112
FXCA62 TJSJ 170854
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
454 AM AST Fri May 17 2024

.SYNOPSIS...
Surface high pressure spread across the central and northeast Atlantic
will maintain a light easterly wind flow becoming more southeasterly
later today an over the weekend. Passing showers will continue across
the local waters and reach portions of the north and east coastal
areas during the rest of the morning hours. Afternoon showers will
likely develop across the central interior and northwestern quadrant
of Puerto Rico. Mostly sunny and fair weather skies are forecast
elsewhere but few showers cannot be ruled out around the U.S. Virgin
Islands. Hot and humid conditions will continue through the weekend
but with another round of Saharan dust forecast to spread across the
region. The potential for afternoon shower development will however
continue in isolated areas each day.

&&

.SHORT TERM...Today through Sunday

Calm weather conditions prevailed during the night and early morning
hours. Doppler radar showed a few passing showers affecting the
local waters and streaming across the southeastern sections of
Puerto Rico, with most showers occurring from Patillas to Maunabo.
Since the shower activity was relatively shallow, rainfall
accumulations remained insignificant. 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 remains on track. The interaction between
two surface high pressures, one in the central Atlantic and the
other over the western Atlantic, will continue to maintain a
southeasterly wind flow across the islands. As a result, patches of
tropical moisture will reach the islands daily. Although instability
will slightly increase as an upper-level trough moves eastward into
the area, the islands can expect an increase in afternoon shower
coverage today. According to high-resolution models and the NMB
model, the bulk of the showers today should be across the
northwestern sections of Puerto Rico and some interior areas.
Additionally, the southeasterly wind flow, combined with clear skies
and high water content, will increase heat index values, especially
across the northern-central coastal areas and most urban and coastal
regions. Residents and visitors can expect warm conditions with high
chances of precipitation.

According to recent model guidelines, the high-resolution model
continues to show increased shower activity for Saturday into
Sunday. As suggested yesterday, this pattern will mainly be driven
by the favorable side of an upper-level trough moving into the
region, positioning a zonal flow over the area. This instability and
the available moisture will enhance periods of strong showers during
the afternoon each day. Additionally, according to the NASA Goddard
Earth Observing System Model V5, a significant pulse of Saharan Air
Layer will filter into the region, resulting in hazy skies across
the islands. Given the expected conditions, the bulk of the showers
should remain localized across the northwestern quadrant and the San
Juan metro area, slightly limited by the presence of Saharan Dust on
Sunday. A weakness in the pressure gradient will result in lighter
to moderate winds, affecting the movement of afternoon showers.
Therefore, the island can experience minor flooding in urban areas,
including small streams and arroyos.

.LONG TERM...Monday through Friday...

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 over land areas particularly
during the afternoon hours. Shower development should be focused
across portions of the interior and west to northwest sections of
Puerto Rico as well as in isolated areas in the San Juan metro.
Mostly sunny skies and hot an humid conditions will persist through
Monday. As a result these conditions will increasing maximum heat
indices, posing a limited to elevated heat threat especially for
the northern coastal and urban areas of Puerto Rico and the adjacent
islands including the U.S. Virgin Islands.

Recent model guidance although there is some discrepancy between
the them both point toward an increasing moist and unstable
weather pattern with increased low level moisture convergence and
instability aloft for most of the period. The GFS model guidance
continued to be the most aggressive and suggests a wetter pattern
during the period. It has so far initialized well with the
present and ongoing pattern and continued to suggest the formation
of a deep layered trough forecast to deepen and move across the
western Atlantic while becoming amplified west of the region while
moving eastward. So far the strongest divergence and most favorable
side of the trough and its associated jet max should reach the forecast
area by Tuesday and continue to affect the region through Thursday
and Friday. With increasing instability along with cooler advective
temperatures aloft and abundant tropical moisture convergence, the
forecast layered precipitable water (PWAT) values are still expected
to exceed 2 inches during the latter part of the period. Consequently,
this wet and unstable period is forecast for the region, with the
moderate to high probability of showers and thunderstorm development
with areas of widespread rains likely, especially during the overnight
and afternoon hours. The anticipated period of excessive and heavy
rainfall will therefore increase the risk of urban and river flooding
especially from Tuesday onwards with the most favorable impacts being
Wednesday through Thursday but activity could continue thorough the
end of the work week. Stay tuned and be prepared, as we will continue
to monitor how this pattern unfolds and make the necessary adjustments
to the long term forecast.

&&

.AVIATION...
(06Z TAFS)

VFR conditions will prevail at all TAF sites during the period.
Winds will increase by 16/15Z from the E-SE at 15 knots, with sea
breeze variations. SHRA and VCTS are forecasted for the mountain
areas, resulting in lower ceilings and VCTS and SHRA across TJPS and
TJBQ. Winds will decrease by 17/23Z, becoming variable with land
breeze variations.

&&

.MARINE...
A surface high pressure across the central and northeast Atlantic
will result in light to moderate east to southeast through the
weekend. Overall, tranquil marine conditions are forecast throughout
the forecast period. Afternoon showers and isolated thunderstorms
could develop and move from western Puerto Rico into the coastal
waters each day.

&&

.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 through the weekend. However, life-threatening rip
currents can often occur in the vicinity of groins, jetties and
piers. In addition, with possible maximum heat indices between
102-107 degrees or higher, along the local beaches, heat
exhaustion will be likely with prolonged exposure. Stay hydrated!


&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 11 AM this morning to 5 PM AST this afternoon
     for PRZ005.

VI...None.
AM...None.
&&

$$

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