504
FXCA62 TJSJ 122112
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
512 PM AST Wed Jun 12 2024

.SYNOPSIS...Weather conditions will become more showery beginning
tonight. This will cool most temperatures somewhat, but after a
second tropical wave moves through on Friday southeasterly flow
will bring back very warm temperatures over the weekend. The heat
watch has been extended for the lower elevations of the north and
west coasts of Puerto Rico for Thursday. Heat advisories may be
required for other areas in the forecast area tomorrow. saharan
dust will become quite noticeable over the weekend.

&&

.SHORT TERM...Tonight through Friday...

The narrow upper level trough over the Mona channel will drift
east across Puerto Rico and the U.S. Virgin Islands tonight and
Thursday, leaving a weak cut-off low in the southern Caribbean Sea
south of Hispaniola. Moisture from a tropical wave has allowed
shower activity--some heavy-- in western Puerto Rico and showers
from the advancing moisture from an old cold front are visible on
NWS NEXRAD with tops around 20 KFt. High pressure at the surface
is maintaining mostly easterly flow, but a tropical wave is
developing in the western tropical Atlantic at about 50 West
longitude and will move into the area Thursday night and Friday
and will take advantage of the moisture associated with the old
cold front boundary advancing into the area to prolong the shower
activity over the area. Although some showers and thunderstorms
will be intense, prolonged periods of heavy rainfall with this
scenario are not expected. Saharan dust is being seen behind the
tropical wave.

.LONG TERM...Saturday through Wednesday.../issued 447 AM AST Wed
Jun 12 2024/

Current model guidance suggests precipitable water (PWAT) at
below normal to normal values this weekend, with only few patches
of more humid air moving through the islands, last remnants of a
past frontal boundary pushed towards the region by a high pressure
system over the Atlantic. This system will also promote
southeasterly surface windflow this weekend. This will steer
warmer tropical air towards our area and promote overall warm
conditions for the islands. Heat related products will likely be
issued for the islands later on. 925 mb temperatures are forecast
to be above normal this weekend. Saharan dust particles will
increase to start the period as well, promoting hazy skies during
the weekend at least. Most available moisture will remain below
850 mb. Although model guidance suggest some moisture over the
mid levels, a more notable increase is suggested by Tuesday and
into Friday. This increase in moisture, will lead to increasing
PWAT values Tuesday and Wednesday, in part due to the possible
passing of a tropical wave. High pressure aloft will also
strengthen over the Western Caribbean through at least the
weekend. Model guidance suggests more easterly winds to start the
workweek as the Atlantic surface high weakens, uncertainty
remains.

&&

.AVIATION...
(18Z TAFS)

VFR conditions are expected at all TAF sites during the next 24
hours except wrn PR arnd and just east of TJMZ where +SHRA will
cont thru 12/23Z with a slgt chc of TSRA. E-ESE at 10-15 knots
accompanied by higher gusts and sea breeze variations, becoming at
10 knots or less overnight.

&&

.MARINE...Winds will continue from the east as an old cold front
boundary and its associated moisture and a tropical wave
approaches the area. Over the weekend winds will become more
southeasterly. The potential for showers and thunderstorms will
increase tonight and continue through Sunday. Seas will remain
benign during the period.

&&

.BEACH FORECAST...The beaches of Saint Croix will have a moderate
risk of rip currents tonight and Thursday. The northwestern and
north central beaches of Puerto Rico will see a moderate risk on
Thursday, with all other areas having only a lower risk.

&&

.CLIMATE...Although the high temperature in San Juan did not
approach record levels this afternoon, the minimum temperature was
still quite warm and if no rain occurs before midnight will likely
set another record with 81 degrees. The previous record was 80 set
in 1980.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Excessive Heat Warning until 6 PM AST this evening for
     PRZ001>003-005-007-008-010-011.

     Excessive Heat Watch from Thursday morning through Thursday
     afternoon for PRZ001-005-008-010.

     Heat Advisory until 6 PM AST this evening for PRZ004-009-012-013.

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

AM...None.
&&

$$

SHORT TERM...WS
LONG TERM....MMC
AVIATION...CAM