030
FXCA62 TJSJ 161514 CCA
AFDSJU

Area Forecast Discussion...CORRECTED
National Weather Service San Juan PR
1114 AM AST Fri Aug 16 2024

.UPDATE...A few minor showers are being seen in the Caribbean
south through west of Saint Croix, otherwise no shower activity
has been noted as of 11 AM AST. The sounding came in considerable
drier, as expected, with only 1.47 inches of precipitable water.
Most of this is due to the ridging that is occurring over the area
from the northeast at lower levels. To our east southeast a weak
tropical wave enhanced by a TUTT low to our east northeast is
throwing moisture northwest toward the area and this moisture is
still expected to arrive on Saturday. The sounding does not appear
particularly conducive to thunderstorms in the west northwest
portion of Puerto Rico with a significant dry layer above the
marine layer, but at this time it does not seem prudent to rule
out local showers and possible thunderstorms there, so will
continue with the inherited forecast slightly diminished.

Rio Guanajibo and Rio Culebrinas have subsided below flood stage,
therefore the flood warnings were allowed to expire. Some roads
near these rivers may still be overtopped with water and closed
today.

Temperatures are running up to 6 degrees cooler today at the same
hour as yesterday due to a rogue line of cirrus developing out of
Hurricane Ernesto. Temperatures may still reach forecast levels
this afternoon, therefore will continue with the heat warnings and
advisories.

&&

.AVIATION...
(12Z TAFs)

VFR conds thru 17/12Z although some mtn topping may occur btwn
17/08-17/12Z in the Luquillo range. SHRA/TSRA psbl VCTY TJBQ/TJMZ
btwn 16/18-16/21Z. Winds SE 6-12kt and lcly higher in sea breezes
especially alg the S coast of PR and the USVI.

&&

.PREV DISCUSSION... /issued 655 AM AST Fri Aug 16 2024/

SYNOPSIS...

Excessive heat hazards will be present today and during the next
few days. There is a chance for afternoon showers and
thunderstorms to develop each day, even during the driest periods,
with increased frequency and intensity during the weekend and
during the second half of the next workweek as tropical waves
affect the region at those times. Pulses of Saharan dust will
continue to reach the region. River flood warnings are in effect
for the Culebrinas and Guanajibo rivers at least through early
this morning.

A moderate rip current risk will return tonight and persist
through most of the weekend for several areas. A high risk of rip
currents is forecast for some areas by Sunday night and into
early next week as swells from Hurricane Ernesto and moderate to
locally fresh winds impact the local waters, also creating choppy
seas for small craft by Sunday into early next week.

SHORT TERM...Today through Sunday...

Satellite imagery and doppler weather radar depicted partly cloudy
to variably cloudy skies overnight, with a mix of high level clouds
spilling over the region from distant Hurricane Ernesto and patches
of low to mid level clouds and few showers moving across the region
while brushing parts of the islands. So far rainfall accumulations
were little to none with these light passing showers. Rivers and
streams along the northern half of Puerto Rico continued slightly
higher than normal but were slowly declining to near normal.
However, in the west section of Puerto Rico, both Culebrinas and
the Hormigueros river remained at high level overnight although
steadily subsiding. With flood waters still remaining over some
major roadways, hazardous conditions for residents and drivers
continue. For this reason a Flood warning remains in effect at
least until later this morning. The early morning low
temperatures were in the low 80s along the coastal areas and in
the mid to upper 70s in higher elevations. Daytime highs today
will range between 89 to low 90 degrees, with maximum heat indices
expected to soar once again into the lower 110s over portions of
the islands where Excessive Heat Warnings and advisories will be
likely. Winds were calm to light and variable but will become
east southeast 10 to 15 mph with sea breeze variations and
occasionally higher gust during the afternoon hours.

