A quick moving tropical wave, enhancing the low-level moisture combined with local and diurnal effects to generate rounds of shower and thunderstorm activity across interior, northern, and western PR.
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The episode as NWS SJU wrote it up.
A quick moving tropical wave, enhancing the low-level moisture combined with local and diurnal effects to generate rounds of shower and thunderstorm activity across interior, northern, and western PR.
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 Jul 18, 11:34 AM AST, 2 hours 46 minutes earlier.
Area Forecast Discussion National Weather Service San Juan PR 1134 AM AST Sun Jul 18 2021 .UPDATE... A tropical wave will produce showers and thunderstorms across Puerto Rico's interior and western sections. The U.S. Virgin Islands can expect quick passing showers from time to time. A drier air mass will move across the region around mid-morning on Monday. && .AVIATION... A tropical wave will bring SHRA/TSRA mainly across the interior and western sections of Puerto Rico. Brief MVFR conditions are possible at TJBQ with SHRA/TSRA between 18/17-23Z. In general, VFR should prevail thru the fcst period. Sfc winds from the east at 15-20 knots with higher gusts and sea breeze variations, turning light and variable after 17/23Z. && .MARINE... Choppy seas will prevail due to winds up to 20 knots. Mariners can expect seas up to 6 feet across most of the local waters, and small craft operators should exercise caution. && .PREV DISCUSSION... /issued 439 AM AST Sun Jul 18 2021/ SYNOPSIS... Showers have increased across the region overnight tonight, as a tropical wave makes its way over the area, and passing showers will continue through the morning. Afternoon convection is expected, with scattered to numerous showers and isolated thunderstorms, especially for northwestern and interior Puerto Rico, as well as downwind of orographic features, including El Yunque and the eastern islands. Drying is anticipated for the early part of the workweek. SHORT TERM...Today through Tuesday... A tropical wave is expected to stream quickly west across the region today. Shower and thunderstorm activity was observed through the overnight hours across the local waters and over portions of the U.S. Virgins Islands, and the northern and eastern sections of Puerto Rico. This activity could continue through the morning hours and further inland across Puerto Rico, as the wave axis moves further west and away of the local area. Precipitable water content (PWAT) is expected to remain at or above normal values through the rest of the day, due to lingering moisture from the wave. This moisture in combination with diurnal and local effects will aid in the development of afternoon convection over the eastern, north central and northwestern sections of PR. Urban and small stream flooding is possible with the heaviest showers. For Monday and Tuesday, a mid-level ridge northeast of the region is expected to promote drier air intrusion and a Saharan Air Layer across the local area. Therefore, drier conditions with hazy skies should prevail in general across the islands. However, shower development is expected each afternoon over western PR due to diurnal effects. East to southeast trades will keep maximum temperatures in the high 80s to low 90s. LONG TERM...Wednesday through Monday... Drier, more stable conditions continue over the region on Wednesday. Ridging aloft will also maintain stability over the area. Even so, some showers are likely due to local effects, mostly in western and interior Puerto Rico. On Thursday, more patches of near-normal moisture will track across the area, which will increase shower activity. With continuing stability, however, organized convection is unlikely, and rainfall amounts will remain low. Confidence in this trend is relatively high. On Friday, a weak tropical wave is forecast to pass by the region, mostly to the south. Some moisture is likely to make it to the region, however, resulting in more showers. There is some uncertainty in the extent to which the moisture will affect the region; model guidance has backed off on the strength and size of this feature in the last 24 hours. Confidence is low to moderate. Drying is expected overnight on Friday into Saturday. However, moisture levels will not drop as substantially as in the early part of the week. Still, precipitable water is likely to be generally below seasonal normals, or maybe on the low end of normal. Following this is increasing moisture, associated with an approaching tropical wave. Meanwhile, aloft, a mid-level trough is forecast to move over the region, with an upper-level trough following it from the east on Saturday. The upper-level trough is forecast to be west and north of the region by Slate Saturday into Sunday, which will help to provide instability over the area. Above normal moisture is expected for Sunday; combined with the improved instability, increases in shower activity are anticipated, both in intensity and coverage. Confidence in the forecast for the weekend is moderate. Lingering moisture will help to sustain showers on Monday. Increasing stability is expected aloft, with ridging in the mid- to upper-levels. A typical shower pattern is expected, though with decreased activity. Confidence is moderate. AVIATION... SHRA/TSRA expected today due to a tropical wave passage. This may cause tempo MVFR conds at times across the USVI/PR terminals through at least this afternoon. Mtn tops obscd and SCT/BKN cigs btw FL030-FL100 possible. Low-level winds ESE at 15-25 kts. VFR conditions and HZ due to Saharan dust expected tonight into Monday. MARINE... Easterly winds up to around 22 knots are expected to continue across the local waters, associated with a wind surge due to a passing tropical wave. This will sustain choppy conditions, with seas of up to 6 feet. Winds will diminish somewhat Monday into Tuesday, and seas will subside slightly as well, likely to 5 feet or less by midweek. There is a moderate risk of rip currents for nearly all local beaches; beaches of western Puerto Rico and some beaches of south central Puerto Rico have a low risk. && .PRELIMINARY POINT TEMPS/POPS... SJU 89 78 89 78 / 60 40 20 10 STT 89 80 89 80 / 40 30 20 20 && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && SHORT TERM...OM LONG TERM....CAM PUBLIC DESK...ICP
Perspectivas sobre Condiciones Peligrosas del Tiempo Servicio Nacional de Meteorologia en San Juan PR 433 AM AST domingo 18 de julio de 2021 AMZ710>745-PRZ001>013-190845- 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- 433 AM AST domingo 18 de julio 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 rafagas fuertes a traves del este, oeste y mitad norte de Puerto Rico. .Inundaciones...Lluvias fuertes pueden resultar en inundaciones urbanas y de riachuelos. .Viento...Se esperan vientos sostenidos en exceso de 20 mph (17 nudos) a traves de la mayoria de las aguas locales. .Oleaje...Se espera oleaje picado de hasta 6 pies a traves de la mayoria de las aguas regionales. .Corrientes Marinas...Existe un riesgo moderado de corrientes marinas para todas las islas. .Dias dos hasta el siete...lunes a sabado El riesgo moderado de corrientes marinas continuara a traves de todas las islas hasta el fin de semana. Se pronostica que la proxima onda tropical se acerque al area el viernes. .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-VIZ001-002-190845- San Tomas, San John e Islas Adyacentes- Santa Cruz-Aguas cercanas del Atlantico y aguas costeras adyacentes del Caribe- 433 AM AST domingo 18 de julio 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 .Tronadas...Existe el potencial de tronadas con lluvias fuertes y rafagas fuertes a traves de las islas hoy. .Inundaciones...Lluvias fuertes pueden resultar en inundaciones urbanas y de riachuelos. .Viento...Se esperan vientos sostenidos en exceso de 20 mph (17 nudos) a traves de la mayoria de las aguas locales. .Oleaje...Se espera oleaje picado de hasta 6 pies a traves de la mayoria de las aguas regionales. .Corrientes Marinas...Existe un riesgo moderado de corrientes marinas para todas las islas. .Dias dos hasta el siete...lunes a sabado El riesgo moderado de corrientes marinas continuara a traves de todas las islas hasta el fin de semana. Se pronostica que la proxima onda tropical se acerque al area el viernes. .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 national radar mosaic for the moment the first event began.
1 flood. Local standard time, as filed.
Ponding of water reported on Highway 22 near Exit 27. The water accumulations caused all but one lane to be impassable.
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, July 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: 975482.