Area Forecast Discussion
National Weather Service San Juan PR
510 AM AST Tue Feb 11 2020
.SYNOPSIS...
Fair weather conditions prevailed over Puerto Rico during the
overnight hours with some passing showers observed across the
local waters and portions of interior Puerto Rico. For tomorrow,
some passing showers can reach the eastern coast of Puerto Rico
and some sections of the U.S. Virgin Islands in the morning hours.
Then, in the afternoon hours scattered to numerous showers are
expected with some isolated thunderstorms over the western,
northwestern, interior, northeastern Puerto Rico, as well Culebra.
&&
...SHORT TERM...Tuesday through Thursday...
The dominant surface high pressure spread across the central and
northeastern Atlantic, will maintain its hold across the region.
This will promote moderate to strong trade winds across the forecast
area through Thursday. This pattern will aid in transporting
frequent patches of moisture with embedded showers across the
islands and regional waters. A short Wave trough now crossing the
west atlantic and moving north of the region today will continue to
erode the ridge aloft, and at the same time reinforce and amplify a
TUTT low and associated trough which will develop and sink
Southwards across the region today through Wednesday.
The fairly strong low level winds will continue to bring quick
surges of shallow moisture across the forecast area, resulting in
periods of moderate to locally heavy rainfall over portions of the
local waters, and along parts of the north and eastern coastal areas
of the islands especially during the early morning hours.
Thereafter, afternoon convection should will be focused mainly over
the central and west sections of Puerto Rico. Due to the strong
steering winds, expect afternoon showers to be fast moving and of
short duration. However due to the proximity of the short wave
trough and the increasing instability aloft, some of the afternoon
convection will be enhanced also aided by daytime heating and local
effects. Periods of moderate to locally heavy rains will be possible
but should be mainly over the west sections of Puerto rico. For the
U.S. Virgin Islands, quick passing morning showers in and around the
islands, will be followed by isolated afternoon convection with only
a slight chance of an isolated thunderstorms forming mainly downwind
of the islands or just offshore. The remainder of the surrounding
islands will enjoy a mostly sunny skies, but an isolated afternoon
shower cannot be ruled out today.
Model guidance suggests increasing instability aloft over the next
couple of days with continued shallow moisture advective pattern
through Wednesday. The increasing instability aloft due to the
amplification of the aforementioned Tutt low across the northeastern
Caribbean, along with good low level moisture transport and speed
convergence, should all support afternoon convection over parts of
the islands. However expect most of the afternoon shower development
to be in the west sections of Puerto Rico, as well as around San
Juan metro in the form of streamers due to the strong steering wind
flow. In addition, the proximity of the Tutt, combined with the
colder mid level temperatures, will provide good instability aloft
and support low level moisture convergence. This will provide a
better chance for enhanced afternoon convection. At this time, do
not anticipate widespread activity, as most of the convection will
be limited and isolated due to the fairly dry conditions aloft. The
formation of isolated thunderstorms in and around the islands will
however remain possible through Wednesday. Therefore, minor urban
and small stream flooding as well as ponding of water on roadways
and poor drainage areas will be possible in isolated areas.
Rapidly improving conditions is forecast for Thursday, as the ridge
will again reestablish and build aloft, leading to stable conditions
and the strengthening of the trade wind cap inversion resulting in
the erosion of low level moisture transport. Passing showers during
the early morning hours will remain possible but less frequent. Any
afternoon convection should be limited to mainly the west interior
sections of Puerto Rico. Mostly fair and sunny weather conditions
expected elsewhere including around the U.S.Virgin Islands on
Thursday.
.LONG TERM...Friday through Wednesday...
A surface high pressure located at western Atlantic will continue to
promote easterly winds across the local region through the long-
term period. The precipitable water values from the GFS guidance
suggest a dry air mass across the region and this can limited the
shower activity across the islands. However, afternoon convection
still possible due to some fragments of moisture embedded in the
trade winds and the local effects. Afternoon convection should be
focused over the central and west interior Sections of puerto
Rico. At mid and upper levels, a Tropical Upper Tropospheric
Trough (TUTT) is located at the east of Puerto Rico. This feature
at the upper levels can enhanced the instability over the region
and shower activity is possible in the afternoon hours across the
local islands for Thursday through Friday. If the model guidance
remain consistent, exist a chance for isolated thunderstorms in
the afternoon hours across Puerto Rico and the U.S. Virgin Islands
as well.
Recent model guidance present a building surface high pressure
located in the northwestern Atlantic and this will move across
the central Atlantic on Monday though at least Wednesday. This
feature will promote a easterly-southeasterly wind flow over the
region and will push some fragments of moisture from the
Caribbean. This moisture will enhanced the shower activity across
the local islands in the afternoon hours for these days.
&&
.AVIATION...
VFR conds to prevail for all TAF, except durg aftn vcty
TJBQ/TJMZ. Mtn Tops obscr ovr ctrl Mtn range and the wrn foothills
due to SHRA till 11/22Z, also vcty ern PR til 11/14Z. Sfc wnds fm
E 10 kts of less bcmg fm E-SE 15-20 kts with ocnl hir gusts along
coastal areas of PR til 11/23Z with sea breeze influences. Max
winds fm NW 60-70 kt btwn FL300-FL350.
&&
.MARINE...
A surface high pressure will continue to promote moderate
easterly wind flow across the regional waters. Therefore, hazardous
marine conditions will continue across the majority of the marine
zones, except across the coastal waters of southern and western
Puerto Rico. Mariners can expect seas up to 8 feet with occasional
seas up to 10 feet across the regional waters. Winds will be from
the east ranging from 15 to 21 kts with higher gusts. There is a
high risk of rip currents for the north-central beaches of Puerto
Rico, as well as the northeastern beach of St. Croix from this
evening.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 85 74 84 73 / 40 50 40 50
STT 84 74 84 73 / 50 40 40 40