474
FXCA62 TJSJ 160919
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
519 AM AST Thu May 16 2024

.SYNOPSIS...
Surface high pressure ridge spread across the west central Atlantic
and weakly induced trough northeast of the region will maintain a
light east to northeast wind flow today. Passing showers steered by
this flow will reach the local waters and portions of the north and
east coastal areas from time to time during the early morning hours.
Afternoon shower development will be possible mainly across the
interior and western sections of Puerto Rico. Mostly fair weather
and sunny skies are forecast elsewhere. An increase in early
morning and afternoon shower activity is expected by Friday and
into the upcoming weekend due to better moisture transport and
instability aloft due to the proximity of a mid to upper-level
trough. Warm and humid conditions will continue for the next
several days especially with winds forecast to become more
southeasterly.

&&

.SHORT TERM...Today through Saturday...

Stable weather conditions prevailed during the night and early
morning hours. Around midnight, Radar Doppler showed a few passing
showers affecting the northeastern sections of Puerto Rico,
resulting in minimal rainfall accumulations. The rest of the area
and the U.S. Virgin Islands mainly observed clear skies and no
rainfall activity. Overnight temperatures were a little warmer,
especially across the coastal areas, ranging in the upper 70s to
lower 80s and mid-70s across the mountains.

The short-term forecast is still on track. A mid to upper-level
ridge will continue to result in drier air aloft and stability. At
the surface, a building high pressure over the central to
northeastern Atlantic will result in an east-southeasterly wind.
Embedded in this wind pattern, as shown by the GOES Satellite-
derived products, patches of moisture with PWAT near normals will
drive a convective pattern in the afternoon across the islands
today. Due to the more southerly surface wind change, shower
activity will concentrate across the interior and northwestern
quadrant of Puerto Rico. Elevated moisture levels and southeasterly
winds will enhance heat indices around 102-107 degrees Fahrenheit
across the northern coastal sections, resulting in a limited heat
threat for most coastal areas and some isolated areas with higher
heat indices near 108 degrees Fahrenheit.

Recent model guidelines and the Hi-res model show increased shower
activity from Friday to Saturday. This pattern will be mainly driven
by the favorable side of an upper-level trough sinking into the region,
positioning a regional zonal flow over the region. This instability
and the available moisture will enhance periods of strong showers
during the afternoon hours from Friday into Saturday. The bulk of
the showers should remain localized across the northwestern quadrant
and the San Juan metro area. A weakness in the pressure gradient
will result in lighter to moderate winds, affecting the movement
of afternoon showers. That said, for Friday into Saturday, the islands
can expect deteriorating weather in the afternoon.

.LONG TERM...Sunday through Thursday...

A surface high pressure system extending from the central and
northeast Atlantic will maintain southeasterly winds across the
region during the initial phase of the period. These winds will
aid in transporting patches of tropical moisture to the region,
and therefore increase the likelihood of shower development each
day,but particularly in the afternoon. Most showers should be
focused across the interior and northwestern quadrant of PR and
and in isolated areas around the San Juan metro area. There will
be plenty of sunshine and hot and humid conditions through the
weekend and into early next week. These conditions will yield
increasing heat indices, posing a limited to elevated heat threat
for the northern coastal and urban areas of Puerto Rico and the
adjacent islands including the U.S. Virgin Islands.

Moisture convergence and instability is expected to gradually
increase by late Sunday and throughout the forecast period
as a deep layered trough is forecast to move across the western
Atlantic and become amplified west of the region while moving
eastward. Recent model guidance continued to suggest, the
strongest divergence and most favorable side of the deepening TUTT
(Tropical Upper Tropospheric Trough) and its associated jet
stream will reach the forecast area from Monday and continue to
affect the region through Thursday. With increasing instability
aloft along with colder temperatures and abundant tropical
moisture convergence,precipitable water (PWAT) values are forecast
to exceed the climatological normals by over 2 inches. Consequently,
a wet period is still forecast for the islands and coastal waters,
with the increasing probability of showers and thunderstorm development
with areas of widespread rains likely especially during the late morning
and afternoon hours. This so far anticipated periods of heavy rainfall
will increase the risk of urban and river flooding from Tuesday through
the rest of the period. Stay tuned as we will continue to monitor how
this unfolds and and make adjustments to the long term forecast if
needed.

&&

.AVIATION...
(06Z TAFS)

VFR conditions will prevail for all TAF sites during the period.
Winds will increase by 16/15Z from the east at 15 knots or less,
with sea breeze variation. SHRA and VCST are forecasted for the
mountain sections, resulting in lower ceilings, and VCTS and SHRA
across TJPS and TJBQ. Winds will decrease by 16/23Z, becoming
variable with land breeze variation.

&&

.MARINE...
A surface high pressure extending from the western Atlantic into
the Central Atlantic will continue to promote light to moderate
easterly winds across the region. With this pattern, seas will
remain up to 4 feet across all the local waters and passages.
These conditions will prevail across the local waters during the
period. Similar tranquil marine conditions are expected for the
coastal areas, where the risk for rip currents will remain low
for the next five days or so.

&&

.BEACH FORECAST...
There is a low rip current risk today across all area beaches of
Puerto Rico and the U.S. Virgin Islands. The risk will likely remain
low for the next several days. However, life-threatening rip currents
can often occur in the vicinity of groins, jetties and piers. In addition,
with possible heat indices between 102-107 degrees, along the local
beaches, heat exhaustion will be possible with prolonged exposure.
Stay hydrated!

&&

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

$$

SHORT TERM...LIS
LONG TERM....RAM
AVIATION...LIS