Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 210125 AAA
AFDSJU

Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
925 PM AST Tue Aug 20 2019

.UPDATE...Showers have dissipated almost completely, except for
several small areas: one in Vega Baja and one in Naguabo.
Conditions still remain favorable for showers to develop as
precipitable water in the 21/00Z sounding from San Juan was 2.23
inches which is the highest this week so far. The main inhibiting
factor seems to be a lack of surface forcing with the land cooling
and only topographical features aiding in lifting from east
northeast to east winds. These winds extend all the way to 100 mb
with 10 to 20 kt and little directional variation.

The GFS still indicates drier air arriving in the U.S. Virgin
Islands no later than noon on Wednesday and arriving in Mayaguez
by 22/03Z. This should shut down most of the convection that gets
going during the middle of the day Wednesday. This is still
expected to be considerable and will likely favor western,
northwestern and interior Puerto Rico as well as the San Juan
Metropolitan area in flow just south of east.

Long Term Note: Precipitable water values now are likely as high
as can be expected any time in the next 7 days. A significant dust
episode is expected over the following weekend.

Some changes in interior temperatures were made, otherwise few
other changes were needed.

&&

.AVIATION...VFR conds expected to persist through 21/16Z except
for brief MVFR CIGs in SHRA in TNCM/TKPK. Aft 21/16Z areas of
MVFR/IFR and mtn obscurations over wrn, nwrn and central PR as
well as downstream from each US Virgin Island. Clrg will commence
aft 21/21Z.

&&

.MARINE...Seas are expected to remain less than 6 feet through
the next 7 days. Isolated thunderstorms may induce locally higher
conditions.

&&

.PREV DISCUSSION... /issued 319 PM AST Tue Aug 20 2019/

SYNOPSIS...Lingering moisture associated with a departing tropical
wave will continue to produce showers and thunderstorms, mainly
across interior and western Puerto Rico through late this
afternoon. A more stable weather pattern is expected Wednesday
through Friday, however, sufficient low-level moisture will remain
in place to result in afternoon convection across interior and
western portions of Puerto Rico. A tropical wave will bring
increased rain chances for Saturday, followed by drier conditions
and Saharan dust for Sunday.

SHORT TERM...Tonight through Thursday...

Showers and thunderstorms will continue to affect mainly interior
and western Puerto Rico through the late afternoon hours before
diminishing through the early evening hours with the loss of
daytime heating. For later tonight, a drier air mass will be
moving overhead. Therefore, more tranquil weather conditions are
expected with just some passing showers expected across the
regional waters and possibly affecting portions of northern and
eastern portions of Puerto Rico and the USVI. More stable weather
conditions are expected Wednesday and Thursday as an upper-level
ridge builds near the area with very dry air expected in the mid
to upper-levels of the atmosphere. This is expected to limit
rainfall activity across the area compared to previous days.
However, patches of low-level moisture embedded within the
easterly trade wind flow will move across the area from time to
time. This will generate some passing showers during the overnight
and early morning hours across the regional waters and portions
of eastern Puerto Rico and the USVI with only very light rainfall
accumulations expected. Then, during the afternoon, the available
low-level moisture will combine with strong daytime and local
effects to generate showers and thunderstorms across portions of
the western half of Puerto Rico.

LONG TERM...Friday through Wednesday...

PREV DISCUSSION... /issued 520 AM AST Tue Aug 20 2019/

A mid to upper level ridge north of the region is expected to
promote fair weather conditions across the islands on Friday.
However, diurnally induced afternoon convection should develop
over western Puerto Rico. By Saturday, a tropical wave is expected
to increase shower and thunderstorm activity across the region.
At the same time a 700mb high pressure moving from the tropical
Atlantic will bring a wind surge behind it and as well a Saharan
Air Layer. Therefore, breezy conditions and hazy skies due to
Saharan dust are expected late in the weekend. At the end of the
long term period, a tropical wave is forecast to increase shower
and thunderstorm activity. Both global models are showing
different solutions at this moment. The GFS has a strong tropical
wave entering the southeastern Caribbean by Monday morning, while
the ECMWF has a slower solution(including the timing of the 700mb
high) and a weak wave at the end of the period.

AVIATION...VCTS are expected across TJSJ/TJPS/TJBQ until 20/22Z.
TEMPO due to thunderstorms is expected for TJMZ until 20/22Z. This
could result in MVFR conditions with possible IFR. VCSH is expected
for the other terminals. FL050 wind flow will continue out of the
east at 10 to 20 knots.

MARINE...Mainly tranquil marine conditions are expected to continue
during the rest of the work week with seas of up to 5 feet and winds
up to 15 knots. However, showers and thunderstorms moving off the
west coast of Puerto Rico during the afternoon may briefly result
in hazardous seas and higher winds. There is a moderate risk of
rip currents for the northern beaches of Puerto Rico through
tonight.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  78  89  78  89 /  30  20  30  30
STT  79  88  78  90 /  30  30  50  30

&&

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

$$

SHORT TERM...WS
LONG TERM....MB


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