Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 220858

Area Forecast Discussion
National Weather Service San Juan PR
458 AM AST Mon Oct 22 2018

.SYNOPSIS...Modest low level moisture and east northeasterly flow
will continue to bring minor showers across the forecast area and
onto windward slopes. Flow shifts to east southeast to southeast
by mid week and weakens to bring slightly warmer temperatures to
the north coast and continued shower activity as patches of
moisture move through.


.SHORT TERM...Today through Wednesday...
A mid to upper level ridge will continue to promote dry air aloft
through the short term period. However, a weak surface trough just
east of the region will increase shower activity today. The best
moisture is forecast to remain across the Caribbean waters through
the evening hours. Under an east to northeast steering wind flow,
scattered to locally numerous showers are expected to move over the
local waters with some reaching portions of the U.S. Virgin Islands
and the eastern sections of Puerto Rico through the morning hours.
This slight increase in low level moisture today will aid in the
development of afternoon showers over sections of the interior and
southwestern PR, some with heavy showers that could lead to urban
and small stream flooding.

On Tuesday and Wednesday, winds are forecast to gradually diminish
and turn more from the east, with a more east to southeast flow by
Wednesday. Enough low level moisture will remain across the forecast
area, leading to passing showers with light rainfall amounts across
the U.S. Virgin Islands and the eastern sections of Puerto Rico
during the night and early morning hours...followed by diurnally
induced afternoon convection along the interior and western sections
of Puerto Rico.

.LONG TERM...Thursday through Monday...
Northwest flow at upper levels continues on Thursday while the
area remains south of a strong upper level jet of the same
direction. High pressure builds over the area over the weekend.
At mid levels high pressure continues over the area and weak
moisture is noted, but the driest air is still above 500 mb. At
lower levels a broad but weak trough is located over and north
northeast of the area on Thursday, but as a deep low off the
southeast coast of the United States strengthens, flow in the
local area turns southeast and the resulting flow is moderately
moist. This will keep showers in the forecast during the period,
but will also allow some warming on the north coast of Puerto
Rico. Since 1000-850 mb thicknesses remain nearly constant,
extreme values are not expected and some sea breezes may modify
the southeast flow expected during the period on the north coast.
Showers in interior and northwestern Puerto Rico are expected
each day and some will likely be heavy resulting in ponding or
very localized urban and small stream flooding. Surface flow
gains a northerly component on Sunday and shower activity will
move off the northwest coastal areas Sunday and Monday. Although
thunderstorms are not in the forecast since columnar moisture
values are generally in decline after Thursday, lifted indices and
divergence aloft do become more favorable, therefore
thunderstorms cannot be ruled out completely. Will look at later
model runs to determine if probabilities are strong enough to add
then Saturday through Monday.


.AVIATION...Mainly VFR conditions are expected to prevail across all
the terminals through the forecast period. However, SHRA across the
regional waters may cause tempo MVFR cigs across the Leeward/USVI
terminals. Between 16z-22z SHRA expected to develop over SW PR,
impacting mainly the vcty of TJPS/TJMZ. East to northeast winds
expected btwn 10-15 kt.


.MARINE...Diminishing winds over the next several days will lead
to lowering seas. Small craft advisories are not expected during
the next 7 days.


SJU  87  78  88  78 /  60  40  40  50
STT  88  78  88  78 /  50  50  40  50




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