Scientific Forecaster Discussion
fxca62 tjsj 201500
Area forecast discussion
National Weather Service San Juan PR
1100 am AST Tue Feb 20 2018
Update...forecast on track, breezy to windy conditions to
continue across the area. A generally fair and stable weather
pattern will continue to prevail with some passing showers
possible across east Puerto Rico as well Vieques, Culebra, and
the USVI. Moisture advection is expected on Wednesday to increase
the chance of shower activity across the area.
Aviation...VFR conds are expected to prevail through the
forecast cycle with brief -shra/rain showers still possible at the
leeward and USVI terminals as well as jsj/jbq. East-northeast winds
15-20 knots with higher gusts, becoming at around 10-12
Marine...surface high pressure north of the region will produce
hazardous seas across the local waters and all passages much of
this week due to fresh to strong east northeast trade winds it is
generating. Expect seas 8 to 10 feet with occasional seas up to
11 feet across the offshore waters today building to 12 feet by
Previous discussion... /issued 517 am AST Tue Feb 20 2018/
Synopsis...high pressure in the western Atlantic will move
southwest while weakening during the week. This will produce fresh
to strong winds that will slowly decrease after mid week. Areas of
moisture will move through bringing episodes of showers to the
smaller islands and windward sides of Puerto Rico. Little change
is expected in the weather pattern for the next 5 to 7 days.
Short term...today through Thursday...
strong surface high pressure over the west Atlantic will continue to
sink into the SW North Atlantic. Fresh to locally strong east to
northeast trade winds will prevail across the region as the
pressure gradient tightens. Trade wind showers will continue to
move at times from the Atlantic waters into the region, affecting
mainly the northern USVI and the northern and eastern sections of
PR. On Wednesday, the remnants of an old front and windy
conditions will increase the frequency of showers across the
islands. Drier air will fill in briefly on Thursday morning but
shower activity will increase once again during the evening hours
as an upper level short wave trof moves just to the east-
northeast of the region.
A Wind Advisory was issued for the USVI and Vieques due to expected
sustained winds around 25 mph. Breezy conditions are expected across
the rest of the region with gusts in the low to mid 30 mph. As winds
are forecast to gradually increase and peak on Wednesday another
Wind Advisory will be required for at least the northern and eastern
municipalities of PR.
Long term...Friday through Wednesday...
fading high pressure in the western Atlantic will continue to
drive moderate to fresh east northeast trade winds across the
area. Patches of moisture will form in the low level flow and
become progressively wetter with strong fluctuations in moisture
through the period generating increasing showers that will remain
brief and shallow but become more frequent and affect more of the
island of Puerto Rico during the period. An upper level jet
around a high building over the western tropical Atlantic will
grow over the area and translate to the northwest maintaining
subsidence at mid and upper levels through the period. Friday
should be the coolest day of the period with the GFS 1000-850 mb
thicknesses increasing afterward. Otherwise little change in the
overall pattern will be noted.
Aviation...mainly VFR conditions are expected to prevail through
forecast period. However, rain showers will continue to move at times early
this morning across tjsj/tist creating possible MVFR cigs. Rain showers
expected to increase once again after 21/00z across the northern PR
terminals and tist. East-northeast winds will prevail at 15-25 knots
with higher gusts after 20/14z. Sea breeze variations are possible
across the southwestern quadrant of PR. Winds back from NE to northwest
and increase from 25 to 40 knots btwn fl170-350.
Marine...winds will be raising seas and short period swell during
the next 48 hours. Only sheltered areas on the western side of
Puerto Rico will escape small craft advisories and small craft
should still exercise caution there for winds. Winds will slowly
subside later in the week, but seas are expected to remain
hazardous in all but the waters least exposed to northeast swell and
seas through the weekend.
Preliminary point temps/pops...
sju 81 75 82 73 / 60 60 60 60
stt 85 73 84 72 / 60 60 60 60
PR...high rip current risk through late tonight for central interior-
Culebra-Mayaguez and vicinity-north Central-Northeast-
northwest-San Juan and vicinity-southeast-Vieques-western
Small Craft Advisory from 8 am Wednesday to 8 PM AST Friday for
Wind Advisory until 6 PM AST this evening for Vieques.
Vi...high rip current risk through late tonight for St Croix.
Wind Advisory until 6 PM AST this evening for St Croix-
St... John...and adjacent islands.
Small Craft Advisory until 6 PM AST Saturday for St...
John...and adjacent islands.
Am...high rip current risk through late tonight for coastal waters of
southern Puerto Rico out 10 nm.
Wind Advisory until 6 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 10 nm.
Small Craft Advisory until 8 PM AST Friday for coastal waters of
southern Puerto Rico out 10 nm.
Small Craft Advisory until 6 PM AST Saturday for Anegada Passage
southward to 17n-Atlantic waters of Puerto Rico and USVI
from 10 nm to 19.5n-Caribbean waters of Puerto Rico from 10
nm to 17n-coastal waters of northwestern Puerto Rico out 10
nm-coastal waters of northern Puerto Rico out 10 nm-coastal
waters of northern USVI and Culebra out 10 nm-coastal
waters of southern USVI Vieques and eastern Puerto Rico out
10 nm-Mona Passage southward to 17n.