977
FXCA62 TJSJ 192100
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
500 PM AST Thu Sep 19 2024

.SYNOPSIS...

Light and variable winds will persist over the next few days,
promoting slow-moving showers and thunderstorms. The risk of
flooding will be elevated nearly every afternoon, while hot
temperatures are expected to continue across the islands.

&&

.SHORT TERM...Tonight through Saturday

Explosive convection prevailed during the afternoon hours, with very
high values of Vertically Integrated Liquid (VIL) and Geostationary
Lightning Mapper (GLM) data indicating strong thunderstorms
featuring frequent lightning, torrential rain, and the potential for
pea-sized hail. Clusters of thunderstorms pulsed in intensity
throughout the afternoon, gradually redeveloping and propagating
westward across interior Puerto Rico. Several Flood Advisories and
Special Weather Statements were issued. Excessive runoff caused
multiple USGS river sensors to reach action stage, with Rio
Tanama briefly hitting minor flood stage. Although showers and
thunderstorms have subsided, excessive runoff will continue
through the evening, with several rivers remaining at elevated
streamflows.

The unstable and wet weather pattern will persist through the rest
of the short-term forecast. Precipitable water content will
fluctuate between 1.8 and 2.2 inches through Friday before dropping
below 1.7 inches on Saturday. Conditions aloft will remain conducive
for deep convection as a TUTT (Tropical Upper Tropospheric Trough)
stays anchored over the northeast Caribbean. Remnants of Gordon in
the central Atlantic will continue influencing the wind pattern
across the forecast area. A low-level col region, currently
southeast of St. Croix, will gradually move northwest and linger
nearby through the weekend. This will promote a mostly light and
variable wind pattern, exacerbating rainfall accumulations due to
slow-moving showers and thunderstorms each afternoon, particularly
on Friday. The 925 mb temperatures will rise, especially on
Saturday. As a result, the combination of the TUTT to the northeast
of the Caribbean, abundant moisture, and strong surface heating will
support diurnally induced convective activity, mainly over interior
Puerto Rico each afternoon, though less widespread on Saturday. A
limited to elevated flooding threat, along with the potential for
frequent lightning and gusty winds, is expected with this activity.

As mentioned above, 925 mb temperatures will exceed the 90th
percentile over the next few days. Therefore, before afternoon
convection, heat index values could reach Heat Advisory or Warning
criteria, particularly in urban and coastal areas. Be sure to drink
plenty of fluids and avoid direct sun exposure during peak hours of
the day.

.LONG TERM...Sunday through Wednesday...

.PREV DISCUSSION... /issued 350 AM AST Thu Sep 19 2024/

The pressure gradient will remain weaken late on the weekend and
early next week, with a low level steering flow coming from the
south-southwest of 1 to 4 knots. These light winds are due to a
series of trough over the western Atlantic. At the surface a small
pocket of moisture is expected to reach the islands. This will
act to fuel afternoon showers and thunderstorms along the
Cordillera Central and then spreading toward the coastal areas of
Puerto Rico. Some of this shower activity, although at a lesser
scale, are also expected for the Virgin Islands.

On Tuesday, the combination of a low pressure system developing
over the western Caribbean, and an upper level trough near the
Bahamas will escort an area of mid to upper level clouds into the
region. Conditions aloft will be favorable for showers and
thunderstorms too, so if enough sunshine filters in, another
active afternoon will be likely for the Cordillera Central, and
with the showers then drifting toward northeastern Puerto Rico and
the San Juan metro area. These showers will later move into the
vicinity of the Virgin Islands.

Late Tuesday and Wednesday, the trade wind will recover, gaining
an easterly component, and at 7 to 11 knots. The unsettle weather
pattern is expected to persist, as an upper level low develops
near the islands. The risk for flooding will be elevated all these
days. Temperatures will be high too, but the excessive risk will
be dependent on how deep and widespread the cloud coverage finally
materialize.


&&

.AVIATION...
(18Z TAFs)

Mainly VFR conditions are expected to prevail across all terminals
through the forecast period. However, SHRA/TSRA over mainland PR may
cause tempo MVFR cigs across the area terminals until sunset. Light
winds with a northerly component will prevail through 20/08z. A
southerly component and lighter winds are forecast for Friday.

&&

.MARINE...

A weak surface low pressure lingering northeast of the region will
continue to shift winds, gradually becoming variable and weakening
throughout the weekend. Showers and isolated thunderstorms will
persist across the regional waters, particularly off the western
coastal waters of Puerto Rico each afternoon.

&&

.BEACH FORECAST...

Low to moderate rip current risk will persist throughout the period.
A long-period northwesterly swell could reach northern exposed
beaches by midweek next week, potentially increasing the rip current
risk at some beaches. For a more detailed forecast, please refer to
the Surf Zone Forecast for Puerto Rico and the U.S. Virgin Islands
(SRFSJU).


&&

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

$$

SHORT TERM/MARINE/BEACH...CVB
LONG TERM...PREV DISC
AVIATION...DSR/RAM
PUBLIC...CAM