755 
FXCA62 TJSJ 242058
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
458 PM AST Wed Aug 24 2022

.SYNOPSIS... 

A trade wind perturbation will bring scattered to numerous 
showers over eastern Puerto Rico, Vieques, Culebra and USVI tonight
through late morning hours. Also, a more active afternoon is 
expected tomorrow due to this increase in moisture. A broad 
tropical wave is forecast to cross the Lesser Antilles around late
Thursday night or early Friday morning, but most of the activity 
is anticipated to remain over the Caribbean waters. The National 
Hurricane Center is actively monitoring this disturbance; 
additional information is available in the Tropical Weather 
Outlook (TWOAT) or www.nhc.noaa.gov.

.SHORT TERM...Tonight through Friday

As previously discussed in the forecast, afternoon showers and 
isolated thunderstorms affected western and interior sections of 
Puerto Rico. However, minimal impacts were observed with this 
activity as most showers fell over rural areas with only some small 
streams running high for a short time. This activity will
gradually diminish after sunset over land areas with the most 
intense moving offshore over the Mona Passage in the evening 
hours. 

The short-term period will be dominated by mid- to upper-level 
ridging for the most part with a surface high pressure system across
north central Atlantic inducing east southeast moderate to fresh 
trade winds over the northeastern Caribbean. Global models also 
suggest relatively drier air within the 500-250 mb layer than in 
previous days with more stable dynamics aloft. The 500 mb 
temperatures are expected to remain between -5 to -4 degrees 
Celsius through late Friday night. This weather pattern will 
hinder deep convective activity to a large extent, except for the 
diurnally induced afternoon convection over land areas and 
streamers downwind of the local islands due to surface heating and
available 1000-700 mb moisture content. 

GOES-16 Total Precipitable water imagery shows a trade wind 
perturbation east of the forecast area, currently over the northern 
Lesser Antilles, with higher moisture content. The perturbation is 
anticipated to start filtering in tonight. Latest imagery from 
the Polar Blended-Percent of Normal Total Precipitable Water (TPW)
indicates between 110% to 125% of above TPW associated with the 
perturbation. This higher moisture and its interaction with the 
local islands and eastern portions of Puerto Rico will enhance 
shower development. Due to unfavorable weather condtions aloft, 
this shower activity is expected to be mostly shallow resulting in 
scattered to numerous showers at most across the aforementioned 
areas. A few very isolated thunderstorms may pop up from time to 
time but mostly over the local waters. According to the Hi-Res 
models, showers will increase across the USVI, Vieques, Culebra 
around 25/03z moving westward towards the east of Puerto Rico. 
This activity will persist through late morning hours increasing 
the potential of ponding of water in roads and poorly drained 
areas. In the afternoon, this increased moisture will promote a 
more active afternoon than today over western and western 
interior. The San Juan/Bayamon metropolitan area could also be 
impacted due to streamers developing downwind of El Yunque. 
Elevated potential will be present for flooding of urban areas, 
roads and small streams. 

A tropical wave currently over 58W is expected to cross the Lesser 
Antilles by Thursday/Thursday night and invade the Caribbean 
waters by Friday. The National Hurricane Center is currently 
monitoring this disturbance with very low formation chance for the
next 5 days. Nevertheless, the disturbance is expected to remain 
mostly south of us, with a slight increase in moisture over the 
local islands. Showers are forecast to increase over eastern 
Puerto Rico and the local islands by Friday morning and deep 
convective activity over western Puerto Rico in the afternoon. The
bulk of shower and thunderstorm activity through is expected to 
remain over the Caribbean waters. Please continue to monitor the 
forecast for the latest information. 


&&

.LONG TERM...Saturday through Thursday...From Prev Discussion...

The weekend will start off with humidity related to a low pressure
system reaching our offshore Caribbean waters on Saturday morning
according to model guidance. This humid airmass includes 
precipitable water values of above 2 inches, which is above normal
values for this time of the year. This disturbance is currently 
being monitored by the NHC, who gave it a low formation chance of 
near 0 percent through the next 48 hours and 20 percent through 
the next five days. This low formation chance of 20 percent 
through the next five days could occur in the Caribbean Sea. 
Nevertheless, the humid air related to this disturbance will 
increase rain chances, especially in the Caribbean Sea. Meanwhile,
a wind surge along with a band of moist air is also forecast to 
reach the region during the late afternoon hours on Saturday. This
band should also have precipitable water values just below 2 
inches. Both of these areas of increased moisture can bring 
increased shower activity as they affect the forecast area on 
Saturday.

GFS model guidance shows humidity related to a tropical wave 
affecting our region from Sunday and into the first half of the 
workweek. This wave no longer has any cyclonic formation chance, 
but current model guidance shows high precipitable water values 
over the region. Late Sunday through early Wednesday still show a 
deep moist layer reaching the upper levels. These disturbances 
promote an increased potential for heavy rainfall, especially to 
start the workweek. Weather conditions in the forecast region will
be tied to the development of these systems, we will continue to 
monitor the future development of theses system throughout the 
coming days and given how the models have been behaving, 
especially for the start of the workweek, direct impacts to our 
region can vary. A drying trend will be observed from late 
Wednesday through the end of the long term forecast period, with 
precipitable water values reaching normal values. For this time 
expect a generally typical weather pattern of passing showers 
mainly over the east and shower activity during the afternoon 
hours mainly over the interior and western Puerto Rico due to 
diurnal heating and local effects.

&&

.AVIATION...
(18Z TAFS)
Issued at 440 PM AST Wed Aug 24 2022

Afternoon SHRA will affect the vicinity of the local terminals 
this afternoon until 24/21Z. TSRA is possible near TJBQ during 
this time. SHRA activity improving this evening, but ISOL SHRA 
could still affect the USVI terminals as well as TJSJ overnight. 
Winds will be from the East for the rest of the afternoon with sea
breeze variations, at about 15 KT. Winds to decrease during the 
night to around 5-10 KT.

&&

.MARINE...
Issued at 440 PM AST Wed Aug 24 2022

Generally moderate easterly winds are expected to prevail due to 
a surface high pressure system over the Atlantic. Quick passing 
showers and isolated thunderstorms producing brief wind gusts will
continue to affect the coastal waters and local passages from 
time to time. Small craft should exercise caution in the local 
Atlantic waters due to locally fresh winds. There is also a 
moderate risk of rip currents for the north-facing beaches of 
Puerto Rico tonight, with a low risk of rip currents elsewhere.

&&

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

$$

SHORT TERM...WS
LONG TERM....CVB
AVIATION...RC