National Weather Service San Juan PR
519 AM AST Fri Oct 27 2023
.SYNOPSIS...Wet and unstable weather conditions along with
hazardous marine and coastal conditions expected through at least
early next workweek. Due to the rainfall expected today onwards,
there is an elevated risk for flooding of urban areas, roads and
small streams. Life threatening flash flooding and mudslides are
possible as well.
&&
.SHORT TERM...Today through Sunday...
Good moisture pooling and low level convergence due to the
combination of a broad surface trough, along with the proximity of a
mid to upper level trough (TUTT) through the upcoming weekend, will
maintain a very moist and unstable weather pattern across the
islands and coastal waters. This will support favorable conditions
for elevated risk of urban and small stream flooding and flash
flooding across portions of Puerto Rico and the U.S. Virgin Islands.
Strong northerly swells generated by Post-Tropical Cyclone Tammy
will continue to affect the regional waters and passages for the
next several days resulting in hazardous seas and dangerous surf-
zone conditions.
Passing showers and isolated thunderstorms now steered by the
southeast low level wind flow will continue to bring periods of
moderate to locally heavy rains to mainly the north and east sections
of Puerto Rico as well as over the adjacent islands and the U.S.
Virgin Islands from time to time. This may lead to ponding of water
on roads and poor drainage areas, as well as the chance for minor
urban flooding in some areas by the early morning hours. For the
rest of today, afternoon showers and thunderstorm development will
be likely over the interior, south and east sections of Puerto Rico,
as well as over portions of the San Juan metro and northwest PR and
around the U.S. Virgin Islands. Variably cloudy skies will prevail
with near normal high temperatures expected today, in the mid to
upper 80s along the coastal areas and mid 70 to low 80s in higher
elevation.
On Saturday and through Sunday, the recent guidance continued to
agree and both suggest increasing moisture convergence and a wind
surge with the approach of a tropical wave and the passage of the
the surface trough axis. However by late Saturday through Sunday,
the mid to upper level trough is to gradually erode and be replaced
by an upper ridge, resulting in more stable conditions aloft. In
spite of this, with abundant amounts of moisture as suggested by
model guidance, the combination of good daytime heating, local
effects and breaks in cloud cover will result in late morning and
afternoon showers and isolated thunderstorm development, some of which
may be enhanced and could lead to locally heavy rains each day.
Enhanced early morning and afternoon convection across the islands
will cause ponding of water on roads and poor drainage areas and may
cause urban and flash flooding as well as flash flooding in the
interior, eastern, southern and north sections of Puerto Rico as
well as the U.S Virgin Islands.
Therefore, an elevated risk for widespread ponding of water,
flooding of urban areas, roads and quick reacting rivers and small
streams will continue. Life threatening flash flooding and mudslides
due to loose and unstable soil conditions will also be likely in
some areas especially over the interior and eastern PR.
.LONG TERM...Monday through Friday...
The GFS model continues to suggest that a wet and unstable weather
pattern will continue through Thursday while the ECMWF model
continues to suggest improved weather conditions by Tuesday. Because
of this, uncertainty remains in the long term forecast. The GFS
model suggests precipitable water (PWAT) values at around and above
2.5 inches across the region to start the workweek, decreasing
somewhat to around 2.2 inches on Tuesday before drier air filters
in on Thursday and Friday. A surface trough will persist over the
area with additional tropical waves arriving from the east.
Southerly flow will persist over the area through Thursday,
helping to lift tropical moisture into the area from the ITCZ
before drier air from the east arrives on Thursday. To start the
period, a mid to upper level ridge will be overhead, with the
upper level trough mentioned in the short term period lifting to
the northeast and away from the region. Although the environment
will remain moist, the driving force for the afternoon convection
will be diurnally induced, as conditions will be more stable
aloft, with additional instability from the lingering surface
trough. Model guidance suggest a mid to upper level trough
building to our east slowly eroding the ridge throughout the
period. Overall, a wet and unstable pattern is forecast at least
through Tuesday with urban and small stream flooding likely and,
with saturated terrain, the potential for flash flooding and
mudslides in areas of steep terrain, uncertainty in the forecast
increases by Wednesday onwards. Please continue to monitor future
updates.
&&
.AVIATION....Prevailing VFR conds durg prd. SCT-BKN lyrs nr
FL025...FL040...FL090. SCT SHRA/Isold TSRA ovr regional waters and
en route btw islands. Light low level east southeast winds BLO FL100
will bring prds of SHRA/-SHRA vcty TJSJ/TIST/TISX til 27/14Z.Mtn top
obscr psbl ovr E PR. SFC wnd calm to lgt/vrb bcmg fm E-SE btw around
10 kts with sea breeze variations aft 27/14Z. FM 27/17Z-23Z
SHRA/Isold TSRA dvlpmnt ovr ctrl interior of PR with VCTS psbl at
TJSJ/TJPS/TJMZ/TJBQ and mstly VCSH at USVI terminals til 27/23Z.
&&
.MARINE...Hazardous marine and coastal conditions will persist
through early next week due to northerly swells. Small Craft
Advisories are in effect for the Atlantic Offshore Waters and
Caribbean Passages until at least late Sunday night due to seas
between 8 and 12 feet. At 5 AM AST the San Juan, Arecibo and
Rincon CariCOOS buoys reported 7-9 feet wave heights with periods
of 12-13s Beachgoers, avoid the Atlantic Coastline in Puerto Rico
and the U.S. Virgin Islands due to hazardous coastal conditions.
&&