Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 140133

Area Forecast Discussion
National Weather Service San Juan PR
933 PM AST Sun Oct 13 2019

.UPDATE...Thunderstorms persisted through the evening hours across
portions of southwestern PR and across the western waters of PR.
However, by 9 PM this activity dissipated and new showers with
isolated thunderstorms were developing across the surrounding
waters of the USVI and over eastern PR. This surge in moisture
will continue through Monday morning and additional showers with
possible thunderstorms are expected across these areas. Updated
short term grids based on the current weather and latest model
guidance. Otherwise, no change from previous discussion.


.AVIATION Update...Mainly VFR conditions expected to prevail
across all terminals. However, SHRA/-TSRA en route from Leeward
Islands into eastern PR may cause brief MVFR conds through Monday
morning. TSRA is expected to develop from 14/16z-22z in and
around TJSJ/TJPS/TJMZ/TJBQ causing tempo MVFR. Winds overnight
will continue at less than 10 kts...increasing from the east
at 10-15 kts after 14/14z with sea breeze variations.


.MARINE Update...San Juan and Rincon buoys are indicating the
northerly swell at 4 feet between 11-13 seconds. This swell is
expected to continue on Monday, creating a high risk of rip
currents along the north coast of Puerto Rico. Seas will range
between 3-5 feet in general and easterly winds will pick up
between 10-15 knots on Monday. Showers and isolated thunderstorms
are increasing tonight between the Anegada Passage and across the
USVI/Eastern PR waters. Mariners across these waters could
experience occasionally lightning strikes and brief gusty winds
with this activity.


.PREV DISCUSSION... /issued 156 PM AST Sun Oct 13 2019/


Showers have developed downstream of El Yunque this afternoon, and
more are expected for central and western Puerto Rico. The threat
for isolated showers will continue throughout the week. However,
by the end of this next week, conditions look more conducive for
more widespread showers and thunderstorms. Much of the Saharan
dust has left the region, but a little bit of haze is still likely
throughout the week.

SHORT TERM...Rest of today through Tuesday...

A weak surface trough will persist just NNE of the region during
the rest of today, while weakening and slowly moving further away.
This trough is to maintain light east to southeast winds across the
forecast area, but is expected to slightly increase through Monday
as the trough is eroded by the building northeast Atlantic ridge.  A
broad Tutt low and associated trough NE of the Leewards will remain
in place through the weekend to maintain the region under the
influence of the upper ridge. However, by late  Monday and into the
early part of the upcoming week, the tutt will amplify while
sinking southwards into the northeastern Caribbean. The trough
axis will relocate just east of the local area by Tuesday. This
amplifying trough will however induce weak low level easterly
pertubations and also weaken the trade wind cap inversion. As a
result, expect an increase in low level moisture advection and
convergence across the forecast area.

Given the current patterns, expect isolated to scattered showers
across the area during the rest of today, with thunderstorm
development still likely mainly across the central interior and
western sections of PR. Minor urban and small stream flooding will
remain possible in isolated areas over PR. On Monday and Tuesday
similar conditions are forecast, but model guidance continued to
suggest a much better chance for shower and thunderstorms development
each day due to the increasing low level moisture advection. In
addition, with the winds are expected to be stronger on Monday and
Tuesday, the afternoon thunderstorm activity should be focused more
more across the western interior and surrounding municipalities.
This will also increase the chance for urban and small stream
flooding across portions of the islands through Tuesday.

LONG TERM...Wednesday through Sunday...

The first part of the long-term forecast looks fairly typical.
Lingering moisture from a dying tropical wave and frontal system
will interact with local terrain effects to produce isolated showers
across the region. Scattered showers can be expected in central and
western Puerto Rico for each afternoon Wednesday and Thursday.
However impacts are not expected to be major.

Heading towards the weekend, the trend for moisture and shower
activity is expected to increase. However, there are key
differences in long range forecast models. A tropical wave is
forecasted to arrive on Friday and last through the weekend. This
is something models do agree on, and it appears likely that this
next weekend will be more active than the current one, starting
Friday. Where things get uncertain is regarding the upper-level
support for rainfall. The European model puts the center of an
upper tropospheric trough just to our west for this period, which
is more favorable for upper level divergence and upward motion.
This could help develop enhance thunderstorms across the region.
The GFS model puts us on the convergent side of the trough,
forecasting it to be to our east for Friday. However, the GFS then
puts it to our west for Sunday. Regardless, next weekend does
look active. Impacts will likely include frequent lightning, gusty
winds, and heavy rain with localized flooding.


VFR durg prd. SCT lyrs nr FL025...FL060 w/Isold SHRA en route btw
local islands. Aftn SHRA/Isold TSRA will persist ovr PR til 13/23Z.
SFC wnds mainly fm E-SE with sea breeze variation bcmg lgt/vrb aft
13/23z. No sig operational wx impacts attm.


We are currently being impacted by a north northwesterly swell from
Tropical Storm Melissa. This storm is far away to the north, and
moving away, but we will continue to see a weak swell through
tomorrow. Wave heights have been around 5 feet in the Atlantic
waters, and will continue at 4-6 feet. However, after Monday the
seas will calm down for the next several days thereafter. Isolated
showers and a few thunderstorms across local waters will be an
ongoing threat.


SJU  77  89  77  89 /  30  40  30  50
STT  78  89  79  87 /  50  40  30  50


PR...High Rip Current Risk through Monday afternoon for North Central-
     Northeast-Northwest-San Juan and Vicinity-Southeast.

     High Rip Current Risk until 6 AM AST Monday for Mayaguez and



LONG TERM....DS is the U.S. government's official web portal to all federal, state and local government web resources and services.