Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 200732
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
332 AM AST Thu Jun 20 2019

.SYNOPSIS...

A tropical wave has moved into the southern Caribbean waters
during the overnight hours. This wave will increase low-level
moisture across the region today. However, the bulk of the
moisture is expected to remain south of the forecast area, but
scattered showers could develop and move across the local islands.
Tonight the moisture rapidly moves west of Puerto Rico thus
giving way to a dry Saharan air mass. The dry Saharan air mass is
forecast to hold across the region through the weekend. Then
another tropical wave is forecast to move into the Caribbean
waters Sunday night into Monday

&&

.SHORT TERM...Today through Saturday....

A tropical wave is forecast to pass south of the local islands
today, which could bring some increase in moisture. The satellite
derived blended total precipitable water imagery indicates that the
moisture associated with the wave reaches latitude 19N and has
precipitable water values almost to 2 inches, moving west. Guidance
is consistent with this, and it shows that the moisture will be
highest across the local waters, USVI, and coastal areas of PR
today. However, there is some Saharan dust and the precipitation
potential seems to be very limited, as suggested by both global and
high resolution models. The main area of convection associated with
the wave is expected to remain to our south, and current satellite
imagery shows the convection (associated with the tropical wave and
an upper trough to our NE) occurring south and southeast of the local
islands at about latitudes 15-16N. Therefore, we toned down the
precipitation potential a little, limiting it mostly to the
northwestern quadrant of PR in the afternoon hours as sea breeze
convergence and available moisture will combine with diurnal heating
to cause some shower development across portions of NW-PR.
Otherwise, we expect a warm to hot day with higher humidity, which
could cause the apparent temperature to reach temps around 105
degrees.

By late tonight into Friday, the Saharan dust concentration will
increase and the available moisture will plummet to around 1.25 or
so inches, which is close to 2 standard deviations below normal for
the month. The very dry air will likely result in fair weather with
hazy skies and warm to hot temperatures. Similar conditions are
expected for Saturday as there is no significant changes expected
between Friday and Saturday except maybe even drier air moving in on
Saturday.

&&

.LONG TERM...Sunday through Friday....

A tropical wave and its associated moisture is forecast to move
into the southern Caribbean waters late Sunday, and continue to
pool over Puerto Rico, and the adjacent islands through Monday.
The increasing low-level moisture combined with local and diurnal
effects will increase showers and possibly isolated thunderstorms
during the afternoon on Monday. Then late Monday the moisture
rapidly moves west of Puerto Rico thus giving way to a dry Saharan
air mass. The dry Saharan air is forecast to hold through the end
of the workweek. Therefore, expect fair weather with hazy skies
to prevail over the region with limited shower activity during
the afternoon.

&&

.AVIATION...

VFR conds expected across the local terminals for the
next 24 hours. However, brief VCSH is expected at TJBQ and TJMZ
between 20-17 and 20-21Z due to SCT SHRA developing across NW-PR.
SFC winds will be from the east at 15 to 20 kts with higher gusts
and sea breeze variations. Some Saharan dust is expected but VIS
will be P6SM.

&&

.MARINE...

Seas will range from 4 to 6 feet with easterly winds of 15 to 20
kts with higher gusts across the north Atlantic waters and local
waters north of Puerto Rico. Across the Caribbean waters seas will
build up to 7 feet due to increasing winds and the passage of a
weak tropical wave. Therefore, small craft advisory is in effect
for the Caribbean waters. Elsewhere, small craft operators are
urged to exercise caution.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  88  78  89  79 /  20  10  10  20
STT  87  79  88  79 /  10  10  10  10

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...Small Craft Advisory until 2 PM AST this afternoon for Anegada
     Passage Southward to 17N.

     Small Craft Advisory until 2 PM AST Friday for Caribbean Waters
     of Puerto Rico from 10 NM to 17N-Mona Passage Southward to
     17N.

&&

$$

SHORT TERM...JA
LONG TERM....TW


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