NWS San Juan, PR wrote its last area forecast discussion before the first event at Jul 16, 10:02 AM AST, 5 hours 6 minutes earlier.
Area forecast discussion, Jul 16, 10:02 AM AST, 5 hours 6 minutes before
Area Forecast Discussion
National Weather Service San Juan PR
1002 AM AST Sun Jul 16 2023
.UPDATE...The forecast is on track. A layer of upper-level clouds
maintained partly to variably cloudy skies. At lower levels, a
patch of moisture streamed across the region from the east,
bringing precipitable water values to above-normal levels around
1.9-2.0 inches, as indicated by the latest sounding data (16/12Z).
The resulting isolated to scattered showers moved over northern
and eastern sections of Puerto Rico and the local islands, with
some moving further inland across the eastern and central
interior. So far, preliminary rainfall totals indicated up to
three-quarters of an inch over eastern Puerto Rico, around Humacao
and Fajardo. Overnight low temperatures ranged from the upper 60s
across higher elevations to lower 80s across lower elevations.
Winds were generally light and variable, gradually increasing to
10-15 mph from the east this morning.
Doppler radar and satellite imagery suggest this same patch of
moisture will continue to bring showers to northern and eastern
Puerto Rico during the next hour or so. After that, weather
conditions will dry as relatively drier air invades the area from
the east. Still, lingering moisture, local effects, and diurnal
heating will combine to support afternoon convective development
with possible isolated thunderstorms, clustering along the western
interior to western sections. Elsewhere, isolated shower activity
is possible. Warm-to-hot conditions are likely again today, with
daytime maximum temperatures peaking into the upper 80s to lower
90s and 102 degrees or higher heat indices across most lower
elevations and other urban areas. The highest heat indices between
108-111 degrees are likely across northwestern Puerto Rico and
Culebra, where a Heat Advisory is in place through 5 PM AST.
Breezy conditions will again materialize across most coastal areas
of Puerto Rico and the local islands through at least 4 PM AST,
with 15-25 mph wind speeds and gusts up to 30-35 mph. As of 10 AM
AST, peak wind gusts have reached 15-22 mph across coastal areas,
and air temperatures have peaked around 84-90 degrees, with heat
indices up to 109 degrees across lower elevations and other urban
areas of Puerto Rico, Vieques, Culebra, and the U.S. Virgin
Islands.
&&
.AVIATION...Mainly VFR conditions will prevail across all
terminals during the next 24 hours. However, afternoon TSRA/SHRA
could result in brief MVFR conditions at TJBQ between 16/18-23Z.
After 17/04Z, increased SHRA activity could bring VCSH and brief
MVFR conditions to TJSJ and USVI terminals. Winds will be mainly
from the E, peaking around 15-20KT with higher gusts and sea
breeze variations by 16/18Z. Lighter and variable winds will
return after 16/22Z.
&&
.MARINE...No changes to the marine forecast. Seas should remain at
5 feet or below during the next few days. Still, locally breezy
conditions, with 10-20 knots winds, may yield choppy marine
conditions for small craft across the coastal waters and local
passages. Please refer to the Marine Discussion below for
additional information.
&&
.PREV DISCUSSION... /issued 416 AM AST Sun Jul 16 2023/
SYNOPSIS...
Issued at 410 AM AST Sun Jul 16 2023
Expect another warm to hot day, especially across the urban and
coastal areas in Puerto Rico and the U.S. Virgin Islands. Pockets
of moisture will bring a few clouds and showers across the
windward sections, followed by afternoon convection across the
southwestern portions. The leading edge of a tropical wave will
bring showers and thunderstorms late tonight into Monday, and its
backside will produce a second round on Tuesday. A warm and dry
Saharan Air Layer (SAL) will promote above-normal low
temperatures, hazy skies, and limited rain activity Wednesday
through at least Friday. A summer-like pattern may evolve for the
following weekend.
SHORT TERM...Today through Tuesday...
