Even though fair weather skies were observed across the area, some directional turning of the winds with height resulted in the formation of funnel clouds just south of Guayama and Arroyo.
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The episode as NWS SJU wrote it up.
Even though fair weather skies were observed across the area, some directional turning of the winds with height resulted in the formation of funnel clouds just south of Guayama and Arroyo.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS San Juan, PR wrote its last area forecast discussion before the first event at Dec 21, 4:35 AM AST, 3 hours 10 minutes earlier.
Area Forecast Discussion
National Weather Service San Juan PR
435 AM AST Fri Dec 21 2018
.SYNOPSIS...Patches of low-level moisture embedded in moderate
easterly trade winds will continue to move across the forecast
area from time-to-time. However, the presence of a mid level high
pressure will limit the intensity and coverage of any associated shower
activity, resulting in mostly fair weather conditions through the
forecast period. A slight increase in moisture content and shower
activity is expected during the weekend, but no significant
rainfall accumulations are expected. A seasonal and relatively
dry weather pattern will return next week.
&&
.SHORT TERM...Today through Sunday...
Seasonal weather pattern had prevailed this week and will continue
through much of the forecast period. Mid/upper level ridge pattern
will hold aloft inducing a strong trade wind cap through the short
term period. At the surface, a high pressure drifting into the north
central Atlantic Ocean will promote a breezy trade wind flow across
the region. In addition, a frontal boundary which is forecast to
move across the western Caribbean and Atlantic ocean today, will
linger off to the west and northwest of the local islands.
Model guidance continue to suggest a drier than normal airmass
through the short term period. However, trade wind perturbations
will bring pockets of moisture/clouds at times, which will lead to
fast moving passing showers across the windward sections of Puerto
Rico and the U.S. Virgin Islands each day. However, moisture content
and shower activity should increase somewhat by the upcoming
weekend. Meanwhile, the high pressure aloft will make difficult
vertical development and afternoon convection over the western and
interior sections of Puerto Rico, but should not be ruled out.
Rainfall amounts should be light to moderate.
.LONG TERM...Monday through Friday...
Patches of low-level moisture embedded in the trade winds will
continue to move across the forecast area through the long term
period. As a high pressure system exists the eastern coast of the
United States and moves over the western Atlantic late Wednesday
night into Thursday, there will be a slight increase in the
moisture transported with these patches. However, a mid level
high pressure that is forecast to hold over the region will limit
the intensity and coverage of any shower activity that develops
over the forecast area. As a result, mostly fair weather
conditions will prevail across Puerto Rico and the U.S. Virgin
Islands. No significant rainfall accumulations are expected.
&&
.AVIATION...Mainly VFR conditions are expected to prevail across all
terminals through the forecast period. However, -SHRA/SHRA possible
this afternoon in and around TJSJ/TJMZ/TJBQ/TIST/TISX. Light to calm
and variable winds, increasing at 10 to 15 knots out from the east
with higher gusts and sea breeze variations after 21/13z.
&&
.MARINE...Choppy/Rough marine continue will continue through at
least this evening across the offshore Atlantic waters and the
Anegada Passage with seas up to 7 feet. Thus, Small Craft
Advisories remain in effect. Small craft operators should exercise
caution across nearshore Atlantic waters and Mona Passage due to
seas up to 6 feet. Elsewhere, seas up to 5 feet are expected.
Moderate easterly winds will prevail today and on Saturday, but
are forecast to increase on Sunday into early next week. A high
rip current risk is in effect for beaches along the north coast of
Puerto Rico, Culebra, and Saint Thomas. Marine conditions will
improve during the weekend.
&&
.FIRE WEATHER...Relative humidity values are expected to drop
into the mid 40s to low 50s today. Although winds are forecast to
range between 10-15 mph, recent KBDI and 10-hour fuel moisture
observations show that a drying pattern continues to dominate the
local islands and given that no significant wetting rains are
expected, elevated fire weather conditions can be expected. As a
result, a Fire Danger Statement has been issue for today,
particularly for the southern coastal plains of Puerto Rico where
KBDI values are near or above 600.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 85 73 85 75 / 10 40 40 30
STT 85 75 84 74 / 10 50 50 30
&&
.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through late tonight for Culebra-North
Central-Northeast-Northwest-San Juan and Vicinity.
