566
FXCA62 TJSJ 220814
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
414 AM AST Wed May 22 2024

.SYNOPSIS...
Unsettled, squally weather is expected to prevail through the end of
the workweek. The risk for flooding, rapid river rises, and
mudslides will be elevated. Conditions will gradually improve by the
weekend, but still with the risk for locally strong rains in the
afternoons for western and northern Puerto Rico. Some Saharan dust
is expected early next week, as well as an elevated heat risk.

&&

.SHORT TERM...Today through Friday...

A significant increase of passing showers were observed across the
Caribbean and Atlantic waters as seen on the radar overnight. It
appears the trend towards unstable conditions are beginning as we
approach wetter days for the rest of the week into the weekend.

This morning an increase of passing showers across southern and
eastern Puerto Rico and also across the USVI is expected as a moist
airmass begins to lift and spread throughout the local islands. This
is associated with an approaching deep-layered trough that is
currently situated between eastern Cuba and Hispanola which will
increase instability across the local area into the weekend. As this
feature induces a surface perturbation south of Hispanola late
tonight, winds are expected to strengthen over the western coastal
waters through Thursday which can bring squally weather conditions.
Precipitable water content is likely to reach above 2.4 inches will
significantly enhance the rainfall activity across the region
through this time. Thunderstorms capable of producing strong winds,
heavy downpours, and frequent lightning remain very possible during
today through Friday.

Currently models are suggesting the heaviest congestion of rainfall
to be across the interior to eastern sections of Puerto Rico,
however elsewhere plenty of rain remains possible. The mid-level jet
segment of this trough begins to move northward into the western
Atlantic Thursday night, which is projected to then cause a surface
low to form north of the region by Friday night. This will cause
surface winds to veer through Thursday into Friday becoming
generally southerly by the end of the week. A situation like this
would bring a good chance for afternoon convection along northern
Puerto Rico, however the influence of a thick cloud layer may
disrupt diurnal heating. Nonetheless, conditions will remain
unstable through this time for Puerto Rico and the Virgin Islands
with an increased risk of flooding.

.LONG TERM...Saturday through Wednesday...

The islands will continue to be under the influence of a moist low
to mid weak southerly flow associated with a trough centered well
north of Puerto Rico by this time. Precipitable water values will
remain above the climatological value for late May, meaning that the
potential for showers will persist. On Sunday, the trade winds will
begin to recover and will gain an easterly component, at about 10 to
12 knots. A mid to upper level ridge centered to the east of the
Leeward islands will begin to push small pockets of drier air, along
with Saharan dust, but the moisture channel just west of Puerto Rico
will also push back, reaching the islands at times. Under this
environment, it is likely that passing showers will continue to form
around the Virgin Islands and eastern Puerto Rico, while strong
afternoon convection is anticipated for the interior and western
Puerto Rico each afternoon. In fact, the Galvez-Davison index
indicate isolated to scattered thunderstorms from Saturday through
Tuesday. The risk of flooding and mudslides will remain elevated for
these areas. Temperatures will once again warm up to the low 90s,
with heat indices above 102 degrees, mainly for the coastal and
urban areas of the islands.


&&

.AVIATION...
(06z TAFS)

VFR conditions will continue, however a rapid increase of shower
activity will bring bring periods of reduced VIS and low ceilings.
Chance of TSRA will increase through the day, worsening during the
afternoon with this activity impacting mostly all terminals. Winds
will blow from SE at 8-15 kts, with stronger wind gusts and sea
breezes variations.

&&

.MARINE...
The interaction of an induced surface trough to the west and a weakening
surface high pressure to the east will promote moderate east-southeast
winds through today. As the surface trough moves closer to the
region, expect increasing shower and thunderstorm activity across
the local waters through early in the weekend. In addition, the
proximity of an amplifying mid to upper level trough just west of
the region will enhance thunderstorm development as well as increase
winds and the potential for squally conditions across the Mona
Passage and Atlantic waters late tonight through at least Friday.
Winds are forecast to diminish once again by Friday night.


&&

.BEACH FORECAST...
Surf heights 4 to 5 increased the rip current risk for eastern St.
Croix. Elsewhere, the rip current risk is low.

&&

.HYDROLOGY...
Periods of heavy showers and thunderstorms are anticipated,
especially today through Friday. The USGS streamflows show that most
rivers are running near normal to above normal. Additionally, the
experimental Puerto Rico Landslide Forecast Network (Slides-PR) are
showing high soil saturation levels. With prolonged episodes of
heavy rain, the risk for flooding, rapid river rises, water surges
and mudslides will be elevated.

&&

.CLIMATE...
The heat spell that has been affecting the islands should appease
today and tomorrow due to abundant cloudiness brought by the deep-
layered trough. So far, for the San Juan Area climate site (LMM
Airport), eight daily warm-minimum temperatures, and three daily
maximum records have been broken for May. To date, it is also the
second warmest May in history (84.0F), after 2020 (84.1). The
month is also running 3.96 inches wetter than normal.

&&

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

$$

SHORT TERM...RC
LONG TERM....ERG