526
FXCA62 TJSJ 220904
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
504 AM AST Sun Sep 22 2024

.SYNOPSIS...

Expect the humidity water values to peak by the late afternoon
and early evening each day of the short term period when showers
and thunderstorms grow over interior Puerto Rico. A typical
pattern of calm nights and mornings, followed by warm- to-hot
afternoons and evenings driven by diurnal heating and weak
steering winds, will prevail in the coming days. Excessive heat
will continue to pose a risk to vulnerable individuals,
particularly those outdoors without proper cooling or hydration.
Additionally, the moderate risk of rip currents will persist
tonight.

&&

.SHORT TERM...Today through Tuesday...

Currently, radar and satellite imagery show scattered showers over
the Atlantic waters and south of the forecast area and isolated
showers over the local Caribbean waters. Isolated thunderstorms
continued to pop off and then dissipate, but tops are not as high as
yesterday and the showers are not reaching the level where their
tops can blow off.  Most clouds and showers have dissipated over
land. Low temperatures this morning are running from the mid 70s to
the lower 80s in the lower elevations and from the upper 60s to the
mid 70s in the mid to upper elevations. Winds have quieted down to 6
mph or less with many areas becalmed.

The TPW product showed a drier strip of air over the local Caribbean
waters. It continues to squelch showers over the area including
Ponce and Saint Croix. This strip is no longer making any northward
progress. Also there is a narrow band of much better moisture just
south of the forecast area that is allowing showers and
thunderstorms to fire off from time to time. Some moisture is also
seen over the Atlantic outer waters in the center of our forecast
area. Lower level winds are quite light with winds 10 kts or less
below 28 kft. This is due to high pressure over the Canary Islands
being blocked by a pair of weakening low pressures between 25 and 30
north and between 40 and 60 west. Weak high pressure can be seen
north of the Mona Passage and across the local area. At mid levels a
broad area of high pressure extends across the tropical Atlantic,
the U.S. Virgin Islands, Puerto Rico and over to Hispaniola. At
upper levels the TUTT low is found over and north of the Leeward
Islands and the 30 knot jet from the north northwest continues to
push across the area today and Monday while increasing to 50 knots
over the eastern Caribbean tonight. Low pressure will bring
troughing to our area Monday night, but its associated low will
remain north of 30 north and will just barely be able to impinge on
the high pressure over the west central Caribbean Sea. Flow at lower
levels and specifically the mean flow between 1000 and 700 mb has
been and will continue to be quite variable, which is one reason why
neither the dry band in our local Caribbean waters nor the band of
moisture just south of it have made no further northward progress.
This lack of winds and the continued moisture over the local islands
is part of the reason heat advisories have been issued for all lower
elevation locations. Some relief from the heat is expected in all
but coastal spaces as showers and thunderstorms, fueled by the
continued humidity, redevelop over the area.

All of these warm-to-hot conditions will hold on, and precipitable
fueled by the continued humidity water values peak late afternoon
and early evening each day of the short term period when showers and
thunderstorms grow over interior Puerto Rico. The GFS 1000-850 mb
thickness values are also seen to increase each day until Tuesday
when the highest value for the week reaches 1426 m.

&&

.LONG TERM...Wednesday through Saturday...

For the long term, we anticipate calm and warm weather conditions
with the development of showers and thunderstorm activity,
particularly in the afternoon hours.

On Wednesday, the steering wind flow will gradually recover to around
10-15 knots as the deep-layered trough continues to exit the
region. The latest model guidance suggests that all the available
moisture should remain below 700 mb through at least Thursday,
meaning that patches of moisture will move across the forecast
area from time to time, resulting in showers reaching the coastal
areas of Puerto Rico and the Virgin Islands each day. The
strongest shower and thunderstorm activity is expected each
afternoon across the Cordillera Central, with the activity then
drifting toward the coast. From Friday into the upcoming weekend,
we foresee that a tropical wave will move well south of Puerto
Rico and the U.S. Virgin Islands. However, we anticipate a slight
increase in moisture across the area, which could enhance shower
development. With the proximity of this tropical wave and the
everyday afternoon shower activity, we forecast occasional ponding
of water on roads and poorly drained areas through Saturday.

Temperature-wise, above-normal temperatures are expected to
persist over the region, with the threat of excessive heat each
day. This heat will mainly impact those without effective cooling,
adequate hydration, or prolonged exposure.

&&

.AVIATION...
(06Z TAFs)

Mainly VFR conditions ovr land. Aft 22/15Z clds and SHRA to dvlp ovr
interior PR followed by SHRA/TSRA with mtn obscurations and MVFR/IFR
conds over interior and wrn PR aft 22/17Z till arnd 22/21Z. TAF
sites will have only brief unfavorable conds. Winds less than 10 kt
with a few hir gusts in mostly sea breezes and nr TSRA till aft
22/22Z. SHRA/TSRA ovr land will dissipate btwn 22/21-23Z. High
density altitudes at TAF sites are expected today due to heat and
humidity.

&&

.MARINE...

A surface low pressure to the northeast of the region over the
Central Atlantic will maintain light and variable winds into the
early part of next week. Showers and isolated thunderstorms will
develop from time to time across the regional waters, particularly
off the western and southern coastal waters of Puerto Rico each
afternoon.

&&

.BEACH FORECAST...

The rip current risk will remain low across the islands through at
least Wednesday, when the moderate risk returns for the northern
coastal areas of Puerto Rico.

&&

.CLIMATE...

Yesterday San Juan tied the record warm minimum temperature for the
day at 81 degrees. Today we will not, as the minimum temperature has
already dropped to 81 degrees, with 82 degrees as the record. Should
the minimum remain at 81 degrees, it will be the second warmest
minimum in its own right. Although it may not seem like there is
much difference, today is the first official day of Autumn and when
the shortening of the daytime hours reaches its greatest speed. The
sun will cross the equator at 8:44 AM AST this morning.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>005-007-
     008-010>013.

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

AM...None.
&&

$$

SHORT TERM/AVIATION...WS
LONG TERM/MARINE/BEACH FORECAST...MMC