856
FXCA62 TJSJ 100852
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
452 AM AST Fri May 10 2024

.SYNOPSIS...Elevated to locally Significant Rainfall Risk today
across the region due to afternoon convection enhanced by
unstable conditions and saturated soils. Yesterday`s flooding
event continues to affect areas near Rio Grande de Manati, Rio
Cibuco, Rio Guanajibo and Rio Culebrinas, where Flood Warnings
are in effect. Patchy fog continues to affect areas of the
interior and light to moderate showers are currently affecting
eastern municipalities. The upper-level trough will continue to
weaken as it moves away from the region. Simultaneously, an upper-
level jet streak will spread across the northeastern Caribbean,
further enabling the development of deep convection through at
least Saturday night. A shift towards southerly winds this weekend
will prompt a warmer trend, further boosting afternoon convective
development. This can also promote limited to locally elevated
heat risk as we head into the weekend. Lingering saharan dust will
persist through at least early next week.

&&

.SHORT TERM...Today through Sunday...

Overnight satellite and radar observations have indicated the
development of showers over the local waters, with some moving
across the local islands and eastern Puerto Rico. Rainfall totals
have peaked at approximately 2 inches between Naguabo and Ceiba,
and around a quarter of an inch in St Croix. Despite clearer
conditions elsewhere, excessive runoff from previous heavy
rainfall has caused Rio Guanajibo, Rio Culebrinas, Rio Grande de
Manati, and Rio Cibuco to remain above the action to flood stage,
leading to continued Flood Warnings for neighboring
municipalities. From around 8 PM AST, satellite-derived data has
also indicated persistent foggy conditions extending from Orocovis
east-southeastward to Cayey. Coastal areas have experienced
temperatures ranging from the mid-70s to around 80 degrees, while
higher elevations have recorded temperatures in the mid-to-upper
60s. Winds have generally been light and variable, influenced by
land breezes.

The upper-level trough will continue to weaken as it lifts out to
the northeast, away from the region. Simultaneously, an upper-level
jet streak (70-90 knots) will spread across the northeastern
Caribbean, further enabling the development of deep convection
through at least Saturday night, with 500 mbar temperatures falling
to around -7 degrees by Saturday evening. Consequently, there is a
potential for isolated to locally scattered thunderstorms, as
indicated by the Galvez-Davison Index. According to the latest model
guidance, the most active period is expected during peak diurnal
heating this afternoon and again on Saturday afternoon.
Nevertheless, anticipate prolonged stormy conditions over the local
waters tonight and again over Saturday night, with some activity
affecting Puerto Rico`s eastern half, Vieques, Culebra, and the U.S.
Virgin Islands. Although conditions may become less favorable for
convective development on Sunday, afternoon showers with isolated
thunderstorm development are still likely.

Surface winds will persist at light to moderate speeds, ranging from
5 to 15 mph, with easterly winds gradually transitioning to south-
southeast by Saturday and Sunday. This shift towards southerly winds
will prompt a warmer trend, further boosting afternoon convective
development. As the prevailing steering flow influences the
distribution of showers, anticipate the highest rainfall totals to
transition from the interior today to the northern sections of
Puerto Rico during the weekend. Furthermore, a southwesterly
component at the 700 mbar level will likely drive some afternoon
activity towards the San Juan metropolitan area and northeastern
Puerto Rico. Given the persistently weak steering flow, moderate to
locally heavy rainfall from slow-moving showers over already
saturated soils will sustain an elevated flooding risk, exacerbating
existing flood situations, and disrupting ongoing recovery and
cleanup efforts.

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...

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...
(06Z TAFS)

SHRA may result in reduced visibilities and lower ceilings across
the USVI terminals and TJSJ through 10/14Z. Afternoon SHRA/TSRA
development will prompt VCSH/VCTS and, thus, possible MVFR
conditions, mainly across PR terminals between 10/16-22Z. The
potential for VCTS and showers will likely increase after 10/22Z for
the USVI and TJSJ sites. Light and variable low-level winds,
increasing to 10-15 knots and influenced by sea breeze variations
between 10/14-22Z. Light and variable winds will return after 10/22Z.

&&

.MARINE...A surface high-pressure across the Atlantic will yield light
to moderate easterly winds through today, becoming more east to
southeast during the weekend. Pulses of northeasterly swell will
continue to spread across the Atlantic waters and Caribbean
Passages for the next few days. Weather conditions across the
local waters are expected to gradually improve towards the
weekend.

&&

.BEACH FORECAST...A High Rip Current risk is in effect for the
northern coastline of Puerto Rico, from Aguadilla to Fajardo,
through this afternoon. A Moderate Risk of Rip Currents is in
effect today for Aguada, Rincon, Culebra and most of the northern
USVI`s coastlines and for the eastern half of St. Croix. Low risk
elsewhere. The risk of rip currents are forecast to gradually
diminish and an overall low risk of rip currents is forecast next
weekend.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through this afternoon for PRZ001-002-005-
     008.

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

$$

SHORT TERM...ICP
LONG TERM....MRR