170
FXCA62 TJSJ 210358 CCA
AFDSJU

Area Forecast Discussion...CORRECTED
National Weather Service San Juan PR
1158 PM AST Fri Sep 20 2024

.SYNOPSIS...

Fair weather is expected to return tonight as conditions become
less conducive to shower activity. A typical pattern of calm
nights and mornings, followed by warm-to-hot afternoons and
evenings driven by diurnal heating and weak steering winds, will
prevail in the coming days. Excessive heat will continue to pose a
risk to vulnerable individuals, particularly those outdoors
without proper cooling or hydration. Additionally, the moderate
risk of rip currents will persist over the next few days.

&&

.SHORT TERM...Tonight through Sunday...

Radar, satellite, and surface analyses indicate generally fair,
warm-to-hot conditions, with skies becoming progressively partly
to variably cloudy. Shower activity began late this morning, with
showers developing over the far eastern and southeastern slopes of
Puerto Rico, followed by showers with strong isolated
thunderstorms developing over and to the north and west of the
Cordillera Central. So far, radar estimates indicated the highest
rainfall totals, around 1.5-2.0 inches, from southern Bayamon
westward to Ciales, prompting the issuance of Special Weather
Statements and Flood Advisories. Surface observations report
consistently hot temperatures up to 93-95F, 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 20 mph along
coastal areas.

The short-term forecast indicates typical precipitable water
(PWAT) levels between 1.7 and 2.1 inches, briefly interrupted by a
drier-than-normal phase, with PWAT falling to 1.4-1.5 inches from
early Saturday into Sunday morning. A tropical upper tropospheric
trough (TUTT) will gradually lift northeastward in the coming
days, allowing the trade wind cap inversion to form and promoting
the entrainment of drier air. While these conditions are generally
unfavorable for thunderstorm activity, peak daytime heating will
still support afternoon thunderstorms. By Sunday night, the
proximity of an upper-level low and increasing wind speeds aloft
could create more favorable conditions for thunderstorm
development. A weak surface low-pressure system to the northeast
will maintain light wind speeds, mainly with a south-southeasterly
component. Despite the moisture, intense heat will continue, with
925 mb temperatures rising more than two standard deviations
above normal for September, potentially requiring Excessive Heat
Advisories or Warnings.

Tonight`s forecast suggests a shift towards more stable conditions
and reduced shower activity, with clearer skies, as the
environment becomes less conducive to shower development. However,
isolated to locally scattered showers remain possible, especially
in the southern and eastern coastal areas of Puerto Rico. Winds
will be light and variable, with overnight temperatures ranging
from the upper 60s in the higher elevations to the lower 80s in
the coastal areas and the local islands.

Calm nights and mornings, followed by warm-to-hot and stormy
afternoons and evenings, are expected to continue through the
weekend. Increased moisture and instability may result in more
showers and isolated thunderstorms by Sunday night into early next
week. For updates on excessive heat, heavy rainfall, or other
potential hazards in the days ahead, visit the Experimental
Graphical Hazard Weather Outlook at
https://www.weather.gov/erh/ghwo?wfo=sju.

.LONG TERM...Monday through Thursday...

From previous discussion issued at 402 AM AST Fri Sep 20 2024

A deep-layered trough will continue to dig just north of the
Caribbean islands. This through will act to maintain a weak
steering flow, with speeds of 1 to 5 knots on Monday and Tuesday.
The through will gradually moves away by mid-week, with winds
gradually recovering speed, but only to around 10 knots. At the
surface, induced patches of moisture will move from time to time
through the area, although there is no particular day with a
great amount of low level moisture. This moisture will act to
generate some showers reaching the coastal areas of Puerto Rico
and the Virgin Islands each day. However, the strongest activity,
with thunderstorms, is expected each afternoon across the
Cordillera Central, with the activity then drifting toward the
coast. Since the winds will be so weak, it is difficult to
determine which area is going to be more affected, aside than the
interior that should receive the bulk of the activity each day.

As has been the case in the past couple of weeks, above normal
temperatures will persist in the area, with the threat of
excessive heat each day. This heat will mainly impact those
without effective cooling or adequate hydration.


&&

.AVIATION...
(18Z TAFs)

SHRA/TSRA over mainland PR may cause tempo MVFR conds at TJBQ/TJSJ
thru 22z. Thereafter, mainly VFR conditions are expected to prevail
across all terminals through the forecast period. Light winds with a
southerly component will prevail through Saturday.

&&

.MARINE...

A weak surface low pressure to the northeast of the region will
maintain weak and variable winds throughout early next week. Showers
and isolated thunderstorms will persist across the regional waters,
particularly off the western and southern coastal waters of Puerto
Rico each afternoon.

&&

.BEACH FORECAST...

The risk of rip currents will remain moderate for the beaches of
northern Puerto Rico and Culebra on Saturday. The risk will drop
to low on Sunday across all local 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/RAM
LONG TERM....ICP
AVIATION...DSR/RAM
PUBLIC...GRS/MNG/CVB