AFDSJU Area Forecast Discussion National Weather Service San Juan PR 258 PM AST Sat Mar 21 2026 ...New SHORT TERM, AVIATION, MARINE, BEACH FORECAST... .KEY MESSAGES... Issued at 209 PM AST Sat Mar 21 2026 * For the rest of the afternoon, isolated showers may develop over interior Puerto Rico and drift northward with the southerly winds. None to limited flood threat. * A more unsettled weather pattern is forecast to develop from late tonight into Sunday, as shower and thunderstorm activity associated with an approaching frontal boundary and its induced surface trough moves into the CWA. These conditions may lead to localized flooding, landslides in areas of steep terrain and lightning hazard. * Residents and visitors are advised to monitor the forecast as conditions evolve and to avoid rivers and flood-prone areas, particularly on Sunday. * A low to moderate risk of rip currents exists across the coastal waters of Puerto Rico and the U.S. Virgin Islands. && .Short Term(This evening through Monday)... Issued at 209 PM AST Sat Mar 21 2026 Fairly calm weather conditions were observed during the morning hours. A few isolated showers developed over the surrounding waters and moved into the southern coastal areas of Puerto Rico under a southerly wind flow; however, rainfall accumulations remained minimal. Currently, the southern extent of a frontal boundary associated with a polar trough is affecting the Dominican Republic. Mid-level conditions remain relatively dry, as indicated by the TJSJ 21/12Z upper-air sounding, which shows mid-level relative humidity near 28 percent. This dryness may initially limit convective development. However, the 700500 mb advected layered precipitable water product indicates that moisture will gradually increase through the afternoon and evening as the frontal boundary approaches the region, allowing for a gradual increase in shower activity. For the remainder of the afternoon, isolated to scattered showers may develop over interior Puerto Rico and drift northward by evening. Despite the increasing moisture, the overall flooding risk is expected to remain limited. As the polar trough and associated frontal boundary continue to approach, the frequency of showers and thunderstorms is expected to increase, particularly late tonight into Sunday. Activity will initially affect northwestern Puerto Rico late tonight into early Sunday morning, followed by additional shower and thunderstorm development over interior areas by midday and into the afternoon. Winds will veer from southerly tonight to southwesterly or westerly Sunday morning, becoming northwesterly to northerly by Sunday afternoon while decreasing in speed. This evolving wind pattern may result in erratic storm motion, especially on Sunday. The reduction in steering winds could also enhance rainfall accumulation over localized areas. Cloud cover will be an important limiting factor for convective development. Although upper-level conditions will become more unstable as the polar trough approaches and the frontal boundary moves across the area, the bulk of the deeper convection is expected to remain north of the islands over the Atlantic waters. Therefore, local weather conditions will depend largely on advective shower activity as well as localized convection driven by diurnal heating and sea breeze convergence, both of which will be influenced by cloud coverage. Nonetheless, conditions should continue to be monitored, as persistent heavy rainfall could result in excessive runoff and rapid river rises. Although relatively dry antecedent conditions may initially mitigate flooding impacts, prolonged rainfall could still lead to flooding in vulnerable areas, particularly in flood-prone locations and along steep terrain where landslides cannot be ruled out. Additionally, any thunderstorm activity may produce gusty winds and frequent lightning. Residents and visitors are advised to monitor the forecast as conditions evolve and to avoid rivers and flood-prone areas, particularly on Sunday. By Monday, conditions will depend on how quickly the frontal boundary moves east of the region. This will likely be a transition day, with lingering showers and thunderstorms possible, mainly across northeastern Puerto Rico, Vieques, Culebra, and the northern U.S. Virgin Islands. && .LONG TERM(Tuesday through Saturday)... Issued at 304 AM AST Sat Mar 21 2026 A broad mid- to upper-level trough will persist across the western Atlantic and northeastern Caribbean through the period, maintaining unsettled conditions across Puerto Rico and the U.S. Virgin Islands. Moisture will remain near to above normal throughout the period. On Wednesday, lighter steering flow and sufficient moisture will favor slow-moving showers, particularly across windward areas during the morning and over the interior and western portions of Puerto Rico during the afternoon. Thunderstorms are possible, with locally heavy rainfall that may lead to urban and small stream flooding as well as frequent lightning. From Thursday onward, moisture will deepen across the region while 500 mb temperatures remain below -8 degrees Celsius, resulting in a more unstable atmosphere. This combination of cold air aloft and abundant moisture through a deep layer will promote more efficient rainfall processes and an increased coverage of showers and thunderstorms each day. Periods of heavy rain will maintain an elevated risk of flooding, especially in areas that receive repeated rounds of precipitation. Gusty winds will be possible in and near heavier showers, while saturated soils will increase the likelihood of flooding, ponding of water on roadways, and minor landslides in steep terrain. Overall, an extended period of unsettled weather is expected from midweek through Saturday, driven by persistent troughing aloft, cold mid-level temperatures, abundant moisture at all levels, and periodic disturbances moving through the region. && .AVIATION... (18Z TAFS) Issued at 209 PM AST Sat Mar 21 2026 Mainly VFR conds expected at the terminals early in the period with - SHRA/VCSH developing aft 21/23Z, becoming more frequent ovr PR terminals, particularly TJBQ, overnight into Sunday. THe passage of a shearline will support SHRA/TSRA development, especially aft 22/12Z across interior and NWRN PR. Periods of MVFR/brief IFR conds possible in SHRA/TSRA, with reduced VIS and lower CIGs (BKN/OVC010- 020). Mtn obsc likely, particularly across interior PR. Winds will veer during the period, from S-SW at 1020 kt with higher gusts thru tonight, becoming W by Sunday morning, then NW-N aft 22/15Z. && .MARINE... Issued at 209 PM AST Sat Mar 21 2026 A pre-frontal trough and associated boundary approaching from the northwest will continue to weaken the pressure gradient, promoting light to moderate south to southwest winds tonight. Periods of unsettled weather will increase shower activity and possible thunderstorm development across the local waters, especially during the overnight hours into Sunday. Localized hazardous marine conditions near storms and gusty winds, particularly across the Atlantic waters and the Mona Passage. Late Sunday into early next week, high pressure building over the western Atlantic will promote the return northeasterly winds and increasing seas, leading to deteriorating marine conditions. && .BEACH FORECAST... Issued at 255 PM AST Sat Mar 21 2026 Mainly, a moderate risk of rip currents will prevail over the next few days along the northern and eastern beaches of Puerto Rico, including Vieques, Culebra, as well as the U.S. Virgin Islands. A moderate risk means that life-threatening rip currents remain possible in the surf zones, and beachgoers should remain cautious. A small pulse of northerly swell may locally increase the risk in exposed beaches. A low risk are expected along the more protected southern beaches. Additionally, increasing moisture and instability will bring periods of showers and isolated thunderstorms posing a danger to those in or near the water, mainly tomorrow, Sunday. Stay weather- aware! For additional information and location specific rip current details, please visit: weather.gov/beach/sju. && .HYDROLOGY... Issued at 255 PM AST Sat Mar 21 2026 The expected increase in moisture associated with the approaching frontal boundary and upper-level trough will result in periods of showers and isolated thunderstorms through the next 24 hours. While relatively dry antecedent conditions may initially limit excessive runoff and delay river responses, the potential for persistent or slow-moving showers, enhanced by weakening winds, could lead to localized ponding of water, urban and small stream flooding, and rapid river rises. The risk of flooding will remain generally limited but could increase in areas experiencing repeated rainfall from late tonight into Sunday. Additionally, isolated landslides cannot be ruled out in steeper terrain where soils may become saturated. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$