341
FXCA62 TJSJ 042058
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
458 PM AST Thu Jul 4 2024

.SYNOPSIS...

The passage of a tropical wave (Invest 96L) will continue to
result in passing showers and thunderstorms across the region
through at least Friday afternoon. A drying trend is expected
during the weekend along with a Saharan Air Layer. Warm to hot
conditions will continue. The next tropical wave will reach the
islands Monday night into Tuesday increasing the potential for
shower activity once again.

&&

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

Satellite and radar observations have indicated predominantly
partly cloudy to cloudy skies across the region throughout the
day. The arrival of a tropical wave resulted in showers and
thunderstorms that began to affect the USVI, Anegada Passage, and
Caribbean Waters early this morning, then moved inland, especially
across eastern and southern Puerto Rico, where approximately 1.0
to 1.5 inches of rainfall fell along with breezy conditions. In
the last hour or so, a thunderstorm have been moving across
Aguada and Rincon leaving around 2.5 inches of rain. A Flood
Advisory was issued for the mentioned municipalities. Daytime
high temperatures were around the 80s in higher elevations and
peaked in the lower to mid 90s across lower and coastal areas of
Puerto Rico and the USVI islands. Heat indices ranged around the
upper 90s to lower 100s, with isolated higher values observed.

At this time, rain activity is mainly over the local waters and
northwestern areas of Puerto Rico. High-resolution models show
additional showers moving tonight and tomorrow morning mainly
across the eastern and southeastern sectors of Puerto Rico and the
islands. The wave`s trailing moisture will linger across the
region through Friday afternoon.

Then, a change in the pattern is anticipated as the arrival of a
drier-than-normal air mass; with precipitable water values below
1.4 inches, will result in more stable conditions. This mass will
be accompanied by the presence of a dense Saharan Air Layer and
warmer-than-usual mid-level temperatures. A surface high pressure
will strengthen and extend into the northeast Caribbean promoting
breezy easterly winds. Available moisture and dust particles will
likely contribute to the development of afternoon and evening
convection, particularly over western Puerto Rico and the
surrounding waters but without flooding concerns at the moment.
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 lower elevations, coastal, and urban areas.
Therefore, it is very likely that Heat Advisories and possibly
Excessive Heat Warnings may be needed. In summary, warm-to-hot
temperatures, dry, and hazy conditions are expected into the
weekend.

&&

.LONG TERM...Sunday through Thursday...
.PREV DISCUSSION... /issued 458 AM AST Thu Jul 4 2024/

The inherited forecast remains unchanged. The latest model guidance
continues to suggest a strong high-pressure system over the
central Atlantic. This weather feature will maintain the wind flow
out of the east-southeast through most of the long-term period.
Precipitable water models show values in the 25th percentile or
below normal climatological levels for this time of the year as a
drier airmass continues to move across the region with low
concentrations of Saharan dust according to the Dust Aerosol
Optical Depth model. Therefore, we foresee dry weather conditions
through Monday. We anticipate warm weather conditions with
temperatures in the 90s along the coastal and urban areas of the
islands and in the upper 70s to low 80s in the mountainous areas.
As mentioned in previous discussions, the mixture of warm
temperatures and the available moisture will result in higher heat
indices across the region, surpassing the mid-100s.

From late Monday into Tuesday, expect a change in weather
conditions as a vigorous tropical wave approaches the local
islands. The precipitable water content analysis shows values
between 1.90 and 2.30 inches on those days. This wave will move
relatively fast across the region, particularly on Tuesday. With
the passage of this tropical wave, expect the passing showers to
increase across the islands during the morning hours, with
moderate to heavy showers and isolated thunderstorms expected
across the western and northwestern portions of Puerto Rico during
the afternoon hours. At this time, mid-week is the transition day
to a drier weather pattern.

&&

.AVIATION...

(18Z TAFS)

BKN layers at FL060-FL080 due to tropical wave crossing the
Caribbean waters. SHRA and TSRA will persist in the Caribbean
waters, reaching the USVI terminals occasionally. Wind gusts of
15- 25 kts can be expected, along with brief periods of reduced
VIS and low ceilings. HZ due to Saharan dust is possible after
04/12Z, but with VIS at P6SM. Main wind flow from the ESE at 12-15
kts after 04/13Z, with stronger gusts and sea breeze variations.

&&

.MARINE...

A tropical wave moving westward will continue to bring showers and
thunderstorms across the regional waters tonight and tomorrow. A
surface high pressure over the central Atlantic will strengthen and
extend into the northeast Caribbean by Friday afternoon through the
weekend, leading to moderate and fresh easterly winds across the
local waters. The next tropical wave will be approaching the islands
by late Monday night into Tuesday increasing the chance of showers
across the local waters.

&&

.BEACH FORECAST...

There is a low risk of rip current tonight along all the coastal
areas of the islands, with the exception of the southwestern
beaches of PR and St. Croix. The risk should remain between low to
moderate through the upcoming weekend. For additional information,
consult the Surf Zone Forecast (SRFSJU) issued by the NWS San
Juan Office.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...Small Craft Advisory until 6 PM AST this evening for AMZ723-733-
     735-741.

&&

$$

SHORT TERM/MARINE/BEACH...YZR
AVIATION...ERG/ICP
LONG TERM....MMC
PUBLIC...ICP/YZR