Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Juan PR
1015 PM AST Friday Jul 31 2015
Update...the afternoon showers across western and interior Puerto
Rico diminished rapidly after sunset as a dry air mass moved
across the region from the east during the early evening hours.
Only few isolated showers were noted across the Caribbean waters
this evening...but not precipitation was observed over land areas.
A Saharan air layer will move across the region overnight...
limiting even more the chances of showers across the local
islands during the weekend. However...some afternoon showers are
possible each afternoon across western and interior sections of
Puerto Rico as the limited moisture combines with daytime heating
and local effects. A weak tropical wave will cross the region
Sunday night and early Monday...slightly increasing the moisture
across the local islands.
Aviation...VFR conds at all taf sites with easterly winds around 5
to 10 knots overnight increasing to 15 kts and higher gusts after
Marine...no changes to previous forecast reasoning.
Previous discussion... /issued 300 PM AST Friday Jul 31 2015/
Synopsis...lingering moisture from the tropical wave is expected
to continue through the local area through this afternoon...then
drying will occur through the weekend with Saharan dust moving
in. Upper trough will continue to the north of Cuba this
weekend...moving south by the early part of the upcoming week. A
weak tropical wave is expected on Monday...mostly dry air expected
thereafter with Saharan dust and a few patches of moisture.
Discussion...many sectors of Puerto Rico received some
rain...with the northwestern quadrant of PR receiving the most
significant rainfall. The lingering moisture combined with the
upper trough helped trigger some showers and thunderstorms across
the local area. Most of the showers were brief but northwest-PR received
over an inch of rain in isolated areas as estimated by the Doppler
These showers are expected to quickly dissipate by this evening
when much drier air and Saharan dust is expected to move in. The
drier air will prevail through the weekend...causing mostly fair
weather with only a slight chance of showers. Some isolated
thunderstorms are still possible though since we do have the upper
trough to the west of the local area...causing some instability.
A weak tropical wave is expected for Monday...this wave should
pass quickly. Some showers and thunderstorms are possible on
Monday but quickly thereafter dry air and Saharan dust will make a
return to the local area. For the rest of the week...nothing but
patches of moisture are suggested by the latest guidance. Passing
isolated to scattered showers at most are expected after the
tropical wave on Monday.
Aviation...rain showers and thunderstorms and rain will continue across north ctrl and west PR until
31/22z. As a result...brief MVFR ceilings in passing rain showers/isolated thunderstorms and rain over
and vcty tjbq and tjmz. Maximum tops west/thunderstorms and rain by the way 450-500...mountain top obscr
possible west/passing rain showers and isolated thunderstorm activity over PR. Few cumulus/towering cumulus en
Route by the way east PR and USVI with passing rain showers...tops 160. Isld thunderstorms and rain also
possible over the offshore Caribbean waters S of PR until 31/22z. Surface wind
from east at 15-20 knots west/higher gust up to 25 kts...becoming from east 5-10 kts
after 22z. L/level wind from east 15-20 kts below 150 become light/vrb above.
Marine...seas will be up to 6 feet and winds up to 20 knots
through the weekend. Small craft should exercise caution.
Preliminary point temps/pops...
sju 78 90 79 89 / 40 10 20 20
stt 79 89 78 89 / 30 10 20 20