Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Juan PR
529 am AST Friday Mar 6 2015
Synopsis...at upper levels...a sharp trough just south of Puerto
Rico and the U.S. Virgin Islands that extends from a strong TUTT
in the western tropical Atlantic will form a weak cut-off low
over the weekend as it retreats to the east. High pressure over
the central Caribbean will then ridge over the area during the
rest of next week.
At middle levels...high pressure will dominate the western Atlantic
just north of the local area for the rest of this week and most of
next. A strong low in the central Atlantic will shift southwest
over the weekend and then weaken in the western tropical Atlantic
over the upcoming week. Both middle and upper levels are very dry
At lower levels...high pressure extending out of the northeastern
central Atlantic will weaken before a low out of the eastern
United States moves into its place later next week. Although trade
winds will weaken slightly over the weekend...stronger winds will
re-develop early next week and continue through Thursday. Patches
of relatively shallow moisture will be carried through the area
for quick passing showers having minor amounts of rainfall. Best
moisture is expected on Monday.
Discussion...light trade wind showers moving rapidly through the
local waters brought up to one or two tenths of an inch to the
waters just north of Puerto Rico and isolated amounts of less than
five hundredths to Caribbean waters. The GFS model shows better
patches of moisture moving through on Saturday morning and much of
the day Sunday and Monday. Most of the showers will be on the
windward slopes of Puerto Rico. Models suggest that showers will
decrease after Monday...but passing showers will continue. Trade
winds are expected to weaken Saturday and then gradually increase
through Wednesday...with winds at 850 mb reaching around 33 knots
by Wednesday. At the surface this will translate into 10 to 20 miles per hour
winds with gusts up to 35 miles per hour.
Middle and upper levels remain quite dry through Friday of next week
with relative humidities at most levels below 10 percent.
Nevertheless...persistent patches of low level moisture will bring
scattered shallow showers to the U.S. Virgin Islands at least
through Tuesday...and likely through much of next week. Some
showers may also develop in the west depending of the exact flow
pattern and convergence during the afternoons...but this is
expected to be more limited. High pressure at middle and upper levels
will serve to limit convection and rainfall amounts...but windward
slopes will see some showers each day.
Aviation...VFR conditions are expected across all terminals for the
next 24 hours. Passing -shra/rain showers embedded on trade winds will
create mainly vcsh at tjsj/tist/tncm/tkpk/tjbq/tisx through the morning
hours. Then...after 06/18z clouds will form over the interior and
western PR. Winds at less than 10 knots at PR/USVI...and around 15
kts at tncm/tkpk with higher gusts...increasing after 06/13z to 15-25
kts with higher gusts in the vicinity of rain showers.
Marine...seas in protected areas should begin to come down now
through Sunday...though small craft will have to exercise caution
even after some small craft advisories expire over the weekend.
Most Small Craft Advisory conditions will re-emerge on Monday as
winds increase and seas roughen again.
Preliminary point temps/pops...
sju 86 74 86 74 / 40 40 40 40
stt 85 74 85 74 / 30 20 30 40
PR...Fire Weather Watch from 10 am AST this morning through Saturday
afternoon for south coastal plains and Mona island.
Coastal waters...Small Craft Advisory until 6 am AST Tuesday for Anegada Passage
southward to 17n-Atlantic waters of Puerto Rico and USVI from
10nm to 19.5n-Caribbean waters of Puerto Rico from 10 nm to
17n-coastal waters of southern USVI Vieques and eastern
Puerto Rico out 10 nm-Mona Passage southward to 17n.
Small Craft Advisory until midnight AST tonight for coastal
waters of southern Puerto Rico out 10 nm.
Small Craft Advisory until 6 am AST Saturday for coastal waters
of northwestern Puerto Rico out 10 nm.
Small Craft Advisory until noon AST Saturday for coastal waters
of northern Puerto Rico out 10 nm.
Small Craft Advisory until noon AST Sunday for coastal waters of
northern USVI and Culebra out 10 nm.