Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Juan PR
1013 am AST Friday Jul 29 2016
Update...partly cloudy skies with passing trade wind showers
were observed moving across the offshore and nearshore waters.
Some of these showers briefly affected the USVI, Vieques and
Culebra during the early to mid morning hours. An area of showers
with moderate to locally heavy rainfall was also observed moving
westward across the eastern half of Puerto Rico during this
period. No significant rainfall accumulation has been reported as
Isolated showers will continue across the eastern portions of
Puerto Rico during the rest of the morning hours causing some
ponding of water on roads and low lying areas. Showers and
thunderstorms are expected to develop across the interior and
western portions of Puerto Rico during the afternoon hours. With
these showers and thunderstorms, expect periods of heavy
rainfall, gusty winds, and lightning activity, as well as ponding
of water of roads and low lying areas and reduced visibilities.
Some Urban and Small Stream flooding is possible across these
Aviation...passing rain showers will continue to affect the leeward,
USVI and eastern PR flying area throughout the morning hours.
Rain showers/thunderstorms and rain with mtn obscuration expected across the interior and west
PR from 16z-22z, periods of MVFR expected for tjmz/tjbq during
that period. Low level winds from the east to southeast at 10-20
kts with higher gusts and sea breeze variations.
seas up to 5 feet and winds up to 15 kts expected across the local
waters today. Hazardous marine conditions posible in and near
strong showers and thunderstorms. No change to the rest of the
previous marine forecast.
Previous discussion... /issued 550 am AST Friday Jul 29 2016/
Synopsis...upper level trough will aid in the development of
showers and thunderstorms today. A strong tropical wave located in
the central Atlantic is expected to move across the region
Saturday night into Sunday. Saharan air layer is expected to
affect the region after the passage of the wave.
Discussion...scattered showers were seen across the coastal
waters overnight. Some of these showers affected the eastern half
of Puerto Rico and the U.S. Virgin Islands. Some showers produced
heavy rainfall in localized areas. This activity is associated
with an upper level trough which will be passing across the area
today. This upper level trough will aid in the development of
showers and thunderstorms mainly over the western interior and
west sections of Puerto Rico. After the passage of this
feature...drier air will move across the region Saturday.
Our attention is focus on a strong tropical wave which is located
near 45 west longitude this morning. This wave is accompany by
very deep tropical moisture. Global models agreed on bringing this
system to the local area Sunday. However...the leading edge of
this system is expected to affect the local region as early as
Saturday afternoon. At this time...looks like the most active
weather will occur Sunday morning into the afternoon...with the
GFS model showing impressive precipitable water values over the
area as high as 2.5 inches. Regardless if the system develops into
a tropical cyclone or not...the potential for very heavy rainfall
and strong gusty are highly possible mainly Sunday afternoon as
the wave moves across the region. In fact...flash flooding can't
be ruled out as a very moist southeasterly winds are expected to
aid in the development of showers and thunderstorms across the
area. We will continue to closely monitor the progress of this
system. Stay tuned.
Aviation...iso-sct rain showers will continue to produce brief periods of
MVFR ceilings across the leeward, USVI and eastern PR taf sites this
morning. Low level winds will shift from east-northeast to east after 29/12z. After
29/17z areas of rain showers/thunderstorms and rain will form across western and northwest PR
with mountain obscurations and periods of MVFR ceilings/visibilities from
29/18z to 29/22z. After 29/18z rain showers and possible thunderstorms and rain will also move
from the east into tncm and tkpk with MVFR likely before 30/00z.
Marine...due to the proximity of the aformentioned tropical
wave..hazardous seas are expected from Saturday afternoon through
at least Sunday afternoon. Therefore...a Small Craft Advisory has
been issued for the offshore Atlantic waters and the Anegada
Passage due to increasing seas up to 8 feet.
Preliminary point temps/pops...
sju 89 79 89 79 / 30 20 20 60
stt 89 80 92 79 / 30 20 20 60
Am...Small Craft Advisory from 2 PM Saturday to 4 PM AST Sunday for
Anegada Passage southward to 17n-Atlantic waters of Puerto
Rico and USVI from 10 nm to 19.5n.