Scientific Forecaster Discussion
fxca62 tjsj 231934
Area forecast discussion
National Weather Service San Juan PR
334 PM AST Thu Mar 23 2017
Synopsis...deep moisture, a shear line to the north of the local
area, and an upper level jet is causing persistent showers across
the local islands. It is expected to be rainy over the general
forecast area with isolated thunderstorms over the waters
expected for the next few days.
Short term...Thursday afternoon through Sunday...a shear line
extends off of a front connected to low pressure in the north
central Atlantic. Strong high pressure around 1036 mb is moving into
the western Atlantic off the coast of Virginia and will cut off the
support for the shearline so that the trough will gradually shift
west but will also allow low pressure to slowly form north of
Hispaniola Friday and Saturday to maintain south flow at lower
levels. In the meantime, an upper level trough is deepening over
Cuba with its associated jet pulling strongly over Hispaniola from
the southwest. As the trough pulls northeast the right entrance
will bring upper level divergence over the area to enhance
convection and isolated to scattered thunderstorms. The tail of the
jet will sweep across Puerto Rico Saturday night and the U.S. Virgin
Islands on Sunday.
Moisture will peak on Saturday afternoon, according to the GFS, and
decline thereafter. This combination of moist flow and upper level
dynamics will enhance the possibility of urban and small stream
flooding, beginning on Friday and increasing on Saturday, and,
should another line of convection form over the U.S. Virgin Islands,
prolonged heavy rain from training cells could ensue there as well.
Long term...the deep moisture will remain over the local area in
the long term as well, the surface low pressure develops north of the
Bahamas, which will keep pulling moisture across our area from the
south for the upcoming week. However, the latest model guidance
indicates that the bulk of the moisture will affect eastern PR
and the USVI mostly. Keep in mind that this is the long term
forecast and that confidence is generally low, in this case
particularly on the exact area where the deepest moisture will be.
Overall, it looks like we will have a rather rainy pattern for the
local islands over the next several days.
Aviation...sct rain showers and brief MVFR cigs streaming across Guadeloupe
and Antigua will encroach upon tkpk and tncm btwn 23/18-24/00z with
clrg and VFR conds expected by 24/08z. Incrsg mstr and rain showers expected
aft 24/08z in tist/tisx will also contain brief MVFR ceilings. Areas of
mtn obscurations will cont now thru at least 24/22z with pds of MVFR
cigs and LCL MVFR-IFR vsbys in +shra. Sfc winds southeast 8 to 14 kts bcmg
SW from fl100-150 10 to 20 kts. Max winds west-southwest 90 kts at fl440, but
reach 48 kt by fl220.
Marine...seas generally of 3-6 feet are expected for the next
few days. Small craft should exercise caution across the Atlantic
waters and local passages. High risk of rip currents across the
north and northwest coast of PR as well as Culebra and the northwest
coast of Saint Thomas. Moderate risk across most of the rest of
the local beaches.
Preliminary point temps/pops...
sju 74 84 75 84 / 50 70 50 60
stt 73 82 73 82 / 60 60 60 60
PR...high rip current risk until 8 PM AST this evening for Culebra-
Mayaguez and vicinity-north Central-Northeast-northwest-San
Juan and vicinity-southeast-western interior.
Vi...high rip current risk until 8 PM AST this evening for
St... John...and adjacent islands.
Am...high rip current risk until 8 PM AST this evening for coastal
waters of northern USVI and Culebra out 10 nm-coastal
waters of southern USVI Vieques and eastern Puerto Rico out