For the rest of the short-term period, a much drier airmass is
expected at least into early Friday with winds becoming more east
northeast in advance of a weak tropical wave and a mid to upper
level trough expected to approach and cross the region from the
east by Saturday afternoon through early Sunday. So far expect
most of the moisture with this wave to pass south of the region,
however some peripheral moisture may converge across the region
bringing intervals of passing clouds and showers to the region.
Recent model guidance and satellite derived layered precipitable
water products both suggest a drier airmass with values dropping
to between 1.35 and 1.65 inches at least until early Saturday,
along with a low to moderate concentration of Saharan dust to
linger across the region through the period. Expect generally fair
weather skies but still a 40-60% chance for afternoon showers
with isolated thunderstorms over northwestern Puerto Rico driven
by significant diurnal heating and sea-breeze convergence. By
Saturday afternoon, more favorable conditions are expected due to
the nearby mid-to-upper level trough and increasing PWAT values
nearing 2.0 inches as the tropical wave cross the eastern
Caribbean and approaches just south of the region. This will
likely aid in increasing the potential for showers and isolated
thunderstorms, with up to a 70% chance of precipitation by
Saturday afternoon and into early Sunday. The forecast then calls
for a limited to elevated risk of excessive rainfall and lightning
hazards for Saturday through early Sunday. Thereafter by Sunday
afternoon onwards, more typical weather conditions are forecast
with east to southeast winds and diminishing moisture content once
again.

Overall warmer-than-normal conditions will persist in the coming
days and coastal and urban areas can expect nighttime lows in the
lower 80s, while higher elevations will see temperatures drop to
around low to mid 70s. Daytime highs will range from the upper 80s
to lower 90s in lower elevations to around 80 degrees in higher
elevations. Maximum heat indices in many coastal and urban
regions may soar above 108 degrees Fahrenheit. These conditions
will present significant health risks and may require the issuance
of Excessive Heat Advisories and/or warnings. It is crucial for
residents and visitors, especially those participating in outdoor
activities, to take necessary precautions to stay safe.

LONG TERM...Monday through Friday...

No major changes to the long term forecast. Model guidance
indicates precipitable water (PWAT) values decreasing to below
normal and normal values to start the workweek, coinciding with a
pulse of saharan dust that will also serve to promote hazy skies.
PWAT values are forecast to further decrease on Tuesday before
gradually increasing to above normal values (above 2 inches)
through the second half of the workweek. This last increase will
be due to a tropical wave that, in addition to increasing
moisture, will increase the potential for enhanced shower activity
across the area. Surface high pressure across the Atlantic will
spread westward on Monday as Ernesto moves farther north. This
will promote more east to east northeast winds through midweek,
becoming more easterly by the second half of the workweek. Heat
risks will persist through the workweek in addition to a generally
typical weather pattern. This pattern will include periods of
passing overnight and early morning showers across windward
sectors of the islands (especially windward sectors of Puerto
Rico), followed by afternoon showers and thunderstorms mainly over
the interior to western Puerto Rico each day. This activity will
be somewhat enhanced, Wednesday onwards.

AVIATION...

(06Z TAFs)

VFR conds at all terminals thru prd. Few passing SHRA ovr regional
waters and en route btw islands. Some -SHRA reaching windward side
of islands but no sig impacts to flight VIS. Low level steering wnds
generally fm ESE 5-15 kts BLO FL120. Sfc wnds calm to light and
variable bcmg fm east to northeast 10-15 kts aft 16/14Z with sea
breeze variations and hir gusts from time to time durg aftn on the
north coast today but some will appear aft 16/15Z. SHRA activity
with chc of isold TSRA in NW and interior PR btwn 16/17-16/22Z.

MARINE...

Hurricane Ernesto, currently around 640 miles north northwest of
San Juan, will continue to generate moderate southeasterly winds
despite its distance. Easterly winds are expected to return over
the weekend. Swells from Ernesto and moderate to locally fresh
winds from a tropical wave will create choppy seas for small craft
early next week.

BEACH FORECAST...

The rip current risk will remain low for all beaches during the
day. However, a moderate risk will return tonight for northwestern
to north-central Puerto Rico. The moderate risk will spread to
other areas during the weekend. A high risk of rip currents is
forecast for some areas by Sunday night and into early next week
as swells from Hurricane Ernesto and moderate to locally fresh
winds from a tropical wave impact the local waters.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Excessive Heat Warning until 5 PM AST this afternoon for PRZ001-
     005-008-010-011.

     Heat Advisory until 5 PM AST this afternoon for PRZ002>004-007-
     012-013.

VI...Excessive Heat Warning until 5 PM AST this afternoon for VIZ002.

     Heat Advisory until 5 PM AST this afternoon for VIZ001.

AM...None.
&&

$$

SHORT TERM/AVIATION/MARINE...WS
PUBLIC...CVB
UPPER AIR...RVT