734
FXCA62 TJSJ 201933
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
333 PM AST Mon May 20 2024

.SYNOPSIS...

The warm-to-hot spell will persist through Tuesday afternoon under
an east-southeasterly wind flow. The mid-level ridge will restrict
widespread afternoon activity across the islands through
Wednesday. However, afternoon convection due to excessive heating
and local effects will impact portions of PR and the USVI daily.
From Wednesday afternoon onward, we anticipate a moist and
unstable weather pattern as above-normal moisture pools over the
islands, interacting with a deep layer trough aloft. Consequently,
a Hydrologic Outlook (ESFSJU) has been issued for Puerto Rico and
the Virgin Islands from Wednesday through Friday due to an
increased risk of flooding.


&&

.SHORT TERM...Tonight through Wednesday...

Calm but warm to hot weather conditions prevailed through this
morning, with above-normal heat indices recorded, particularly
across the urban and coastal areas from the San Juan Metropolitan
Area to Northwest Puerto Rico. This is where an Excessive Heat
Warning has been issued. The excessive heating has led to strong
afternoon convection across isolated areas. The rest of PR and the
US Virgin Islands have observed mostly clear skies under an east-
southeasterly wind flow.

Sea breeze-driven afternoon and evening convection will produce
downpours across the northwest quadrant of Puerto Rico for the
rest of this afternoon into the evening. A 60 to 80 percent
chance of rain is expected for those locations.

Excessive heating due to the combination of warm air temperatures
and available moisture will result in warmer-to-hot heat indices
for the urban and coastal sections in Puerto Rico and the US
Virgin Islands tomorrow afternoon. Therefore, we may issue Heat
Advisories or Excessive Heat Warnings for that period tomorrow
early morning. Once again, afternoon convection may develop near
the San Juan Metropolitan Area, the interior, and the northwest
quadrant under the east-southeasterly wind pattern. Downpours may
result in ponding of water or flooding of poorly drained areas in
these locations.

An approaching deep-layer trough aloft will increase instability
across the region. Additionally, moisture from a trade wind
perturbation and plenty of tropical moisture lifted from the
southern Caribbean will pool above-normal moisture content over
the islands. Therefore, a Hydrologic Outlook (ESFSJU) was issued
from Wednesday through Friday for Puerto Rico and the Virgin
Islands due to an increased risk of flooding.

.LONG TERM...Thursday through Monday...PREV DISCUSSION.../issued
416 AM AST Mon May 20 2024/

On Thursday, the islands will be squeezed between a surface high
pressure over the central Atlantic and a surface trough/low over
Hispaniola. Plenty of moisture will stream from the Caribbean Sea
into the islands, with precipitable water values above 2.5 inches
possible (above climatology). Additionally, pockets of vorticity
at the mid levels will be advected into the islands, hence
conditions will favor the continuation of showers and
thunderstorms over the region into Friday. It is not that it will
rain all the time, but these showers could be heavy from time to
time, maintaining elevated the risk for flooding, mudslides and
rapid river rises. Also, early on Thursday, as the low lifts
northward, a swath of winds will move over the region, bringing
gusts of 20-25 knots with the heavy showers and thunderstorms.

By the weekend, trailing moisture from the low, now expected to
be well north of Puerto Rico will continue to impact the region.
Although the rain is not expected to be as widespread, it could
aid in the development of convection for the interior, north and
west of Puerto Rico in the afternoon. Some showers will also move
in the vicinity of the Virgin Islands.

On Sunday and Monday, some improvement is anticipated, mostly for
the Virgin Islands and eastern Puerto Rico, but conditions will
remain very favorable for afternoon activity, again for the
interior, northern and western Puerto Rico.

The heat risk was maintained below the limited criteria for
Wednesday and Thursday, mostly because of the cloud coverage, but
it is expected to be back again Friday onward, when the sun peeks
once more across the northeastern Caribbean archipelagos.


&&

.AVIATION...
(18Z TAFS)

VFR conds expected during the next 24 hours. However, SHRA in and
around TJSJ/TJBQ may cause brief MVFR conds thru 20/22z. Sfc winds
ESE arnd 14 kt with stronger sea breeze influences til 20/22Z
followed by winds ovr land less than 6 kt in land breezes. VCSH ovr
the USVI and ern PR aft 20/22Z. Aft 21/14Z sea breezes will return
with winds up to 15 kt in ESE flow. Trace HZ due to Saharan dust,
but vsby should remain P6SM.

&&

.MARINE...

A surface high pressure across the central and northeast Atlantic
will drive the trade winds from the southeast at moderate speed.
A trough will develop south of Hispaniola, increasing shower and
thunderstorm activity across the local waters Wednesday onward.
This trough may increase winds across the Mona Passage and
Atlantic waters Wednesday night into Thursday.

&&

.BEACH FORECAST...

The risk for rip currents is low, however, life-threatening rip
currents often occur in the vicinity of groins, jetties, reefs,
and piers.

&&

.HYDROLOGY...

A significant increase in moisture caused by an approaching mid
to upper level trough, and a developing surface low pressure
system will result in an enhanced risk for flooding later this
week, especially between Wednesday afternoon through Friday
evening, across Puerto Rico and the Virgin Islands.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Excessive Heat Warning until 6 PM AST this evening for
     PRZ001-005-008.

PR...Heat Advisory until 6 PM AST this evening for
     PRZ002>003-007-010>013.

VI...Heat Advisory until 6 PM AST this evening for VIZ001-002.

AM...None.

&&

$$

ICP/CAM/MRR