096
FXCA62 TJSJ 151707 AAA
AFDSJU

Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
107 PM AST Mon Jul 15 2024

.UPDATE...A few showers have developed around the northern U.S.
Virgin Islands. These are moving with the lower level flow at
about 11 knots from the east (086 degrees). A streamer has also
formed west of Saint Croix. Owing to the much drier air above the
marine layer, cloud formation has been delayed. Precipitable water
values were 1.14 inches. Some moisture is expected to come in over
the southeast corner of the forecast area. The air over western
Puerto Rico is quite dry, but it is still too early to rule out at
least some showers and a thunderstorm during the late afternoon.
Some adjustments have been made to PoPs due to an approaching
trough/tropical wave. Minor adjustments were also made to the
temperature grids.

&&

.AVIATION...
(12Z TAFS)

VFR conds expected to continue for the next 24 hours except for
some SHRA activity in wrn interior PR btwn 15/18-21Z and ovr El
Yunque aft 16/08Z. E flow to gradually bcm ESE 10-15 kts. Aft
15/22Z land breezes at less than 10 kt are expected. Winds aft
16/14Z 10-15 kt with gusts to 23kt in sea breezes.

&&

.PREV DISCUSSION... /issued 456 AM AST Mon Jul 15 2024/

SYNOPSIS...
Tutt and induced low level trough will continue to cross the region
today trough Tuesday, with trailing moisture to affect the area as the
east northeast winds become more southeasterly by midweek. Hot, hazy
and humid conditions will persist with another extensive plume of
Saharan dust forecast to affect the region by mid week. Good daytime
heating along with local sea breeze variations and the proximity of
the upper trough will aid in enhancing afternoon shower and thunderstorm
development particularly across the interior and southwest sections
of Puerto Rico, as well as over the coastal waters and local passages
today.

SHORT TERM...Today through Wednesday...

Current satellite derived precipitable water (PWAT) values indicate
a dry air mass approaching the region from the northeast. Lows last
night were in the upper 70s to low 80s at lower elevations and in
the upper 60s to low 70s in higher elevations and valleys. Surface
winds were generally from the east northeast at 5 to 10 mph. Showers
moved across the area during the overnight hours, leaving mostly
minimal accumulations over northern, interior and eastern Puerto
Rico since midnight. Radar estimated rainfall indicate up to 0.20 to
0.24 in at localized sectors of north-central and western interior
Puerto Rico since midnight.

Throughout the day, drier air will plummet available PWAT to below
normal values for this time of the year. PWAT values could even
decrease to 2 standard deviations below normal, around an inch. For
today, only patches of more humid air reaching the region, and
afternoon convection can promote local areas of more normal PWAT
values. Instability will be present, however, as a TUTT and an
induced surface trough continue to cross the region today through
Tuesday. Daytime heating, sea breeze convergence and these features
will aid in enhancing afternoon shower and thunderstorm development.
Surface winds will be from the east northeast veering and becoming
lighter from the east later in the afternoon. Winds will then
continue to veer to become east-southeasterly, while also
increasing, by Tuesday and Wednesday as the trough retrogrades
westward and a surface high-pressure continues to builds over the
central Atlantic. Current model guidance indicate PWAT values
increasing and reaching above normal values, above 2 inches, by late
Tuesday morning and persisting throughout most of the period.
Afternoon showers and thunderstorms today will concentrate over the
interior to the southwestern quadrant of Puerto Rico. For tomorrow
and Wednesday, afternoon activity should focus mainly on the
northwestern quadrant of Puerto Rico as steering winds become more
east-southeast. The risk of minor urban and small stream flooding
will persist in isolated areas mainly over the central interior and
west sections of Puerto Rico steered by low level winds. Patches of
moisture and showers will affect some windward sectors of the
islands during the overnight to morning hours each day.

Lingering, but overall decreasing, Saharan dust will be present
today and tomorrow. However, a more prominent Saharan Air Layer
(SAL) will approach the islands by Wednesday morning, prompting hazy
skies and decreased air quality to end the short term period.
Aerosol Optical Depth (AOD) values can reach around 0.40. This can
exacerbate respiratory conditions in immunocompromised, vulnerable
and sensitive groups. Maximum temperatures will be in the upper 80s
to around the low 90s in lower elevations of the islands. The
Saharan dust can also inhibit nighttime cooling (especially with
more prominent dust like on Wednesday), prompting warmer than normal
nights. Heat indices in coastal, urban and lower elevation areas can
reach above 108 degrees Fahrenheit. This will be further exacerbated
by east-southeast winds from tomorrow onwards. Hot temperatures and
high humidity may cause heat illnesses.

LONG TERM...Thursday through Tuesday...
Recent model guidance suggests a strong surface high pressure will
remain anchored across the Atlantic and north of the region to
maintain moderate to strong east southeast winds on Thursday. Winds
are then to become more east northeast by Friday into the following
weekend as a tropical wave will enter the eastern Caribbean and
cross the forecast area. This is expected to bring increasing
moisture and instability to the region along with a wind surge and
moderate to strong southeasterly winds by Saturday afternoon, as
the tropical wave crosses the region. Accompanying the wave and
thereafter another extensive plume of dense Saharan dust is forecast
to quickly spread across the region Sunday through Monday then
gradually diminish from Tuesday onwards. By then winds are to become
more easterly with yet another Tutt and induced surface trough
forecast to approaches from the east.

All in all the most impactful period will so far be late Thursday
through Saturday, when enhanced overnight and afternoon convection
is expected due to the tropical wave along with good moisture
convergence and cooler advective temperatures aloft. This will be
followed by hazy/dusty conditions and a somewhat drier airmass
due to the forecast dense Saharan dust particulates which will
maintain hot and humid conditions. Temperatures are expected to
continue slightly above normal and elevated to significant maximum
daytime heat indices are forecast to be reached at times in some
coastal and urban areas. Marine conditions area also expected to
deteriorate by Friday and into the weekend due to the tropical wave
and the increasing trade winds. Therefore precautionary statements
and small craft advisories may be required for all or portions of
the local waters and passages by that time.

AVIATION...
(06z TAFs)

Mainly VFR conditions across the terminals. Winds will be ENE, bcmg
E later in the afternoon then ESE by tomorrow. VCSH AND VCTS are
forecasted for the interior and SW PR, including TJPS, at around
15/17Z-22Z. This can promote brief MVFR conditions. Brief VCSH
across other terminals possible during the period. Lingering but
decreasing saharan dust present, visibility P6SM.

MARINE...
Surface high pressure across the western Atlantic will promote
light east to northeast winds today, then become more southeasterly
and increase to moderate on Tuesday and become breezy by midweek
due to the arrival of a tropical wave and accompanying wind surge.
As the wind increases, choppy seas will be likely across the local
waters and passages, resulting in seas up to 7 feet and winds up
to 20 knots. Small craft advisories and precautionary statement
will be required by then.

BEACH FORECAST...
The rip current risk will be low today, however life-threatening
rip currents often occur in the vicinity of groins, jetties and
piers. By Tuesday and for the remainder of the workweek, there
will be a moderate risk of rip current for most beaches and
therefore life-threatening rip currents will be possible in the
surf zone.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>003-005-
     008-010-012-013.

VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.

AM...None.

&&

$$

SHORT TERM/MARINE/AVIATION...WS
PUBLIC....GRS
UPPER AIR...MNG