Area Forecast Discussion
Issued by NWS San Juan, PR

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
000
FXCA62 TJSJ 211956
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
356 PM AST Thu Mar 21 2024


.SYNOPSIS...

Expect, the increase in moisture content through Saturday. A
frontal system is expected to bring showers and possibly
thunderstorms by late Sunday into early in the weekend. Tranquil
weather is expected Wednesday onward, but with deteriorating
marine conditions.

&&

.SHORT TERM...Tonight through Saturday...

In the morning, mostly calm conditions and cloudy skies prevailed
over Puerto Rico and the U.S. Virgin Islands. The cloudiness
began to filter over the northwestern quadrant of Puerto Rico
around 6 AM AST and proceeded to spread across the islands
throughout the day. By the afternoon, showers started developing
across the Cordillera Central/interior municipalities of Puerto
Rico. The light easterly winds, diurnal heating, and sea breeze
variations contributed to the formation of these showers. The
maximum temperatures today ranged in the mid to upper 80s along
the coastal areas, while over the higher terrains, they stayed in
the upper 70s to low 80s.

For the rest of today, expect the moisture content to extend from the
surface through the mid-levels, accompanied by some shower
activity across the islands. The wind flow overnight should be out
of the east-northeast and gradually increase its speed by
tomorrow. The overnight minimum temperature should remain in the
70s along the coast and in the 60s across the mountainous areas.

The latest model guidance continues to suggest above-normal
climatological moisture content through at least the end of the
workweek. This above-normal moisture content will persist through
at least Saturday as the frontal boundary stalls just north of the
forecast area. At this time, models show some stability at the
mid to upper levels from tomorrow onwards as a weak ridge develops
in the next few days. The mid-level temperatures are foreseen to
remain around -6 and -4 degrees Celsius through Saturday.
Therefore, it is less likely that thunderstorm activity will form
across the area during the short-term period. As mentioned in
previous discussions, the pressure gradient over the local area
will tighten as a strong low-pressure system exiting the Eastern
Seaboard pushes against a surface high-pressure system over the
northern Atlantic. Consequently, winds will shift more easterly,
and its speeds will increase through Saturday. Although moisture
levels will not fluctuate much for the rest of the forecast
period, the faster steering winds will reduce rainfall
accumulations each afternoon. Overall, the dominating weather
pattern from Friday onwards will be the breezy conditions with
limited threats of flooding, if any, in the presence of shower
activity across the area.

&&

.LONG TERM...Sunday through Thursday...
//from previous discussions//

As a cold front approaches from the west, winds from the surface
up to 750 mb are expected to shift from the southeast. This will
result in very warm or hot temperatures across the region, with
heat indices likely above 102 degrees for many areas in northern
and western Puerto Rico, and above 90 degrees elsewhere in low-
elevated areas. Moisture associated with the front will increase
on Sunday night and early in the workweek, with values above
normal for this time of year. As the frontal system approaches,
instability will also increase, thus widespread showers and
isolated thunderstorms are anticipated. The risk for urban and
small stream flooding will be elevated. It is worth mentioning
that, even with some difference in timing and coverage, the global
models agree in the generally wet pattern for early next week.

After the frontal system moves, the winds will shift from the
northwest, bringing a cooler and drier air mass across the
islands. Conditions will begin to dry out on Tuesday, although
with some lingering showers, and even additional shower
development over southeastern Puerto Rico and near Saint Croix.
It should be enjoyable on Wednesday and Thursday, with lot of
sunshine and temperatures in the low 80s in the afternoon hours.
Regardless, small and shallow pocket of moisture may still linger
around, so a few showers cannot be ruled out during this period.

&&

.AVIATION...
(18Z TAFS)

The BKN-OVC layer at 100-130 has dissipated over the local area
although this clds remain at that level. Lcl +SHRA with MVFR conds
are forming over the Cordillera Central but more ltd heating and
northerly flow have so far kept dvlpmnt well blo yesterday. Expect
some +SHRA to form in NW PR btwn TJPS and TJBQ til arnd 21/21Z. Sfc
winds are sea breezes of less than 12 kts incrg to 10-15 kt later
aft 21/18Z. Winds to bcm land breezes less than 10 kt aft 21/22Z.
Sfc winds will see an increase in easterly flow aft 22/14Z and sea
breezes will return. Max winds WNW 60-72 kts btwn FL400-480. Aft
22/17Z SCT-NUM SHRA dvlp in wrn/swrn PR with areas of MVFR/IFR conds
and mtn obscurations.

&&

.MARINE...
Gentle to moderate east-northeast winds will continue as a cold
front lingers north of the area tonight. A surface high pressure
building over the western Atlantic will yield moderate to locally
fresh easterly winds on Friday, and thus, causing deteriorating
marine conditions during the weekend. These winds will push
fragments of moisture over the local waters.

&&

.BEACH FORECAST...
The rip current risk is currently moderate for north-central
Puerto Rico, but all north-facing beaches in Puerto Rico and the
U.S. Virgin Islands will see this risk increase to moderate too by
tonight and stretching into the weekend. Breaking waves should be
around 5 feet in this areas. The risk will remain low for the
beaches in the Caribbean Sea.

&&

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

$$

SHORT TERM...MMC/ICP
LONG TERM....ERG
MARINE...MMC
BEACH FORECAST...MMC
AVIATION...ICP
PUBLIC DESK..YZR


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.