Area Forecast Discussion
Issued by NWS San Juan, PR
FXCA62 TJSJ 260135
Area Forecast Discussion
National Weather Service San Juan PR
935 PM AST Sat Mar 25 2017
.UPDATE...Weakening mid to upper level trough across the western
Atlantic and into the central and eastern Caribbean will continue
to enhance shower and thunder activity across the forecast area.
Shower and thunder activity overnight should be focused across
the local waters with a few of them probably brushing the eastern
coastal areas of PR as well as USVI. On Sunday afternoon with
plenty of sunshine, locally induced showers and thunderstorms are
expected over and north of the Cordillera Central including the
San Juan Metro Area.
.AVIATION...Brief periods of MVFR conds will remain possible at
the Leeward and USVI terminal overnight. SHRA/TSRA expected aft
26/16z in and around JMZ/JBQ and JSJ. Light and variable winds
are expected overnight.
.MARINE UPDATE...Although seas will subside overnight and into
Sunday, rough conditions will continue to prevail across the
local waters and all passages through early next week. This will
continue to result in dangerous breaking waves and hazardous surf
conditions. A High Surf Advisory is in effect through 6 AM AST
Sunday with a High Rip Current Risk through Sunday afternoon.
Latest observations across the nearshore waters continue to
suggest seas 8 to 10 feet at around 12 seconds.
.PREV DISCUSSION... /issued 339 PM AST Sat Mar 25 2017/
SYNOPSIS...The deep moisture and trough in the upper levels will
maintain cloudy skies across the local area with showers over the
local waters. SFC low pressure, north of Hispaniola will move
north and east slowly for the next few days, causing some clearing
SHORT TERM...Today through Tuesday...
The area of shower activity had moved to the east of Puerto Rico by
1:30 PM AST. Clouds were also thinning over the western interior
leaving a chance for thunderstorms to grow over the area still.
Showers or light rain will continue to affect the U.S. Virgin
Low pressure at the surface and in the lower levels less
than two hundred miles north of Punta Cana is now the primary driver
of the moist flow from the south across Puerto Rico and the U.S.
Virgin Islands. The axis of that moisture is presently across
Vieques and Culebra, and will move eastward to the Leeward islands
by Monday as the low moves to about 500 miles north of Puerto Rico.
During the same periods precipitable water will drop from 1.93 to
around 1.5 inches. As a result available moisture and instability
will decrease and shower activity and cloud cover will decrease.
Because the low will intensify as it moves north, the pressure
gradient over the local area will drop to nearly zero in a col. This
will allow sea breezes to form during the day and keep temperatures
from reaching 90 degrees over the greater Metropolitan area due to
the south winds that we have had under cloudy skies at present. The
MEX MOS is handling this well and it would explain why it is
forecasting only 82 degrees for a high on Wednesday with much
less moisture after forecasting 86 degrees today. The U.S. Virgin
islands will also see less moisture but it will be delayed about 24
hours, and the more significant drying will be delayed until
Tuesday. A few thunderstorms may still be possible during the next
several days--especially in the interior of Puerto Rico with more
sunshine on Monday.
LONG TERM...The long term guidance continues to suggest that
drier air will filter in on Wednesday and should remain fairly dry
for the rest of the workweek with winds becoming northerly on
Wednesday and Thursday then shifting to an easterly wind on Friday
an for the upcoming weekend. This general pattern should cause
mostly fair weather with isolated shower activity. Temperatures
late this upcoming week are expected to be pleasant in the low 80s
across the lower elevations with also lower dew point
temperatures, at this time the long term guidance suggest dew point
temperatures in the mid 60s across the San Juan metro and the
USVI, which is at 5-7 degrees than what we have been observing
these past few days.
AVIATION...Some clrg is occurring wrn PR leaving isold -TSRA
psbl til 25/22z nr TJMZ/TJBQ. Mtn obscurations will persist thru
at least 26/02z. SHRA is migrating to the NE across TIST/TISX,
but no MVFR conds yet. A band of mstr will apch TNCM/TKPK from
the SSE arnd 25/22z and may generate areas of MVFR conds for CIGS
BLO 025 til 26/06z. Thereafter expect VFR. SFC winds, SE-S 5 to
15 kt with local sea breeze effects along the north coast of PR
MARINE...Small craft advisory was extended for the local
offshore Atlantic waters when taken into consideration the latest
guidance and observations. The models have been under estimating
the northerly swell and the local buoys are reporting seas of 9-11
feet across the nearshore waters to the north and northwest of
PR. Because of these wave heights and the current period of 10-12
seconds, a high surf advisory was issued and it still remains in
effect until 6 AM AST Sunday. There is a high risk of rip currents
across the north, northeast and western coasts of PR as well as
many beaches in Vieques, Culebra, and Saint Thomas.
.PRELIMINARY POINT TEMPS/POPS...
SJU 73 82 74 84 / 20 50 40 50
STT 72 82 73 83 / 50 50 50 50
High Rip Current Risk through Sunday afternoon for Culebra-
Mayaguez and Vicinity-North Central-Northeast-Northwest-San Juan
and Vicinity-Southeast-Southwest-Western Interior.
High Surf Advisory until 6 AM AST Sunday for Culebra-Mayaguez and
Vicinity-North Central-Northeast-Northwest-San Juan and Vicinity-
High Rip Current Risk until 6 AM AST Sunday for Vieques.
High Rip Current Risk through Sunday afternoon for
St.Thomas...St. John...and Adjacent Islands.
High Surf Advisory until 6 AM AST Sunday for St.Thomas...St.
John...and Adjacent Islands.
Small Craft Advisory until 2 PM AST Sunday for Anegada Passage
Southward 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-Mona Passage
Southward to 17N.
Small Craft Advisory until 2 AM AST Sunday for Caribbean Waters
of Puerto Rico from 10 NM to 17N-Coastal Waters OF Southwestern
Puerto Rico out 10 NM-Coastal Waters of Southern USVI Vieques and
Eastern Puerto Rico out 10 NM.
Small Craft Advisory until 8 PM AST Sunday for Atlantic Waters of
Puerto Rico and USVI from 10 NM to 19.5N.