Area Forecast Discussion
Issued by NWS San Juan, PR

Home |  Current Version |  Previous Version |  Text Only |  Print | Product List |  Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
FXCA62 TJSJ 150048

Area Forecast Discussion
National Weather Service San Juan PR
848 PM AST Fri Jun 14 2024

.UPDATE...Little change to the current package. Have removed haze
from the weekend forecast. Although some Saharan dust will enter
the area the model has backed off on the amount expected and the
RGB Dust product shows most of the dust is to our south. This will
allow better chances of rain, but quantities are still expected
to be light except where intense thunderstorms form in western and
interior Puerto Rico.

Have raised the local overnight temperatures for tonight in the
vicinity of San Juan on the basis of persistence, but will hold
onto the high temperatures tomorrow area-wide on account of the
south southeast winds expected in the area tomorrow. The heat
watch is continued. For more details see the NPW Weather Message.


.AVIATION...Numrs SHRA and isold TSRA will linger thru 15/04Z
tonight--mainly ovr NW and W PR where MVFR/IFR conds may be
expected. Then VFR conds should prevail through 15/17Z. Aft 15/17Z
SHRA/TSRA will redvlp in NWrn PR with MVFR/IFR conds some of which
may affect TJBQ. Wind flow of 6-12 knots will turn to the SE
overnight, but land and sea breeze patterns will override this
flow during the midday hours. VSBYs in Saharan dust are expected
to remain above 10 miles thru the period.


.CLIMATE...Clouds and showers prevented temperatures from warming
to yesterday`s heat, but San Juan`s minimum temperature today is
likely to remain at 82 degrees, breaking the record warmest
minimum for the date of 80 degrees set in 2023 and three previous


.PREV DISCUSSION... /issued 410 PM AST Fri Jun 14 2024/


An induced surface trough passing through the region will promote
passing showers and afternoon convection tonight through tomorrow,
however mid to upper- level ridging will limit widespread
rainfall. A plume of Saharan dust of moderare to high
concentration is forecast to spread across the region, causing hot
and hazy conditions that last through the second half of the
weekend into the next workweek. An Excessive Heat Watch is in
effect for tomorrow.

SHORT TERM...Tonight through Sunday...

A surface trough will exit the region overnight, heading
northwestward which will begin a veering of the surface to lower
level winds to a south-southeasterly direction by tomorrow lasting
through Sunday. Meanwhile, a mid to upper-level ridge continues to
anchor across the central Caribbean, providing overall stable
conditions. A dry airmass containing Saharan dust begins to move
northward across the eastern Caribbean basin late Saturday into
Sunday, however plenty of moisture hung up from the short wave
trough will sit over the area keeping rain possible in the forecast
through the weekend. Most of the expected rainfall will be across
eastern Puerto Rico through the day and night in the form of passing
showers, however during the afternoons convection is likely across
northern central to northwestern Puerto Rico with the southerly
component in the wind flow.

The combination of suspended Saharan dust particulates and the mid
to upper level ridging will favor subsidence and good compression of
the warm air in the low layers throughout this weekend. Daytime high
temperatures will be in the low 90s especially along the north and
west coastal areas with maximum heat indices expected to again
exceed 100 degrees, resulting in excessive heat impacts particularly
along the west north and east coastal and urban areas. Currently a
Heat Advisory is in effect for most of the islands tomorrow besides
the interior and southern coastline of Puerto Rico.

LONG TERM...Monday through Thursday...
/from previous discussion/

Although a decrease in saharan dust is forecast to start the
workweek, another pulse will continue to promote hazy conditions
through most of the long term period. Current model guidance
suggest Precipitable Water (PWAT) values at generally normal
values to start the period with a patch of below normal PWAT
values reaching the area on Tuesday. For Wednesday and Thursday,
columnar moisture will increase once again to normal to above
normal values as moisture related to a surface trough and a
possible wave, although uncertainty remains. A developing upper
level trough near the area will also increase instability. Although
model guidance suggest moisture over the mid levels, a more
notable increase is suggested by Wednesday. Most available
moisture to start the workweek will remain below 850 mb,
however. East to east- southeast surface winds should affect the
islands through Thursday, when they become more easterly and
increase. This will steer warmer tropical air towards the islands,
promoting overall warm to hot conditions. Heat related products
will possibly be issued for the islands to start the period. 925
mb temperatures are forecast to be above normal to start the

(18Z TAFS)

Scattered SHRA expected to prevail across eastern PR and TJBQ
through 22Z, with periods of reduced VIS and low ceilings possible.
Additional activity expected to increase after 22Z for the Caribbean
waters and USVI terminals as well. Winds are shifting from the ESE
at around 10-12 knots, becoming more from the south above the SFC to
FL050 on 15/12Z. Hz due to Saharan dust is expected to increase
after 12/16Z for the USVI, but VIS should remain at P6SM.


Surface high pressure over the Atlantic and an induced trough over the
area will promote light to moderate east to southeast winds.
Moist air will allow showers and isolated thunderstorms to develop
over portions of the local waters tonight through the weekend. Winds
will continue to diminish and veer, becoming more southeasterly
through the weekend with increasing Saharan dust expected across the
region through Sunday.


PR...Excessive Heat Watch from Saturday morning through Saturday
     afternoon for PRZ001-002-005-008-010-012-013.

VI...Excessive Heat Watch from Saturday morning through Saturday
     afternoon for VIZ001-002.