836
FXCA62 TJSJ 150049
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
849 PM AST Wed Aug 14 2024

.UPDATE...

Rainfall over portions of the interior and central Puerto Rico
continued during the evening hours. Rainfall totals across these
areas were between 6 and 10 inches. Runoff from this excessive
rainfall and any additional shower activity over Puerto Rico will
cause rivers and streams to remain elevated through the night and
lead to flash floods. Mudslides are also expected in areas of
steep terrain where soils are saturated. Several major rivers and
tributaries are still above flood stage across western and north
central Puerto Rico. Therefore, a Flash Flood Watch remains in
effect for mainland PR through the rest of the overnight and early
morning hours.

Across the USVI, Vieques and Culebra, the threat for any significant
rainfall has diminished and the Flash Flood Watch was cancelled.


&&

...Corrected Tropical Storm Ernesto to Hurricane Ernesto in
previous discussion....

.PREV DISCUSSION... /issued 402 PM AST Wed Aug 14 2024/

SYNOPSIS...At 2 PM AST Hurricane Ernesto was 225 miles northwest
of San Juan and moving northwest at 16 mph. The tail of Ernesto is
pulling across western Puerto Rico and our western Atlantic
waters with scattered showers elsewhere. Some thunderstorms
linger. Conditions will improve but southerly flow will cause
excessive heat into the weekend. Moisture will return to normal or
slightly above normal levels beginning on Friday.

SHORT TERM...Tonight through Friday...

Hurricane Ernesto will continue to be the dominant influence on
the weather through Thursday. The tail of Ernesto will pull across
western Puerto Rico and the local western Atlantic waters this
evening, but the GFS shows moisture retreating with the storm to
the north northwest except for isolated convection later tonight
and Thursday. Moisture at 850 mb drops off rapidly tonight and at
higher levels on Thursday. Considerable drying will be seen on
Friday up through 300 mb. As the convergence into Ernesto moves
with the storm to the northwest, a ridge from the surface to 700
mb will move in over the area to reinforce drying and subsidence.
This will also greatly decrease the amount of rainfall expected
Thursday and Friday. At upper levels a very weak ridge of high
pressure will develop over and just north of the area between TS
Ernesto and another low pressure currently negatively tilted over
52 west longitude to our east.

LONG TERM...Saturday through Thursday...

A surface high ridging over the area and the northeastern
Caribbean on Saturday will shift northwest into the Bahama Islands
by Monday from a quasi stationary high over the Azores. An
easterly wave near 35 west on Saturday will move toward the area
to about 55 west on Tuesday. At this time it is not carrying a
significant amount of additional moisture and will travel well
south of Puerto Rico. Better moisture will move into the area
behind a weaker wave closely following that will bring showers to
the area. Since precipitable water values will continue during the
long-term period in the range of 1.4 to 2.0 inches and flow will
be mostly southeast over the weekend and turning more easterly
during the week, a more typical weather patter will prevail for
Puerto Rico and the U.S. Virgin Islands with bands of better
moisture even if excessive heat persists early in the period.
This will mean early morning showers in eastern Puerto Rico
followed by afternoon showers and thunderstorms in western and
interior parts of the island. For the U.S. Virgin Islands only
isolated to scattered showers are expected with limited amounts of
rainfall occurring. Some lines of convection are likely during
this period extending with the flow to the west northwest or west
of Culebra, Vieques and the U.S. Virgin Islands.

AVIATION...

(18z)

IFR/MVFR conditions are forecast to prevail across TJBQ until at
least 14/21z as the rainbands from Hurricane Ernesto move out. For
the rest of the PR TAF sites, mainly VFR conds, at times MVFR, will
prevail with brief SHRA/TSRA and low CIG thru 14/22z, while USVI TAF
sites can expect mostly VFR conds with VCSH.  Mostly VFR conds for
all TAF sites aft 15/00Z. Winds will remain mainly from the S-SE up
to 15 knots and gusty winds up to 28 kts, a more SE wind flow is
forecast at around 15/03Z.

MARINE...Hazardous seas will slowly retreat with Hurricane
Ernesto and small craft advisories are not expected after Thursday
at 6 PM AST through Thursday of next week. Nevertheless subsiding
swell from Ernesto will continue to arrive in the local area now
through Tuesday. Showers and thunderstorms will continue--
especially in the northern waters--through Thursday also with
diminishing frequency.

BEACH FORECAST...Rip current risk will remain moderate to high
over all beaches tonight and moderate on Thursday. Due to the
swell returning from TC Ernesto, the rip current risk over the
weekend will remain moderate on some northern beaches on Friday.

HYDROLOGY...Many rivers across Puerto Rico are in flood stage and
some in western Puerto Rico will rise further. The Flash Flood
watch has been extended through 8 AM AST. All rivers, if they
have not done so by now, should crest soon or overnight.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Flood Watch until 8 AM AST Thursday for PRZ001>011.

     High Rip Current Risk until 6 AM AST Thursday for PRZ003-008-012-
     013.

VI...None.
AM...Small Craft Advisory until 6 AM AST Thursday for AMZ711-712-741-
     742.

     Small Craft Advisory until midnight AST tonight for AMZ716-723-
     726-733-735.

&&

$$

DSR