Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 250117

Area Forecast Discussion
National Weather Service San Juan PR
917 PM AST Mon Jul 24 2017

.UPDATE...Latest satellite and radar data indicates that a band of
moisture with trade wind showers is moving across local Atlantic
waters. This band of showers is expected to affect at times the
USVI and the north and east sections of Puerto Rico through
Tuesday morning. No change from previous forecast. Saharan dust
across the forecast area is expected to gradually diminish through
Tuesday and the next tropical wave moves on Wednesday.


.AVIATION Update...Mainly VFR conds expected across all
terminals. However, trade wind showers increasing from the
northeast can result in VCSH and tempo SHRA across TIST/TISX and
TJSJ overnight. SHRA/TSRA expected to develop on Tuesday afternoon
across much of the interior and western sections of PR. Latest
25/00z TJSJ upper air sounding indicated ENE low level winds at
8-19 kts.


.MARINE Update...Latest buoy data indicates seas below 4 feet and
east northeast winds between 13-17 knots across the coastal waters
of the islands. Seas up to 5 feet and winds up to 20 knots
expected mainly across the offshore waters and passages. A
moderate risk of rip currents continues across the north and
southern coast of the islands.


.PREV DISCUSSION... /issued 435 PM AST Mon Jul 24 2017/

SYNOPSIS...Afternoon heating produced a few thunderstorms with
heavy rain in western Puerto Rico. Moisture will increase later
tonight to spread showers across the area from the east northeast
early Tuesday morning and during the day. On Wednesday a tropical
wave will cross through to increase the coverage and amount of
rain. Scattered showers typical for the time of year will continue
through early next week as areas of moisture move through, from
the east southeast.

SHORT TERM...Tonight through Thursday
High pressure at the surface just southeast of the Azores will
shift to within 1500 miles northeast of the area to continue the
easterly trade winds across the area. A tropical wave will move
through the area Wednesday morning to exit to the west on
Wednesday night. Local flow will then become east southeast.
Tonight, moisture increases from the east northeast having
wrapped around high pressure north northeast of the area at 700 mb
and from the east at lower levels. The tropical wave on Wednesday will
enhance this interaction with better showers and thunderstorms in
the area. Increase shower activity will begin later tonight and
then continue in the southeast flow through Thursday. A weak
trough will extend eastward across the area from a TUTT to our
northeast, but divergence aloft is limited to Tuesday evening and
Wednesday afternoon and will have a limited effect. Thunderstorms
that do develop Tuesday and Wednesday afternoons will likely
produce areas urban and small stream flooding in portions of
western and northwestern Puerto Rico from heavy rain. Thursday
will have showers and thunderstorms also in the same areas but
flooding will be more localized.

LONG TERM...Friday through Monday
After a dry slot on Thursday night another tropical wave moves
across the area on Friday, then alternating bands of moist and
drier air move through as disturbances from the east travel
through. The TUTT low will be northeast of the area on Friday with
an upper level trough hanging over or just east of the Lesser
Antilles off of the TUTT. That trough will break off and travel
through the Caribbean south of the area Sunday and Sunday night,
but will have little effect until Monday when a weak southerly jet
crosses through and winds at 700 mb increase to 30 knots
according to the GFS solution. No significant features are
expected tonight through early next week in the local area.

AVIATION...VFR conds expected through 25/16Z. Saharan Dust is
present so there is haze but visibilities will be P6SM. 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 24/18Z and 24/22Z. Winds will
lighten up after 24/23Z to about 10KT with mostly dry conditions.
Some SHRA/TSRA possible around TJMZ tomorrow after 25/16Z.

MARINE...Winds are keeping precautionary statements in the the
local waters as seas remain 5 feet or less. At this time small
craft advisories are not expected through the first of August.
Rip currents will remain at low to moderate risk. Thunderstorms
may become strong over the local waters during the wave passages
Wednesday and Friday to generate locally higher seas and gusty


SJU  80  89  79  88 /  60  30  30  30
STT  80  90  79  88 /  60  50  40  30




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