616
FXCA62 TJSJ 172129
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
529 PM AST Wed Jul 17 2024

.SYNOPSIS...Favorable conditions aloft and above-normal moisture
levels will maintain the potential for showers and isolated
thunderstorms early tonight. However, a drier-than-normal airmass
and a dense Saharan dust spreading across the northeastern
Caribbean will bring more stable weather and hazy skies by
Thursday. By Friday and into the weekend, a passing tropical wave
will increase the likelihood of showers and isolated
thunderstorms, raising flooding concerns. Despite the expected
increase in cloud cover and high rain chances, warm-to-hot
conditions will persist.


&&

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

Satellite and radar observations have indicated predominantly
sunny to partly cloudy and hazy skies throughout the region, with
occasional showers affecting eastern Puerto Rico and afternoon
convective development over northwestern Puerto Rico. Daytime high
temperatures varied significantly, peaking in the lower 80s in
higher elevations and climbing to the mid-90s in lower elevations.
Meanwhile, heat index values soared above 108-112 degrees,
especially in the lower elevations of northwestern to north-
central Puerto Rico, where Excessive Heat Warnings and Advisories
were in effect. In the U.S. Virgin Islands, daytime highs ranged
around 88 to 94 degrees, with heat indices in the mid 100s. Winds
were generally from the east to east-southeast, with stronger
gusts and sea breeze effects leading to 15-25 mph speeds. Wind
gusts in the 30-40 mph range were reported across northeastern
Puerto Rico, Culebra, and the northern U.S. Virgin Islands.

The short-term forecast period features several key elements: a
series of TUTT lows and surface-induced features, a Saharan Air
Layer (SAL), and a tropical wave. Instability generated by a TUTT
low to the north and above-normal moisture levels across the
region will maintain the potential for showers and isolated
thunderstorms tonight. The highest risk is expected for eastern
Puerto Rico and the local islands, where these storms may bring
heavy downpours, frequent lightning, and gusty winds. As a result,
there are excessive rainfall risks or flooding concerns, with
potential impacts ranging from water ponding on roads, poorly
drained areas, and urban and small stream flooding. Residents and
visitors are advised to stay updated on weather conditions.

By late tonight and continuing through the workweek, a dense SAL
will cause a moderate to severe dust event, worsening health
conditions for immunocompromised individuals, vulnerable
populations, and those sensitive to air quality issues. This event
will also result in hazier skies and reduced visibility. Along
with an expected drier-than-normal airmass, with precipitable
water values dropping near or below seasonal thresholds, expect a
gradual decrease in convective development by Thursday. Dust
concentrations are expected to decrease by Friday with the arrival
of a tropical wave and its associated moisture field, supporting
an increase in showers with thunderstorm activity.

Warmer-than-usual conditions will persist in the coming days.
Coastal and urban areas can expect nighttime lows in the lower
80s, while higher elevations will see temperatures drop to around
70 degrees. Daytime highs will range from the lower 90s in lower
elevations to around 80 degrees in higher elevations. With
abundant moisture in the air, heat indices in many coastal and
urban regions may soar above 108 degrees Fahrenheit. These
conditions present significant health risks, requiring the
issuance of Excessive Heat Advisories and Warnings. It is crucial
for residents and visitors, especially those participating in
outdoor activities, to take necessary precautions to stay safe.

.LONG TERM...Saturday through Thursday...

From previous discussion issued at 513 AM AST Wed Jul 17 2024

Strong surface high pressure will remain the dominant feature across
the Atlantic and north of the region. This will maintain moderate
east to northeast winds through Saturday as a tropical wave and associated
moisture field enters the eastern Caribbean and crosses the forecast
area. This increasing moisture will bring unstable conditions to the
region followed by a wind surge and moderate to strong southeasterly
winds by Saturday afternoon and early Sunday as the tropical wave
exits the region. Trailing the wave an extensive layer of dense
Saharan dust will quickly spread across the region the rest of the
weekend and linger through Monday then diminish from Tuesday onwards.
Low level winds are then expected to become more easterly as another
Tutt and induced surface trough is forecast to approach the region
from the east. All in all Hot, humid and hazy conditions will likely
be the dominant weather pattern through the weekend and into the early
part of the following week with some locally and diurnally induced
afternoon convection each day.

Recent model guidance continued to initialize well and suggests the
most impactful period to be Friday through Saturday due to increasing
moisture convergence and instability. That said there will be a better
chance for enhanced overnight and afternoon convection as the tropical
wave and somewhat cooler advective temperatures aloft are across the
region. As previously mentioned considerably hazy conditions and fairly
drier airmass will follow due to the dense layer of Saharan dust particulates.
Local temperatures will remain slightly above normal with maximum daytime
heat indices forecast to be elevated to significant at times especially
along the coastal and urban areas. Marine conditions area also forecast
to deteriorate through the weekend due to the tightening of the local
pressure gradient and a wind surge which will increase the tradewinds.
Precautionary statements and or small craft advisories may therefore
continue for portions of the local waters and passages due to the
choppy wind driven conditions.

Sunday through Monday are forecast to be the driest period with typical
summertime weather and hot and hazy conditions. Isolated to scattered
locally and diurnally afternoon convection expected mainly over the
west and northwest sections of Puerto Rico. By late Monday through
the rest of the period, the upper ridge will erode as a Tutt low is
forecast to retrograde across the region and low level moisture gradually
increases. This will again increase the chance for more frequent overnight
passing showers followed by afternoon convection across parts of the
islands.

Based on the Tropical Weather Outlook from the National Hurricane
Center...Tropical cyclone formation is not expected during the next
seven days.


&&

.AVIATION...
(18z TAFs)

SHRA/TSRA cont ovr NW-PR, thus JBQ has a TEMPO thru 1720z due to
possible MVFR conds. VFR conds will prevail thru ,most of the
forecast. Hazy skies will return, as an extensive area with
suspended Saharan Dust Particles arrive this evening into tomorrow,
limiting SHRAs. However, TSRA/SHRA may develop during the afternoon
(18/18z-22z). Winds 10-15 kt from the ESE with gusts to 27 kt in sea
breeze variations. Winds bcmg ESE-SE around 10 kt aft 17/22Z in land
breezes, but easterly, gusts and sea breeze return aft 18/13z..


&&

.MARINE...

Strong surface high pressure anchored across the west central Atlantic
will yield moderate to locally fresh easterly winds. Consequently,
choppy to hazardous seas for small craft are likely across the local
waters and passages. Precautionary statements will continue and
Small Craft Advisories have been issued for portions of the local
waters, beginning Thursday night. Showers with isolated thunderstorms
are expected to affect the local waters from time-to-time.


&&

.BEACH FORECAST...

For the remainder of the week, 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...None.
VI...None.
AM...Small Craft Advisory from midnight Thursday night to midnight
     AST Friday night for AMZ711-723.

&&

$$

SHORT TERM...CAM/CVB
LONG TERM....ICP
AVIATION...CAM/CVB
PUBLIC...GRS/YZR