242
FXCA62 TJSJ 092143
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
543 PM AST Thu May 9 2024

.SYNOPSIS...

Elevated Rainfall Risk continues this afternoon across the
islands due to convection enhanced by unstable conditions and
saturated soils. Yesterday`s flooding event continues to affect
areas near Rio Guanajibo and Rio Culebrinas, where Flood Warnings
are in effect. Weather conditionsare expected to improve in the
coming days as the upper-level trough moves away and the amount
of precipitable water decreases to more typical levels. However,
conditions will remain marginally conducive to deep convective
development and susceptible diurnal heating influence. Winds are
forecast to become more southerly into the weekend, this can a
promote limited to locally elevated heat risk during the weekend.

&&

.SHORT TERM...Tonight through Saturday...

During the morning period, doppler radar indicated showers and
isolated thunderstorms affecting the local waters and eastern
Puerto Rico, where approximately 1 to 2 inches of rain fell.
Flood Advisories were issued for municipalities of that area.
Around noon, convection started to develop across the Cordillera
Central and have been spreading across the island. Rainfall totals
based on radar estimates have ranged between 2 and 4 inches.
Several Flash Flood Warnings and Flood Advisories were issued. The
Flood Warnings for area rivers (Rio Culebrinas and Rio Guanajibo)
continue in effect through atr least tonight and will be extended
if necessary.

For tonight, activity should gradually diminish, but according to
the latest model guidance, additional shower activity will likely
develop and concentrate across the local waters, north central
and eastern Puerto Rico, Vieques, Culebra, and the U.S. Virgin
Islands.

The forecast remains on track. Based on the latest models, the
upper-level trough should move away from us and the amount of
moisture across the region decreases to more typical levels based
on climatology (around 1.8 to 2.0 inches). For that reason,
weather conditionsare expected to improve in the coming days,
however conditions will remain marginally conducive to deep
convective development due to local effects. Although widespread
rains are no longer expected, scattered to locally numerous
showers and isolated thunderstorms are likely during afternoons.

By Saturday, the high pressure across the Atlantic will change
our surface wind flow. Low-level winds are expected to veer from
easterly to south/southeasterly. Under this pattern, a warming
trend will be likely with temperatures in the upper 80s to mid 90
across coastal and urban areas. Heat indices between 100-107
Fahrenheit or even higher are possible in localized areas during
the weekend, especially during the daily maximum heating, across
the USVI and PR`s urban and coastal areas. In summary, more stable
conditions are anticipated by the weekend, but typical afternoon
convection are still posible. Warmer temperatures and heat
indices are very likely.

&&

/LONG TERM...Sunday through Thursday...
.PREV DISCUSSION... /issued 518 AM AST Thu May 9 2024/

Model guidance continues to suggest a mid- to upper-level ridge
building over the region to start the long term forecast period.
This feature could potentially promote more typical weather.
Precipitable water (PWAT) values are forecast to generally stay at
normal values, 1.5 to 2.0 inches, through the period as patches of
moisture continue to advect into the region. Sunday appears to be
the wettest day, with high end normal values to possibly above
normal values under southerly steering flow. With veering flow
backing to become more easterly by Monday afternoon, PWAT values
should remain at normal values through the rest of the period. Some
exceptions could be during afternoon convection, were PWAT can
increase, and on Wednesday morning, when model guidance suggests a
patch of drier air reaching the islands. Most available moisture
will reach up to 700 mb on Sunday morning before gradually
decreasing and only reaching up to 850 mb by Wednesday. The NASA
aerosol optical thickness (AOT) guidance continues to suggests the
presence of an air mass with Saharan dust particles to start the
period, which may extend into next week.  Diurnal heating, local
effects and sea breeze variations will continue to promote up to
strong afternoon convection, steered by southerly winds on Sunday
and by more easterly winds for the rest of the period, 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. The Galvez-Davidson Index
(GDI) suggest that Sunday will be the day with stronger afternoon
convective activity. 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.

&&

.AVIATION...
(18Z TAFS)

SHRA/TSRA will continue to affect the area, causing mountain
obscuration across the Cordillera Central gradually moving towards
the coast resulting in VCSH mainly across TJSJ and TJPS.
Generally VFR conditions expected the rest of the overnight
period. Light and variable low-level winds, increasing to 10-15
knots and influenced by sea breeze variations between 09/14-22Z.
Then, light and variable winds will return later tonight.

&&

.MARINE...

A surface high-pressure across the Atlantic will yield light to
moderate easterly winds through tomorrow, 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...

There is a High Risk of Rip Currents tonight and Friday for the
northern coastline of Puerto Rico, from Aguadilla to Fajardo, and
a Moderate Risk for Culebra, northern and western St. Thomas and
eastern St. Croix. There is a 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.

&&

.HYDROLOGY...

There is a significant flood risk across the local islands due to
saturated soils and river running well above normal values.
The Flood Warnings for area rivers (Rio Culebrinas and Rio
Guanajibo) continue in effect through atr least tonight and will
be extended if necessary.

&&

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

VI...None.

AM...None.

&&

$$

YZR/LIS/MMC/CVB