National Weather Service San Juan PR
458 AM AST Tue Jul 14 2020
.SYNOPSIS...Drier air along with Saharan dust will support
relatively fair weather conditions with limited shower activity
favoring portions of southwestern Puerto Rico under a generally
east-northeast steering flow. A wet and unstable weather pattern is
expected by Wednesday and continue through the weekend as a TUTT
and associated surface induced trough, followed by an easterly
disturbance stream across the region. Both features are expected
to enhance the potential for shower and thunderstorm development.
&&
.SHORT TERM...Today through Thursday...
A vertical stack high pressure and a dry air mass will inhibit rain
activity across the islands today. Under this weather pattern,
warming will happen once again across the islands with heat indices
between 100-107 degrees Fahrenheit. African dust particles still in
the atmosphere and somewhat hazy skies. Rainfall activity, if any,
will be wind-driven during the morning, and limited afternoon
convection over the southwest quadrant of mainland PR and downwind
from the U.S. Virgin Islands.
The high pressure system will erode with the arrival of a TUTT-Low
Wednesday into the rest of the work-week. A moist air mass is
forecast to replace the current air mass. This pattern will slowly
increase the frequency of showers and thunderstorms Wednesday and
Thursday. Based on the current model guidance, a wet and unstable
weather pattern is likely after today. However, the best chance for
organized convection in the short-term seems to be late Wednesday
night into early Thursday.
.LONG TERM...Friday through Tuesday...
Model guidance continues to suggest that a relative wet weather
pattern will dominate the long-term forecast period. Increased
low-level moisture and somewhat favorable conditions aloft are
expected as a retrogressive TUTT and induced surface perturbation
streams across the area by Friday. Drier air will briefly move in
by late Friday night, but an easterly disturbance and associated
plume of moisture is forecast to arrive on Saturday afternoon and
affect the area through Monday. Both features will enhance the
potential for shower and isolated thunderstorm development across
the region, following the typical seasonal shower pattern.
However, the best chance for organized convection in the long-
term is expected between Sunday afternoon and Monday afternoon
with peak moisture content around 2.0-2.2 inches and 500 mbar
temperatures ranging between -5 and -7 degrees Celsius. Drier air
will quickly move in by Monday night into Tuesday, resulting in
fair and stable weather conditions with limited shower activity
thereafter.
&&
.AVIATION...VFR conds expected across the local terminals.
Although hazy skies will continue, P6SM vis will prevail. Passing
-SHRA/SHRA will affect the terminals of JSJ/IST/NCM/KPK at times.
Because winds are from the NE, SHRA/isol-TSRA will develop over
the southwest quadrant of PR and downwind from the USVI terminals
btwn 14/17-23z. NE winds will increase at 15 to 20 knots with
higher gusts and sea breeze variations after 14/13z.
&&
.MARINE...Small craft operators are urged to exercise caution due
to choppy seas up to 6 feet and/or winds up to 20 knots across the
offshore Atlantic waters, coastal waters of southern Puerto Rico
and the Mona Passage. Elsewhere, relative tranquil marine
conditions are expected with seas up to 5 feet and winds up to 15
knots. A moderate risk of rip currents continues for beaches
along the north and south coast of Puerto Rico, eastern half of
Vieques, and most beaches in Culebra and Saint Croix.
&&
.FIRE...A Fire Danger Statement has been issue for elevated
fire danger conditions across the southern coastal plains of
Puerto Rico today. Although wetting rains are expected across
southwestern Puerto Rico this afternoon, the timing of showers
affecting the area may occur after relative humidity drop to fire
danger thresholds. Elsewhere in the southern coastal plains,
limited to no shower activity is expected with relative humidity
expected to drop into the mid 40s to low 50s today. Winds will
favor a east-northeast flow at 15-20 mph with higher gusts. Based
on the forecast and current soil/moisture conditions, elevated
fire danger criteria is expected today.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 90 79 91 77 / 40 50 40 60
STT 90 78 91 78 / 30 30 40 60