Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 220840
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
440 AM AST Fri Mar 22 2024


.SYNOPSIS...
Showers are expected to develop this afternoon across the interior
and southwestern Puerto Rico, where an elevated risk of flooding
exist. The weekend will be hot and breezy, but a cold front will
bring wetter conditions on Monday and Tuesday, with cooler
temperatures and drier conditions after that. A northerly swell is
expected to arrive on Tuesday, deteriorating marine and beach
conditions

&&

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

Fairly calm conditions prevailed during the night, with only a few
very isolated showers observed across windward coastal areas. A
similar weather pattern is anticipated for the remainder of the
morning.

Today, a surface high-pressure system north of the Atlantic will
gradually shift eastward, inducing more east-northeast winds across
the region. Concurrently, a robust low-pressure system forming over
the Carolinas will slowly progress eastward, moving towards the
Atlantic waters and exerting pressure against the aforementioned
high-pressure system. This interaction will result in a tightening
pressure gradient over the northern Caribbean, thereby increasing
wind speeds as well. Moisture levels at low to mid-levels will
remain above normal throughout the day, as remnants of a frontal
boundary continue to enhance moisture convergence over the local
area. Marginal instability will persist, with 700-500 mb lapse rates
gradually increasing to normal levels as the day progresses.
Consequently, we can anticipate a similar afternoon to yesterday,
with showers and very isolated thunderstorms developing as diurnal
heating combines with marginal instability across Puerto Rico.

The notable difference today will be the distribution of
precipitation, as east-northeast winds will drive showers primarily
over the interior and southwestern regions of Puerto Rico.
Additionally, due to faster steering winds, showers will move more
swiftly than in previous days. Nonetheless, areas affected by these
showers and thunderstorms should remain vigilant, as urban and small-
stream flooding could still occur.

For the remainder of the short-term forecast, the potent deep-
layered low-pressure system and its associated frontal boundary will
gradually approach our forecast area. While recent model cycles
indicate a slightly more northerly track for the low, our weather
conditions will still be closely tied to the evolution of these
features. Winds are anticipated to shift more towards the east-
southeast by Saturday and then become southerly by Sunday. At the
925 mb level, temperatures are forecast to surge above the 99th
percentile, significantly surpassing the climatological normals for
this time of year. This could potentially result in record-breaking
temperatures if cloud coverage doesn`t disrupt the diurnal heating
pattern.

Moisture levels are forecast to remain above normal on Saturday and
near normal on Sunday, with dynamics expected to be marginal and
possibly increasing by Sunday. On Saturday, anticipate some
afternoon showers, primarily over interior and northwestern Puerto
Rico. However, Sunday`s weather conditions will heavily depend on
the track of the low-pressure system. According to model guidance,
shower and thunderstorm activity on Sunday is likely to be clustered
over the Atlantic waters. With a southerly flow in place, it appears
that the northern half of Puerto Rico and the northern US Virgin
Islands will be most affected. Nevertheless, there is a possibility
that the majority of the activity will remain over the waters.
Regardless, anticipate shower and possible thunderstorm activity
affecting northern Puerto Rico in the afternoon, possibly shifting
to windward coastal areas of the islands during the evening hours.
Stay tuned as this event unfolds, possibly impacting the local
islands through early next week.

&&

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

The workweek should begin on a wet note, as a cold front
approaches the local islands from the west. The system appears to
preserve definitions all the way up into the mid-levels of the
atmosphere, so conditions should be favorable for periods of heavy
rain and isolated thunderstorms on Monday and Tuesday. First, on
Monday, the areas that should begin to receive the greatest
amounts of rain will be the north and west of Puerto Rico, but
the moisture field will eventually move into the southern half of
Puerto Rico, reaching the Virgin Islands late Monday or early
Tuesday. The global models agree on this frontal passage early in
the week, although the ECMWF is a little faster, ending most of
the rain by late Monday.

Moving into the middle of the week, columnar moisture decreases,
with precipitable water values likely dropping below 1.0 inch, and
a trade wind cap inversion developing around 850 mb. Therefore,
the potential for showers will decrease considerably, and will be
limited to small patches occasionally reaching the islands. The
wind flow will remain from the northwest for most of the week, and
fairly light. Temperatures will be a little cooler too during this
period, but with highs still reaching the mid-80s.



&&

.AVIATION...
(06Z TAFS)

Mainly VFR conditions expected across the local terminals. Winds
will continue from the E below 10 kts through 22/13z, increasing to
up to 15 kts and shifting more ENE aft 22/14z, with higher gusts and
sea breeze variations. Passing SHRA could affect TJSJ/TISX/TIST
after 23/00z, resulting in VCSH.

&&

.MARINE...
A surface high pressure building over the western Atlantic will
yield moderate to locally fresh easterly winds, causing
marine conditions to deteriorate during the weekend. These winds
will push fragments of moisture from a frontal boundary north of the
islands into the local waters. Marine conditions are expected to
deteriorate further on Tuesday as a long-period northerly swell
arrives.

&&

.BEACH FORECAST...
Breezy conditions are expected in the beach areas, especially in
the afternoon hours. The risk of rip currents is now moderate for
the north-facing beaches in Puerto Rico and the U.S. Virgin
Islands.

&&

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

$$

SHORT TERM...CVB
LONG TERM....ERG


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