-
southeasterly direction and remain very light. The latest models
suggest that the area of convergence associated with the upper-
level trough will be over the region, which may help to limit rain
chances. However, daytime heating and local effects will continue
to enhance afternoon convection in some areas, particularly in
the interior, northwestern, and north-central Puerto Rico. In
contrast, the U.S. Virgin Islands is expected to experience mostly
isolated showers.

Hot conditions are likely to persist throughout this period, with
temperatures potentially reaching the upper 90s and heat indices
remaining elevated. Temperatures at 925 mb will remain well above
normal through the weekend. The combination of high temperatures
and moisture could create hazardous heat conditions, particularly
in urban and low-elevation areas. Therefore, it is very likely
that Heat Advisories and possibly Excessive Heat Warnings will be
issued daily for some areas. Residents and visitors should 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)

VFR conds will cont across the area thru 18/15Z except in the vcnty
of isold SHRA that are expected to dvlp in WNW PR btwn 17/18-22Z and
sct SHRA alg the N coast of PR east of Dorado aft 18/09Z. No
significant impacts expected at TAF sites. Wind flow is ESE 10-16 kt
with sea breeze influences and gusts up to 25 kt but will bcm less
than 10 kt aft 18/00Z with land breezes. Sea breezes return with E
flow 10-15 kt aft 18/14Z. Max winds E-ESE 15-23 kt btwn FL011-105.


&&

.MARINE...

A weak surface high-pressure system will maintain gentle to
moderate easterly winds. Beginning Tuesday, winds will shift from
the northeast and gradually weaken, becoming more variable by mid-
week. Increased shower activity is expected over the Atlantic
local waters and Caribbean passages tonight. There is also a
possibility of a few thunderstorms off the western coast of Puerto
Rico each afternoon. Overall, seas will range from 3 to 5 feet,
occasionally reaching up to 6 feet in most local waters, with lower
seas along coastal areas over the next few 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...Excessive Heat Warning until 5 PM AST this afternoon for PRZ001-
     005-008-010.

     Heat Advisory until 5 PM AST this afternoon for PRZ002-003-007-
     011>013.

VI...Excessive Heat Warning until 5 PM AST this afternoon for VIZ002.

     Heat Advisory until 5 PM AST this afternoon for VIZ001.

AM...None.
&&

$$

SHORT TERM...WS/CVB
LONG TERM....ICP
AVIATION...WS/CVB
PUBLIC...MRR/MMC