Area Forecast Discussion
Issued by NWS San Juan, PR
FXCA62 TJSJ 280146
Area Forecast Discussion
National Weather Service San Juan PR
946 PM AST Thu Oct 27 2016
.UPDATE...Except for a heavy shower moving onshore between Ceiba
and Fajardo all of the showers in the area are over the local
waters, with the heaviest within 30 nm of Saint Thomas/Saint John.
Showers have been moving northwest since the wave passage earlier
today. The showers north of Barceloneta are now dissipating. The
28/00z sounding showed a modestly unstable atmosphere with
precipitable water of 1.93 inches and a lifted index of around
minus 5. But the inhibiting layer extends from the surface to
about 2300 feet so showers over land are not forming. Also the
500 mb layer showed relative humidities of 1 percent so showers
will be greatly hindered for now above that layer. Nevertheless
local islands will see isolated showers in the band of activity
behind the wave now over the Dominican Republic.
.DISCUSSION...Activity over the Leeward Islands and extending
northwest are still on track to move into the U.S. Virgin Islands
and Puerto Rico early Saturday morning between 29/06 and 29/12z,
ahead of another trough. Winds will become even more southeasterly
after Saturday setting up the trend for southerly flow and the rainy
pattern mentioned below.
.AVIATION...VFR conds expected through 28/16Z across the local
terminals. VCSH expected for TISX/TIST and TNCM/TKPK overnight.
Winds will be mainly light and variable overnight for the PR and
USVI terminals but TNCM and TKPK are expected to observe E-ESE
winds at around 5-8 KT. Winds are expected to increase after
28/13Z from the E-ESE at around 10KT with sea breeze variations.
Afternoon convection across PR could cause at least VCTS for TJSJ,
TJMZ, and TJBQ after 28/16Z and mtn obscurations over most of PR.
.MARINE...Seas will generally begin to subside over the area with
small craft advisories not expected before next Thursday if then.
.PREV DISCUSSION... /issued 143 PM AST Thu Oct 27 2016/
SYNOPSIS...A seasonable weather pattern will prevail Friday and
into the upcoming weekend with locally induced showers and Tstorms
under a mid to upper level ridge pattern. Rainy conditions possible
next week as the aforementioned ridge pattern weakens and a trough
develops north of the area. Meanwhile, tropical wave will continue
to move away from the local islands tonight.
DISCUSSION...As a tropical wave moves away from the forecast
area this evening and tonight, the chance of showers and isolated
Tstorms will decrease. However, continue to expect shower and
thunder development over and west of the Cordillera Central through
the afternoon hours followed by a few passing showers across
windward areas overnight.
Friday through the upcoming weekend, a seasonable weather pattern
is expected to prevail with locally induced afternoon showers and
thunderstorms each day. Showers and Tstorms will be focused across
central and northwest PR as well as portions of the San Juan Metro
Area under a gentle southeast wind flow. Although widespread
convection is not expected at this time, urban and small stream
flooding still possible Friday and the upcoming weekend due to
already saturated soils and slow moving showers/Tstorms.
The ridge aloft will then erode next week as a broad upper level
trough develops across the tropical Atlantic and into the Caribbean
basin. This will result in deep tropical moisture pooling across
the forecast area Tuesday-Saturday with a rainy weather pattern
AVIATION...Afternoon SHRA/TSRA are possible till 27/22z across
the interior and West Half of PR. This activity could produce
brief MVFR conds at TJPS/TJMZ/TJBQ. Elsewhere frequent VCSH are
possible. Easterly winds at 10kt during the afternoon and evening,
becoming light and variable overnight.
MARINE...A moderate swell NNE of 4-6 ft at 10 sec will continue
to move across the local Atlantic Waters and Caribbean Passage
over the next few days. Seas will subside somewhat during the
weekend and early next week. Seas are expected to remain below
advisory levels through the forecast period. However...Small
craft operators should exercise caution through Friday due to
seas up to 6 feet across the offshore Atlantic Waters.
.PRELIMINARY POINT TEMPS/POPS...
SJU 79 90 78 89 / 20 20 40 40
STT 80 88 79 87 / 30 30 40 40