832
FXCA62 TJSJ 131629
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1229 PM AST Mon May 13 2024

.UPDATE...

Based on the latest observations, the Heat Advisory was extended
to southwestern Puerto Rico, Vieques, Culebra, St. Thomas and St.
John. The heat indices surpassed 108 degrees in Lajas, and was
close in the other areas. Hot conditions are expected to prevail
through late this afternoon.

&&

.PREV DISCUSSION... /issued 441 AM AST Mon May 13 2024/

SYNOPSIS...
Stable conditions during the morning hours and a southeasterly
component in the wind flow will promote warmer conditions across
the northern coastal areas of Puerto Rico. Warmer conditions are
forecast to begin around 10 AM, which will be more significant for
the coastal areas. Given that, a heat advisory was issued for the
northern coastal municipalities. Residents are urged to stay
hydrated and follow the local law enforcement recommendations.
Typical weather is forecast for the short term, with mostly stable
weather conditions in the morning, followed by afternoon showers
across mountain sections. An upper-level trough is forecast to
increase shower activity late in the upcoming weekend.

SHORT TERM...Today through Wednesday...

Overnight observations from satellite and radar have indicated
mainly clear to partly cloudy skies, with a few isolated showers
remaining over the local waters. From around 11 PM AST and
continuing through the night, satellite-derived data indicated
persistent foggy conditions along the Sierra Cayey and Sierra
Luquillo. Overnight low temperatures ranged from the upper 60s
across higher elevations to the lower 80s across northeastern Puerto
Rico and the local islands. Winds were generally light and variable,
influenced by land breezes.

Expect drier-than-normal weather conditions during the short-term
forecast period, with heightened rains favoring a typical seasonal
pattern each afternoon. As the jet streak moves northward and away
from the area, a mid-level high pressure system will start extending
into the northeastern Caribbean, gradually establishing a trade wind
cap inversion and enhancing drier air entrainment. Consequently,
anticipate a reduction in moisture levels, with precipitable water
values dropping from typical to below typical thresholds or from
approximately 1.8 inches to around 1.5 inches through Tuesday
evening, as moisture becomes confined below the 700 bar layers. From
Tuesday night onward, precipitable water values will rise again to
typical moisture levels of around 1.6-1.8 inches. Light
concentrations of Saharan dust will persist throughout the forecast
period.

Despite conditions becoming less conducive to convective development
and fluctuations in moisture levels, afternoon showers with possible
isolated thunderstorms are still probable, even during the driest
periods on Tuesday. As a result of diminishing area coverage and
intensity, the risk level for excessive rainfall hazards will
decrease from elevated today, with urban and small stream flooding
likely, to limited on Tuesday. While there is a limited risk on
Wednesday, the potential for flooding could increase with future
forecast updates as moisture levels return to normal thresholds. As
light winds shift from east-southeasterly today, afternoon activity
will concentrate on central to western sections of Puerto Rico.
However, as winds become more easterly and speeds increase to 10-15
mph on Tuesday and Wednesday, activity will likely focus on far
western sections of the island.

Due to a southeasterly component in the prevailing wind pattern,
temperatures along the coast and in urban areas today could reach
the upper 80s to mid-90s. Conversely, higher elevations might see
highs ranging from the upper 70s to mid-80s. With plenty of moisture
throughout the region, heat indices could surpass 102 degrees
Fahrenheit in most coastal and urban areas, with some sections
likely experiencing extended periods of 108 degrees or higher across
western, northern, and eastern Puerto Rico and Saint Croix.
Consequently, a Heat Advisory has been issued, effective from 10 AM
to 5 PM AST this afternoon. Although model guidance indicates
slightly cooler 925 mb temperatures in the following days, warmer
conditions will persist.

LONG TERM...Thursday through Monday...

A typical weather pattern will be present for most of the long
term. On Thursday, at the surface, east-southeast winds promoted
from a surface ridge extending across all the central Atlantic
will continue to inject the islands with tropical moisture.
According to the late model guidance, PWAT values from Thursday to
Friday will remain near the climatological normals. Both model
guidances (GFS and ECMWF) at the upper levels agreed on an upper-
level trough just in the region. Although the present moisture
will keep high chances for general shower activity, the stability
aloft provided for the convergent side of the TUTT will permit
only localized showers resulting from the local and diurnal effect
across the interior sections, the San Juan metro area, and the
northwestern quadrant of Puerto Rico.

As the upper feature moves out, moisture availability will increase
even more by Saturday into Sunday when surface winds turn again
from the southeast in response to a building high pressure just
over the northwestern Atlantic. This influx of moisture and the
warm temperatures over the land areas in the afternoon hours will
enhance periods of moderate to heavy showers with isolated
thunderstorms. Given the moderate southeasterly wind pattern, the
showers will be mostly over the interior and northwestern
sections; according to the Galvez-Davison Index (GDI-GFS), the
shower activity is forecast to remain very localized.

Unfavorable weather conditions are forecast for the upcoming week
as upper-level conditions change. By late Sunday into Monday, an
upper-level trough will move just over the region, leading to
divergence aloft and colder temperatures at 500 MB, enhancing
vertical development for shower activity. Uncertainty remains
present due to the forecast period. However, the islands can
expect increased episodes of widespread shower activity in the
afternoon.

AVIATION...
(06Z TAFS)

Mainly VFR conditions across most local terminals during the next 24
hours. Afternoon SHRA/TSRA may result in brief MVFR conditions,
with periods of reduced visibility and lower ceilings, across
TJSJ/TJBQ between 13/16-23Z. Light to calm and variable winds
increasing to 8-12 knots and turning more from the east with sea
breeze variations between 13/13-23Z.

MARINE...
Surface high pressure extending over the northeastern Atlantic
into the Central Atlantic will continue to promote east-
southeasterly winds today. Therefore, winds will remain at 15
knots or less, with some gusty winds near the afternoon showers
across some coastal sections. Seas are forecast to remain up to 4
feet across most local waters and even lower across the coastal
areas. From late Tuesday night onward, pulses of northeasterly
swell will spread across the regional waters.Afternoon convection
may result in thunderstorms across the coastal sections of north
and western PR each day.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001-002-005-
     008-010>013.

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

AM...None.
&&

$$

ERG/GRS/YZR