High Rip Current Risk until 6 AM AST early this morning for
Mayaguez and Vicinity.
VI...High Rip Current Risk through this afternoon for St.Thomas...St.
John...and Adjacent Islands.
AM...Small Craft Advisory until 2 PM AST this afternoon for Anegada
Passage Southward to 17N.
Small Craft Advisory until 8 PM AST this evening for Atlantic
Waters of Puerto Rico and USVI from 10 NM to 19.5N.
&&
SHORT TERM...CAM
LONG TERM....ICPPerspectivas sobre Condiciones Peligrosas del Tiempo Servicio Nacional de Meteorologia San Juan PR 520 AM AST viernes 21 de diciembre de 2018 AMZ710>745-PRZ001>013-220930- San Juan y Vecindad-Noreste-Sureste- Este Interior- Norte Central-Interior Central-Ponce y Vecindad- Noroeste- Oeste Interior-Mayaguez y Vecindad-Suroeste-Culebra- Vieques-Aguas cercanas y lejanas a la costa del Atlantico y Caribe- 520 AM AST viernes 21 de diciembre de 2018 Esta perspectiva sobre las condiciones peligrosas del tiempo es para Puerto Rico y las aguas costeras adyacentes del Atlantico. .Dia Uno...Hoy y esta noche .Viento...Se esperan vientos sostenidos que excedan las 20 mph a traves de las aguas costeras durante horas de la tarde. .Oleaje...Se espera oleaje picado hasta 6 pies a travas de las aguas del Atlantico. Se espea que el oleaje incremente hasta 7 pies a traves de las aguas abiertas del Atlantico y el Pasaje de la Anegada. .Corrientes marinas...Existe riesgo alto de corrientes marinas para la mayoria de las playas del norte de Puerto Rico y Culebra. El riesgo de corrientes marinas tabien sera moderado para la mayoria de las playas del noroeste, sureste y suroeste de Puerto Rico y Vieques. .Dias dos a siete...sabado a jueves El riesgo de corrientes marinas sera moderado a lo largo de algunas playas de Puerto Rico, Culebra y Vieques. Se pronostica que otra marejada llegue a las aguas del Atlantico el domingo. Modelos maritimos indican un incremento en el oleaje para mitad de semana la proxima semana. .Comunicado sobre informacion reportada por observadores... No se anticipa activacion amplia de observadores, sin embargo, se les exhorta a los observadores individuales a reportar corrientes marinas al Servicio Nacional de Meteorologia en San Juan. AMZ715-725-VIZ001-002-220930- San Tomas, San John e Islas Adyacentes-Santa Cruz- Aguas cercanas del Atlantico y aguas costeras adyacentes del Caribe- 520 AM AST viernes 21 de diciembre de 2018 Esta perspectiva sobre las condiciones del tiempo peligrosas es para las Islas Virgenes Americanas y las aguas costeras adyacentes. .Dia Uno...Hoy y esta Noche .Vientos... Se esperan vientos sostenidos que excedan las 20 mph a traves de las aguas costeras durante horas de la tarde. .Oleaje...Se espera oleaje picado hasta 6 pies a traves de las aguas del Atlantico. Se espera un aumento en el oleaje hasta 7 pies a traves de aguas abiertas del Atlantico. .Dias dos a siete...viernes a miercoles El riesgo de corrientes marinas sera moderado a lo largo de la mayoria de las playas de las islas. Se pronostica que otra marejada llegue a las aguas del Atlantico el domingo. Modelos maritimos indican un incremento en el oleaje para mitad de semana la proxima semana. .Comunicado sobre informacion reportada por observadores... No se anticipa activacion amplia de observadores, sin embargo, se les exhorta a los observadores individuales a reportar corrientes marinas al Servicio Nacional de Meteorologia en San Juan.
The San Juan radar (TJUA), 23 km away, was scanning. The volume nearest the first event began at 11:46 UTC.
1 funnel cloud. Local standard time, as filed.
Two funnel clouds observed at 3 to 4 miles along the southern coast of Guayama-Arroyo Municipalities.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Puerto Rico in 2018Every state in 2018This day in weather historyWarnings in Puerto Rico, December 2018
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 794430.