866
FXCA62 TJSJ 040118 AAA
AFDSJU

Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
918 PM AST Wed Jul 3 2024

.UPDATE...The leading edge of the convection associated with the
tropical wave moving westward across the Caribbean is just
entering the southern-most boundary of our CWA.Lightning is now
relatively rare there, but it is anticipated that as the east
southeasterly flow meets the east northeast flow across the local
area that convection will again increase. Precipitable water over
Puerto Rico is 1.66 inches indicated by the TPW product and
confirmed by the 00Z sounding from 04/00Z launched from San Juan,
Puerto Rico.The precipitable water just to the south, however, is
between 2.1 and 2.3 inches, confirming the models` forecast that
plenty of moisture is in this tropical wave rapidly traversing the
Caribbean. This is not expected to be a strong performer rain-wise
though. 850 mb relative humidity is divided into bands that will
represent areas of the best rainfall. Also instability increases
only after the best moisture has already passed--on Friday
morning. The envelope of moist air will represent a 32 hour respite
from the Saharan dust as levels will drop to one third the
amounts expected to return after it has passed. Expect even worse
dust beginning late Friday night and continuing into Saturday.
Temperatures will still seem hotter or as hot on Thursday due to
the moisture except where showers bring cooling.

Current forecast is on track and no changes have been introduced
except to delay the arrival of isolated thunderstorms to the area.

&&

.PREV DISCUSSION... /issued 444 PM AST Wed Jul 3 2024/

SYNOPSIS...Drier-than-normal conditions and Saharan Dust will
maintain stable weather and hazy skies for most of the next 5-7
days. However, the passage of a tropical wave (Invest 96L) tonight
will disrupt this pattern, increasing the likelihood of showers
and isolated thunderstorms across the region through at least
Friday. Even with the anticipated increase in cloud cover and high
chances of rain, warm to hot conditions will continue.

SHORT TERM...Tonight through Friday...

Satellite and radar observations have indicated predominantly
sunny to partly cloudy and hazy skies throughout the region, with
occasional showers bringing minimal rainfall to eastern Puerto
Rico. Daytime high temperatures varied significantly, peaking
around 80 degrees in the higher elevations and climbing to the
mid-90s in southern and southwestern Puerto Rico. Meanwhile, heat
index values soared above 108 degrees, especially in the lower
elevations of western, southern, and northern Puerto Rico, where
Heat Advisories were in effect. In the U.S. Virgin Islands,
daytime highs ranged around 88 to 92 degrees, with heat indices
in the lower 100s. Winds were generally from the east to east-
northeast, with stronger gusts and sea breeze effects leading to
speeds of 15-25 mph in Puerto Rico and 10-20 mph in the U.S.
Virgin Islands.

A drier-than-usual air mass, with satellite-estimated precipitable
water values as low as 1.4 inches, and the presence of the Saharan
Air Layer, will keep conditions generally stable. Later tonight
into Thursday morning, moisture from the approaching tropical
wave, Invest 96L, will begin to affect the forecast area. As this
tropical wave moves through and favorable upper-level conditions
develop, with 500 mb temperatures gradually decreasing to around
-7C by Thursday night, we can expect an increase in showers and
isolated thunderstorms. These storms may bring heavy downpours,
frequent lightning, and gusty winds from the 4th of July holiday
into Friday morning. Consequently, there will be a moderate to
high risk of excessive rainfall, particularly over Puerto Rico`s
southern and eastern regions, as well as Vieques, Culebra, and the
U.S. Virgin Islands. Potential impacts include ponding of water
on roads and in poorly drained areas and urban and small stream
flooding. Localized flash floods are possible. Residents and
visitors are advised to stay updated on weather conditions.

By Friday afternoon, weather conditions will begin to stabilize
due to the arrival of a new, drier-than-normal air mass with
precipitable water values below 1.4 inches. This transition will
be accompanied by the presence of the Saharan Air Layer and
warmer-than-usual mid-level temperatures. Despite these
stabilizing factors, the 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. However, significant flooding impacts are
unlikely.

Even with the anticipated increase in cloud cover and high chances
of rain, warmer-than-usual conditions will persist in the coming
days. Coastal and urban areas can expect nighttime lows in the
lower 80s, while higher elevations will see temperatures drop to
around 70 degrees. Daytime highs will range from the lower 90s in
lower elevations to around 80 degrees in higher elevations. With
abundant moisture in the air, heat indices in many coastal and
urban regions may soar above 108 degrees Fahrenheit. These
conditions present significant health risks, leading to the
issuance of Excessive Heat Advisories. It is crucial for
residents and visitors, especially those participating in outdoor
activities, to take necessary precautions to stay safe.

LONG TERM...Saturday through Wednesday...

From previous discussion issued at 512 AM AST Wed Jul 3 2024

A strong high pressure located in the central Atlantic will maintain
the wind flow from the east southeast for most of the long-term
period. Dry conditions are expected to prevail for the first half
of the period as a drier airmass moves into the region with
moderate to high concetrations of Saharan Air Layer (SAL). Hazy
skies will dominate until at least Monday, resulting in lower
visibilities. Very hot conditions are expected too, with
temperatures rising to the 90s along the coastal and urban
portions of the islands. This in combination with the available
moisture will result in higher heat indices across the region
surpassing the mid 100s. By the second half of the period a
vigorous tropical wave is anticipated to approach the local
islands. The precipitable water content analysis suggest values
between 1.90 to 2.15 inches from Tuesday to Wednesday. This wave
will move relatively fast across the region on Tuesday. Moderate
to heavy showers and isolated thunderstorms are expected across
the western and northwestern portions of Puerto Rico during the
afternoon hours. The US Virgin Islands can expect passing showers
throughout the morning hours and isolated shower activity during
the afternoon. After wave passage, a drier pattern will establish
and will persist through the week.

AVIATION...

(00Z TAFS)

HZ due to Saharan dust is expected to prevail through at least 04/13Z,
but VIS will remain at P6SM. A tropical wave will increase SHRA
and TSRA across the Caribbean, USVI, and TJSJ terminals begg aft
04/09. Periods of reduced VIS and low ceilings are anticipated.
Winds will be strong, from the SE at 15 kts, with gusts around 25
kts through the forecast period. SHRA/TSRA will incrs aft 04/16Z--
mainly across wrn and interior PR.

MARINE...

Fresh to strong easterly winds will persist into Thursday,
initially driven by the distant Major Hurricane Beryl and
subsequently by a tropical wave moving westward across the eastern
Caribbean. These conditions will create hazardous marine and
coastal conditions, particularly affecting the Caribbean Waters
and Passages. The showers and thunderstorms associated with the
tropical wave will exacerbate these hazardous conditions for small
craft through Friday. While wind speeds are expected to decrease
from Friday onwards, localized hazardous marine conditions may
still occur.

BEACH FORECAST...

As Major Hurricane Beryl moves further away from the local
islands, coastal conditions will improve. However, a high risk of
rip currents is expected tonight for Saint Croix, influenced by
the approaching tropical wave. In other areas, a moderate risk of
rip currents will persist in the coming days. For more
information, consult the Coastal Hazard Message (CFWSJU) issued by
the NWS San Juan office.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...High Rip Current Risk until 6 AM AST Thursday for VIZ002.

AM...Small Craft Advisory until 6 PM AST Thursday for AMZ723-733-735-
     741.

&&

$$

SHORT TERM/AVIATION...WS
MARINE/LONG TERM....ICP
UPPER AIR/PUBLIC...RVT