Tropical Discussion - International Desks
NWS Weather Prediction Center College Park MD
805 AM EDT Thu Mar 13 2025

Weekly Discussion for Puerto Rico and the USVI 13 March 2025

Summary: Unseasonably wet conditions are expected during the next
several days in association with a slow-moving surface front
interacting with dynamics in the base of upper troughs propagating
in the region. A new front is likely to enter the region by
Tuesday-Wednesday next week, but confidence on the evolution is
too low at this time.

Large Scale Analysis and Current Conditions: The large scale
pattern shows a wide area of enhanced upper convergence over the
Eastern Pacific, extending into the Caribbean, partially in
association with the MJO. This is in phase with the presence of a
dry air mass over the entire Caribbean, which is favoring
seasonally dry conditions over most of the basin. However, Puerto
Rico, the VI and Hispaniola are experiencing a much wetter pattern
given the influence of a slow-moving surface front and the
associated moist plume. The plume associates with total
precipitable water (TPW) values nearing 1.8 inches, which are well
above the normals fexpected for mid-March. Current satellite
imagery show convection associated with the front extending along
20N 63W, Aguadilla and into the Isla Saona region in the southeast
Dominican Republic. The San Juan radar indicates moderate rain
showers occurring off the northwest and north-central coasts of
Puerto rico, while weaker showers are entering coastal areas.

Forecast through late Saturday: Unseasonably wet conditions are
expected due to the front moving little. TPW is expected to remain
above 1.4 inches through Saturday, which is expected to yield to
the largest accumulation during this period. The largest rainfall
amounts are likely to occur on Friday, when interactions with the
base of an upper trough are likely to favor the largest rainfall
rates and a peak in the chance for thunderstorms. On Thursday,
low-level northeasterly trades will likely enhance precipitation
along the entire north coast of Puerto Rico during the morning and
along the central and southern cordillera in the afternoon/early
evening. Frontal convection and precipitation is forecast to reach
the northern VI and Vieques/culebra during the afternoon evening
and persist into overnight hours. Overall, expect maxima in the
0.5-1.5 inch range, although very isolated higher amounts are
possible due to orographic interactions. On Friday, enhancement by
the upper trough will favor deeper convective development and a
peak in accumulation in interior portions of northern Puerto Rico.
In the low-levels, weaker winds and a trough will stimulate
development in central and western Puerto Rico. Expect maxima in
the 1-1.5 inch range, and very isolated amounts approaching the
2-2.5 in range in thunderstorms. On Saturday, a return of the
northeasterly trades will transport the moist plume southward,
yielding to a drying trend in western Puerto Rico. Expect the
largest accumulations in interior portions of eastern Puerto Rico
where expect maxima in the 1-1.5 inch range.

Sunday and Monday: During Sunday into Monday, the advection of a
drier air mass from the north will favor a general decrease in
precipitation amounts. Yet northeasterly low-level winds will
favor scattered showers in most of northeast, eastern and central
Puerto Rico as well as the western VI, where expect maxima in the
0.4 - 0.6 inch/day.

Tuesday and Wednesday: A new front is expected to approach from
the north sometime between Tuesday and Wednesday. Yet, model
discrepancies on the evolution of this system are significant. At
the current time, it appears that the largest rainfall amounts
with this new front are likely during Wednesday, especially given
interactions with a potent upper trough forecast to extends its
axis somewhere near Hispaniola. This positive interaction between
the frontal moist plume and the divergent tier of the upper trough
could favor moderate rainfall amounts. Yet, confidence is too low,
plus both seasonality and large scale upper convergence are
against a significant rain event.

Galvez...WPC (USA)