880
FXCA62 TJSJ 112033
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
433 PM AST Sat May 11 2024

.SYNOPSIS...

A distant mid to upper-level trough will remain close enough to
keep marginally unstable conditions through the rest of the
weekend. With southeasterly winds and available moisture, heat
indices could exceed 102 degrees Fahrenheit, potentially reaching
around 108-110 degrees in localized coastal areas of northern
Puerto Rico over the weekend. Lingering saharan dust will persist
through at least early next week.

&&

SHORT TERM...Tonight through Monday...

Afternoon convection began after 2PM today and by this time,
4:27PM, a line of thunderstorms stretches from Utuado to Catano
with up to 2 inches of rainfall detected according to radar
estimates. Rainfall will continue through the rest of the
afternoon into the early evening hours for these areas, however
conditions should improve into the night.

A distant mid-to-upper level through will remain close
enough to keep marginally unstable conditions during the weekend.
In the meantime, an associated upper-level jet streak (70-90
knots) will continue to spread across the northeastern Caribbean,
further enabling the development of deep convection through late
tonight, with 500 mbar temperatures falling to around -7 degrees.
Consequently, there is a potential for thunderstorm development,
as indicated by the Galvez-Davison Index. Stormy conditions are
possible late tonight over the local waters, with some activity
affecting Puerto Rico`s eastern half, Vieques, Culebra, and the
U.S. Virgin Islands.

With the lingering moist airmass, strong winds aloft associated
with a upper-level low northeast of the region, and a weak lower
level steering flow from the south-east all combines to allow
unstable weather to prevail this evening through Monday,
especially during the afternoon hours. Conditions are expected to
improve over the next few days however afternoon showers with
isolated thunderstorms on Sunday and Monday are still likely.
Wind speeds will persist at 5-15 mph from the southeast, shifting
to an easterly flow on Monday. This shift will relocate the areas
with the potential for the highest rainfall totals from northern
sections of Puerto Rico on Sunday to western sections early next
week.

Temperatures may soar along the coast and urban areas into the upper
80s to mid-90s, while higher elevations could see highs in the upper
70s to mid-80s. With abundant moisture across the region, heat
indices could exceed 102 degrees Fahrenheit, potentially reaching
around 108-110 degrees in localized coastal areas of northern Puerto
Rico over the weekend mainly during the hours between 10am and 3pm

&&

LONG TERM...Tuesday through Saturday...
/from previous discussion/

The NASA aerosol optical thickness (AOT) guidance continues to
suggest lingering Saharan dust particles to start the period.
Model guidance to suggest a mid- to upper-level ridge affecting
the region by the start of the period, promoting more typical
weather with afternoon convection due to diurnal and local
effects. However, model guidance suggests an upper trough closing
in on the islands from the northwest by Friday and moving across
through Saturday. Precipitable water (PWAT) values are forecast to
generally stay at normal values, between 1.5 to near 2.0 inches,
through the workweek as patches of moisture advect into the
region, Friday and Saturday look to be the wettest days with high
end normal values to possibly above normal values. Light to
moderate easterlies are forecast through the period as a surface
high moves into the western Atlantic. Most available moisture
should reach up to 850 to 800 mb for the period, with model
guidance suggesting additional available moisture through the mid
levels by Friday and Saturday. Limited Heat Risk (heat indices
from 102 to 107 degrees Fahrenheit) or even higher, especially
during the daily maximum heating, will persist across several
lower elevation sectors of the islands where no significant rain
is observed. This diurnal heating, local effects and sea breeze
variations will continue to promote up to strong afternoon
convection, steered by easterly winds and possibly enhanced by the
upper through on Friday and Saturday. This will promote
convective activity mainly over sectors of the interior to western
half of the Puerto Rico and downwind of the local islands. Above-
normal sea surface temperatures can also promote nighttime
showers across windward sectors of the islands, patchy fog is also
forecast for areas of the interior during the early morning
hours.

&&

.AVIATION...
(18Z TAFS)

SHRA/TSRA expected to develop north of Cordillera Central and near
TJBQ, dissipating around 11/23Z. Periods of reduced VIS and lower
ceilings expected. SHRA will increase across the Caribbean waters
and near the USVI terminals after 00Z. Winds are light, out of the
SW at 5-7 kts, switching from the SE around 06z. After 12Z, speeds
will pick up to 9-11 kts. SHRA/TSRA will develop again after 12/17Z
for NW PR, impacting TJBQ.

&&

.MARINE...

A surface high pressure across the central Atlantic and a building
low in the western Atlantic will allow southeasterly winds to prevail through
the weekend. A weak northerly swell will continue to fade across the
Atlantic waters and passages. Strong showers and thunderstorms over
the waters can result in locally higher seas and gusty winds.

&&

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

&&

$$

CAM/MMC/RC