Area Forecast Discussion
Issued by NWS San Juan, PR

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000
FXCA62 TJSJ 210900
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
500 AM AST Mon May 21 2018

.SYNOPSIS...As a mid to upper level trough (TUTT low) to the
northeast of the area weakens, a ridge aloft will build across
the forecast area and hold much of this week. Moderate to fresh
easterly winds to continue as a surface high remains across the
central Atlantic. Locally induced afternoon showers expected each
day with the chance for shower and thunder activity increasing by
the end of the week.

&&

.SHORT TERM...Tonight through Wednesday...

Partly cloudy skies were observed during the overnight hours and
this morning with a few passing showers across the coastal waters
and portions of eastern, northern and interior of Puerto Rico.
Rainfall accumulations were less than a tenth of an inch.

Two features that have been anchored across the region are still
present which are the surface high pressure in the North Atlantic
and the TUTT low to the northeast of the Leeward Islands. The
surface high will continue to bring brisk winds across the local
island and most of the Caribbean through the short term period. The
TUTT low northeast of the Leeward Islands will continue to roam
aimlessly through Wednesday and have no effects on the local
weather. No significant changes to the short term forecast due to
the lack of forcing in the lower and upper levels of the atmosphere
combined with the lack of low level moisture.

SJU local model and skew T shows precipitable water values ranging
from 1.26 to 1.50 over the next several days comparing these values
to climatology they rank near the 25th percentile. Analysis of
relative humidity from 1000 to 850 mb, 700 to 500 mb, and  500 to
250 mb, only the 1000 to 850 mb levels have relative humidity values
greater than 60 percent. A seasonable weather pattern is expected
to prevail during the short term period with a few isolated
showers embedded in the trades. Low level moisture will increase
on Wednesday; as a result, an increase in showers across the area.

.LONG TERM...Thursday through Tuesday...

A mid to upper level ridge will hold across the forecast area
through early the upcoming weekend. This feature is expected to
slowly erode Sunday onwards as a mid to upper level trough (TUTT)
develops across the central Atlantic and into the eastern Caribbean.
Amplifying trough will also result in moisture advection. At
lower levels, a broad high will promote east southeast winds much
of the forecast period.

If forecast holds, expect locally induced afternoon showers with
isolated thunderstorms possible across western areas of Puerto
Rico Thu-Sat with fair weather expected elsewhere. Warm to hot
temperatures particularly along the northern slopes of Puerto
Rico likely. As the ridge aloft erodes and upper trough amplifies
across the eastern Caribbean, the chance for showers and
thunderstorms (organized convection) will increase across the
forecast area on Sunday.

&&

.AVIATION...VFR conditions are expected to prevail through 22/00Z
across all the terminals. SCT SHRA are possible near terminals TJBQ
and TJMZ by 21/15z. Surface winds will continue from the east between
15-20 kts with some sea breeze variations.

&&

.MARINE...Choppy marine conditions will continue to prevail across
most of the offshore waters during the next few with seas 4 to 6
feet and winds 20 knots or less. Small boat operators should exercise
caution.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  86  77  86  77 /  40  40  20  30
STT  87  77  87  77 /  20  20  20  20

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through late tonight for Northwest-Western
     Interior.

VI...None.
AM...None.
&&

$$

SHORT TERM...TW
LONG TERM....OM


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