National Weather Service San Juan PR
438 AM AST Wed Mar 20 2024
.SYNOPSIS...
Issued at 435 AM AST Wed Mar 20 2024
Today and tomorrow, a pre-frontal trough will lead to an uptick in
shower activity across the area. Slow steering winds will contribute
to increased rainfall accumulations. As a result, a limited to
elevated flooding threat can be expected, particularly over interior
Puerto Rico. Looking ahead, a strong low-pressure system and
associated frontal boundary are forecast to impact the local islands
by early next week. However, uncertainty persists regarding the
timing, location, and eventual rainfall amounts associated with
these features.
&&
.SHORT TERM...Today through Friday...
Clear skies prevailed across Puerto Rico under a very light wind
flow. The current satellite-derived precipitable water imagery show
a dry air mass engulfing the local islands, with a moisture
gradient over the local Atlantic waters. This moisture is associated
to a cold front now over the Bahamas and Cuba, and a prefrontal
trough near eastern Hispaniola. As the day progresses, the moisture
field will approach the area, with precipitable water values
climbing to near normal values for this time of year. The low
pressure associated with the cold front will maintain the gradient
loose, hence winds will remain on the light side. In fact, the low
level steering flow is expected to be around 2 to 8 knots today.
Since skies are clear, enough sunshine is expected to warm up the
surface, with local effects kicking in, resulting in shower activity
developing over the Cordillera Central in Puerto Rico. Due to the
light winds, these showers are not expected to move much, but should
drift mostly toward the south and west. Some minor flooding cannot
be ruled out.
As the workweek ends, a ridge building over the western Atlantic
will cause the winds to shift from the northeast, and strengthen
some. On Thursday, the low level steering flow will be from the
northeast at around 8-10 knots, increasing to nearly 15 knots by
Friday. This ridge will push more moisture from the front into the
local islands. A shortwave trough is also expected to cross the
region on Thursday, so conditions aloft will allow for better
dynamics for shower generation. Regardless, on both days, showers
are expected to develop over the interior, moving toward the south
and west of Puerto Rico. The chances of rain are better on Thursday,
as some patches of drier air are expected to move over the area on
Friday, limiting the coverage of the rainfall activity.
In terms of temperatures, 925 mb values are still above normal for
today, although not as high as yesterday. Still some urban areas
along northern Puerto Rico could experience values above 102 degrees
Fahrenheit. Heat indices should be at 90-95F over the Virgin
Islands. Temperatures should cool down a little for Thursday and
Friday, but with highs still reaching the mid and upper 80s.
&&
.LONG TERM...Saturday through Wednesday...
Moisture availability from the remnants of a frontal boundary will
continue to stream across the region this weekend. Moisture levels
will be near to above climatological normals. The pressure gradient
will tighten as a strong low-pressure system exits the Eastern
Seaboard and pushes against a surface high pressure over the
northern Atlantic. This will promote breezy wind conditions from the
east-southeast. Shower activity will be mostly shallow during this
period, moving quickly due to fast-steering winds. This could
somewhat inhibit rainfall accumulations across the islands.
Nevertheless, ponding of water on roadways and in poor drainage
areas is possible.
Model guidance continues to suggest a frontal boundary approaching
the region from the northwest by late Sunday into early next week.
This front is associated with the aforementioned strong low-pressure
system. Although models have been consistent in the development of
these features, timing, and position continue to differ, not only
between models but also between each model cycle. The 20/00z GFS
model shows the upper-level features a little bit east and north of
the local islands compared to previous runs. However, the surface
low meanders over Puerto Rico and the U.S. Virgin Islands. The
20/00Z ECMWF now places these features a bit closer than before,
with both models indicating the low closest to the islands by
Wednesday. The limiting factor will be moisture availability. Models
suggest a frontal passage (FROPA) by late Tuesday. After the frontal
passage, moisture levels are expected to plummet from approximately
1.60 inches to less than 1.0 inches.
As such, uncertainty persists regarding when the frontal boundary
will be nearest, whether it will pass over the islands, and the
amount of moisture it will bring into the area. This uncertainty
arises from both the front itself and potential moisture convergence
over the area ahead/southeast of the front as it interacts with the
deep-layered low-pressure system.
In summary, stability is expected to decrease from late this weekend
into midweek of next week as the deep-layered trough and associated
frontal boundary approach from the northwest. Moisture convergence
over the region is likely to occur early next week, with the most
probable timing being late Monday into Tuesday. By Tuesday, the
frontal boundary is forecast to be near or over the islands, further
enhancing shower and thunderstorm activity. However, the full
impacts of this frontal boundary remain unclear. Shifting and slow
steering winds will influence the location of the strongest shower
activity, with the extent of wind shifts largely depending on
whether or not a frontal passage occurs. At most, a limited to
elevated flooding threat can be expected, with urban and small
stream flooding likely, and isolated flash flooding in some areas.
Landslides in the most vulnerable places cannot be ruled out.
&&
.AVIATION...
(06Z TAFS)
Issued at 435 AM AST Wed Mar 20 2024
Mainly VFR conditions expected to prevail. SHRA are expected to
develop after 17Z across the Cordillera Central, spreading toward
the south and west. This activity could reach TJPS from 17-22z,
with reduced VIS and low ceilings possible. Winds are on the light
side, out of the ENE at 10 knots or less.
&&
.MARINE...
Issued at 435 AM AST Wed Mar 20 2024
Light east to southeast winds will continue today as the surface
high pressure builds over the eastern Atlantic and a cold front
approaches the northeast Caribbean from the western Atlantic. The
cold front will induce a pre-frontal trough off to the northwest of
PR today. The frontal boundary will linger north of the region
through the second part of the work week, while the moderate to
locally fresh trade winds will push fragments of moisture over the
local waters. Another surface high pressure will build over the
western Atlantic, increasing the winds and seas during the weekend.
&&
.BEACH FORECAST...
Issued at 435 AM AST Wed Mar 20 2024
Today, winds will be calmer, resulting in a low rip current risk
across all beaches of the local islands. However, winds are expected
to increase by the end of the workweek, raising the rip current risk
to moderate once again, mostly across the northern exposed beaches
of the islands.