059
FXCA62 TJSJ 230730
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
330 AM AST Thu May 23 2024

.SYNOPSIS...

Unsettled, squally weather is expected to prevail through the end of
the workweek, with elevated risks for flooding, rapid river rises,
and mudslides. Wet weather conditions will persist throughout next
week as a tropical wave approaches from the southeast by Monday,
maintaining above-normal moisture across the northeastern Caribbean.
Additionally, some Saharan dust is expected by early next week,
along with an elevated heat risk.

&&

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

Numerous showers and thunderstorms continued across the eastern
Puerto Rico out to the USVI, providing a rainy night. The best
rainfall overnight was seen across St. Thomas and John with multiple
thunderstorms passing by during these early morning hours.

Unstable, wet conditions will prevail into the weekend as a deep
layered trough moves through the Caribbean basin then lifting into
the southwestern Atlantic by Saturday. As the mid to upper jet of
the trough sits over Hispaniola over the next 24 hours, a surface
induced trough begins to form to the southwest of the region
causing the veering of the surface winds to a southerly flow by
Friday. A moist airmass entered the region yesterday and as winds
veer, another plume of moisture sourced from northern South
America will move into the area, keeping precipitable water values
high in the upper 2nd percentile. This moisture, unstable
dynamics aloft due to the proximity of the mid to upper-level
jets, and moderate surface wind flow will maintain a pattern of
numerous passing showers, especially for the eastern half of the
region, including eastern mainland Puerto Rico, Vieques, Culebra,
and the USVI. Squally weather with thunderstorms are possible at
times which can bring strong winds and heavy downpours.

This afternoon convection will increase shower activity with the
eastern sections most likely to receive the bulk of rain.
Thunderstorms are also likely to spawn across northwestern Puerto
Rico, however due to cloud coverage convection over land may be
reduced. However, expect a 70 to 90 percent chance of rain for all
of the island through this afternoon. Passing showers will
continue through the night into Friday and Saturday with a similar
pattern prevailing. The greatest threat during this time is
flooding due to consecutive showers and thunderstorms capable of
producing frequent lightning and strong winds. Conditions do look
slightly less unstable Friday through Saturday, yet the previous
days of rainfall and ongoing activity will keep an elevated risk
of flooding for most of the area. Also if cloud coverage begins to
lighten on Saturday, heavy afternoon convection is very likely.

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

Model guidance indicates that the wet period will persist through
the long-term forecast. Sunday remains very moist, with precipitable
water values above 2.0 inches and ample instability aloft. A mid-
level ridge just east of the forecast area is expected to move over
us by late Sunday into early Monday, coinciding with a slight
reduction in low-level moisture, which will cause precipitable water
values to drop to almost 1.8 inches. However, this "reprieve" won`t
last long as a tropical wave moves from the southeast over the
islands by late Monday, causing moisture content to surge above 2.0
inches once again, persisting through the long-term forecast.
Instability aloft will be marginal, as weak ridging will alternate
with a series of weak short-wave troughs throughout the week.
Therefore, the combination of above-normal moisture and marginal
instability will maintain the flood risk across the area.

The National Blend of Models indicates approximately a 3-in-5 (60%)
chance for thunderstorm activity and a 1-in-5 (20%) chance for 24-
hour rainfall accumulations to exceed an inch each day of the long-
term forecast, particularly across western/northwestern Puerto Rico
under an east-southeast steering flow. The Galvez-Davison Index also
suggests a similar scenario, with a high potential for observing
isolated to scattered thunderstorms capable of producing heavy
rainfall. With saturated soils and elevated streamflows from
previous rainfall, the flooding risk will remain elevated.
Therefore, we could expect urban and small stream flooding, as well
as flash flooding and landslides in areas of steep terrain.

Another hot episode is expected on Sunday and Monday, accompanied by
suspended Saharan dust particulates by early next week. Therefore,
in areas with limited showers, hot and hazy conditions will prevail.
The combination of high moisture content and high temperatures will
produce hazardous heat index values, particularly in coastal and
urban areas, possibly reaching Heat Advisory criteria once again.


&&

.AVIATION...
(06z TAFS)

SHRA/TSRA will spread across USVI, mainly TIST, and
eastern/southern PR terminals this morning. Therefore, MVFR to
brief IFR conds are possible across TIST/TISX/TJSJ. A deep layered
trough over Hispaniola will continue to cause TSRA across the
Mona Passage through the forecast period, impacting the regional
terminals with VCTS/VCSH. Low-level winds ESE up to 21 kt blo
FL080, bcmg S/SW abv and increasing with height.

&&

.MARINE...

The interaction of an induced surface trough and a surface high
pressure to the northeast will promote moderate to locally fresh
east-southeast winds. In addition, the proximity of a deep layered
trough close to the Dominican Republic will continue to enhance
thunderstorm development with the potential for squally conditions
across the regional waters and local passages through at least
Friday. Winds are forecast to diminish once again by Friday night.


&&

.BEACH FORECAST...

Low to moderate rip current risk will persist throughout the period.
In the presence of moderate rip current risk, there is still the
possibility of life-threatening rip currents in the surf zone.
Please remember to heed the advice of the local beach patrol and
flag warning systems.


&&

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

$$

SHORT TERM/AVIATION...RC
LONG TERM/MARINE/BEACH FORECAST....CVB