Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 121544

Area Forecast Discussion
National Weather Service San Juan PR
1144 AM AST Mon Apr 12 2021

.UPDATE...Another rain-free morning was observed across Puerto
Rico and the U.S. Virgin Islands. However, low level clouds were
developing during the late morning hours across mainland PR due to
the sea breeze convergence. The 12/12z SJU upper air sounding
indicated light to moderate ESE winds below 850 mb and westerlies
above with plenty of drier air in the low-to-mid levels.
Precipitable water content today is at 1.06 inches, and any shower
development this afternoon should be light and over the interior
of Puerto Rico. The main threat today continues to be along the
northern beaches of PR, Culebra and St. Thomas due to life
threatening rip currents.


.AVIATION...Mainly VFR conditions are expected to prevail across
all terminals during the next 24 hours. The 12z TJSJ upper air
sounding indicated ESE winds up to 14 kt below 4000 ft.


.MARINE...A 4-6 feet long period northerly swell will continue to
impact our Atlantic waters and passages through at least early
Tuesday morning. Small craft operators are urge to exercise
caution across these waters. East to southeast winds up to 15
knots are expected to continue for the next several days. This
swell is causing breaking waves between 5-8 feet and occasionally
higher along the northern beaches of Puerto Rico, Culebra and St.
Thomas. Therefore, there is a high risk of rip currents through at
least Tuesday for those areas.


.FIRE WEATHER...There is a Fire Danger Statement(RFDSJU) in effect
through this afternoon for southern and western coastal areas of
Puerto Rico. Relative humidities were between 45%-55% and winds
were just under 15 mph earlier this morning along southern PR.


.PREV DISCUSSION... /issued 454 AM AST Mon Apr 12 2021/

SYNOPSIS...Generally fair and stable weather conditions with
limited shower activity, if any, will prevail through Tuesday.
This scenario will support elevated fire danger conditions.
Local conditions will shift to a more wetter and unstable weather
pattern beginning on Wednesday as above average moisture moves
into the region. Simultaneously, warmer temperatures can be
expected as winds become more southeasterly. Choppy seas and
hazardous surf zone conditions will continue today.

SHORT TERM...Today through Wednesday...

Dry air interacting with a mid level ridge will promote stable
weather conditions with sunny skies today. Model suggest little to
no rainfall over the forecast area. Therefore, shower activity, if
any, could be expected in the afternoon over parts of interior
Puerto Rico and across the Caribbean waters. Winds are shifting now
more to the east southeast. Therefore, expect slightly warmer
daytime temperatures across the region, compared to what has been
observed in the past couple of days.

For Tuesday, the aforementioned ridge will start to erode and
moisture will increase somewhat. The chance for isolated
thunderstorms remains marginal as latest model guidance shows
moisture content below normal values and the dynamic aloft looks
less favorable. Therefore, sunny skies will be observed across the
islands during the morning hours. Once the moisture embedded in the
southeasterly flow reaches the islands in the afternoon, partly
cloudy skies with isolated to scattered showers will be noted mainly
over parts of interior and eastern interior of Puerto Rico. Rainfall
activity is expected to lead to minor rainfall accumulations.

For Wednesday, a large polar trough will pass to our north during
the day. Therefore, dynamics aloft look more favorable for the
development of isolated thunderstorms. Also, models continue to
suggest a westerly component in the wind flow pattern from 250-
700mb, but light southeasterly winds at sfc. This weather scenario
will interact with a surge in moisture advecting across the local
islands. Thus, diurnally induced afternoon showers with isolated
thunderstorm activity will focus mainly over parts of interior and
eastern interior Puerto Rico. Showers as well as isolated
thunderstorms over the San Juan metro area cannot be ruled out. This
activity could result in ponding of water over roadways and in poor
drainage areas and possible localized urban and small stream
flooding with the heaviest and most persistent rains.

LONG TERM...Thursday through Tuesday...

Recent model guidance continues to suggest that a somewhat wetter
and unstable weather pattern will continue on Thursday, but now a
bit drier than previously expected and lasting through late
Friday night since the area of enhanced moisture is expected to
stall over the region. A delay in the peak of moisture is once
again presented, suggesting highest model-estimated precipitable
water around 1.80 inches by Friday evening. Over land, the bulk of
the activity will favor southern and eastern sections of Puerto
Rico, as well as the US Virgin Islands during the overnight and
early morning hours, shifting to the interior and northern
sections of Puerto Rico in the afternoon, with the potential for
isolated thunderstorm development since 500 mb are expected to
fall as low as -8 degrees Celsius. This scenario, under a
generally weak steering flow, presents the potential for ponding
of water to localized urban and small stream flooding through
late Friday night.

Thereafter, global models are now in strong agreement
with a drying tendency dominating the local weather conditions
Saturday into early next week, with peak drying expected on
Monday. So far, this will result from a mid-to-upper ridge moving
into the area, strengthening the trade wind cap while limiting
any chance for organized shower development. Nevertheless, limited
afternoon convection cannot be ruled out. As conditions dry out
and the general wind flow becomes more southeasterly, expected
warmer temperatures that could reach the lower to mid 90s with
possible heat indices into the upper 90s to lower 100s by Friday

AVIATION...Mainly VFR conditions are expected to prevail across TAF
sites during the next 24 hours. Winds will be from the ESE at 10-15
kt, with sea breeze variations through 22z. Then, light and variable
winds are expected overnight.

MARINE...A weak northerly swell spreading across the Atlantic
waters and local passages will continue to result in choppy seas
through tonight. Recent buoy observations (41043) suggest the
peak of the swell will occur early this morning, with seas in the
outer Atlantic waters approaching but not quite reaching Small
Craft Advisory conditions. As a result, small craft operators are
urged to exercise caution across these waters.

Beachgoers,  are urged to exercise caution when visiting the north-
facing beaches of Puerto Rico, Culebra and the northern US Virgin
Islands due to the potential of dangerous breaking waves and
expected dangerous rip currents conditions. In fact, a High Risk of
Rip Currents remains in effect for these areas through at least
Tuesday afternoon.

Marine and surf conditions are expected to gradually improve on
Tuesday, with tranquil seas prevailing through the end of the

FIRE WEATHER...A generally dry and stable weather pattern with no
chance for significant wetting rains is expected today, with the
potential for further deterioration of soils and fuels. Relative
humidities are expected to fall into the upper 30s to lower 40s,
under a generally east to southeast wind flow that may remain
below 15 mph with higher gusts. However, sea breeze variations
may generate locally higher winds around 15 mph or higher between
the late morning and early afternoon hours. Since recent KBDI and
10-hour fuel moisture reports have remained above 600 and as low
as 8 per cent, respectively, this scenario will support elevated
fire danger conditions. Therefore, a Fire Danger Statement has
been issued due to elevated fire danger conditions expected across
the southern coastal plains and hills, as well as the west coast
of Puerto Rico.


SJU  85  73  88  75 /   0   0  30  20
STT  84  74  85  74 /   0  20  20  30


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

     High Rip Current Risk through late tonight for Mayaguez and

VI...High Rip Current Risk through late tonight for St.Thomas...St.
     John...and Adjacent Islands.



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