Passing showers moved across the islands overnight, leaving various
amounts of rain, mostly below half an inch as moist. Skies were
filled with cirrus clouds and the temperatures dropped into the low
70s for higher elevations and around 80F for coastal regions. Winds
were generally from the east-northeast between 10 and 15 mph.
For today, a typical pattern of passing showers and afternoon
convection is expected as patches of moist air moves across the
northern half of the region. Skies filled with cirrus clouds are
expected to linger through the early part of the day as upper-level
southwest flow pushes them over the islands. By the afternoon,
diurnal heating and local effects are likely to generate convection,
with the greatest rainfall anticipated across western Puerto Rico.
Eastern sections may receive a noticeable amount of rainfall as
well due to streamer development.
Overnight a tropical wave begins to enter the region with the axis
crossing during the morning hours. Increased precipitable water
values will bring a noticeable amount of showers and
thunderstorms and there will be a slight veering of the winds to
more of an east- southeast direction. Most of the activity will be
seen across the eastern half during the morning hours followed by
wide spread coverage including western Puerto Rico. There is a
possibly for isolated flooding for affected areas. The moisture
from the wave will linger throughout Tuesday, bringing more rounds
of showers and thunderstorms for the area until late Tuesday
night when the arrival of dry air with a Saharan Air Layer begins
a trend of hazy conditions.
LONG TERM...Thursday through Sunday...
GFS indicated the presence of an African Easterly Wave
propagating into the Northeast Caribbean Wednesday through Friday.
In addition, NASA GEOS-5 Aerosol Optical Thickness (AOT)
suggested an extensive/dense Saharan Air Layer (SAL) through that
same period that may result in hazy skies across Puerto Rico and
the U.S. Virgin Islands, with the event's peak on Thursday.
Suspended Saharan Dust Particles may even last through Friday or
Saturday. Also, at the surface, a high pressure across the
Atlantic Ocean will tighten the local pressure gradient promoting
somewhat windy trades on Wednesday and possibly Thursday.
Regardless of the hot and dry air mass over the islands, the trade
winds may advect moisture pockets, producing passing showers
daily across the windward sections and local waters. Though near
to below-normal moisture will prevail through the long term,
extensive heating, local topography, and sea breezes may result in
periods of afternoon or evening thunderstorms across western P.R.
each day.
Several factors could play an active role in contributing to
above-normal high temperatures across the islands during the
second part of the work-week, among them a mid-level high pressure
promoting subsidence over the Northeast Caribbean, warmer-than-
normal low temperatures due to the SAL.
A summer weather pattern may hold for the next weekend, with the
typical trade wind showers followed by afternoon convection across
the western locations. &&
AVIATION...
(06Z TAFS)
Issued at 410 AM AST Sun Jul 16 2023
Mainly VFR conditions will prevail across all terminals during
the next 24 hours. However, afternoon TSRA/SHRA could develop
across the interior and southwest portion of PR btwn 16/18-23z.
Winds will remain from the E-ENE, peaking around 14-20KT with
higher gusts and sea breeze variations. Lighter and variable winds
will return after 16/23Z. Starting around 17/04z, the arrival of
a tropical wave may bring TS to eastern terminals.
MARINE...
Issued at 410 AM AST Sun Jul 16 2023
Expect calm marine weather conditions with a pocket of moisture
resulting in passing showers across the local waters. Seas will
range at 3 to 5 feet across the offshore waters and less than 4
feet elsewhere. The winds will blow from the east to east-
northeast at 10 to 15 knots, but sea breeze may increase them
along the coastal areas.
The subtropical high pressure north of the region will promote
moderate to locally strong trade winds Monday through at least
Thursday, resulting in choppy seas across the islands. A tropical
wave will bring inclement weather Monday and Tuesday.
&&
.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ005-008-010-
012-013.
VI...None.
AM...None.
&&
SHORT TERM/AVIATION...ICP
PUBLIC/UA...ERG