Area Forecast Discussion
Issued by NWS San Juan, PR
FXCA62 TJSJ 290056
Area Forecast Discussion
National Weather Service San Juan PR
856 PM AST Fri Apr 28 2017
.UPDATE...Several Urban and Small Stream Flooding were issued for
numerous municipalities along the northern, central and western
sections of Puerto Rico late in the afternoon and early this
evening as showers and thunderstorms persisted across the region.
Some showers will continue to affect the eastern half of Puerto
Rico as well as Vieques, Culebra and the U.S. Virgin Islands
overnight. For Friday and Saturday a similar weather pattern is
expected with showers in the morning across windward areas
followed by afternoon showers and thunderstorms across western and
interior sections of Puerto Rico, however intensity and coverage
will increase as a trough establishes north of the area.
.AVIATION...SHRA will move westward across the U.S. Virgin
Islands and eastern Puerto Rico overnight. This activity will
affect mostly TISX and TJSJ with brief periods of SHRA until
29/06Z. VCSH expected elsewhere across the local flying area.
Expect mtn obscurations and areas of MVFR/IFR in SHRA/TSRA after
29/16Z. Latest TJSJ sounding indicated an easterly wind flow up to
20 knots from the SFC to around 14k feet, becoming light and
variable from 14-25k feet and then westerly and stronger aloft.
.MARINE...Seas remained choppy at 3 to 6 feet. Seas will continue
to increase slightly up to possible 7 feet by Sunday, diminishing .
Monday night and thru the rest of the week over the Atlantic.
Winds will continue at 15 to 20 knots with occasionally higher
gusts. A moderate risk of rip currents expected across most of the
.PREV DISCUSSION... /issued 614 PM AST Fri Apr 28 2017/
SYNOPSIS...The approaching and development of a Tutt and area of low
pressure north of the region will continue to increase the chance for
showers and thunderstorm development each day across the region through
the weekend. A more seasonable weather pattern will prevail by the latter
part of next week. However a moist southeasterly wind flow should return
by the following weekend.
SHORT TERM...Tonight through Monday...
Clouds and showers are becoming more numerous toward the east where
precipitable water values exceed 2 inches across the Windward Islands.
This moisture is expected to continue moving west across the Caribbean
Sea and will raise moisture levels here today through Sunday.
Moisture diminishes somewhat to around 1.7 inches on Monday although
this is still considered reasonably moist.
Upper levels will play a much larger role in the development of
convection beginning Saturday. An upper level low 640 miles north of
Samana, Dominican Republic at 28/18z will drop south to within 270
miles of Arecibo, Puerto Rico by 30/18Z. On Saturday night an upper
level jet on the upstream side of the trough south of the low will
develop 70 knot winds and create strong divergence in its left exit
which will move over Puerto Rico on Sunday. The trough passage is
expected to enhance convection and areas of urban and small stream
flooding on Saturday and the left exit region at upper levels will
also further enhance convection on Sunday when urban and small
stream flooding will likely increase. Although greatly diminished,
upper level conditions will still remain favorable to any convection
that forms on Monday and although moisture will be reduced, some weak
convergence at 700 mb in an east-west line across Puerto Rico
will still be present to favor the formation of showers and thunderstorms.
LONG TERM...Tuesday through Saturday...
TUTT will linger just north of the forecast area through the
middle of next week. As this feature gradually weakens by Wednesday
and into early Thursday, a mid to upper level ridge is forecast to
to build and spread westward across the forecast area. The ridge
aloft is then expected to hold through at least the following weekend.
At lower levels, a high pressure across the north central Atlantic
will shift south and eastward across the Atlantic Wednesday through
Thursday. The moderate easterly trade winds will therefore continue
to transport tropical moisture from the south and east across the
region beginning Friday and continuing through Saturday under the
prevailing east to southeast wind flow.
During the early part of next week, due to the proximity of the
Tutt and good moisture advection, still expect periods of showers
and thunderstorms across E PR and USVI during the evening and
early morning hours. This will be followed by periods of enhanced
convection with showers and thunderstorms across portions of central
and west PR during the late morning and afternoon hours. Isolated
to scattered convection can also be expected in and around parts
of the San Juan metro area. As the ridge builds aloft, expect a
gradual decrease in moisture advection Thursday of next week.
However, expect a return of a moist southeasterly wind flow by
Friday therefore increasing the chance for shower activity across
the region once again through Saturday.
AVIATION...SHRA now dvlpg over wrn PR and isolated SHRA ovr nrn
coastal plains including VC TJMZ/TJSJ. Expect mtn obscurations and
areas of MVFR/IFR in SHRA/TSRA aft 28/16Z. Tops to FL400 psbl.
LLVL winds E-ESE 10-20 kt up thru FL125. Expect sea/land variations
at the sfc. SHRA will continue byd 29/02Z as moisture increases
especially in ern PR and arnd the USVI. Maximum winds WNW 45 knots
at FL410 at 29/06Z.
MARINE...Seas remained choppy at 3 to 6 feet. Seas will continue
to increase to 6 to 7 feet by Saturday night. Seas diminish after
Monday night and thru the rest of the week over the Atlantic and
after Sunday in the Caribbean where maximum seas should remain below
7.0 feet. winds will continue at 15 to 20 knots with occasionally
.PRELIMINARY POINT TEMPS/POPS...
SJU 76 83 75 83 / 50 50 50 50
STT 75 83 74 83 / 50 50 50 50