527
FXCA62 TJSJ 240827
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
427 AM AST Fri May 24 2024


SYNOPSIS...

Cloudy conditions will the chance of showers across eastern Puerto
Rico continues through today with an increase of rain during the
afternoon hours. Wet weather conditions are expected to persist
throughout next week as a tropical wave approaches from the
southeast by Monday. In addition, hot days are expected over this
weekend with heat indices in the upper 100s mainly for coastal
sections.

&&

.SHORT TERM...Today through Sunday...

The night was tranquil for the most part over land areas, with some
isolated to scattered showers occurring over the local waters,
affecting windward coastal areas of Puerto Rico and the US Virgin
Islands at times. The strongest activity was located over the
western quarter of the offshore Atlantic waters, with heavy showers
and strong thunderstorms associated with the deep-layered trough
that had been affecting the Dominican Republic in the past few days.
This activity will gradually approach the area during the morning
hours.

The forecast had to be adjusted slightly to account for some of the
latest tendencies in the recent model cycles. Challenges arise due
to the forecast area being situated between the deep-layered trough
to the northwest and a mid- to upper-level ridge southeast of the
region. The TJSJ 24/00z sounding showed dry air within the 850-500
mb layer, with mid-level relative humidities at 28%. This, along
with ridging aloft, inhibited convective activity in the last 18-24
hours.

However, the 700-500 mb CIRA Layered Precipitable Water (LPW)
product shows a broad area between the Dominican Republic and Puerto
Rico with high moisture content gradually making its way towards us.
This is partly eroding the mid-level ridge while increasing mid-
level relative humidities across the region. At the same time, the
850-700 mb CIRA LPW shows a moist slot over the USVI moving
northwestward. The latest model guidance suggests increased moisture
convergence and instability as these features approach today,
promoting the development of shower and thunderstorm activity,
particularly over the northern half of Puerto Rico and smaller
islands this afternoon.

For this weekend, the mid-level ridge will continue to move eastward
as the deep-layered trough weakens and settles in. Marginal
instability aloft will persist, with precipitable water content
exceeding and remaining above the 90th percentile of the
climatological normal. Sufficient dynamics will combine with well
above normal moisture every day, potentially leading to heavy
showers and isolated thunderstorms, mostly during the afternoon
hours. A weak southeast steering flow will cause convective activity
to cluster mostly over interior and northwestern sections of Puerto
Rico, with streamers developing downwind of the smaller islands and
El Yunque into the San Juan metro area. Slow-moving showers could
enhance rainfall accumulations in flood-prone areas. Therefore, we
can expect a limited to elevated flooding threat to persist through
the weekend.

Southerly winds will persist, advecting warm temperatures over the
local islands through the short-term forecast. Extensive cloud
coverage moving in will somewhat inhibit temperatures from
increasing substantially today. However, we can still anticipate
heat index values above 100 F across coastal and urban areas. But,
for this weekend, heat index values are projected to reach Heat
Advisory Criteria in those areas during peak hours of the day before
shower activity begins. Therefore, it`s important to stay hydrated
if engaging in outdoor activities.

&&

.LONG TERM...Monday through Friday...

Model guidance indicates that a wet pattern will persist through the
long-term forecast due to a moist airmass across the region as a
tropical wave approaches from the east. As this tropical wave
moves over the islands by late Monday, causing moisture content
to rise above 2.0 inches, an increase of unstable weather is
expected to last through the long term period. Weak ridging aloft
throughout the week will help keep conditions somewhat decent,
yet the continuation of the passage of weak short-wave troughs
through Friday will maintain above-normal moisture and marginal
instability.

The National Blend of Models hints for heavier rainfall during the
afternoon hours particularly across western/northwestern Puerto Rico
under an east-southeast steering flow. The Galvez-Davison Index also
suggests a similar scenario, with a high potential for observing
isolated to scattered thunderstorms capable of producing heavy
rainfall each day. With saturated soils and elevated streamflows
from previous days of rainfall, the flooding risk will remain
elevated. Therefore, we could expect urban and small stream
flooding, as well as flash flooding and landslides in areas of steep
terrain.

Hot days are also to be expected with heat indices reaching in the
100s to 110s through the week due to the high RH values and heating
from the summer sun, therefore heat advisories or warnings may be
issued during this time. Saharan dust particulates are expected to
arrive by early in the week. Expect hot and hazy conditions to
prevail across sections with less rainfall.

&&

.AVIATION...
(06z TAFS)

Areas of SHRA and Isold TSRA over Hispaniola will move close to TJBQ
aft 24/10Z. Aft 24/16z, SHRA and VCTS could affect all remaining
terminals. These features will reduce VIS and CIG possibly producing
MVFR/IFR conds and mtn obscurations. Sfc winds ESE 10-15 kts with
higher gusts near SHRA till 24/22Z.

&&

.MARINE...

Surface high pressure across the central and northeast Atlantic and
an induced surface trough just northwest of the region will promote
moderate to fresh southeast winds through tonight. In addition, the
proximity of a deep layer trough over the Dominican Republic will
aid in enhancing thunderstorm development across portions of the
regional waters and local passages through at least today. Winds are
forecast to diminish overnight and then become more easterly over
the weekend.

&&

.BEACH FORECAST...

Low to moderate rip current risk will persist throughout the period.
In the presence of a moderate rip current risk for St. Croix,
there is still the possibility of life-threatening rip currents in
the surf zone. Please remember to heed the advice of the local
beach patrol and flag warning systems.

&&

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

&&

$$

SHORT TERM...CVB
LONG TERM....RC