National Weather Service Today's Weather Impact Levels

105
FXCA62 TJSJ 090112 AAA
AFDSJU

Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
912 PM AST Sun Sep 8 2024

.UPDATE...SHRA have developed inland from the northeast coast of
Puerto Rico from Luquillo to Manati. Showers and isolated
thunderstorms will continue over the local waters--mainly over the
Caribbean--moving west northwest at about 20 knots. Better
moisture is forecast to arrive aft 09/06Z. Winds from the
previous run of the GFS looked better than the 18Z run and so they
were kept. No changes have been introduced to the afternoon
package since it was issued, therefore the zones will not be re-
issued.

&&

.AVIATION...
(00Z TAFs)

As the tropical wave moves toward PR, SHRA and clusters of SCT
TSRA are forming. Altho some SHRA are forming alg the NE coast of
PR just inland, most SHRA/TSRA will cont to form ovr the lcl
waters. Flow is E-ESE at abt 10 kt at the sfc-- lighter ovr land
with land breeze influences. VFR prevails except for some MVFR in
SHRA and some IFR in and arnd TSRA especially south of ern PR.
SHRA/TSRA will cont thru 09/14Z and then dvlp ovr land thru
09/20Z, bringing widespread MVFR/IFR to wrn and interior PR. Mtn
obscurations. Aft 09/14Z some sea breezes are expected at 10-15
kts.


&&

.PREV DISCUSSION... /issued 346 PM AST Sun Sep 8 2024/

SYNOPSIS...

Expect shower and thunderstorm activity during the next few days
as the tropical upper tropospheric trough (TUTT) and a broad
tropical wave continues to move across the area. Southeasterly
winds and a high- moisture environment will also contribute to
excessive heat risks. The National Hurricane Center (NHC) is
monitoring an area, currently in the central tropical Atlantic,
with a medium cyclonic formation chance in the next 48 hours and
a high formation chance in the next seven days.

SHORT TERM...Today through Tuesday...

Doppler radar observed isolated shower activity across eastern
Puerto Rico and the U.S. Virgin Islands in the morning. Around 10
AM, more well-developed showers began streaming across the eastern
sections of Puerto Rico, affecting Luquillo, Rio Grande, and
Fajardo. Afterward, the shower activity started to develop across
most of the eastern sections of Puerto Rico, the interior, and
the northwestern regions. Intense shower activity resulted in
rainfall accumulations between 1 and 3 inches, with the highest
amount observed in San Sebastian, where radar estimates indicated
around 3 inches. Although cloud cover persisted throughout the
morning, maximum temperatures climbed into the upper 80s to lower
90s, with heat indices ranging from 108 to 110 degrees
particularly along the southern coastal areas.

Unsettled and wet weather conditions will prevail for the rest of
the forecast period. The upper-level low will continue to linger
across the islands, resulting in an unstable pattern conducive to
the development of showers. Meanwhile, at the surface, a tropical
wave moving westward into the region will provide abundant moisture,
with PWAT values of 2.1 inches as indicated by derived GOES
satellite images. This combination will trigger active afternoon
showers and variable weather conditions during the late and early
morning hours. Given these expected conditions, the risk of
excessive rainfall will remain elevated, mainly for Puerto Rico.
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. Across the U.S. Virgin
Islands, minor flooding may occur on roads and in poor drainage
areas as shower activity increases.

Temperature-wise, the islands can expect warmer conditions, with
maximum temperatures in the upper 80s to lower 90s across most urban
and coastal areas. Heat indices could exacerbate health issues,
particularly in vulnerable populations. Given the forecast
conditions, heat advisories and warnings are likely to be issued.

LONG TERM...Wednesday through Sunday...
//from previous discussion//

An upper level trough will be present northeast of the local islands
to start the period, before gradually weakening throughout the
workweek. By the end of the week, a mid-level ridge will reduce
instability aloft. Precipitable water (PWAT) values will continue
mainly above 2.1 inches for the start of the period, 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 generates
afternoon convective activity, increasing the risk of flooding
during the afternoon for sectors of interior and western Puerto
Rico. Lines of convection (showers and thunderstorms) 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
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) is monitoring an area, currently in the
central tropical Atlantic, with a medium cyclonic formation chance
through the next seven days. While it`s too early to assess any
potential impact on our forecast area, please continue to remain
vigilant and monitor future forecast updates from the NHC and from
WFO San Juan.

AVIATION...
(18z)

SHRA and TSRA are resulting in MVFR conditions across
all TAF sites. Winds will continue from the E-SE at up to 15 knots
with gusts. TSRA and SHRA may lead to lower ceilings across the
mountains and reduced visibility. Another round of SHRA with TS is
forecast for all eastern TAF sites from 09/09Z to 09/15Z.

MARINE...

A surface high pressure across the Atlantic will promote gentle to
moderate easterly winds over the next few days. An upper-level
trough to our north and a broad tropical wave moving in from the east
will boost shower and thunderstorm activity during the next few
days. Thunderstorms will affect the coastal waters of western Puerto
Rico each afternoon.

BEACH FORECAST...

A moderate rip current risk is present along the northern and
eastern-facing beaches of Puerto Rico and Culebra. Low risk
elsewhere, however, life-threatening rip currents often occur in
the vicinity of groins, jetties, reefs, and piers.

&&

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

&&

$$

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

NWS SJU Office Area Forecast Discussion