461
FXCA62 TJSJ 182010
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
410 PM AST Sat May 18 2024

.SYNOPSIS...

Hazy skies due to suspended Saharan dust and hot weather conditions
will continue through the rest of the weekend and early next week.
However, shower with isolated thunderstorm development is likely
each afternoon, mainly across the interior to western portions of
Puerto Rico and downwind of the local islands. A wet and unstable
weather pattern is still expected by midweek of next week as a
deep-layered trough approaches from the west and combines with
abundant tropical moisture, heightening flood threat risks.


&&

.SHORT TERM...Tonight through Monday...

Very early in the morning, a line of heavy showers, with possibly an
embedded very short-lived thunderstorm, developed across Fajardo,
Luquillo, and Rio Grande, leaving close to an inch and a half of
rainfall accumulations, mostly over Fajardo. By early afternoon,
another round of heavy showers and thunderstorms developed over the
interior and western/northwestern areas of Puerto Rico. Some of
these storms were strong, featuring frequent lightning, gusty wind
conditions, and torrential rains. This activity is expected to
diminish by this evening as the heating effects, which fuel this
convection, subside. Additionally, very hot temperatures were
experienced today, and in combination with high moisture content,
heat index values once again exceeded 100F. In some coastal and
urban areas, heat index values even surpassed 108F.

The inherited forecast remains on track as the weak mid-level trough
with its axis over the Leeward Islands continues moving eastward.
Sunday will see weak ridging aloft, accompanied by some warm air
advection, which may somewhat inhibit convective activity across the
area. However, marginal instability at upper levels seems to return
on Monday as another weak mid-level shortwave trough moves over the
forecast area. This will promote favorable dynamics aloft, with 500
mb temperatures falling from -5 degrees on Sunday to close to -7
degrees Celsius, leading to increased lapse rates.

Moisture content at low levels will remain near climatological
norms. Therefore, a similar weather pattern can be expected for the
remainder of the short-term forecast, with Sunday possibly
experiencing diminishing shower activity during the afternoon hours.
On Monday, a larger area of convective activity is expected, mainly
over the interior and northwestern quadrant of Puerto Rico, as well
as downwind of El Yunque and smaller islands, as dynamics aloft
become conducive to thunderstorm development.

Southeasterly winds will persist, bringing warmer temperatures along
with an increase in suspended Saharan dust particulates.
Consequently, hazy skies and hot temperatures are expected, with
coastal and urban areas likely requiring Heat Advisories for the
next few days. Stay tuned and Stay Hydrated!

.LONG TERM...Tuesday to Saturday...

From previous discussion...416 AM AST Sat May 18 2024

A seasonal pattern will continue through Tuesday as a high pressure
across the surface of the central Atlantic allows a diurnal trade
wind pattern to prevail. However on Wednesday onward through the
weekend, the development of a mid to upper-level trough moving
across the Caribbean basin eastward towards Puerto Rico will begin
to cause tropical moisture to rise up from the ITCZ and spread
across the region. A surface low will also begin to develop due to
this feature, appearing to gain structure by Friday north of the
region. Current model guidance has the dirtiest part of the
upper- low remaining over Hispanola through this event, however
the influx of moist air with high precipitable water values and
southerly flow will cause unstable, wet conditions for the local
islands. Mostly all sections of the islands have a chance to see
some impressive rainfall during this event. Thunderstorms are
likely as well due to strong winds aloft allowing ventilation to
occur.

The anticipated period of excessive and heavy rainfall will
therefore increase the risk of urban and river flooding especially
from Tuesday onwards with the most favorable impacts being Wednesday
through Thursday but activity could continue throughout the end
of the work week so tuned and be prepared, as we will continue to
monitor how this pattern unfolds and make


&&

.AVIATION...
(18Z TAFS)

VFR conds to prevail over the entire period except for SHRA and
isold TSRA ovr NW PR til arnd 18/21Z. Some mtn obs and MVFR/IFR
conds could briefly occur. Sfc winds ESE arnd 12 kt with stronger
sea breeze influences til 18/21Z followed by winds ovr land less
than 6 kt in land breezes. VCSH ovr the USVI and ern PR aft 18/22Z.
Aft 19/14Z sea breezes will return with winds up to 13 kt in ESE
flow.


&&

.MARINE...

A surface high pressure across the central and northeast Atlantic
will result in light to moderate easterly winds through the weekend.
Tranquil marine conditions will therefore prevail throughout the
forecast period. Afternoon showers and isolated thunderstorms could
move from western Puerto Rico into the coastal waters each day.


&&

.BEACH FORECAST...

There is a low rip current risk for today across all area beaches
of Puerto Rico and the U.S. Virgin Islands. The risk will likely
remain low through the weekend. However, life-threatening rip
currents can often occur in the vicinity of groins, jetties and
piers. In addition, with possible maximum heat indices between
102-107 degrees or higher, along the local beaches, heat
exhaustion will be likely with prolonged exposure. Stay hydrated!


&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001-002-005-
     008-010-012.

VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.

AM...None.
&&

$$

SHORT TERM...CVB/ICP
AVIATION...ICP
PUBLIC...MMC/MRR