Area Forecast Discussion
National Weather Service San Juan PR
454 AM AST Thu Apr 18 2019
.SYNOPSIS...A fair weather pattern is expected to continue into
the upcoming weekend as a ridge aloft holds overhead. Strong
surface high pressure is expected to build across the north-
central Atlantic and tighten the local pressure gradient,
resulting in moderate to fresh easterly trade winds across the
region through Sunday. Ridge aloft is expected to erode completely
by early next week as a mid to upper-level trough amplifies
across the western and central Atlantic.
.SHORT TERM...Today through Saturday...
High pressure is building in the western Atlantic north of Puerto
Rico and will move east southeast during the period. It is forecast
to become a 1036 mb high by Saturday (20/18Z). This high will
gradually increase the easterly trade wind flow over the area
through the period. Low pressure will cut off from a trough in
the mid Atlantic today near 40 degrees north 39 degrees west and
migrate southward during the period. The low will adopt a front
associated with the main low and under pressure from the high in
the north its tail will approach Puerto Rico and the U.S. Virgin
Islands, such that by late Saturday it will be about 350 miles
north northeast of San Juan. Patchy moisture will continue to
yield scattered light showers across the area in the usual diurnal
pattern of night and early morning showers in the eastern portion
of Puerto Rico and mostly around the U.S. Virgin Islands over the
local waters, followed by moderate afternoon showers in western
and interior Puerto Rico.
At mid levels, high pressure between the Bahama Islands and eastern
Cuba, will drift to Hispaniola by Friday night and join with a ridge
to the north north east of Hispaniola. On Friday a low will spawn on
the eastern side of the ridge and drop south to east of the Leeward
Islands by Saturday as a reflection of the upper level low mentioned
below. The combination of the ridge and the developing low will
maintain light northerly flow over the local forecast area at mid
levels, but also maintain very low mid level relative humidity.
At upper levels, northwesterly flow will continue on the back side
of the upper level trough now near 55 west northeast of the local
area. Low pressure cuts off in the east central Atlantic on Friday
and another low develops on the southwest side of this low on
Saturday about 600 miles northeast of Saint John. These will both
begin to affect the area in the long term period.
Convection is capped just above 700 mb for the entire period, but
not as strongly as in previous days. This will allow convection to
grow a few thousand feet more than in the last few days, but the dry
mid levels and lack of forcing at upper levels will generally
suppress any shower activity beyond the shallow passing showers of
late that are moved by quickly by the increasing trade wind flow.
.LONG TERM...Sunday through Friday...
Current indications are that the remnants of the frontal
boundary, associated with the surface and 700 mb lows that will be
located northeast of the region, will move over the area on
Monday. This feature would increase the low-level moisture and
therefore, the shower activity. Then, as the mid to upper-level
trough amplifies across the western and central Atlantic, a strong
jet between 80 to 100 knots at 250 mb is expected to develop just
north of the area and remain near the area through the end of the
week. Meanwhile, as the surface and 700 mb low meander northeast
of the region throughout the period, the low-level flow will be
from the northeast. These lows will also serve to advect patches
of low-level moisture into the region from time to time from the
northeast. Even though the upper-level forcing may turn more
favorable as a result of the strong jet present aloft, the main
limiting factor for any significant rainfall activity will
continue to be the very dry air that will still be present in the
mid to upper-levels throughout the period. Nevertheless,
sufficient low-level moisture will still be available to generate
isolated to scattered showers across the northern coast of PR and
the northern USVI during the overnight and morning hours followed
by showers across interior and southern portions of PR during the
afternoon hours due to daytime heating and local effects.
.AVIATION...Iso-sct SHRA cont across the waters surrounding the
north and south coasts of PR with few arriving over land. Sfc winds
are E and will incr to 10-22 kt aft 17/14Z with gusts to 27kt along
the N and S coasts of PR. Aft 18/17Z SHRA are to dvlp downstream
from TISX, El Yunque and over wrn PR with lcl MVFR conds nr TJMZ and
mtn obscurations. Tops mostly blo FL150. Conds imprvg aft 18/22Z.
Max winds WNW 50-60 kt btwn FL360-480.
.MARINE...Seas between 3 and 5 feet will continue to prevail
across the regional waters through this afternoon. Winds will be
out of the east up to 15 knots across most areas. However, across
the nearshore Atlantic and Caribbean waters, winds up to 20 knots
are possible due to local effects. Therefore, small craft
operators should exercise caution across these waters. Seas are
expected to increase to up to 6 feet by tonight as the winds will
increase and will remain choppy into the upcoming weekend. In
addition, a small northerly swell will arrive by tonight and
affect mainly the Atlantic waters.
For beachgoers, there is a moderate risk of rip currents for the
beaches located across the coastline of northern, southeast, and
southwest PR as well as across the north and east facing beaches
of Vieques, Culebra, and Saint Croix.
.PRELIMINARY POINT TEMPS/POPS...
SJU 87 75 87 75 / 30 50 40 40
STT 86 75 87 76 / 20 30 40 40