FXCA62 TJSJ 221426
Area Forecast Discussion
National Weather Service San Juan PR
1026 AM AST Fri Sep 22 2023
.UPDATE...Despite adjusting for higher rainfall totals expected
from afternoon convective development for today and Saturday, the
forecast is on track. Persistent tropical moisture, with higher
moisture levels of around 2.01 inches with the most recent
sounding (22/12Z) reports compared to yesterday`s, a persistent
weaker steering flow (5-15 knots), and strong diurnal heating
influence after a sunny morning will lead to another active
afternoon. Again, this activity will likely cluster along the
interior, steered by generally east-southeasterly winds, spreading
across most of Puerto Rico in the late afternoon. Limited to
moderate excessive rainfall impacts are likely, ranging from
urban and small stream flooding to localized flash floods and
mudslides, mainly in areas that experienced significant rainfall
totals during previous days across the interior to northwestern
Puerto Rico. In fact, looking at the 7-day preliminary radar-
estimated rainfall totals and soil moisture data from the USGS,
portions of Utuado, Adjuntas, Lares, Maricao, Las Marias, and San
Sebastian remain at a higher excessive rainfall risk. Still,
limited flooding impacts are possible across most of Puerto Rico.
Across the U.S. Virgin Islands, if not from showers streaming
across the islands, shower development over western sections of
the islands could result in limited flooding impacts.
Today`s high temperatures are forecast to peak into the lower
90s, with heat indices peaking into the mid 100s across most lower
elevations and urban areas in Puerto Rico and the U.S. Virgin
Islands. Heat index values of 108 degrees or higher are likely
across urban and coastal areas of western and northern Puerto
Rico, Vieques, Culebra and all of the U.S. Virgin Islands, where
a Heat Advisories are in effect. Winds will remain from the east
to east-southeast, peaking at 10-15 mph with gusts up to 20-30 mph
and sea breeze variations. Higher winds are possible near the
.AVIATION...Mainly VFR conditions will prevail during the next 24
hours. Trade wind SHRA could result in brief MVFR conditions
across most terminals throughout the period, except for TJBQ.
In the meantime, afternoon TSRA/SHRA could also result in brief
MVFR conditions, mainly across TJSJ and TJBQ between 22/16-22Z.
Surface winds will remain from the east-southeast at 10-15 knots,
becoming light and variable after 22/22Z.
.MARINE...No changes to the marine forecast. Based on the most
recent buoy observations, seas remain 2-4 feet across coastal
waters, conditions favorable for small craft.
.PREV DISCUSSION... /issued 501 AM AST Fri Sep 22 2023/
SYNOPSIS...A tropical wave will continue to move across the area
today. Heavy rainfall over saturated soils will lead to urban and
small stream flooding and mudslides in areas of steep terrain.
Heat indices between 108 and 111 degrees are expected across most
urban and coastal areas of PR and the USVI. Latest guidance
regarding a tropical wave over the Eastern Tropical Atlantic
suggest that development into a tropical system is likely during
the next couple of days, and that it should remain over the open
waters through next week.
SHORT TERM...Friday through Sunday...
Above normal moisture continues to engulf the northeastern
Caribbean. This moisture is associated mainly with a tropical wave,
but additionally, there is an upper level low northeast of the
Caribbean, and a surface reflection is also evident on satellite
imagery. As a result, showers have been arriving during the night
hours across the U.S. Virgin Islands and eastern Puerto Rico. Some
lightning has been observed as well. Fortunately, these showers have
not leaved significant rainfall accumulation, although a few
sections near Naguabo and Humacao collected nearly half an inch. The
showers will continue into the early morning hours, leaving wet
The trade winds will be from the east with a southeast component
today, with speeds near 13 knots. These winds are mainly driven by a
surface high pressure over the central Atlantic. There is plenty of
moisture for another active afternoon today. The high resolution
models, as well as the National Blend of Models, show widespread
showers covering the interior and west, as well as portions of the
San Juan metro area. These areas, especially the west, have been
receiving plenty of rain in the past few days, hence the soils are
saturated and some rivers are running high. The risk for urban and
small stream flooding, as well as rapid river rises and mudslides
will be elevated for today. Lightning and gusty winds are also
possible with the strongest activity. Across the Virgin Islands,
Vieques, Culebra and eastern Puerto Rico, passing showers will
continue to move too.
Later in the period, a mid to upper level ridge strengthen, capping
moisture at the lower levels of the atmosphere. The frequency of
showers should diminish, but it won`t be completely dry, as patches
of low level moisture will continue to move in these days. Small
concentrations of dust will be also available this weekend. For each
day, the usual pattern of passing showers in the east in the morning
and afternoon activity for the west will prevail.
In terms of temperatures, 925 mb temperatures continue to be above
the climatological values for this time of year. With plenty of
humidity near the surface, heat indices again will be above 108
degrees for many areas in the west and northern municipalities of
Puerto Rico, Vieques, Culebra and all of the U.S. Virgin Islands.
There is a chance that some areas that receive strong rains in the
afternoon could experience a relief from the scorching heat, at
least later in the day, but it will be steamy everywhere else.
LONG TERM...Monday through Friday...
Precipitable water content is expected to increase between 2.00-2.25
inches on Monday due to a tropical wave passage. At this moment,
Monday looks to be the wettest day of the long term period.
Thereafter, a northeasterly wind flow is expected to prevail
through much of the period, with diurnally induced afternoon
showers developing over portions of central and southern PR each
day. An advective pattern should prevail during the night and
early morning hours, favoring passing showers along the northern
and eastern sections of PR, as well as across the USVI. Meanwhile,
a tropical wave with a high development chance over the Eastern
Tropical Atlantic is now expected to remain over open waters
through the end of the week. However, we will continue to monitor
the progress of this system as it can produce hazardous seas and
life-threatening rip currents once again by the second part of the
AVIATION...SHRA will continue across the local waters, and around
the USVI and TJSJ terminals. After 16-17Z, SHRA and TSRA should
develop along the Cordillera Central, and W-NW PR. Periods of
reduced VIS and low ceilings can be anticipated. SHRA could reach
the vicinity of TJSJ and , and the USVI terminals too. Winds will
be from the ESE at 8 to 13 kts, with stronger gusts.
MARINE...Seas are expected to range between 2 and 5 feet through
early next week. Easterly winds should continue between 10 and 15
knots through the period. A moderate risk of rip currents will
continue for all beaches of the islands through the weekend. Seas
are expected to increase by midweek next week.
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001-005-008-
VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.
NWS SJU Office Area Forecast Discussion