002
FXCA62 TJSJ 302043
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
443 PM AST Thu May 30 2024

.SYNOPSIS...

The primary concern for the short-term period will continue to be
heat, at least through this weekend. Diurnally induced afternoon
showers and thunderstorms are expected each day, mainly across the
interior and western Puerto Rico. Additionally, some Saharan dust
is expected by tomorrow and will persist through the weekend,
contributing to hazy skies and an elevated to moderate heat risk.

&&

.SHORT TERM...Tonight through Saturday...

As anticipated, thunderstorm activity developed by early afternoon,
primarily affecting the northwestern quadrant of Puerto Rico
leaving around 1 to 2.5 inches of rain. Strong thunderstorms
eventually moved over the local waters. Weather conditions are
expected to improve after sunset due to diminishing surface
heating.

Once again, sweltering temperatures were observed today as
southerly winds persisted, advecting warmer temperatures and
combining with higher-than-normal moisture across the area due to
the passage of a tropical wave, although the bulk of moisture
remained mostly south throughout the day. Passing showers are
expected to move in over windward coastal areas this evening and
late tonight as the tropical wave continues its westward movement.

The overall synoptic pattern for the remainder of the short-term
forecast suggests that the current typical weather pattern will
persist. On Friday, a slight decrease in moisture content is
expected, with precipitable water values dropping to slightly
below 2.0 inches. However, by Saturday, a slight increase in
precipitable water is anticipated due to the approach of another
tropical wave currently situated at 40W. This pattern will
maintain moisture content within the 75th and 90th percentiles, or
slightly higher than the climatological norm, throughout the
period. Dynamics aloft will remain seasonable, with 500 mb
temperatures staying close to -5 and -6 degrees Celsius, and both
850-700 mb and 700-500 mb lapse rates remaining near normal for
this time of year.

Therefore, under an east-southeast steering wind flow, expect
showers to move over eastern/southeastern portions of Puerto Rico
and smaller islands during the morning and night hours, with
convective activity clustering over the northwestern quadrant of
Puerto Rico each afternoon. Streamers may develop downwind of the
smaller islands and El Yunque into the San Juan metro area. Model
guidance suggests that the highest rainfall accumulations are
expected during the afternoon shower activity, with southwestern
and south-central coastal areas of Puerto Rico observing the least
amount of rainfall accumulations throughout the period. However,
the GEOS-5 model suggests the arrival of suspended Saharan dust
particulate over the forecast area by Friday through the weekend,
which could slightly suppress shower development in some areas,
bringing hazy skies across the northeastern Caribbean.

The 925 mb temperatures will remain well above normal for this
time of year through the weekend. High temperatures combined with
above-normal moisture could promote hazardous heat conditions,
particularly across coastal and urban areas. Therefore, it is very
likely that Heat Advisories and possibly Excessive Heat Warnings
may need to be issued daily for some of these areas.

&&

.LONG TERM...Sunday through Thursday...
.PREV DISCUSSION... /issued 455 AM AST Thu May 30 2024/

There will still be some Saharan dust over the region as the
Saharan Air Layer weakens during the weekend. A tropical wave is
forecast to cross the islands on Sunday. This will bring an
increase in shower activity first across the U.S. Virgin Islands,
then spreading across Puerto Rico during the peak time of
afternoon convection. Some thunderstorms could be strong over
western PR.

For next week, a mid-to upper-level trough is expected to move
from the western Atlantic into the northeastern Caribbean, while a
deep layered ridge builds and hold just east of the Leeward
Islands. This will promote divergence aloft and a south to
southwesterly wind flow at lower levels. In addition, a surface
front associated to the upper trough will sink southwards but
should remain above 20N. However, pooling of moisture is expected
in general across the northeastern Caribbean through the long term
period with global models suggesting between 2.10-2.40 inches of
precipitable water content over the local area, peaking by
midweek.

The southerly winds and high humidity content will also promote
warm to hot conditions across the islands, particularly across St.
Croix and most coastal municipalities of PR.

&&

.AVIATION...

(18z TAFs)

SHRA/TSRA will remain possible across NW-PR thru this evening.
VFR conditions will prevail across the local flying area. However,
the trade winds will bring pulses of SHRA from time to time. A
SAL will arrive later tomorrow, lowering VIS, remaining P6SM. SFC
wnds mainly fm E/ESE at 15 kt or less with sea breeze variations,
bcmg calm to lgt/vrb aft 30/23Z. Expect E/ESE winds at 10-15 kt
with sea breeze variations and ocnly hir gusts aft 31/14Z.

&&

.MARINE...

A moderate east to southeast wind flow will prevail across the
region over the next few days. A tropical wave will continue to
move far to the south of Puerto Rico and the U.S. Virgin Islands
between tonight and early Friday, increasing winds and seas across
the Caribbean Offshore Waters. Thunderstorms are expected to
develop once again tomorrow, between the late afternoon and
evening hours across the western waters of Puerto Rico.

&&

.BEACH FORECAST...

A low risk of rip currents will continue tonight across most
beaches of Puerto Rico, St. Thomas, and St. John while a moderate
risk is expected across Vieques, Culebra, and St. Croix. The risk
of rip currents is expected to remain between low and moderate
throughout the upcoming weekend.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

$$

SHORT TERM/MARINE/BEACH FORECAST...CVB/YZR
LONG TERM...DSR
AVIATION...CAM/RAM