542 
FXCA62 TJSJ 061550 AAA
AFDSJU

Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
1150 AM AST Wed Oct 6 2021

.UPDATE...A band of drier air is moving into the northeast and
southeast corners of our forecast area at the moment but some
moisture is still holding out over the local outer Atlantic and
Caribbean waters where some showers are still present. Very little
adjustments were need in the grids. The latest sounding from San 
Juan at 06/12Z showed air with less than 15 percent relative
humidity between 13 and 25 thousand feet and weak capping from
8500 to 15 thousand feet. This should greatly limit convective
activity today. Nevertheless in areas of favorable local
convergence showers and an isolated thunderstorm are still
possible, so have retained isolated afternoon thunderstorms in the
forecast for western and interior Puerto Rico. 

&&

.FIRE WEATHER...Conditions along the southwest coast of Puerto
Rico have become drier with winds above critical levels. A Fire
Danger Statement has been issued.

&&

.AVIATION...VFR conditions expected to prevail across the local 
terminals through the forecast period except in the Leeward
Islands where CIGS near 020 will occur with incrg hir lyrs also. 
Aft 06/16Z for PR, sct SHRA and ISOLD TSRA psbl across western 
PR. ENE winds of 15 to 20 kts with gusts up to 25 kts and sea 
breeze variations til arnd 06/22Z, becoming less than 12 knots 
with land breezes. Maximum winds NW 26-36 knots btwn FL380-480.

&&

.MARINE...Seas will continue to steadily diminish, and expect that
small craft advisories scheduled to be taken down at 2 PM AST will
be allowed to expire. All seas should be less than 7 feet aft 8 AM
AST Thursday. Winds have already begun to subside somewhat.

&&

.PREV DISCUSSION... /issued 435 AM AST Wed Oct 6 2021/ 

SYNOPSIS...

A ridging pattern aloft will hold until Friday. An increase in 
moisture content due to remnants of Victor will aid in the 
development of showers and thunderstorms Thursday and Friday. 
Choppy to hazardous marine conditions will continue at least 
through early Thursday.

SHORT TERM...Today through Friday...

At lower levels, a surface high pressure north of the area will 
continue to promote east to east northeast winds. This will drive 
patches of low-level moisture into the area from time to time, 
resulting in brief showers across the local waters, USVI and 
portions of eastern and northern Puerto Rico during the overnight 
and early morning hours. A weak mid level ridge will continue to 
hold over the area through Friday. An upper level ridge will weaken 
as an upper low starts moving in on Thursday, and by Friday, the 
upper low will weaken and become a weak trough with its axis over 
the local islands. This will promote somewhat stable conditions, 
hindering widespread development of deep convection. That being 
said, scattered to locally numerous showers are forecast across 
parts of the local forecast area, but only isolated thunderstorms 
are expected. This overall pattern, in combination with the daytime 
heating and local effects, will cause shower, and isolated 
thunderstorm activity, across western Puerto Rico during the 
afternoon hours. However, due to the relatively fast steering flow, 
rainfall activity is not expected to be significant, therefore, the 
flooding potential is expected to remain low, though ponding of 
water in poor drainage areas is expected.

As far as temperatures, we can expect near normal highs and lows, 
possibly within 2 degrees of normal. Highs in the upper 80s to low 
90 across the lower elevations, with lows in the mid 70s to near 80. 
The higher elevations could observe highs and lows about 5 to 10 
degrees cooler than the lower elevations, even slightly cooler in a 
few isolated areas. The heat index values could reach the low 100s 
across some areas in the lower elevations each day through the end 
of the workweek.

LONG TERM...Saturday through Thursday...

A seasonal weather pattern is expected for this upcoming weekend 
with lingering moisture across the area through Sunday. A weak upper-
level trough over the area will maintain some instability aloft 
with 500 mb temperatures remaining between -7 to -6 degree 
Celsius. However, the mid-level ridge will make a comeback with 
dry air filtering in at these levels. This will mainly support
shallow convection and reduce coverage in thunderstorm activity. 
Therefore, isolated to scattered showers are expected during night
hours and each morning across the local waters, USVI, and eastern
PR. Meanwhile, diurnal heating and local effects will produce 
afternoon showers and isolated thunderstorms mainly across the 
central and western sections of PR. 

Model guidance diverge for next Monday throughout the week. Both GFS 
and ECMWF agree on the existence of a deepening cutoff low over 
Florida. However, discrepancies start with GFS having a much 
stronger upper low moving eastward while settling west of the 
forecast area and persisting through the week. Meanwhile ECMWF has
the low weakening into a trough and staying a bit farther west. 
At low levels, a vigorous tropical wave will stream across eastern
Caribbean with precipitable water values increasing to near and 
above 2.0 inches across Puerto Rico and U.S. Virgin Islands by 
early Monday. GFS also combines this event with the upper low 
pulling deeper moisture from the ITCZ into the forecast area 
supporting a very moist and unstable environment throughout the 
week. This suggests a broad coverage of shower and thunderstorm 
activity across the local forecast area through the end of the 
workweek. On the other hand, ECMWF has the tropical wave passing 
through but downplaying on the moisture pulling from the ITCZ, 
hence delaying deeper moisture in the air column and appearing 
much later in the workweek supporting a more seasonal weather 
pattern for most days. Therefore, confidence in the forecast is 
low. We will need to closely monitor the evolution of this 
forecast as there still exist the potential for a significant rain
event by next week.

AVIATION...VFR conditions expected to prevail across the local 
terminals through the forecast period. However, SHRA will be 
observed through at least 06/12Z, causing -SHRA at the local 
terminals. That being said, no significant impacts are expected. In 
the afternoon, after 06/16Z SHRA and ISOLD TSRA expected across 
western PR, causing VCTS at TJBQ. Winds will continue from the ENE 
at 10 KT or less through 06/12Z, increasing to 15 to 20 kts with 
gusts up to 25 kts and sea breeze variations after 06/14z. 

MARINE...

Choppy to hazardous seas between 5 to 8 feet due to wind driven 
seas and a northeasterly swell are expected to continue across 
most of the regional waters through early Thursday. Therefore, 
there is a Small Craft Advisory in effect until Thursday morning. 
There is a high risk of rip currents across the northern and 
eastern beaches of PR, Culebra, eastern beaches of St.Croix and 
northern St. Thomas .

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  90  77  90  79 /  50  60  50  30 
STT  90  79  90  79 /  50  50  40  40 

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through Thursday afternoon for Culebra-
     North Central-Northeast-Northwest-San Juan and Vicinity-
     Southeast.

VI...High Rip Current Risk until 6 PM AST this evening for St Croix-
     St.Thomas...St. John...and Adjacent Islands.

AM...Small Craft Advisory until 2 PM AST this afternoon for Anegada 
     Passage Southward to 17N-Coastal Waters of Northern USVI 
     and Culebra out 10 NM-Mona Passage Southward to 17N.

     Small Craft Advisory until 2 AM AST Thursday for Coastal Waters 
     of Northern Puerto Rico out 10 NM.

     Small Craft Advisory until 8 AM AST Thursday for Atlantic Waters 
     of Puerto Rico and USVI from 10 NM to 19.5N.

&&

$$

SHORT TERM...WS
LONG TERM....CS
PUBLIC DESK...RVT