National Weather Service San Juan PR
500 AM AST Tue Apr 9 2024
.SYNOPSIS...Lingering moisture from a cold front will promote
showery weather conditions today. A slight improvement in weather
conditions is likely by midweek as drier-than-normal air moves in.
However, the remnants of the cold front will return by the end of
the workweek, increasing the chances for rainfall activity again.
Starting tonight, moderate to locally strong winds and a dominant
long-period northerly swell will deteriorate marine and beach
conditions. High surf conditions are possible on Wednesday.
&&
.SHORT TERM...Today through Thursday...
At surface level, a low pressure system over the northeastern
Atlantic has a cold front extending through the Atlantic, with its
remnants and related moisture affecting the islands and moving
southwards of the region, and a weak surface trough is near the
area. Over the western Atlantic, a surface high pressure system
continues to build, this system will spread eastward towards the
central Atlantic through the period. As an upper trough moves away
from the region, the mid to upper levels remain relatively stable
and available moisture extends to around 600 mb. Satellite
derived precipitable water (PWAT) show above normal values across
the islands and local waters. Normal to above normal PWAT values
will continue during the period, as moisture remnants from this
frontal boundary and prefrontal trough continue to be steered by
moderate to fresh northeasterly winds. This moisture will continue
moving southwards, but maintain above normal moisture over the
islands today, promoting variable to showery weather across most
of the region.
Like during the overnight hours, cloudiness and shower activity will
continue to be steered into windward sectors of the island from the
Atlantic Waters, promoting variable to showery weather. Afternoon
convection is still forecast, particularly across the interior to
southwestern Atlantic, although continued cloudiness during the
morning hours can serve to limit it. This activity however, can
still result in ponding of water on roads and poorly drained areas,
as well as urban and small stream flooding, especially with the
heaviest activity. On the bright side, heat risk no longer poses a
hazard. Temperatures are forecast to be cooler under these up to
breezy NE winds with 925 mb temperatures at below normal to normal
values for this time of the year due to the colder air mass behind
the front sinking southwards.
More normal PWAT values are forecast for Wednesday and then a drying
pattern is forecast on Thursday with PWAT values and overall
moisture decreasing. By Thursday night and into the long term
period, however, another patch of moisture, remnants of the past
frontal boundary, will filter into the region. Thursday will be the
driest day in the period (and the week). Wednesday and Thursday,
shower activity will affect windward sectors with more limited
convective activity over interior to SW PR, this activity will be
even more limited on Thursday.
.LONG TERM...Friday through Tuesday...
Model guidance suggests precipitable water values at or above
normal levels, except between Saturday evening and Monday morning.
This wet pattern results from distancing the mid-level ridge early
in the period, followed by the proximity of an upper-level trough
and associated jet early next week. The general steering flow at
lower levels should remain from the east throughout the period,
generated by a broad surface high pressure spreading across the
North Atlantic. However, as this feature weakens, wind speeds are
projected to weaken from 15-25 mph to 5-15 mph early next week.
In the expected scenario, heightened passing shower activity is
likely on Friday and from Monday onwards, affecting northern and
eastern sections of the local islands, with less frequent to
limited passing shower activity during the weekend. Afternoon
convective development is likely each, even during the driest
periods, with the highest impact across the western interior to
western Puerto Rico. Given the uncertainty in the forecast
associated with the timing of the wettest periods and the effects
of the upper-level trough, the excessive rainfall risk was kept at
low levels. However, the risk could elevate with a more direct
influence, and flooding impacts could start as early as Tuesday.
Breezy conditions will prevail through the weekend, leading to
limited wind impacts, mainly across coastal areas. Temperature
variations, in turn, should remain within seasonal levels
throughout the period.
&&
.AVIATION...
(06Z TAFS)
VFR conditions across the terminals, with possible MVFR conditions
from shower activity/low clouds. As SHRA from the Atlantic continue
to reach the islands, expect this activity affecting
TJSJ/TJBQ/TIST/TISX during the morning to afternoon hours. Afternoon
convective SHRA possible across the interior to SW PR, affecting
TJPS. Surface winds from the NE at 15-20 kts with higher gusts,
especially near stronger shower activity, and sea breeze variations.
&&
.MARINE...
An upper-level trough crossing the region along with a weak surface
trough approaching the area, will maintain the chance for showers
with isolated thunderstorm development today. Moderate to fresh
north to northeast winds will promote choppy seas today. Starting
this evening and continuing throughout the workweek, fresh to
locally strong winds and another dominant long-period northerly
swell will promote hazardous seas for small craft. For details,
refer to the Marine Weather Statement (MWWSJU).
&&
.BEACH FORECAST...
The rip current hazard risk will remain low to moderate today.
However, increasing winds and the arrival of a dominant long-
period northerly swell will deteriorate beach conditions starting
this evening, mainly across the north-facing beaches of Puerto
Rico. On Wednesday, these conditions will extend to the beaches of
Rincon and Aguada in western Puerto Rico, Culebra, and the
northern U.S. Virgin Islands. Large breaking waves exceeding 10
feet, possibly generating high surf and hazardous swimming
conditions, as well as localized coastal flooding and beach/dune erosion,
are possible on Wednesday. For details, refer to the Coastal
Hazard Message (CFWSJU) and Surf Zone Forecast (SRFSJU).