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Area Forecast Discussion (AFD)
358 FXCA62 TJSJ 200729 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 329 AM AST Wed May 20 2026 ...New SHORT TERM, LONG TERM, AVIATION, MARINE, BEACH FORECAST, FIRE WEATHER... .KEY MESSAGES... Issued at 324 AM AST Wed May 20 2026 * Breezy, to locally windy, east to east-southeast winds will continue through much of the week, steering periods of fast moving showers towards windward sectors. This can cause unsecured items to blow around. * Afternoon showers and isolated thunderstorms are expected each day across interior and west to northwest Puerto Rico limited to elevated flooding risk) due to daytime heating, local effects, and nearby troughing. Lines of showers can also develop downwind of the local islands and El Yunque. * Breezy conditions will also sustain choppy seas, hazardous marine conditions and up to a moderate risk of rip currents. A Small Craft Advisory remains in effect for the offshore Atlantic through at least Thursday afternoon. * A limited heat risk will persist this week, with heat indices ranging from the mid 90s to the low 100s across urban and coastal areas each afternoon. * Instability should gradually increase Saturday onwards, as an upper-level trough extending into the tropics may shift eastward and linger north of the CWA while moisture content increases. && .Short Term(Today through Friday)... Issued at 324 AM AST Wed May 20 2026 Current satellite derived precipitable water (PWAT) values indicate below normal to normal columnar moisture for this time of the year. PWAT values are currently at around 1.40 inches over the USVI, Vieques and Culebra, and 1.45 to 1.66 inches over PR. Shallow moisture and quick moving showers have continued to be steered towards the region by breezy east to east-southeast winds. At least minimal accumulations since midnight have been detected over the eastern half of PR (including north-central and south-central PR), Vieques, Culebra and the USVI. The highest accumulations from midnight to 3 AM AST have been over Yabucoa with up to a radar estimated around 0.6 inches of rain. Patchy fog was also detected over sectors of interior PR. Similar to the last few days, lows have been in the low 70s to locally near 80 over urban and coastal areas of the islands, and in the upper 50s to the 60s at higher elevations of PR. A surface high pressure over the western to central Atlantic will tighten the local pressure gradient and promote breezy, to locally windy, east to east-southeast winds, with sea and land breeze variations. This flow will sustain an advective pattern, with moisture and fast-moving showers, moving over the region. PWAT values will be at normal values for most of the period (1.50 to 1.90 inches) with model guidance suggesting that a patch of drier air can reach the islands overnight to Thursday afternoon, prompting a dip in PWAT values to below normal values. PWAT values can also reach above normal values (above 2 inches) during afternoon convection, each day. A mid- to upper- level trough NW of the islands will remain during the period, adding some instability and allowing the available moisture to reach the mid-levels. However, patches of drier air in the mid- levels will also be present. The breezy, to locally windy, east to east-southeast steering flow will continue to result in quick moving showers advecting towards windward sectors through the workweek and in periods of variable to showery weather (with locally higher gusts). This can also result in ponding of water over roads and poorly drained areas, with a chance of urban and small stream flooding. An afternoon convective pattern will also develop each day as diurnal heating, local orographic effects and sea breeze convergence fuel this activity. Convective showers and isolated t-storms are forecast to develop over interior towards W to NW Puerto Rico, resulting in a limited to elevated flooding risk. Flood advisories might need to be issued each afternoon. Lines of showers can also develop downwind of El Yunque and the USVI, towards eastern PR. The breezy, to locally windy, steering flow can also help limit rainfall accumulations as showers and t-storms move faster than usual. However, this flow can result in unsecured items blowing around. Current model guidance suggests that 925 mb wind speeds will continue at high-end normal to above normal values through the rest of the period. Current model guidance suggests that 925 mb temperatures will remain at seasonal levels today and Friday and at high-end normal values to slightly above normal on Thursday. Maximum temperatures will reach the upper 80s to lower 90s at urban and coastal areas, while heat indices can exceed the upper 90s and 100 degrees, during the afternoon hours. This will result in a limited heat risk. The east to east-southeast flow will also steer low