Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxca62 tjsj 261453 

Area forecast discussion...updated
National Weather Service San Juan PR
1053 am AST sun Mar 26 2017

Update...two northeast-southwest bands of showers persisted
overnight, but the one over the northwest corner of Puerto Rico
has now completely dissipated. Although light rain has persisted
along the southeast coast of Puerto Rico, the other band has
remained mainly offshore over the near shore waters, across
Vieques and the waters between it and the Puerto Rico Mainland and
across Culebra. The band has moved little since dawn this
morning. Models suggest that convection there could continue to be
favored even into the afternoon. Also most models favor
convection to develop over interior Puerto Rico as the afternoon

The precipitable water in morning sounding came in almost one
third inch drier than yesterday at 1.61 inches and several layers
at 16 kft and 20 kft came in with 15 percent or less of relative
humidity. Minor thin layers of clouds were indicated in the
sounding at 18 kft and 27 kft. Some convective energy potential
for afternoon convection is indicated but it all but vanishes at
the top of an inversion at 18 kft. Little convection will be able
to top 30 kft today. No change to general discussion below. Minor
short-term grid edits for: warmer temperatures on the north coast
due to additional sunshine, to take out the showers this morning
out of western and northern Puerto Rico and to increase pops
around stationary band of showers mentioned above.


Aviation...VFR conditions prevail. Areas of thunderstorms and rain are expected to
initiate arnd 26/18z and last until 25/21z over PR north of the
Cordillera Central, areas of mtn obscurations some of which have
not cleared from this morning will persist til 26/02z. Areas of
MVFR arnd tjbq, tjmz and tjsj. Otherwise taf sites to continue VFR
except for brief MVFR ceilings in rain showers arnd tncm/tkpk btwn
25/19-26/09z. Winds Alf S-SW 10 to 20 kt up thru fl150. Max winds
west-southwest 70 kts at fl400 decreasing slightly thru 27/18z.


Marine...seas came down during the early morning hours and so was
able to cancel high surf advisories for breaking waves of 10 feet
or more in the morning update package. Also the high risk of rip
currents is now moderate in the northern U.S. Virgin Islands. The
only remaining Small Craft Advisory is in the local outer
Atlantic waters and will come down around 2 PM AST. Models show 7
foot seas returning to the northwest corner of the local outer
Atlantic waters returning Monday night.


Previous discussion... /issued 645 am AST sun Mar 26 2017/

Synopsis...a surface low over the Atlantic waters between
Hispaniola and PR will continue to move north northeast of the
forecast area for the next few days. A surface high will build
across the western Atlantic and into the local area by midweek.
Drier air at mid levels is expected to fill in today and persist
through the end of the week. Light southerly winds will continue
until Monday, becoming northerly and increasing through Thursday.

Short through Tuesday...a line of showers and
strong thunderstorms persisted across the Atlantic waters
northwest of Puerto Rico overnight. This activity will continue to
move to the northeast approaching the northwest tip of Puerto
Rico during the morning hours. Showers were also observed over
south central sections of Puerto Rico as well as across Vieques,
Culebra and eastern Puerto Rico overnight. A few light shower
affected the U.S. Virgin Islands.

A developing surface low pressure system to the north of Hispaniola
this morning will continue to bring a moist and unstable southerly
wind flow across Puerto Rico and the U.S. Virgin Islands today. Less
cloud cover is expected today across the region and therefore
diurnal heating in combination with sea breeze convergence and other
local effects will induce the development of showers and
thunderstorms along and north of the Cordillera Central,
including the San Juan Metro area and especially during the
afternoon and early evening hours. Urban and small stream flooding
are possible over northern Puerto Rico. As the low moves further
northeastward and strengthens over the Atlantic waters, moisture
will begin to decrease slowly across the local islands by Tuesday
and thereafter. However, a few thunderstorms may still be possible
during the next several days due to lingering moisture in
combination with local effects--especially in the interior
sections of Puerto Rico. Due to the expected southerly wind flow,
warmer than normal temperatures are expected across the northern
coastal municipalities of Puerto Rico during the next few days.

Long term...Wednesday through Sunday...a surface low moves into
the north central Atlantic on Wednesday and the ridge across the
western Atlantic builds north of the islands through the end of
the week. This will result in a cooler northerly wind flow over
the islands and fair weather conditions through the middle of the
week. East to southeast winds return from Friday through Sunday
with moisture increasing from the southeast...apparently
associated with remnants of the surface low/frontal boundary.

Aviation...brief periods of MVFR conditions are expected across
the Leeward Islands and USVI taf sites this morning. Rain showers/thunderstorms and rain
are expected to develop along and north of Cordillera Central this
afternoon inducing periods of MVFR or even IFR conditions after
26/16z in and around tjmz, tjbq and tjsj. Latest tjsj sounding
indicated a south southwesterly wind flow at all levels, but very
strong aloft.

Marine...small craft advisories continue in effect today for the
Atlantic waters and passages. Northerly swell will continue to
subside through this evening. High surf advisory condtions for
breaking waves of 10 feet or more will continue for the north
facing beaches of PR and a high risk of rip currents will continue
through at least the overnight hours across the northwest to
north coast of PR, Culebra and St. Thomas. Beach goers should
avoid these areas and exercise caution if going to any other
beaches of the islands.


Preliminary point temps/pops...
sju 87 74 88 77 / 50 40 50 40
stt 83 75 85 75 / 60 50 50 40


Sju watches/warnings/advisories...
PR...high rip current risk through late tonight for Mayaguez and
vicinity-north Central-Northeast-northwest-San Juan and
vicinity-southeast-western interior.

High rip current risk until 6 PM AST this evening for Culebra.

Am...Small Craft Advisory until 2 PM AST this afternoon for Atlantic
waters of Puerto Rico and USVI from 10 nm to 19.5n.


National Weather Service Glossary of Abbreviations