988 
FXCA62 TJSJ 291320
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
Issued by National Weather Service Miami FL
920 AM AST Fri Sep 29 2017

.UPDATE...
Made some minor tweaks to the forecast mainly Sky/Pop/Temp. Rain
chances will increase beginning late morning. Short range models
develop scattered showers and storms in the next few hours with
locally heavy rainfall possible. Flash Flood Watch and Rip Current
Statement look good. Updates sent.


&&


.PREV DISCUSSION... /issued Issued by National Weather Service Miami FL/ 

AVIATION...
For the 12Z TAFs....Showers and thunderstorms are expected to
increase in coverage through the morning and afternoon hours.
Predominate VFR will give way to tempo MVFR as convection develops
and impacts the terminals. Light to moderate east winds will be
the rule, perhaps gusty in and around convection. Indications are
that convection will last into the evening hours across VI and 
towards the end of the TAF period invading PR.

PREV DISCUSSION... /issued Issued by National Weather Service Miami FL/ 

.PERIODS OF HEAVY RAINFALL MAY CAUSE FLASH FLOODING ACROSS
PUERTO RICO AND THE U.S. VIRGIN ISLANDS...

SHORT TERM...
Current satellite imagery shows an upper-level low moving slowly 
westward across the northern Caribbean Sea with a large plume of 
tropical moisture surging northward across the Leeward Islands
this morning. Radar imagery from the northern islands showed 
convection earlier from Antigua to St. Kitts, but they have mostly
died out since. As mid-level winds turn southeasterly this 
morning, deep moisture will overspread the U.S. Virgin Islands and
Puerto Rico through the evening with precipitation chances 
quickly increasing throughout the day and into Saturday. Locally 
heavy rainfall is expected and additional flash flooding is 
possible, which would hinder recovery efforts. A Flash Flood Watch
continues for the entire forecast area. See the Hydrology section
for additional information and potential impacts.

There is a high risk of rip currents through Friday along the
northern Atlantic coastline of Puerto Rico and St. Croix with a
moderate risk on the southern coast. The Rip Current Statement 
will continue through Friday afternoon due to dangerous northerly
swells continuing.

LONG TERM...
As the upper-level low continues westward away from Puerto Rico, 
deep tropical moisture will remain in place through Monday, 
keeping the chance of showers and thunderstorms in the forecast. 
Though the best forcing will be associated with the upper low 
this weekend, locally heavy rainfall potential will continue 
through early next week, but there is lower confidence on amounts
at this point. The rest of the week, modest moisture from the 
surface to around 700mb will warrant generally 30 to 40% PoPs each
day. Light surface winds will turn to the ESE/SE Thursday and 
Friday, putting the best moisture convergence in the 
western/interior portions of Puerto Rico and in the higher terrain.

Temperatures will be near or below normal through much of the
forecast period with expected cloud cover and precipitation
chances.

AVIATION...
Moisture will increase substantially Friday morning through the 
afternoon with showers and thunderstorms expected to increase in 
coverage across much of the area. Have kept VFR conditions 
prevailing across the western areas on this package but MVFR or 
lower ceilings will certainly be possible, especially across the 
eastern and interior portions of Puerto Rico and the U.S. Virgin 
Islands with periods of heavy rainfall also possible.

HYDROLOGY...
Deep tropical moisture will overspread the islands this morning
from east to west and continuing this weekend. Rainfall amounts 
of 2 to 4 inches with locally higher amounts are likely each day 
with precipitable water values nearing or exceeding 2.00". Soils 
are already saturated and most of the rivers and small streams are
running above or well above normal levels, therefore there is a 
high risk of river and small stream flooding as well as mudslides 
in areas of steep terrain. Urban flooding is also likely across 
low-lying areas as well as poor drainage areas where water pumps 
are not working at full capacity.

A FLASH FLOOD WATCH has been issued for all islands beginning
Friday morning.

MARINE...
Moderate easterly winds and seas will dominate across the area 
through the weekend. Northerly swells continue to push into the 
northern sections of the islands where small craft will need to 
exercise caution. Showers and thunderstorms will become numerous 
beginning Friday with winds and seas locally higher in and around 
stronger convection.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  83  79  82  80 /  70  40  70  50 
STT  82  80  81  80 /  70  50  70  50 

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through Saturday morning for Mayaguez and 
     Vicinity-North Central-Northeast-Northwest-San Juan and 
     Vicinity-Southeast-Western Interior.

     Flash Flood Watch through late Sunday night for Central Interior-
     Culebra-Eastern Interior-Mayaguez and Vicinity-North 
     Central-Northeast-Northwest-Ponce and Vicinity-San Juan and 
     Vicinity-Southeast-Southwest-Vieques-Western Interior.

VI...High Rip Current Risk through Saturday morning for St Croix.

     Flash Flood Watch through late Sunday night for St Croix-
     St.Thomas...St. John...and Adjacent Islands.

AM...Flash Flood Watch through late Sunday night for Coastal Waters 
     OF Southwestern Puerto Rico out 10 NM-Coastal Waters of 
     Northern USVI and Culebra out 10 NM-Coastal Waters of 
     Southern Puerto Rico out 10 NM-Coastal Waters of Southern 
     USVI Vieques and Eastern Puerto Rico out 10 NM.

&&

$$

SHORT TERM...99
LONG TERM....99