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000
FXCA62 TJSJ 050144
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
944 PM AST Sun Jun 4 2023

.UPDATE...Hot temperatures will continue during the next several
days, with heat indices ranging from 105 to 115 degrees or higher
across most coastal and urban areas of the islands. Therefore, an
Excessive Heat Watch remains in effect through at least Tuesday
afternoon. Drier air with Saharan dust is expected to gradually
filter from the east during the next few days, promoting hazy
skies and limiting shower development across the islands. Isolated
showers could move at times across the windward areas of the
islands.


&&

.AVIATION...Mainly VFR conditions are expected to prevail across
all terminals during the next 24 hours. The latest TJSJ sounding
indicated southerly winds up to 21 kt blo FL100. HZ due to Saharan
dust is expected to filter across the region during the next
couple of days.


&&

.MARINE...Latest buoy observations across the coastal waters of
the islands indicated seas between 1-4 ft, with the highest seas
across the Caribbean waters. Winds were from the south up to 12
kt. A low risk of rip currents continues across all beaches of the
islands.


&&

.PREV DISCUSSION... /issued 446 PM AST Sun Jun 4 2023/

SYNOPSIS...

Breezy southerly winds will prevail through the period. Due to
the combination of high temperatures and the moisture that the
southerly flow will continue to bring, high heat indices are
expected each day of the short- term period. A drier pattern is
forecast to start late tomorrow into Tuesday as winds become more
southeasterly and more Saharan Dust filters in.

SHORT TERM...Tonight through Tuesday...

Sunny skies with warm conditions prevailed across most islands
today. Daily high temperatures fluctuated around the 90s in the
coastal areas, while in the mountains, they ranged in the 80s.
By 10:38 AM AST, the temperature in the San Juan metro area was
92 degrees. Today we tied the record of 94F from 2022. Today heat
indices were around 105 to 116 degrees Fahrenheit in areas where
the Heat Advisory and the Heat Warning were in effect.

Currently, models suggest a deep layer of high pressure over the
eastern Atlantic and a deep layer trough centered near Cuba
dominating the area. Combined weather features will enhance
southerly flow at the mid to low levels through at least the
beginning of the workweek. This flow will keep bringing some
moisture into the region for the next few days across Puerto Rico
and the U.S. Virgin Islands, keeping it around normal moisture
content. Under this pattern, dangerously hot conditions will
prevail, and we anticipate heat indices surpassing 100 degrees
across the low elevations by the end of the forecast period.
Therefore, the Excessive Heat Watch for the northern coast of
Puerto Rico was extended to Tuesday afternoon. By the beginning of
the workweek, conditions will become unfavorable for shower
development as the ridge holds stronger over the area.
Nonetheless, passing showers across the southern municipalities of
Puerto Rico cannot be ruled out. Additionally, Saharan dust will
increase in concentration, resulting in hazy skies, with higher
concentrations on Monday and Tuesday.

LONG TERM...Wednesday through Sunday...FROM previous
discussion...

No major changes to the long term forecast. A surface high pressure
northeast of the region will promote more southeasterly winds and
steer a Saharan Air Layer (SAL) over the islands. Model guidance
indicates higher concentrations on Thursday, where Aerosol Optical
depth could reach more than 0.20 over the islands, above the 75th
percentile. Saharan dust will continue to linger for most of the
period, promoting hazy skies. A mid- to upper- level ridge will
dominate the region and mostly dry air will be present above 850 mb.
This will limit convective development and promote stability over
the islands. In terms of Precipitable Water (PWAT) values, these
should remain at normal to below normal for this time of the year
with the driest day being Thursday, with PWAT values around or below
an inch. Model guidance now seems in more agreement with an increase
in moisture for Friday. Frontal moisture remnants that moved around
the surface high pressure will be steered towards the region on
Friday promoting an increase in PWAT values. Patches of both drier
and humid air will reach the region during the weekend but an
overall drying pattern will be observed. The seasonal weather
pattern will vary in intensity depending on moisture content; on
days with more moisture, passing showers across windward sectors of
the islands and afternoon convective activity due to diurnal heating
and local effects will be more likely. High heat indices are
expected each day, reaching 105 degrees to the 110s, especially
across lower elevations of northern and western Puerto Rico.

AVIATION...Mainly VFR conditions will prevail for all the TAF sites.
Passing SHRA expected to move at TJPS and TISX through the
period, with brief periods of reduced VIS and low ceilings
possible. HZ due to Saharan dust is expected, but with VIS
remaining at P6SM. Winds will be from the south at 13-20 kts, and
stronger gusts, diminishing at 05/06Z.

MARINE...No changes were made into the marine forecast. Seas
will remain around 1- 5 feet across all the local waters. Some
localized choppy conditions are forecast, mainly due to moderate
to locally fresh southeasterly winds.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Excessive Heat Watch through Tuesday afternoon for PRZ001-005-
     008.

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

$$

DSR/ICP