The combination of a mid and upper-level trough and a surge in low-level moisture resulted in heavy showers and isolated thunderstorms across most of interior Puerto Rico.
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The episode as NWS SJU wrote it up.
The combination of a mid and upper-level trough and a surge in low-level moisture resulted in heavy showers and isolated thunderstorms across most of interior Puerto Rico.
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 May 28, 10:13 AM AST, 5 hours 47 minutes earlier.
Area Forecast Discussion National Weather Service San Juan PR 1013 AM AST Fri May 28 2021 .UPDATE...A surge of low-level moisture is currently moving between the U.S. Virgin Islands and eastern PR. Cloudiness and showers were noted through the morning hours across the USVI, Vieques, Culebra and the eastern half of Puerto Rico. For this afternoon, this surge in moisture in combination with the sea breeze convergence and daytime heating will result in shower and thunderstorm development once again across portions of the interior and western PR, and as well as downwind from el Yunque into San Juan and vicinity. The main threat will be urban and small stream flooding, and localized flash flooding can not be ruled out due to slow-moving showers(steering winds are 5-10 kts from the east). No change needed from the previous forecast. && .AVIATION...Mainly VFR conditions are expected to prevail across all the terminals through much of the forecast period. However, SHRA/TSRA developing this afternoon over mainland PR may result in tempo MVFR conditions across the PR terminals fm 16z-22z. The 12z SJU sounding indicated east winds up to 14 kt blo 5000 ft. && .MARINE...A small northerly swell will continue across the Atlantic waters through at least tonight. This will cause a moderate risk of rip currents along the northern beaches of Puerto Rico and Culebra. Light to moderate southeast trades will prevail today. Seas should range up to 5 feet across the offshore waters and up to 4 feet across the nearshore waters of the islands. && .PREV DISCUSSION... /issued 430 AM AST Fri May 28 2021/ SYNOPSIS...A seasonal weather pattern, dominated by overnight and early morning showers moving over windward areas from the waters followed by enhanced afternoon showers and isolated thunderstorms clustering along the interior, will prevail during the next few days. Since soils are saturated, this could result in urban and small stream flooding and possible mudslides near areas of steep terrain. A slightly wetter pattern is expected for the next work week following a similar shower pattern. Tranquil marine conditions will prevail through early next week. SHORT TERM...Today through Sunday... Showers continued over the nearshore Atlantic waters around central Puerto Rico and also around Saint Croix in the Caribbean. A few of these showers were momentarily heavy, but were brief. Fredericksted reported only traces. Today, upper levels will see the approach of a weakening ridge between several cut-off lows. The first low was the one that passed on Thursday, the next is over the Bahama Islands. That low is weakening and is expected to pass well north of the area on Sunday morning. At mid levels, high pressure will come to dominate the west central Atlantic over the next several days. At the surface the pressure pattern changes very little with high pressure maintaining a presence just west of the Azores. This high will drift further west by the end of the weekend. Low level flow will alternate between east and east southeast to maintain normal to slightly above normal temperatures area wide, and slightly warmer on Puerto Rico's northern coast. Although precipitable water values will remain fairly steady, patches of mostly low level moisture will cause fluctuations between 1.4 and 1.75 inches. This in turn will cause showers move onshore each day after midnight on the east coast, and showers and thunderstorms in the west and central portions of Puerto Rico. Passing showers will occur in the U.S. Virgin Islands but accumulations will be light and generally brief. Heavy amounts will occur over Puerto Rico during the afternoon periods but amounts will be light on the south coast and near the coast on the northern coasts. 500 mb temperatures will cool about a degree and a half today through Sunday which could lead to somewhat more vigorous thunderstorms, but convective activity will still have moisture as the limiting factor. Nevertheless, a weaker wind flow will allow heavy rains to linger longer in some areas that would produce urban and small stream flooding. A slight improvement in visibility values is expected now through Tuesday as Atlantic air begins to replace Saharan Air over the local area and the amount of dust in the air decreases. LONG TERM...Monday through Saturday... Model guidance continues to suggest a near to above normal moisture supporting a generally seasonal shower pattern. That is, periods of overnight and morning showers moving from the waters over eastern Puerto Rico and the US Virgin Islands, followed by locally and diurnally induced shower development across the interior and western sections of Puerto Rico and portions of the San Juan metropolitan area. Nevertheless, slight variations in the intensity, spread, and frequency of the activity can be expected due to fluctuations in moisture content and the evolution of mid to upper-level conditions, as well as the timing of key driving features streaming across the region. At the surface, high pressure over the north-central Atlantic will maintain a generally moderate easterly flow through the period. Under this flow, the passage