992 
FXCA62 TJSJ 131506
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1106 AM AST Mon Sep 13 2021

.UPDATE...

No changes to inherited forecast, which is tracking well. An area
of enhanced moisture is tracking over the area, with precipitable
water values in excess of 2 inches - the 12Z upper air sounding 
showed PWAT of 2.06 inches. The TUTT is west/northwest of the 
islands, supporting good instability over the area, leading to 
more active weather. The sounding corroborates the high 
instability over the area, with surface CAPE of 3849 J/kg, and 500
hPa temperature of -6.8C. Also contributing to the risk for urban
and small stream flooding is a weakened steering flow over the 
area. Yesterday, the steering flow was almost 20 knots, and today 
it is around 12 knots, so showers are likely to not move as 
quickly.

&&

.AVIATION...

No chg to previous AVN. Prevailing VFR but showers and isolated 
tstorms will continue to develop and move west northwest across 
the Atlantic waters and passages btwn Eastern PR and the Northern 
Leeward islands. SHRA expected to increase in and around PR and 
the USVI due to the presence of an upper low and tropical wave 
moisture. SHRA/ Isold TSRA dvlpmnt will cause VCSH/VCTS at 
TJSJ/TJBQ TJMZ/TJPS til 13/23Z. Wnds fm E-SE 10-15 kts w/ocnl hir 
gusts and sea breeze variations bcmg lgt/vrb aft 13/22Z. Passing 
SHRA/ Isold TSRA fcst across the flying area into tonight. SCT 
ocnl BKn lyrs nr FL025...FL050..FL100.and btw FL200-FL250.

&&

.MARINE...

Winds remain generally out of the east at up to 15 knots across the 
local waters. Seas are at 5 feet or less, and will subside, becoming 
generally 3 feet or less by midweek.

There is a moderate risk of rip currents for beaches of Culebra, 
northern Puerto Rico, eastern Vieques, and northern and eastern St. 
Croix.

&&

.PREV DISCUSSION... /issued 436 AM AST Mon Sep 13 2021/ 

SYNOPSIS...
Unstable conditions aloft will enhanced once again the afternoon 
shower activity over the local area. As a result, urban and small
stream flooding is expected with the heaviest activity, mainly
over Puerto Rico. For Tuesday into Wednesday, the unsettled 
conditions continue as above normal moisture continues over the 
region and combine with the upper level trough.

SHORT TERM...Today through Wednesday...

Essentially an unsettled and rainy pattern is expected in the
short term. An upper trough is still causing instability across 
the local area, as it remains nearly stationary to the north of 
Hispaniola. An increase in moisture will occur gradually, and the 
shower and thunderstorm activity will gradually increase in the 
early morning hours. Therefore, today's forecast has showers and 
thunderstorms in the morning for eastern PR as well as areas near 
and around the USVI. Also, the mid levels temperatures are 
slightly colder than normal, and are expected to get about a 
degree colder by this evening, from -7 to -8, which would 
contribute to enhanced thunderstorm potential this afternoon and 
into Tuesday. For this reason, the forecast for today has showers 
with isolated to scattered thunderstorms across several sectors of
PR, with a risk of urban and small stream flooding. Eastern PR is
expected to observe the most rainfall starting around the 
mid-morning hours into the early afternoon, while central and 
western PR would have the peak of the rainfall in the afternoon 
hours. The USVI could observe moderate showers and nearby 
thunderstorms as well, but the rainfall is expected to cause 
mainly ponding of water on roadways and poor drainage areas. Most 
of the activity near the USVI is expected to be in the morning 
hours, with a brief break in the early afternoon, then passing 
showers and isolated thunderstorms again in the late afternoon 
hours. 

The upper trough will remain nearly stationary today into Tuesday,
but it is forecast to be over Cuba by Wednesday, which it could
still cause instability over the local area, but perhaps less of
an impact than what is expected today and Tuesday. The available 
moisture will pool over the local area, which will cause 
above normal moisture through Wednesday. The model guidance is 
still suggesting the precipitable water values to remain near 2 
inches. This pattern is expected to continue causing rainy 
conditions with thunderstorms as the instability of the upper 
trough will combine with the deep moisture. Any diurnal heating
and sea breeze convergence over PR would cause areas of enhanced
thunderstorm potential. Some areas across southern PR have only a
slight chance of showers over the next few days. The USVI will 
have mainly passing shower activity, but some of the showers and 
thunderstorms could be heavy for brief periods. 

The daytime high temperatures are forecast to be near 90 degrees 
once again today across the lower elevations of PR and the USVI, 
with heat index values in the low 100s. However, given the amount of 
moisture and possible cloudiness expected today, it would not be 
surprising for the daytime high temps to peak in the upper 80s. 

LONG TERM...Thursday through Monday...

An upper level high pressure will be to the north of the local
area starting in the long term period. In addition, at the mid 
levels, a high pressure will be present, resulting in relatively 
stable conditions in the mid and upper levels. Meanwhile, at the 
surface, a weak trough located well to northwest will maintain 
plenty of tropical moisture with precipitable water values above 
climatological normals. As a result, for Thursday, expect 
afternoon convection with isolated thunderstorms mainly over 
western and northwestern sections of Puerto Rico. For late 
Wednesday into Friday, according to NASA Dust Extinction Aerosol 
Model, low to moderate concentrations of Saharan dust will 
move through the local islands. As a result, hazy skies are 
possibles over the forecast area.

For the upcoming weekend, a drier pattern is expected, due to a 
drier airmass that filters into the region. Therefore, this 
airmass, in combination with stable conditions aloft, will 
result in mainly fair weather. However, patches of low level 
moisture, the local effects, and diurnal heating can cause 
isolated to scattered showers across western Puerto Rico. Monday 
is expected to be a transition day, as the model guidance
suggests that the available moisture will increase, due to an
approaching tropical wave, currently being monitored by the 
National Hurricane Center.

AVIATION...SHRA is gradually increasing, and will cause at least  
VCSH or VCTS across the USVI terminals early this morning, while
VCTS is forecast at TJSJ after 13/14Z. Winds will be light and 
VRB through 13/13Z, but gusty near the SHRA/TSRA, otherwise winds
will increase to around 10-15KT from the east. Sea breeze 
variations in wind direction expected at TJPS after 13/13Z. VCTS 
possible at TJPS and TJBQ after 13/15Z. Winds will decrease after
13/23Z. Overall, passing SHRA/TSRA are expected across the local 
flying area today and tonight.

MARINE...

Across the regional waters, tranquil marine conditions will
prevail for the next several days. Seas of up to 5 feet are 
expected. Winds are expected to be from the east, up to 15 knots.
For today and tomorrow, isolated showers and thunderstorms will 
persist over the local waters. There is a moderate risk of rip 
currents for the northern beaches of Puerto Rico, as well as the
beaches located over the eastern tip of Culebra, Vieques, and St.
Croix.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  89  78  86  77 /  60  40  50  50 
STT  88  79  89  79 /  60  50  60  60 

&&

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

$$

SHORT TERM...RAM
LONG TERM....CRS/ERG
PUBLIC DESK...RVT