Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 240108

Area Forecast Discussion
National Weather Service San Juan PR
908 PM AST Fri Mar 23 2018

.UPDATE...The numerous showers that affected Puerto Rico in the
afternoon had dissipated by sunset, but it wasn`t before leaving
over one inch of rain across many areas, particularly across
southwestern Puerto Rico. After the showers dissipated, fair
weather remained across the local area with variably cloudy skies.

Some minor modifications in the short term forecast were made in
order to account for the latest observations and guidance. Even
though there is plenty of moisture, the shower activity is
expected to be limited early tonight but the chances of
significant rain will increase on Saturday during the day.


.AVIATION...VFR conds expected at all terminals overnight and
through 24/16Z. SHRA/TSRA possible after 24/16Z, which could
affect areas near the terminals in PR. Winds will be easterly up
to 10 knots overnight, increasing to 10-15kt after 24/13Z with sea
breeze developing.


.MARINE...Seas will be up to 6 feet for the rest of tonight and
Saturday, increasing up to 7 feet across the offshore Atlantic
waters from Saturday night into Sunday.


.PREV DISCUSSION... /issued 517 PM AST Fri Mar 23 2018/

Tropical moisture will continue to pool over the region through
Tuesday. A shearline and upper level jet will provide support for
isolated thunderstorms through late Sunday. Several Urban and
small stream flood advisories needed to be issued due to the
heavy rain over portions of southwest, western interior and
northwest Puerto Rico. The potential for urban and small stream
flooding will remain across the same areas through late Saturday.
Early next week surface ridge builds in from the west with
isolated showers possible mostly during the day.

SHORT TERM...tonight through Monday...
Low level moisture will continue to periodically move across the
region from the east. A weak front that will soon turn into a
weak shearline will remain over the local islands through late
Monday. As a result showers with isolated thunderstorms across
portions of southwest, south central Puerto on Saturday. Similar
to today expect streamers to effect the islands early in the
morning and the afternoon so other areas can also expect showers
on Saturday.

Low level winds will be east southeast through tonight,
then turning to the east northeast tomorrow. On Sunday winds will
be out of the northeast. An upper level jet segment crosses the
region while supporting divergence aloft along with the good low
level moisture convergence and local effects could cause Urban and
small stream flooding possible over parts of north, central and
western Puerto Rico on Saturday afternoon.

Sunday, a drier air mass will begin to impinge on the region late
in the afternoon but, showers will be possible over western,
southwest part of the cordillera. Troughiness at the low, mid and
upper levels will still reside over Puerto Rico and the U.S.
Virgin islands with northeast winds which will drag drier air into
the region and thwart showers activity through early Tuesday.

LONG TERM...PREV DISCUSSION... /issued 522 AM AST Fri Mar 23 2018/
By Sunday, a frontal boundary will induce an east to northeast
wind flow across the islands. Moisture pooling will combined with
a shortwave trough, which is expected to enhance shower and
thunderstorms activity. GFS Galvez-Davidson index suggested a
good potential for this activity across the Atlantic Waters, north
and east Puerto Rico, and the northern U.S. Virgin Islands during
the night and early morning hours. Then showers and thunderstorms
are expected to move across the interior and southwestern
sections through the afternoon hours. This moist and unstable
atmosphere is expected through Monday. A surface high pressure
spreading from the west to central Atlantic will push the frontal
boundary over the islands until dissipates through around mid
week. Based on the latest model guidance, the islands will move
into a wet and unstable pattern.

A second frontal boundary is forecast to approach the islands as
a Polar trough amplified Tuesday and Wednesday. The proximity of
this deep trough will increase instability aloft and provide good
ventilation. The combination of low level convergence, favorable
upper level dynamic and plenty of tropical moisture will support
an unsettled weather pattern through at least Thursday.

By the second part of the upcoming week, model guidance are
suggesting a a gradual improvement as a vertically stacked ridge
builds once again across the region. Under this weather pattern
expect little or no shower activity with pleasant temperatures.

AVIATION....VFR conds expected at all terminals overnight and
through 24/15Z. BKN sky at around FL060 will improve after 24/00Z,
then mostly cloudy skies across PR could cause CIGS at FL060 or so,
the USVI should have FEW or SKC. VCSH for TJMZ to improve after
23/23Z and only light and brief SHRA expected overnight across the
local terminals. SHRA/TSRA possible after 24/16Z, which could affect
areas near the terminals in PR.

Seas will increase to 5 - 6 feet on Saturday with east northeast
winds of 5 to 12 kts. A northernly swell will move into the local
waters Saturday and will remain through early Monday. A bigger
swell event is expect early next week due to a cut of low deeding
in the east Atlantic waters. A Small Craft Advisory is in effect
for the Atlantic offshore waters tonight until late Sunday.


SJU  74  86  74  85 /  70  70  50  50
STT  73  85  74  84 /  70  70  50  50


AM...Small Craft Advisory until 9 PM AST Sunday for Atlantic Waters
     of Puerto Rico and USVI from 10 NM to 19.5N.



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