Tropical Discussion - International Desks
NWS Weather Prediction Center College Park MD
858 AM EDT Thu Mar 20 2025

Weekly Discussion for Puerto Rico and the USVI 20 March 2025

Large Scale Analysis: Over the region during the week, the Madden
Julian Oscillation (MJO) is currently in a convergent phase over
the Caribbean Basin, and model guidance shows that this pattern is
expected to continue into the following week. This will favor less
ventilation for deep convection and precipitation values are
expected to remain low and close to trace amounts in Puerto Rico.
However, the CFS model is suggesting an atmospheric Kelvin Wave to
propagate from the west into the Caribbean Basin early next week
and could provide some upper level support for heavier
precipitation once it reaches the north-central Caribbean.

Currently, the north Caribbean is under an weak mid to upper level
trough that is quickly propagating eastward on Thursday and in the
lower levels, a weak stationary front is weakening over Puerto
Rico and the Greater Antilles. The amount of available moisture in
the region has remained below or at around 1.2 inches in
precipitable water and it will slowly increase into Friday with a
weak shear line that will meander into the region, increasing the
amount of precipitable water in the region to about 1.4-1.6 inches
by Friday. On Thursday, expect precipitation totals from a trace
amount to less than a quarter of an inch. By Friday, the upper
level conditions begin to shift to a ridging pattern in the mid to
upper levels, however a weak trade wind cap is expected during the
day. Low level winds begin to shift from the east by the end of
the day and the amount of available moisture increases, as
previously mentioned. Under these conditions, expect precipitation
totals to increase on Friday to more than 0.5 inches in the
Central Cordillera, as well as in the eastern portion of the
island. Localized heavier precipitation amounts reaching 1 inch
are possible in the Central Cordillera.

By Saturday, as the shear line and frontal boundaries have
weakened, the easterly trade winds are dominating the lower
levels, and weak moist plumes are traveling over the north
Caribbean and are expected on Saturday over the islands. Amounts
of precipitable water could exceed 1.6 inches throughout the day.
A weak trade wind inversion continues over Puerto Rico, while the
mid level ridge continues to organize over the Caribbean Basin. On
Saturday, expect preciptiation totals to reach between 0.5-1
inches in the western region of Puerto Rico, while the eastern
portion could remain at maxima of around 0.5 inches with a chance
for higher amounts. Into Sunday, there is model disagreement in
the amount of available moisture over the region. Models are
disagreeing in how much precipitable water a moist plume to the
north will reach Puerto Rico and how close it will reach the
island. Other conditions remain similar, such as the mid-level
ridge centered over Puerto Rico, which may favor subsidence in the
region. The trade wind cap is expected to remain weak, as well.
There is some agreement that the northwest portion of Puerto Rico
may see more available moisture than the southeast. Precipitation
amounts may remain at around 0.5 inches, primarily in the
northwest regions of Puerto Rico.

At the beginning of the following work week, drier conditions are
expected on Monday, as dry air is advected into the region. The
mid to upper level ridge has strengthened, as well as the trade
wind cap over Puerto Rico. Starting on Tuesday through Thursday,
however, a new upper level trough is propagating south and east
from the southern United States, while the Kelvin Wave is expected
to enter the central Caribbean by the middle of the week. The mid
to upper level ridge is expected to weaken over this time and,
with it, the trade wind cap over Puerto Rico is also expected to
weaken. The winds in the lower levels are expected to continue
from the east, while moist plumes along the trade winds are coming
from the east into the north Caribbean. There is model confidence
that Wednesday or Thursday may see one of these plumes favor
preciptiation totals exceeding 1 inch over the eastern portion of
Puerto Rico. The lack of confidence is in the timing, as the moist
plume may enter the region early on Wednesday or later in the day
into Thursday.

Castellanos...WPC (USA)