Tropical Discussion - International Desks
NWS Weather Prediction Center College Park MD
826 AM EST Thu Feb 27 2025

Weekly Discussion for Puerto Rico and the USVI 27 February 2025

In the larger scale, the CFS model guidance is suggesting an upper
divergent MJO phase for the next several days across the Atlantic
basin and the Caribbean. That said, the weather model guidance is
suggesting mainly shallow convection and in general brief periods
of rainfall over PR/USVI over the next several days. The current
forecast suggests that Friday would be the day with the most
rainfall coverage between today and the following Wednesday, but
there is some indication that next Tuesday into Wednesday could
have decent rainfall and thunderstorm potential, though one must
consider that it is far in the forecast period and thus confidence
is very low.

The available moisture will be up and down over the next several
days, with brief peaks of above normal moisture followed by brief
periods of below normal moisture. That said, Friday into Saturday
is expected to have a bit more prolonged period of above normal
moisture, with precipitable water peaking near 1.7 inches on
Friday evening, which is close to 2 standard deviations above
normal. Thereafter, moisture will decrease through Saturday into
Sunday to below normal levels, but it will peak again on Sunday
afternoon for a brief period. This general pattern of patches of
moisture moving by will allow early morning shower activity over
the USVI and eastern PR, while shallow afternoon convection over
western PR may develop in areas of sea breeze convergence.

As mentioned previously, the diurnal convection is expected to be
shallow for the most part, and thunderstorm potential seems low
until perhaps next Tuesday into Wednesday. This is due to the
current and expected mid and upper level patterns over the local
area. In the upper levels, there is a ridge to the east and
northeast of the local islands, while a trough moves east and
passes through the greater Antilles today into Friday. The trough
is weakening, however, and will be passing a bit north of the
local islands late tonight into Friday. By the weekend, a broad
trough in the upper levels will have its axis over the central
Caribbean, causing some diffluence over the local area into early
next week, then the upper trough axis will move over the local
islands on Tuesday into Wednesday.

The upper level pattern would normally be conducive to some
instability and thunderstorm potential, but the patterns in the
mid levels are working against it. At this time there is a
mid-level high pressure across the central tropical Atlantic, to
the northeast of the Leewards. The high pressure extends to the
southwest into the northeast Caribbean. This is keeping the
mid-level trough further west over Hispaniola, while PR/USVI
remains under the influence of the mid-level high pressure. This
will then keep deep convection from developing today and Friday
into Saturday, and thus the reason why mainly shallow convection
is expected, even with the deeper moisture moving in. The
mid-level high pressure will meander over the area into very early
next week, only moving a bit east by Tuesday, where a mid-level
trough then moves in, associated with the upper-level trough
expected for the same time period. This is the reason why there
could be some thunderstorm formation during this time.

The increase in moisture expected for Friday into Saturday will be
due to a low-level trough that will be moving in. This trough will
have enhanced relative vorticity and moisture convergence. The
relative humidity in the low levels will be above normal on Friday
into Saturday as well, but the relative stability in the mid
levels will likely keep convection to be shallow, though there may
be good rainfall coverage over the islands. In terms of rainfall
amounts, up to an inch or so could be expected today across the
eastern third of PR as well as portions of central to western PR
today, while under a half an inch is forecast elsewhere in PR and
across the USVI. Friday into Saturday, once again up to an inch or
so of rain can be expected over PR, though the coverage area may
include many areas from eastern PR into central and northwestern
PR. The northern USVI as well as Vieques and Culebra may observe
0.5 to an inch of rain during this period as well. Thereafter, the
amount of rain in the forecast decreases significantly, with daily
max values across northwestern PR being near a half an inch due to
diurnal convection, while other areas across PR and the USVI could
observe a few hundredths to just over a tenth of an inch of rain
each day. Once again, rainfall potential could increase on Tuesday
into Wednesday, but confidence is low given how far it is in the
forecast period.

Alamo...WPC (USA)