of various weak easterly waves along with a gradual increase in the available moisture is expected. For Monday, mid to upper-level ridging will maintain a trade wind inversion over the region. This may be strong enough to inhibit organized deep convection, but not strong enough to suppress shallow development. Even so, intense diurnal heating may aid in the development of a few isolated thunderstorms in the afternoon. On Tuesday and continuing through the rest of the workweek into the weekend, an amplifying trough to the west will push the ridge eastward and away from the local area. As a result, expect the erosion of the trade wind inversion along with a significant increase in tropical moisture as high as 2.10 inches by Friday afternoon into Saturday. This scenario will support enhanced shower and isolated thunderstorm development, resulting in the potential for urban and small stream flooding and possible mudslides near areas of steep terrain, especially with the strongest activity each afternoon. Although global models have favored a wetter pattern during the last few days and the forecast follows this trend, there is still low to medium confidence on whether it will fully materialize to the extent suggested by the models. AVIATION...Mainly VFR conditions are expected to prevail across all terminals in the USVI but only until 28/16Z in PR. Aft 28/16Z SHRA/TSRA will devlp ovr wrn and interior PR and downstream from El Yunque causing areas of mtn obscurations and periods of MVFR conds. This will most affect TJMZ/TJBQ with VCSH/VCTS at TJSJ/TJPS. Low level winds will continue from the east up to 15 kts, with gusts to 25 kts in sea breezes, then less than 8 knots aft 29/01Z. Maximum winds W 50-65 knots btwn FL380-480 strongest at FL450. MARINE...Tranquil marine conditions with seas at 5 feet or below and east winds up to 15 knots will prevail during the next few days. There is a moderate risk of rip currents for north-facing beaches of Puerto Rico and Culebra. Elsewhere, a low risk will continue. && .PRELIMINARY POINT TEMPS/POPS... SJU 88 78 90 77 / 40 40 30 30 STT 88 76 88 77 / 30 30 30 40 && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && DSR/OMS/CRS
Perspectivas sobre Condiciones Peligrosas del Tiempo Servicio Nacional de Meteorologia en San Juan PR 343 AM AST viernes 28 de mayo de 2021 AMZ710>745-PRZ001>013-280745- San Juan y Vecindad-Noreste-Sureste-Este Interior-Norte Central- Interior Central-Ponce y Vecindad-Noroeste-Oeste Interior- Mayaguez y Vecindad-Suroeste-Culebra-Vieques-Las aguas costeras cercanas y lejanas a la costa del Atlantico y Caribe- 343 AM AST viernes 28 de mayo de 2021 Esta perspectiva sobre las condiciones peligrosas del tiempo es para Puerto Rico y las aguas costeras del Atlantico. .Dia Uno...Hoy y esta noche .Tronadas...Existe el potencial de tronadas con lluvias fuertes y vientos en rafaga breves esta tarde a traves de sectores del oeste e interior de Puerto Rico. .Inundaciones...Lluvias fuertes pudieran resultar en inundaciones urbanas y de riachuelos a favor del viento desde El Yunque y en el oeste e interior de Puerto Rico. .Corrientes Marinas...Existe un riesgo moderado de corrientes marinas para las playas del norte de Puerto Rico desde Rincon hasta Ceiba y Culebra. .Dias dos hasta el siete...sabado a jueves Con un aumento en la humedad durante la proxima semana, existe el potencial de inundaciones urbanas y de riachuelos cada tarde en el oeste e interior de Puerto Rico y a favor del viento desde El Yunque. El riesgo moderado de corrientes marinas para las playas del norte de Puerto Rico desde Rincon hasta Ceiba continuara durante los proximos 7 dias. El riesgo de corrientes marinas en la costa sur de Puerto Rico aumentara el jueves. .Comunicado sobre informacion reportada por observadores... No se espera activacion amplia de observadores. Favor de reportar cualquier informacion sobre vientos, oleaje y corrientes marinas al Servicio Nacional de Meteorologia en San Juan mientras siguen las indicaciones de las autoridades locales y estatales. AMZ715-725-VIZ001-002-280745- San Tomas, San John e Islas Adyacentes-Santa Cruz- Aguas cercanas del Atlantico y aguas costeras adyacentes del Caribe- 343 AM AST viernes 28 de mayo de 2021 Esta perspectiva sobre las condiciones peligrosas del tiempo es para Islas Virgenes Estadounidenses y las aguas costeras adyacentes. .Dia Uno...Hoy y esta noche .Corrientes Marinas...Existe un riesgo moderado de corrientes marinas para el extremo este de Santa Cruz esta noche. .Dias dos hasta el siete...sabado a jueves El riesgo moderado de corrientes marinas a lo largo de las costas de Santa Cruz aumentaran a moderado temprano la proxima semana. .Comunicado sobre informacion reportada por observadores... No se espera activacion amplia de observadores. Favor de reportar cualquier informacion sobre vientos, oleaje y corrientes marinas al Servicio Nacional de Meteorologia en San Juan mientras siguen las indicaciones de las autoridades locales y estatales.
The San Juan radar (TJUA), 24 km away, was scanning. The volume nearest the first event began at 19:57 UTC.
1 thunderstorm wind, 1 debris flow, 1 heavy rain. Local standard time, as filed.
Fallen tree over powerline, obstructing a lane on Road PR-851, Sector Roberto Resto.
Mudslides reported along Road PR-941, sector El Lago.
One adult and one minor were trapped due to a water surge that occurred along Charco Azul. The two people were eventually rescued.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Puerto Rico in 2021Every state in 2021This day in weather historyWarnings in Puerto Rico, May 2021
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: 962024, 962025, 962027.