689
FXCA62 TJSJ 102018
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
418 PM AST Fri May 10 2024

.SYNOPSIS...
Unstable conditions continue to bring the risk of flooding across
the local islands through tonight and the weekend. Excessive
rainfall over saturated areas will further elevate the risk for
flooding, mainly across the interior to northern sections of
Puerto Rico. Winds will become southeasterly over the weekend,
bringing warmer than usual temperatures. Marine conditions are
improving as a northerly swell begins to fade.

&&

.SHORT TERM...Tonight through Sunday...

A cluster of showers started to form across the coastal waters
between Vieques and St. Croix around 10am this morning, bringing
over 2 inches of rain to the eastern tip of the island. After this
showers started to form across the interior to northern sections
of Puerto Rico and continued through the afternoon. Various areas
such as San Juan, Arecibo, and San Sebastian received between 2-4
inches of rain with multiple Flash Flood Warnings issued due to
flooding impacts. Winds were generally from the east around 10 mph
with seabreeze variations.

Abundant moisture and a lingering mid to upper-level trough will
continue to bring unstable weather conditions through the weekend. A
jet streak segment of the upper-level trough will remain over the
islands tonight through Saturday night, which will assist the
development of convection meanwhile causing 500mb temperatures to
drop. Due to this, rainy conditions with thunderstorms will continue
to effect the area with flooding remaining a threat across
sections with prolonged or heavy rainfall. The afternoon hours
continue to have the best chance of bringing strong showers mainly
for the mainland island of Puerto Rico due to diurnal heating and
local effects. The light to moderate easterly surface flow, with
easterly winds gradually transitioning to south-southeast by
Saturday and Sunday, and upper-level westerly wind will allow
showers to form across most sections of Puerto Rico with the
strongest activity across the interior to northern sections.

The weakening of the upper-level trough on Sunday and a reduction of
jet wind speeds will allow conditions to slightly improve compared
the the previous days. However, there is still a high chance for
afternoon convection to bring locally heavy rainfall from slow-
moving showers over already saturated soils which will sustain a
flooding risk. By tomorrow, due to the southerly component in the
wind temperatures may soar into the upper 80s to mid-90s across
coastal and other urban areas, while higher elevations could see
highs in the upper 70s to mid-80s. With plenty of moisture across
the region, heat indices could exceed 102 degrees Fahrenheit,
potentially reaching around 108-110 degrees in localized coastal
areas of northern Puerto Rico over the weekend.

&&

.LONG TERM...Monday through Friday...
/from previous discussion/

No major changes to the long term forecast. Model guidance to
suggest a mid- to upper-level ridge building over the region by
the start of the workweek, potentially promoting more typical
weather with afternoon convection due to diurnal and local
effects. Precipitable water (PWAT) values are forecast to
generally stay at normal values between 1.5 to near 2.0 inches,
through the workweek as patches of moisture advect into the
region. ESE winds to start the period will back to become more
easterly by late Monday through the rest of the period as a
surface high moves into the western Atlantic. Most available
moisture should reach up to 850 to 800 mb for the period. The NASA
aerosol optical thickness (AOT) guidance continues to suggests
the presence of an air mass with Saharan dust particles lingering
to start the period, this could extend through the period. Limited
Heat Risk (heat indices from 102 to 107 degrees Fahrenheit) or
even higher, especially during the daily maximum heating, will
persists across several lower elevation sectors of the islands
where no significant rain is observed. This diurnal heating, local
effects and sea breeze variations will continue to promote up to
strong afternoon convection, steered by east-southeast winds on
Monday, backing to become more easterly winds for the rest of the
workweek, promoting convective activity mainly over sectors of the
interior to western half of the Puerto Rico and downwind of the
local islands. Above-normal sea surface temperatures can also
promote nighttime showers across windward sectors of the islands,
patchy fog also forecast for areas of the interior during the
early morning hours.

&&

.AVIATION...
(18Z TAFS)

SHRA and TSRA are resulting in lower ceilings and VRB winds at
TJBQ/TJSJ/TJPS where TEMPO MVFR conditions conditions through 21Z.
SHRA/TSRA will continue to affect the area, causing mountain
obscuration across the Cordillera Central. SFC winds will continue
light from the E-SE and VRB near the showers with gusty winds. After
23Z, only VCSH are expected for all the eastern sites. Winds are
expected to bcm light at VRB again after 11/03Z. Another round of
SHRA/TSRA is forecast for 11/18Z for all the TAF sites.


&&

.MARINE...
A surface high pressure across the central Atlantic and
a building low in the western Atlantic with allow easterly wind
tonight then southeasterly wind through the weekend. Weak pulses
of northeasterly swell will begin to fade Atlantic waters and
Caribbean Passages over the next few days. Strong showers and
thunderstorms over the waters can result in locally higher seas
and gusty winds.


&&

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

$$

DSR/MMC/RC