concentrations of saharan dust towards the islands during the period. && .Long Term(Saturday through Wednesday)... Issued at 324 AM AST Wed May 20 2026 No major changes were introduced to the long-term forecast. As mentioned in the previous discussion, a broad surface high pressure will linger over the Western Atlantic through most of the period, promoting E-ESE winds and resulting in breezy to locally windy conditions across the CWA. Instability should gradually increase Saturday onwards, as an upper-level trough extending into the tropics may shift eastward and linger north of the CWA. According to the latest model guidance, moisture content will increase as well, with PWAT values approaching near-normal levels (up to 2.0 inches). Additionally, the latest Grand Ensemble shows members tending to wetter conditions for the first part of next workweek (difference around 0.2 - 0.3 inches), increasing confidence. Under the influence of the trough, mid-level temperatures may become slightly cooler than normal (near -9 degrees Celsius), while winds aloft may become stronger and allow ventilation. Additionally, the latest Galvez- Davison Index (GDI) solutions dont rule out the potential for isolated to scattered thunderstorms across the islands each day, becoming frequent Monday onward during afternoon into night hours. Hence, windward sections of the islands can expect passing showers moving each day, while afternoon showers and thunderstorms will likely concentrate over western/northwestern Puerto Rico. Due to strengthening winds, showers should remain progressive, with rainfall accumulations likely to result in ponding of water over roadways and poorly drained areas, including minor flooding. Nevertheless, urban and small streams flooding cannot be ruled out over the aforementioned areas. As mentioned in the previous discussion, warmer than normal temperatures and abundant moisture content will increase the likelihood of heat indices exceeding 100 degrees Fahrenheit. Although a limited risk is expected across low elevations and urban areas of the islands, individuals sensitive to heat must take necessary precautions, such as remaining hydrated and avoiding long sun exposure. && .AVIATION... (06Z TAFS) Issued at 324 AM AST Wed May 20 2026 Mainly VFR conditions across the terminals. Breezy, to locally windy, E to ESE winds up to 15-22 kts after 20/13Z, with higher gusts and sea breeze variations, decreasing after 20/23Z. these winds will steer rounds of -SHRA/VCSH towards windward terminals throughout the period. Btwn 20/16-23Z, SHRA/+SHRA, with isolated TS, will develop across interior to W-NW PR, possibly resulting in brief MVFR conditions for TJBQ. Lines of -SHRA/VCSH can also develop from the local islands and El Yunque, affecting TJSJ. && .MARINE... Issued at 324 AM AST Wed May 20 2026 Marine conditions will remain somewhat hazardous through the next several days as strong high pressure over the Atlantic continues to support moderate to fresh easterly winds across the regional waters. Winds will occasionally reach locally strong levels, resulting in choppy to rough seas at times. Small Craft Advisories remain in effect through at least Thursday afternoon for the offshore Atlantic waters. In addition, isolated afternoon and early evening thunderstorms may develop each day across the western coastal waters of Puerto Rico, producing locally higher winds and seas near storms. && .BEACH FORECAST... Issued at 324 AM AST Wed May 20 2026 Moderate rip current risk conditions are expected to persist across most local beaches through much of the week and into the upcoming weekend due to persistent breezy conditions and ongoing wind-driven seas. The highest risk will continue along north and east-facing beaches. Beachgoers should exercise caution, avoid swimming alone, and remain near lifeguard-protected beaches whenever possible, as life-threatening rip currents can develop even under moderate risk conditions. && .FIRE WEATHER... Issued at 324 AM AST Wed May 20 2026 Increasing moisture today could prevent RH values from decreasing below threshold values for prolonged periods; however, rainfall should again remain limited across the southern plains, allowing relative humidity to drop to critical fire weather thresholds in the region. Additionally, sustained winds of 15 to around 20 mph, with higher gusts, combined with elevated KBDI values, will favor the rapid spread of wildfires across the southern coastal plains. The low 7-day percent of normal rainfall will also persist across the southeastern coastal plains. Fire management partners are once again urged to remain vigilant. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...Small Craft Advisory until 6 PM AST Thursday for AMZ711. && $$ SHORT TERM/AVIATION/FIRE WEATHER/KEY MESSAGES...MRR LONG TERM....MNG MARINE/BEACH FORECAST...ICP
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