Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
567 FXCA62 TJSJ 152041 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 441 PM AST Sat Jun 15 2024 .SYNOPSIS... An Excessive Heat Watch is in effect from 10 AM to 6 PM tomorrow for the western, northern, and eastern coastal areas of Puerto Rico including Culebra, Vieques, and the USVI. Please follow the recommendations from the local health department to avoid heat exhaustion. Passing showers followed by afternoon convection is expected tomorrow as plenty of moisture remains in the area, brought in by the southeasterly lower level wind flow. && SHORT TERM...Tonight through Monday... A surface trough will continue to move north of the region tonight with trailing bringing passing showers overnight with the prevailing southeast low level winds. A mid to upper level ridge will hold just to our west, keeping conditions somewhat stable. Expect this overall pattern to persist through tomorrow along with a light to moderate plume of Saharan Dust and suspended particulates expected to continue crossing the region causing slightly hazy conditions tomorrow. Although drier air will filter in across the region tonight along with the Saharan dust, there will enough moisture available to aid in the development of afternoon showers and isolated thunderstorms during the afternoon hours. With fairly light winds and local sea breezes, most of the expected rainfall should be focused across eastern, central and northwest sections of Puerto Rico steered by the southeast wind flow. The suspended Saharan dust particulates, along with the warm southerly flow and presence of the mid to upper level ridging will continue to favor subsidence and support above normal daytime high temperatures in the low 90s especially along the north and west coastal areas. That said, maximum heat indices are expected to exceed 100 degrees once again, resulting in excessive heat impacts especially along the west, north and east coastal and urban areas. These conditions will also remain likely on Sunday. Therefore a Heat Watch will remains in place for tomorrow. Please refer to the latest Non-Precipitable Weather Message(NPWSJU) issued by WFO SJU PR for additional information on these heat impacts. && LONG TERM...Tuesday through Saturday... /from previous discussion/ A deteriorating weather trend is forecast for the first part of the long-term period. A trough will build just over the western Atlantic at the upper and mid- levels. The presence of the mid- to upper-level feature will decrease the 250 MB height and turn the temperatures at 500 MB colder, ranging from -7 to -8 degrees. This increases the potential for more widespread lightning activity on Tuesday into Wednesday. According to the global model guidances (GFS and ECMWF), surface conditions will also be favorable for shower activity due to plenty of tropical moisture embedded in the east-southeasterly wind flow across the region. Given the expected conditions from Tuesday into Wednesday, the islands will experience passing showers in the morning hours over the eastern sections, the U.S. Virgin Islands, and the local waters, followed by widespread shower activity across the western interior. So far today, the rainfall threat is mostly concentrated across the western interior, with ponding of water in roadways and poor drainage areas. From Thursday into Saturday, plenty of surface moisture will continue to stream across the local islands, keeping the potential for convection in the afternoon, especially across the mountains and the western interior. Although plenty of moisture will remain in place, more stable conditions will be present at the upper level, provided by an upper-level ridge. A similar temperature pattern is expected across the islands by the first part of the long term. According to the model guidances, 925 MB temperature, the trend calls for warmer conditions, resulting in heat indices rounding 108 to 112 F degrees. Residents and visitors are urged to stay informed of possible advisories or warnings due to Excessive heat. && .AVIATION... (18Z TAFS) Scattered SHRA expected to prevail across northern half of PR and TJBQ through 22Z, with periods of reduced VIS and low ceilings possible. Additional activity expected to increase after 18Z for the Caribbean waters and USVI terminals as well. Winds are from south above the SFC to FL050 on 15/12Z. Hz due to Saharan dust is expected to increase after 15/16Z for the USVI, but VIS should remain at P6SM. && MARINE... Surface high pressure over the Atlantic and an induced trough north of the area will promote light to moderate east to southeast winds. Moist air will allow showers and isolated thunderstorms to develop over portions of the local waters tonight, resulting in localized hazardous marine conditions. Winds will continue to diminish and veer, becoming more southeasterly through tomorrow. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Excessive Heat Watch from Sunday morning through Sunday afternoon for PRZ001-002-005-008-010-012-013. VI...Excessive Heat Watch from Sunday morning through Sunday afternoon for VIZ001-002. AM...None. && $$ ERG/YZR/RC