Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 211551 AAA
AFDSJU

Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
1151 AM AST Thu Mar 21 2024

...New UPDATE, AVIATION...

.UPDATE...
Issued at 1132 AM AST Thu Mar 21 2024

A layer of mid clouds has overspread most of Puerto Rico and a
fringe of this layer will reach the northern U.S. Virgin Islands
this afternoon. Up to now this has completely stifled convection
in the lower layers with only a few if any lower clouds being
reported. We will monitor this closely, but heavy rains will
depend on whether the sun can penetrate the broken to overcast
mid-layer and produce sufficient surface heating for there exists
some uncertainty.

The high temperature at San Juan has been 86 degrees so far--so
forecast maximum temperatures are close to predicted values and
therefore cannot rule out convection forming during the afternoon.

&&

.AVIATION...
(12Z TAFS)
Issued at 1132 AM AST Thu Mar 21 2024

A BKN-OVC layer at 100-130 has impeded convection in the lwr lyrs
so far leaving VFR conds nearly everywhere. Convection is still
expected later this afternoon with lcl +SHRA and MVFR conds. Sfc
winds are sea breezes of less than 12 kts incrg to 10-15 kt
later aft 21/18Z. Winds to bcm land breezes less than 10 kt aft
21/22Z. Max winds WNW 60-72 kts btwn FL400-480. Aft 22/17Z SCT-NUM
SHRA dvlp in wrn/swrn PR with areas of MVFR/IFR conds and mtn
obscurations.

&&

.SYNOPSIS...
Showers are expected to develop this afternoon under a light wind
flow. This activity is expected to be focused over the interior
and southern Puerto Rico. Temperatures will be on the warm side
this weekend, with heat indices above 102 degrees possible. Then,
a frontal system is expected to bring showers and possibly
thunderstorms by late Sunday into early in the weekend. Tranquil
weather is expected Wednesday onward, but with deteriorating
marine conditions.

&&

.SHORT TERM...Today through Saturday...

A stationary front located north of Puerto Rico triggered strong
convective activity over the Atlantic waters. Satellite imagery
depicted robust outflows from this convective activity, which moved
southward toward northern Puerto Rico earlier in the night. As these
outflows interacted with the land breeze, a line of showers formed
just off the coast of San Juan. However, the line dissipated fairly
quickly, leaving only isolated showers over the area during the
night hours. Rainfall amounts between a tenth to half an inch were
recorded in isolated areas of the San Juan metropolitan coastal
area. Some activity persists over the northwestern portions of the
Atlantic waters due to this frontal system. However, for the
remainder of the morning hours, not much precipitation can be
expected, with only a few showers occurring here and there.

Satellite-derived precipitable water data indicates PWAT values in
the range of 1.6 to 1.7 inches, which exceed the 90th percentile of
the climatological normal, suggesting above-normal levels. This
moisture level is expected to persist through at least Friday as the
frontal boundary stalls just north of the region. Moisture levels
will slightly diminish to 1.5 inches on Saturday but are still
expected to remain mostly above the 75th percentile of the
climatological normal.

In terms of dynamics, the proximity of the frontal boundary will
promote some low-level instability, with 850-700 mb lapse rates
expected to remain above normal today and decrease slightly
tomorrow. However, stability at the mid- to upper levels is
anticipated to be slightly greater compared to previous days as a
weak ridge develops in the next few days. The 500 mb temperatures
are forecast to increase slightly to -5 and -4 degrees Celsius today
and tomorrow. Consequently, the 700-500 mb lapse rates are expected
to remain mostly below normal levels. This will somewhat hinder
thunderstorm activity across the area. Although thunderstorms were
not included in the forecast as most of the activity appears to be
shallow, one or two short-lived thunderstorms may still occur over
the Atlantic waters and in mountainous areas of Puerto Rico, mostly
this afternoon.

Rainfall accumulations, on the other hand, may be enhanced today due
to light northeastern steering winds promoting slow-moving showers.
Model guidance suggests shower activity initiating over the
Cordillera Central after 18z today and slowly drifting southward,
affecting Ponce and adjacent municipalities through at least 23z.
Because of the potential prolonged duration of these showers, a
limited to elevated flooding threat is possible. Therefore, urban
and small-stream flooding can be expected, with isolated flash
flooding not being ruled out.

The pressure gradient over the local area will tighten as a strong
low-pressure system exiting the Eastern Seaboard pushes against a
surface high-pressure system over the northern Atlantic.
Consequently, winds will shift more easterly and speeds will
increase through Saturday. Although moisture levels are not expected
to fluctuate much for the rest of the forecast period, faster
steering winds will reduce rainfall accumulations during afternoon
convective activity. Therefore, the weather pattern will become
breezy with a limited threat of flooding, if any, in the presence of
shower activity across the area.

&&

.LONG TERM...Sunday through Thursday...

As a cold front approaches from the west, winds from the surface
up to 750 mb are expected to shift from the southeast. This will
result in very warm or hot temperatures across the region, with
heat indices likely above 102 degrees for many areas in northern
and western Puerto Rico, and above 90 degrees elsewhere in low-
elevated areas. Moisture associated with the front will increase
on Sunday night and early in the workweek, with values above
normal for this time of year. As the frontal system approaches,
instability will also increase, thus widespread showers and
isolated thunderstorms are anticipated. The risk for urban and
small stream flooding will be elevated. It is worth mentioning
that, even with some difference in timing and coverage, the global
models agree in the generally wet pattern for early next week.

After the frontal system moves, the winds will shift from the
northwest, bringing a cooler and drier air mass across the
islands. Conditions will begin to dry out on Tuesday, although
with some lingering showers, and even additional shower
development over southeastern Puerto Rico and near Saint Croix.
It should be enjoyable on Wednesday and Thursday, with lot of
sunshine and temperatures in the low 80s in the afternoon hours.
Regardless, small and shallow pocket of moisture may still linger
around, so a few showers cannot be ruled out during this period.

&&

.AVIATION...
(06Z TAFS)

Mainly VFR conditions expected across most terminals. Winds will
remain lgt and vrbl til 21/13z, then increase up to 12 kt and shift
more ENE aft 21/14z, with sea breeze variations. Mtn obs psbl aft
21/18z with MVFR for TJPS through 21/22z as SHRA moves towards the
terminal. PROB30 group added for TJPS as brief IFR/LIFR cannot be
ruled out during this period.

&&

.MARINE...
Light easterly winds will continue as a cold front lingers north of
the area today. A surface high pressure building over the western
Atlantic will yield moderate to locally fresh east- northeast winds
starting Thursday night, and thus, causing deteriorating marine
conditions during the weekend. These winds will push fragments of
moisture over the local waters.

&&

.BEACH FORECAST...
The rip current risk is currently moderate for north-central
Puerto Rico, but all north-facing beaches in Puerto Rico and the
U.S. Virgin Islands will see this risk increase by tonight and
stretching into the weekend. Breaking waves should be around 5
feet in this areas. The risk will remain low for the beaches
in the Caribbean Sea.

&&

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

&&

$$

SHORT TERM/AVIATION...WS
UPPER AIR/PUBLIC....MRR


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