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 18 19 20 21 22 23 24 25 26 27 28 29 30
FXCA62 TJSJ 231547

Area Forecast Discussion
National Weather Service San Juan PR
1147 AM AST Wed May 23 2018

.UPDATE...So far this morning, cloudiness has started streaming
off the USVI and off the Luquillo Mountain Range with a few
showers. Clouds have also started to develop across mainland PR
with sea breeze convergence causing the typical cloud cover across
western PR. The latest guidance is still suggesting showers and
isolated thunderstorms to develop across Western PR in the
afternoon hours. The latest observations are also in alignment
with the inherited forecast. For that reason no changes were made
to the inherited forecast.

For further details, the satellite derived precipitable water
values are consistent with the model guidance. It appears that
there will be some moisture advection this afternoon that will
help in the development of the shower activity this afternoon. The
12Z sounding indicated only 1.29 inches of precipitable water,
which once again is consistent with the model guidance. The
surface high pressure across the Atlantic is expected to continue
causing easterly winds through the rest of the workweek, and the
upper trough to the NE of the local area will slowly move
northeast as it weakens. Overall, the latest guidance still
indicates a fairly persistent weather pattern.


.AVIATION...VFR conds expected across the local terminals through
23/17Z. SHRA/TSRA expected to develop across the western sections
of PR, so TEMPO TSRA with MVFR conds on TJMZ and TJBQ TAF. Winds
from the east at 10-15 KT with a few gusts and sea breeze
variations. Winds to decrease after 23/23Z and weather is also
expected to subside by this evening.


.MARINE...Small craft should exercise caution across the local
waters since seas will be up to 6 feet and winds up to 20 knots.


.PREV DISCUSSION... /issued 458 AM AST Wed May 23 2018/

SYNOPSIS...TUTT low northeast of the Leeward Islands will begin
to move eastward towards the Central Atlantic next few days.
Upper level ridge will establish itself across the forecast area.
Tropical wave will approach the local region Sunday into Monday
next week.

SHORT TERM...Today through Friday...

A TUTT low located northeast of the Leeward Islands will begin to
move eastward towards the Central Atlantic while at the same time
fill in today and Thursday. At the same time, this will allow a mid
to upper-level ridge to establish itself across the forecast area.
Even though this ridging pattern will establish itself across the
forecast area, it will not be sufficiently strong enough to suppress
shower activity across the region during the next couple of days as
there will still be enough low-level moisture in place. The
morning hours should feature brief passing showers across portions
of eastern PR and the USVI. With the low-level flow expected to
be from the east-southeast, the afternoon shower activity should
be concentrated across the northwest quadrant of PR as well as
across portions of the San Juan metro area downwind from El
Yunque. Some of these showers during the afternoon could be heavy
enough to result in ponding of water on roadways and in low-lying
areas as well as some localized minor urban and small stream
flooding. Thunderstorm coverage, as it has been during the last
couple of days, is expected to remain very isolated as the mid
levels of the atmosphere remain very dry.

On Friday, the aforementioned ridge is expected to erode and this
will allow for deeper moisture to pool in across the forecast area.
Therefore, an increase in the coverage and intensity of showers is
expected. Thunderstorm potential will also increase given that the
mid-levels will also moisten up. Under a southeasterly wind flow,
the heaviest showers will be concentrated along and north of the
Cordillera Central.

LONG TERM...Saturday through Thursday...

A moist southeast wind flow will continue to dominates the region
on Saturday, maintaining good chances for showers and
thunderstorms across the area. For Sunday and Monday, the
combination of a tropical wave with and upper level trough promise
to bring an increase in low level moisture both days. As a
result, a wet pattern is anticipated for early next week. A drier
air mass is forecast to encompass the region by later in the
forecast period.

AVIATION...Mainly VFR conditions expected to prevail across all
terminals through 23/16z. +SHRA and possibly some isolated TS
expected to develop between 23/16-22z over western PR, creating MVFR
conditions at times across TJMZ and TJBQ. VCSH also possible across
TJSJ between 23/16-20Z. Winds will be from the E-SE at around 15kts
with sea breeze variations and occasional gusts through 23/22z.

MARINE...Seas between 4 to 6 feet with occasional seas up to 8
feet will continue to prevail across the local waters through at
least Thursday. As a result, small craft should exercise caution
across most local waters.


SJU  87  76  87  77 /  40  30  30  30
STT  87  77  87  77 /  20  30  30  30




LONG TERM....99 is the U.S. government's official web portal to all federal, state and local government web resources and services.