923
FXCA62 TJSJ 172059
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
459 PM AST Sun Nov 17 2024

.SYNOPSIS...
Shower and thunderstorm activity will continue tonight as another
cold front crosses the region. A drier air mass will reach the
region early this workweek. Moisture content will gradually increase
by the latter part of the workweek. A long-period northerly swell
will create hazardous coastal conditions along the north-facing
beaches of PR and the US Virgin Islands.

&&

.SHORT TERM...Tonight through Tuesday

Passing showers were seen today due to moisture associated with
another cold front crossing the region. Showers and thunderstorms
were seen across the region and the local waters during the
afternoon hours. Based on Doppler radar estimates, localized
rainfall accumulations got up to 1 inch in San Juan, Carolina, and
Santa Isabel. Daytime temperatures remained in the mid to upper
80s across the coastal areas and slightly colder in the
mountains.

Shower activity is expected to continue over the northeast and
eastern sections of PR early tonight. Due to saturated soils and
river levels running high, the risk of flooding will remain elevated
for these areas, with high chances of urban, river, and small-stream
flooding with flash floods and landslides. Temperatures are expected
to drop to mid-70s in the urban and lower terrains while the higher
terrains can expect temperatures from low to mid-60s.

A broad surface high pressure over the western Atlantic will bring a
dry air mass early this workweek. The model guidance suggests that
relative humidity in the 850 mb will plummet below 10 percent, with
Precipitable Water (PWAT) values below the climatological normal.
Additionally, warmer than normal 500 mb temperatures will diminish
deep convection activity. With ENE wind flow bringing cooler
temperatures, stable conditions will prevail at least until the
latter part of the workweek.

.LONG TERM...Wednesday through Saturday...
...from previous discussion issued at 446 AM Sun Nov 17 2024...

Below normal precipitable water values (PWAT) are forecast to
continue by midweek. Current model guidance has PWAT values of 0.9
to 1.2 inches, well below to below normal values for this time of
the year. This is due to a cooler and drier air mass filtering
into the region under northerly flow behind the frontal boundary
mentioned at the end of the short term period. Mid to upper level
ridging will move in from the southeastern U.S. into the western
Caribbean, maintaining a trade wind cap inversion around 850 mb,
with all the low level moisture trapped below this level. This low
level moisture as well as 925mb temperatures are forecast to be
below normal to start the period. Generally low probability of
precipitation (POP) will remain, except just southeast of Saint
Croix, where remnant moisture of the cold front will linger.

Increasing moisture will then be experienced Thursday through
Saturday as winds veer to become more easterly on Thursday and
gain a southerly component Friday and Saturday. This change in
wind direction will be due to a surface high moving from the east
to central Atlantic. A moisture plume will be brought in from the
east and south, increasing the potential for showers and
afternoon convection. Model guidance suggest that the trade cap
will not be present by late Friday and Saturday leaving high
relative humidities at all levels. PWAT values are forecast to be
at around 2.25 in. Highs will once again reach the upper 80s to
low 90s across lower elevations of the islands.

&&

.AVIATION...
(18Z)
Conds are mostly VFR, but in areas of SHRA/RA
in the USVI, OCNL MVFR is seen--mainly due to CIGs. Isold SHRA
have returned to the interior of PR movg SE at arnd 7 kts. This
line extends to the SE coast of PR at Yabucoa, and may move S. At
this time models suggest cooler LLVL flow from the north or
northeast will cause moisture to pile up against the mountains and
produce light to possibly mdt rain. Cld tops will begin to come
down however. VSBYs may be reduced in some areas to as low as 4 SM
alg the N coast of PR and mtns will be obscured. The SHRA will
cont thru abt 18/02Z ovr land (turning to
-RA thereafter), but persist ovr the lcl waters with sct TSRA thru at
 least 18/04Z. Expect VFR conds south coast. LLVL winds are
 mostly N-NE less than 12 kt over PR/USVI, but some sea breeze
 influences were detected alg the S coast of PR. Maximum winds NW
 70 kt arnd FL435.

&&

.MARINE...

A subsiding long-period northerly swell affecting the local
waters will maintain high seas across the offshore Atlantic waters
at least until tomorrow after. Seas will remain up  to 7 feet
diminishing tomorrow afternoon. After that, a broad surface high
pressure will dominate the surface pattern with a moderate
northerly winds across the islands. Seas will remain up to 5 feet
across most of the local waters, increasing once again by Thursday
when another northerly swell affect the local waters.


&&

.BEACH FORECAST...

Recent data bouy at San Juan reported seas of 5.2 feet and 13
seconds and more energy coming in. Therefore breaking waves
across the northern coastal areas will remain between 8 to 13 feet
across the northern areas. The next high Tide for San Juan, at la
Puntilla will be at 11:13 AM of 2.06 ft on Monday. Given the
expected conditions, the coastal flood will remain in effect until
noon on Monday and the High Surf Advisory until 6 PM AST on
Monday.


&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Coastal Flood Advisory until noon AST Monday for PRZ001-002-005-
     008.

     High Rip Current Risk through Monday afternoon for PRZ001-002-
     005-008-010-012.

     High Surf Advisory until 6 PM AST Monday for PRZ001-002-005-008.

     High Rip Current Risk through late tonight for PRZ013.

VI...High Rip Current Risk through Monday afternoon for VIZ001.

AM...Small Craft Advisory until noon AST Monday for AMZ711-712-716-
     741-742.

&&

$$

SHORT TERM...MNG
LONG TERM....LIS
AVIATION...WS

NWS SJU Office Area Forecast Discussion