Area Forecast Discussion
National Weather Service San Juan PR
457 AM AST Mon Feb 24 2020
A weak frontal boundary across the Atlantic waters is causing
showers across the local waters. The frontal boundary will
continue to produce showers across the local waters today; this
activity will then spread inland during the afternoon. High
pressure will move over the region by mid-week and hold through
Sunday. The high pressure will cause relatively tranquil weather,
however an isolated showers can`t be ruled out during the
afternoon. Sunday, another frontal boundary is expected to move
close to the area, and bring an end to the relatively tranquil
weather for Puerto Rio and the U.S. Virgin Islands.
.SHORT TERM...Today through Wednesday...
A frontal boundary is providing plenty of moisture across the
islands. This front will linger and dissipate over the region
through the short term. Although there is high uncertainty about
where is going to be observed the rain activity, the available
moisture will result in cloudy skies and rainy conditions across
many parts of the islands. However, this activity should remain
mainly over the waters, and from the northwest to northeast portions
of Puerto Rico and the northern U.S. Virgin Islands. Shower activity
will spread across the interior and southern sections by mid-morning
into the afternoon hours. The less potential to observe rain
activity is in St Croix. Therefore, we are forecasting fair weather
for them. As the day progresses, the available moisture in
combination with surface convergence and mountain effects, will aid
in the development of afternoon showers over the interior, west and
northern half of PR. Under light steering winds periods of prolonged
moderate to locally heavy rains will lead to urban and small stream
flooding across portions of the islands.
As the aforementioned frontal boundary dissipate, a surface high
pressure which is forecast to move eastward into the Central
Atlantic Ocean will promote an advective pattern by Wednesday.
Therefore, winds will return from the east and the typical trade
.LONG TERM...Thursday through Tuesday...
Relatively stable weather is prog to prevail through most of the
long-term period as a result of ridging at the surface and aloft.
As the ridges hold across the area, they will also transport a
drier air across Puerto Rico and the U.S. Virgin Islands, and
increase the trade wind cap through Saturday Afternoon. However,
moisture confined between 1000 to 700 MB, local, and diurnal
effects could produce a few showers during the afternoon. Showers
that do develop will be short-lived and produce light rainfall
Saturday will be a transition day as a frontal boundary across the
western Atlantic begins to weaken the surface and mid-level
ridge, which will allow moisture to slide underneath the ridges.
The moisture increase will aid in the development of showers
during the early morning across eastern and northern Puerto Rico,
and the U.S. Virgin Islands. Then showers across the western and
interior during the afternoon. Sunday, a short-wave, and jet-steak
are forecast to round the best of the trough mentioned above. As
a result, the frontal boundary will continue to move south and
near the local waters late Sunday night into Monday morning.
The GFS shows the frontal boundary draped over Puerto Rico by Monday
afternoon, and lingering through Tuesday morning. Additionally,
the GFS shows precipitable water values ranging from 1.5 to 2.0
inches Monday through early Tuesday morning. The moisture
increase, low-level convergence, adequate instability, and
northerly wind flow will aid in the development of scattered to
numerous showers Monday through Tuesday morning.
By Tuesday afternoon, current guidance shows the frontal boundary south
of Puerto Rico, and precipitable water values decreasing
throughout the day. Nevertheless, lingering moisture will combine
with local and diurnal effects to induce scattered to isolated
showers during the afternoon. At this time, thunderstorm
development looks bleak due to warm 500 MB temperatures and
limited upper-level divergence over Puerto Rico and the U.S.
Virgin Islands. However, a few isolated thunderstorms may develop
across the Atlantic, and Caribbean waters were current guidance
has the highest upper-level divergence.
SHRA/-SHRA will affect the terminals of N/W-PR/St Thomas
as a frontal boundary stall near the region. Therefore, expects
ceilings btwn FL030-FL070 at JBQ/JMZ/JSJ/IST through much of the
fcst period. SHRA will slowly move into the interior and S-PR after
24/15z, impacting JPS. ISX/NCM/KPK can expect tranquil weather
conditions. Winds will continue light and VRB, increasing around 10
knots with a NE component and sea breeze variations after 24/13z.
A northerly swell and winds up to 20 kts will cause hazardous
seas through the middle of the week. There is a small craft
advisory in effect for the Atlantic waters, Mona Passage, Anegada
Passage and the northern near-shore waters of Puerto Rico.
Additionally, there is a high risk of rip currents for the
northern, eastern and western beaches of Puerto Rico. Also, there
is a high risk of rip currents for Saint Thomas, Saint John and
the beaches of Culebra. Winds will be from the northeast to east-
northeast ranging from 10 to 20 kts.
.PRELIMINARY POINT TEMPS/POPS...
SJU 82 73 82 73 / 70 40 60 50
STT 83 73 83 74 / 10 50 50 20