Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 251518
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1118 AM AST Tue Jul 25 2017

.UPDATE...Moisture is gradually increasing across the local area,
and with that the shower activity has also been increasing. As of
now, the northern USVI has observed brief heavy showers and
eastern PR has had some showers move in from the east. This shower
activity is expected to continue through today with showers and
possible thunderstorms developing in the afternoon. The afternoon
showers and thunderstorms are expected to be more persistent in
the interior and western to southwestern sections of PR but the
northern slopes and the San Juan metro could also observe some
isolated thunderstorms.

&&

.AVIATION...VCSH across the local terminals in the morning except
TJMZ with TEMPO MVFR possible at IST. SHRA/TSRA to develop across
W-SW PR could cause VCTS at TJMZ and TJPS with tempo TSRA at TJMZ
very possible. Weather to improve over PR after 25/23Z but VCSH
may continue through tonight across the rest of the local
terminals.

&&

.MARINE...Most of the local waters will have winds of up to 15
knots with up to 20 knots across the waters just south and
southwest of PR. The seas will be mainly 3 to 5 feet today.


&&

.PREV DISCUSSION... /issued 505 AM AST Tue Jul 25 2017/

SYNOPSIS...Ridge aloft will continue to erode as a TUTT low
which is now located across the central Atlantic shifts westward
and relocates northeast of the area. A broad surface high will
continue to hold across the central Atlantic. There is a tropical
wave east of the Lesser Antilles which is expected to move across
the Caribbean waters Wednesday.

SHORT TERM...Tuesday through Thursday...

A humid air mass displaced the dry air mass with Saharan Dust
Particles. This patch of moisture brought showers and clouds across
the islands overnight. Under a northeasterly wind flow, shower
activity is expected to continue into the morning hours mainly
across the surrounding waters and the windward sections of the
U.S. Virgin Islands and Puerto Rico. Afternoon convection is
expected across the interior and southwestern sections of Puerto
Rico. As a result, ponding of water in roads and low lying areas
as well as urban flooding are possible this afternoon.

Moisture pooling will continue as a tropical wave is forecast to
move across the islands Wednesday. Also, instability is forecast
to increase due to the proximity of an upper level trough/ TUTT
low, which combined with a drop in the regional winds, diurnal
heating and local effects will provide a favorable environment for
the development of T-storms and showers during the afternoon hours.
Under a humid environment, the typical showers are expected to
continue each night and early morning.

LONG TERM...Friday through Wednesday...

A weak mid level ridge is expected to hold across the forecast
area Saturday through Wednesday. At lower levels, a broad high
pressure across the central Atlantic will yield east to east
southeast winds through the forecast period. Precipitable water
is expected to remain near the normal range which is 1.75 inches
Friday through at least late Monday. As an easterly disturbance
moves across the eastern Caribbean on Tuesday, deep tropical
moisture is expected to reach the local islands. However, deep
tropical moisture across the area will be short-lived as a drier
air mass moves into the forecast area on Wednesday.

Therefore under the lack of upper level forcing and moisture
content near the normal range; continue to expect a seasonable
weather pattern with trade wind showers across the USVI and E PR
at times, as well as showers and thunderstorms developing across
west sections of Puerto Rico each afternoon. At this time, the
day with the highest chance for showers and thunderstorms is
Tuesday associated with the disturbance moving across the area.

AVIATION...VFR conds expected through 25/16Z. Winds will
continue to be from the East to ESE for the rest of today with
sea breeze variations at about 15KT with occasional gusts. VCSH
possible at TJMZ between 25/18Z and 25/22Z. Some SHRA/TSRA
possible around TJMZ after 25/16Z.

MARINE...Tranquil marine conditions will continue to prevail
across the forecast area with seas 3 to 5 feet and occasional
seas up to 6 feet. Winds 15 to 20 knots to continue.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  88  79  90  81 /  50  40  30  40
STT  90  79  89  80 /  50  40  40  40

&&

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

$$

SHORT TERM...JA
LONG TERM....99


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