138 
FXCA62 TJSJ 161624 AAA
AFDSJU

Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
1224 PM AST Sun Oct 16 2022

...New UPDATE, AVIATION...

.UPDATE...
Issued at 1211 PM AST Sun Oct 16 2022

Showers formed before noon in the usual places, including over
Guaynabo, southern San Juan and Trujillo Alto. Coamo and Utuado 
also are beginning to see showers. An approaching upper level 
trough and a low level trough from the other side will make things
interesting during the next 36 to 48 hours. Will especially be 
watching the Culebrinas river if thunderstorms reappear this 
afternoon. Sounding results were inconclusive due to a software 
error, but moisture has increased during the last 24 hours. Urban 
and small stream flooding are just a few hours away. Still some
time to enjoy the low risk of rip currents at the beaches.

&&

.SYNOPSIS...

Near-normal moisture and increased instability will result in an 
active afternoon today and tomorrow, with the typical shower 
activity increasing through this time. Showers and thunderstorms are 
expected for western and interior portions of Puerto Rico, 
especially. A light steering flow will result in slow-moving 
showers, which could lead to locally high rainfall totals, 
increasing the potential for flooding conditions to develop. 
Decreasing moisture and stability are expected for Tuesday into the 
end of the week. 

&&

.SHORT TERM...Today through Tuesday...

The available moisture is expected to remain fairly steady today and 
tomorrow, with precipitable water near 1.8 inches but perhaps 
peaking to 1.9 inches on Monday. Then, a slight decrease to around 
1.7 inches is forecast. We also have a diffluent pattern in the 
upper levels and an upper trough moving in from the NW, which will 
cause an increase in instability by this afternoon and through 
Monday. The slightly above normal moisture is causing widely 
scattered showers across the local waters in the early morning 
hours, some of which also affected portions of eastern PR and will 
continue for the rest of the day. Brief showers could also affect 
the USVI, though none have so far tonight. In the afternoon hours, 
the available moisture is expected to combine with the local effects 
and diurnal heating to cause showers and thunderstorms to develop. 
The central to western sections of PR as well as portions of the San 
Juan Metro have the best chance of showers, with central to western 
Puerto Rico having more persistent and longer lasting showers and 
thunderstorms. This activity, combined with the shower activity 
observed in the past few days, will likely cause some could cause 
urban and small stream flooding, while isolated flash flooding will 
not be ruled out. Elsewhere in PR, scattered showers with isolated 
thunderstorms are forecast, and some ponding of water in areas of 
poor drainage could be observed. Across the USVI, mainly scattered 
showers could be observed, as well as lines of showers streaming off 
the islands.

For Monday, there is the potential for another round of persistent 
thunderstorms over PR, due the approaching upper trough from the 
northwest, which will be in a favorable position to cause 
instability over the area. This is in addition to the above normal 
moisture with precipitable water values reaching 1.9 inches.
Therefore, showers and thunderstorms are forecast over PR on Monday 
afternoon, while scattered showers are forecast elsewhere in the 
local area. The thunderstorms on Monday may persist over any one 
area simply because the winds are very light, so the thunderstorms 
that due develop in areas of convergence over PR will move very 
slowly. This pattern will once again cause at least urban and small 
stream flooding, but some isolated flash flooding should not be 
ruled out. By Tuesday, the moisture decreases slightly and the upper 
trough continues moving to the SE, with the local islands being in 
the subsident side of the trough by Tuesday afternoon. This will 
limit shower and thunderstorm development to locally induced 
thunderstorms over PR, which will be once again across the central 
to western sections of PR, while isolated to scattered showers are 
forecast elsewhere.

As far as temperatures, daytime high temperatures will be near 
normal, with highs in the upper 80s to low 90s across the lower 
elevations. The heat index values will continue reaching or 
surpassing the 100s degrees Fahrenheit during short term period, 
especially across north central Puerto Rico.

&&

.LONG TERM...Wednesday through Sunday...

Ridging is expected in the mid- to upper-levels, maintaining 
stability over the region through at least the end of the workweek. 
Meanwhile, drier than typical air will be over the area into Friday, 
as well, though patches of near-normal (on the low end of the normal 
range) moisture are likely to pass through the region from time to 
time. Still, local effects will combine with available moisture to 
result in showers and isolated thunderstorms in a typical pattern 
over the region. Forecast confidence is medium, largely due to 
uncertainties in the direction of a slow steering flow and the 
timing of moisture patches.

Forecast certainty decreases at the end of the week. Both the Euro 
and GFS suggest that an upper-level low will approach from the 
northeast, absorbing the upper-level low that will have been 
effectively stationary east of the Windward Islands over the 
previous days. This trough will then affect the region, resulting in 
an increase in shower activity. However, there is disagreement on 
timing, and just how close the low will get to the area. As such, 
there is the potential for active weather next weekend, but it is 
essentially equally likely that there will be seasonally-typical 
conditions. Forecast confidence becomes low by late Friday/early 
Sunday.

&&

.AVIATION...
(12Z TAFS)

VCTS is expected at TJBQ after 16/17Z. TJSJ may have VCTS at some
point between 16/17Z and 16/20Z, as SHRA have already dvlpd 
south of the terminal in the VCNTY. Winds will be generally 
easterly 10-15 knots with sea breeze influences. 

&&

.MARINE...

Winds of up to around 10 to 15 knots will continue across the local 
waters. Seas will remain generally 4 feet or less for the offshore 
Atlantic waters, and 3 feet or less across the rest of the waters. 
There is a low risk of rip currents.

&&

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

$$

SHORT TERM...WS
LONG TERM....GRS
AVIATION...ERG