Scientific Forecaster Discussion
fxca62 tjsj 262055
Area forecast discussion
National Weather Service San Juan PR
455 PM AST Fri may 26 2017
Synopsis...weakening ridge aloft and lesser Saharan dust across
the region, allowed better transport of trade wind showers and
convective development across the coastal waters and parts of the
islands today. TUTT low and associated trough across the Lesser
Antilles will lift northwards across the region over the weekend.
Broad high pressure ridge across the central Atlantic will hold and
allow better transport of trade wind moisture across the forecast
areas overnight and through the weekend.
Short term...rest of today through Sunday...
recent satellite imagery and sal products all suggests the Saharan
dust has diminished or moved away from the region. The weakening
of the mid to upper level ridge across the region has also allow
for better moisture transport in the easterlies and for afternoon
convective development especially over Puerto Rico. Expect activity
over land areas to diminish after sunset. However late evening and
overnight passing showers will remain possible over portions of the
coastal waters, with some brushing the east coastal sections some
of the the islands by early morning.
On Saturday through Sunday the gradual increase in the trade wind
moisture will allow for showers and isolated thunderstorm development
each afternoon. The activity however should be focused mainly over
parts of the central interior and west sections of Puerto Rico. Isolated
streamer like convection will remain possible over the east interior
and parts of the San Juan Metro each day.
The TUTT is expected to lift north and east of the region through Sunday
while gradually weakening. Conditions aloft should therefore be less
favorable for significant convective development. However, isolated
to scattered afternoon showers will remain likely mainly over western
PR with lesser activity over the adjacent islands and the USVI.
Long term...Monday through Saturday...
TUTT low will linger north and east of the region, while a short wave
trough will move eastward across the west and southwest Atlantic. The
trough is forecast to become amplified just north and west of the region
Monday through Tuesday of next week. The deepening of this mid to upper
level trough along with the approach of a weak tropical wave will increase
the chance for showers and thunderstorms across the forecast area especially
during the early part of next week. So far models all in agreement on
the increase in precipitable water to near two inches across the area
through Tuesday of next week. This should result in increased potential
for convective development and instability at least until the middle
of next week.
For the rest of the week another surge of Saharan dust is expected
to accompany or trail the aforementioned wave. This should allow
for a gradual depletion of the trade wind moisture transport and
thus lesser chance for widespread convection. However, lingering
moisture across the islands as well as the proximity of the upper
trough will allow for afternoon convection each day mainly over
parts of the interior and west sections of Puerto Rico. By then
only isolated showers are forecast elsewhere.
Aviation...mtn top obsc observed over the Cordillera Central of
PR with rain showers/thunderstorms and rain resulting in scattered-broken ceiling btwn fl020-fl080
at tjbq/tjmz and possibly at tjsj til 26/23z. Tempo included for
tjmz due to this activity. Elsewhere isold rain showers psbl at all other
taf sites til 26/23z. Easterly winds 10-15 knots with higher gusts,
bcmg calm to light and variable aft 26/23z. VFR conds durg rmdr
Marine...mariners can expect seas of 5 feet or less and winds 15
knots or less overnight, but increasing to 15 to 20 knots over
some of the local waters on Saturday. There is a moderate risk of
rip currents for the northwest to northeast beaches of Puerto Rico,
as well as some of the north facing beaches of Culebra and Saint
Thomas and Cramer Park in Saint Croix overnight through Saturday
Preliminary point temps/pops...
sju 78 90 77 89 / 40 40 20 30
stt 78 88 77 89 / 40 40 30 30