617 
FXCA62 TJSJ 130055
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
855 PM AST Sun Sep 12 2021

.UPDATE...Although the reasoning behind the forecast and analysis
has not changed, some adjustments were made to the weather grids 
following the most recent model guidance and observations. 
Although drier air has filtrated the 750-500 mb layer as depicted 
in the 13/00Z sounding, and radar Doppler indicated a decrease in 
shower and thunderstorm across the area, enhanced tropical 
moisture associated to a passing tropical wave now moving into the
eastern Caribbean interacting with an upper level low to our 
northwest will set the conditions to support further shower and 
thunderstorm development later tonight and continuing through at 
least Monday night. In fact, model guidance suggests precipitable 
water values around 2.10 inches with 500 mb temperatures between 
-7 and -8 degrees Celsius between 13/16Z and 14/08Z. This will 
definitely lead to another active afternoon, with shower and 
thunderstorm development favoring the interior and western 
sections of Puerto Rico, as well as portions of the San Juan 
metropolitan area. In the meantime, shower and thunderstorm 
activity is expected to favor the regional waters during the 
overnight and early morning hours, moving into eastern Puerto Rico
and the US Virgin Islands from time to time. Moderate to locally 
heavy rains over already saturated soils will result in ponding of
waters to urban and small stream flooding and possible mudslides 
near areas of steep terrain. Overnight lows are forecast to drop 
into the lower 70s to around 80 degrees Fahrenheit, while daytime 
highs are expected to peak into the lower 90s with heat indices 
reaching the lower to mid 100s. Winds will continue from the east,
but at lower speeds during the overnight and early morning hours,
increasing to 10-20 mph with higher gusts during the day. 


&&

.AVIATION...The combination of a tropical wave and an upper level 
low moving across the region through Monday will promote shower and 
thunderstorm development with VCSH/VCTS at most terminals during the 
next 24 hours. The focus of the activity will affect Leeward, USVI 
and TJSU terminals through 13/12Z and after 13/22Z, and all PR 
terminals between 13/15-22Z. This may result in brief MVFR 
conditions with SCT-BKN layers between FL025-050 and reduced 
visibilities. Winds will be light and variable through 13/12Z, 
turning from the ESE at 10-20 knots with occasionally higher gusts 
thereafter.


&&

.MARINE...No changes to the marine and surf zone grids. Please
refer to the Marine discussion below for more information. 


&&

.PREV DISCUSSION... /issued 454 PM AST Sun Sep 12 2021/ 

SYNOPSIS...A Tropical Upper Tropospheric Trough (TUTT) low will
continue to move westward and stall over Hispaniola through early 
next week. A tropical wave will continue to pull further away from 
the Mona Passage tonight. A surface trough will move across the 
local area on Monday from the east. Precipitable water content is 
forecast to remain above 2 inches through the first part of the 
workweek. A mid-level ridge and a Saharan Air Layer should promote 
drier air and hazy skies during the second part of the workweek.

SHORT TERM...Today through Tuesday...

The TUTT-low is expected to meander over Hispaniola through the
rest of the short term period. This will continue to promote
unstable conditions and provide divergence aloft to enhance shower
and thunderstorm activity across the local area. Furthermore, a
surface trough east of the Windward Islands will bring a surge of
low-level moisture on Monday, and linger through Tuesday.
Therefore, showers and thunderstorms are expected each day across
portions of the regional waters and enhanced in the diurnal cycle
over the mountain ranges of PR, as well across the San Juan metro
area and western PR. Urban and small stream flooding is expected
due to saturated soils across much of PR. Across the USVI,
scattered showers are expected mainly, but thunderstorms across
the nearby coastal waters could move at times over land areas and
cause ponding of water in poor drainage areas.

LONG TERM...Wednesday through Sunday...

From Previous Discussion /issued 428 AM AST Sun Sep 12 2021/ 

An induced surface trough is expected to develop just east of the 
Bahamas by Wednesday. This feature will drag additional deep 
tropical moisture into the area with precipitable water values 
expected to range between 2.0-2.2 inches. This in combination with
a weakening TUTT expected to be located over the island of 
Hispanola will continue to result in favorable conditions for 
additional enhanced shower and thunderstorm activity, with the 
potential for urban and small stream flooding across areas that 
receive the heaviest and most persistent rainfall activity. 

Thursday is expected to be a transition day. As the aformentioned 
TUTT moves further westward, a mid and upper-level ridge will begin 
to build overhead. This feature will be introducing drier air aloft. 
However, abundant low-level moisture will still remain in place
that another round of afternoon convection appears likely, mainly
over the northern half of Puerto Rico. The ridge aloft pattern 
will hold through the rest of the long term period, limiting the 
potential for widespread convective activity. Nevertheless, 
sufficient low-level moisture will still be present to combine 
with strong daytime heating and local effects to generate 
afternoon convection, mainly across the western half of Puerto 
Rico. 

A Saharan Air Layer, according to the NASA Dust Extinction Aerosol 
Model, will be over the area between late Wednesday and Friday 
bringing light to moderate dust concentrations and somewhat hazy
skies. Dust concentrations look to diminish by next weekend as 
the Saharan Air Layer moves away.

AVIATION...The combination of a tropical wave and a upper low 
moving across the region thru Monday will promote shower and Isold
Tstorm dvlpmnt with VCSH/VCTS at most terminals during rest of 
the period. Winds will be mainly fm ESE 10-15KT with occasionally 
hir gusts, becoming light and variable aft 12/23Z. SCT-BKN lyrs nr
FL025... FL050... FL100. BKN ocnl OVC btw FL200-FL280. Incr cloud
development ovr the ctrl mountain range of PR will cause brief 
Mtn Top Obscr with aftn convection. Max Tops btw FL300-FL350 
w/TSRA. VCSH/VCTS psbl at most terminals til 12/23Z. SFC wnds will
bcm lgt/vrb aft 12/23Z.

MARINE...Seas across the coastal waters of Puerto Rico and the
USVI should range between 2-4 feet during the next several days.
Across the offshore waters and passages, seas are expected to
range in general between 3-5 feet, with up to 6 feet on Monday
across the offshore Atlantic waters. Trade winds in general should
range between 10-15 knots, with locally higher winds at times
across the Anegada Passage and offshore Atlantic waters.

A moderate risk of rip currents will prevail across most of the
north and east facing beaches of the islands through at least
Tuesday.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  77  89  78  89 /  50  60  40  50 
STT  79  88  78  91 /  60  60  40  60 

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

$$

SHORT TERM...ICP
LONG TERM....DS
PUBLIC DESK...FRG