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330 FXCA62 TJSJ 151908 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 308 PM AST Mon Sep 15 2025 .KEY MESSAGES... * A tropical wave combined with a TUTT low expected around midweek, will increase the frequency of showers and thunderstorms throughout much of the workweek. As a result, the risk of flooding will remain elevated. Additional hazards include rapid river rises, landslides, and frequent lightning. * The Virgin Islands will also experience an increase in showers and thunderstorms, potentially leading to water surges in drainage guts, ponding on roadways, occasional lightning, and isolated landslides as the tropical wave approaches. * Increasing heat risk is forecast again toward the end of the week and into the weekend. A limited heat risk is expected today and over the next few days. * A moderate risk of rip currents is in effect for the northern and eastern beaches of Puerto Rico, as well as Culebra and St. Croix through Friday. A low risk is expected in other areas. Similar conditions are likely to continue through most of the workweek. && .SHORT TERM...Rest of the afternoon through Wednesday... Variable weather conditions prevailed through the morning hours as expected due to the arrival of an easterly perturbation over the region. Shower and thunderstorm activity initially clustered mainly over the surrounding waters. However, as the morning progressed, activity expanded in coverage, with several streamers developing downwind of the islands and mountainous terrain. Despite the increased coverage, showers and thunderstorms have been moving fairly quickly. Some thunderstorms have produced frequent lightning and gusty winds. This activity will persist through the evening hours over land areas. During the night hours, showers and thunderstorms could develop again over the waters, particularly as troughiness aloft persists, with some of this activity brushing windward coastal areas. However, any activity overnight should move fairly quickly. A similar pattern is expected on Tuesday, with afternoon convection enhanced by diurnal heating, sea breeze convergence, and local effects. However, showers and thunderstorms will tend to move more east-southwestward, shifting the highest activity over the interior and western to southwestern Puerto Rico as winds become more east- northeasterly. By Wednesday, weather conditions are expected to become significantly more active. A deep-layered trough northeast of the region will gradually move southwest and meander across the area. Model guidance suggests mid-level vort max will be near or over the region by Wednesday night. In addition, cool air advection at mid- levels will lower the 1000-500 mb thicknesses and steepen lapse rates between 850-700 mb and 700-500 mb. This feature will interact with a tropical wave (with axis currently near 52.6W) to create highly favorable conditions for heavy showers and scattered thunderstorms. Therefore, expect increasing areal coverage of rainfall on Wednesday with an elevated flooding threat. Urban and river flooding are possible, with landslides and rockfalls expected in areas of steep terrain. Frequent lightning and gusty winds could accompany stronger thunderstorms, potentially downing tree limbs and blowing around unsecured objects. .LONG TERM...Thursday through Monday... .PREV DISCUSSION... /issued 505 AM AST Mon Sep 15 2025/ By Thursday, unsettled weather conditions are expected to prevail due to the combined influence of a tropical wave and a TUTT low moving across Puerto Rico and the Virgin Islands. Latest model guidance suggests precipitable water (PWAT) values will remain above normal for this time of yearranging between 2.0 and 2.25 inches. These atmospheric features will maintain high rain chances and enhance thunderstorm development, supported by 500 mb temperatures near average for mid-September. As a result, urban and small stream flooding is likely to persist daily, and some rivers may reach flood stage. In the Virgin Islands, periods of showers and thunderstorms are expected to cause water surges along guts and drainage channels, ponding on roadways, and isolated urban flooding. Additional hazards include landslides, frequent lightning, and gusty winds. By Friday, drier air is forecast to move into the region, decreasing PWAT values to near normal levels, and dropping further to below normal over the weekend and into early next week. Rainfall activity will diminish, but localized afternoon convection, particularly in northwestern Puerto Rico, will still be possible due to local effects and diurnal heating. As cloud cover decreases and rain subsides, winds will shift from the southeast, and 925 mb temperatures are forecast to peak at nearly two standard deviations above normal, especially on Friday and Saturday. This pattern will elevate the heat risk across the region. Heat Advisories or even Extreme Heat Warnings may be issued. Residents and visitors are strongly advised to stay hydrated, avoid prolonged sun exposure during peak heat hours, and take extra precautions, particularly those without access to adequate cooling or hydration. This forecast scenario assumes the current guidance remains consistent for the tropical wave being monitored by the National Hurricane Center, which currently has a 30% chance of formation in the next 48 hours and an 80% chance over the next 7 days. If development continues as expected, the system may track northwestward toward the northeastern Caribbean. We will continue to monitor this closely and provide updates throughout the workweek. && .AVIATION... (18Z TAFs) SHRA/TSRA will continue across PR terminals through 15/23Z. Periods of MVFR, briefly IFR, with Mtn Obsc across the Cordillera Central are possible. USVI terminals will see VCSH/VCTS through the period, mainly downwind of islands. Frequent LTG and gusty winds possible with stronger TSRA. Overnight, SHRA/TSRA developing over waters may brush windward coastal terminals, but moving quickly. On Tue, aftn SHRA/TSRA expected mainly across W/SW PR with MVFR conds possible. Winds ESE at 10-15 kts with sea breeze variations and higher gusts, bcmg light/VRB aft 23Z, increasing again 10-15 kts from ENE by 16/14z with stronger gusts near SHRA/TSRA. && .MARINE... A surface high pressure system located over the central and eastern Atlantic, in combination with a low-level perturbation, will promote moderate to locally fresh east to southeasterly winds. This surface perturbation will also increase the likelihood of showers and thunderstorms across the waters and local Caribbean passages. Small craft are urged to exercise caution across the regional waters. A stronger tropical wave is expected to approach by midweek, enhancing shower coverage. && .BEACH FORECAST... The moderate rip current risk across the north and east-facing beaches of Puerto Rico, Culebra, and St. Croix will prevail through Friday. Similar conditions are expected to prevail most of the workweek so beachgoers are advised to exercise caution at all times as a moderate risk means life-threatherning rip current are possible in the surf zone. A low risk should prevail elsewhere. Additional hazards include afternoon thunderstorms. Remember: if you hear thunder, seek shelter inside of a safer place immediately. && .HYDROLOGY... The disturbance that will move in today, and the tropical wave coming in on Wednesday will maintain the risk of flooding elevated for most of the area in Puerto Rico and the Virgin Islands. The latest Root Soil Saturation product from PRAGWATER, and also from the National Water Model shows saturated soils along the western half of Puerto Rico, and also for the extreme east and southeast. Also, the 7-day average streamflow from the USGS show that many rivers are running near normal or well above normal. Therefore, it is likely that any upcoming rain will quickly turn into runoff, enhancing the risk of flooding, rapid river rises, and landslides. For the Virgin Islands, periods of heavy rain will cause water surges along drainage guts, and ponding of water on roadways. Urban flooding could develop too, while landslides in areas of steep terrain cannot be ruled out. Although an elevated flood risk is shown everyday of the week, the higher risk is on Wednesday and Thursday due to the above normal moisture from the tropical wave and increased instability aloft from an upper level low. Conditions will begin to improve by Friday. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM/AVIATION...CVB HYDROLOGY...ERG LONG TERM...YZR MARINE/BEACH FORECAST...MMC
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