037 FXCA62 TJSJ 211901 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 301 PM AST Wed May 21 2025 .KEY MESSAGES... * Afternoon showers and thunderstorms are expected daily across the Cordillera Central of Puerto Rico, creating a limited to elevated risk of flooding rains. * The warm to hot period will extend into tomorrow, Thursday, but will linger, in less magnitude, throughout much of the forecast period. * An unstable weather pattern may return by early next week. && .SHORT TERM...Tonight through Saturday... Skies remained mostly clear today across Puerto Rico and the U.S. Virgin Islands, allowing for strong diurnal heating. As a result, maximum heat index values reached between 100 and 110F, particularly across urban and coastal areas. Winds prevailed from the east-southeast at 5 to 15 mph, with locally higher gusts observed near passing showers. Doppler radar detected isolated thunderstorm development downwind of La Sierra de Luquillo during the early afternoon, followed by additional convective activity along the Cordillera Central later in the day, particularly from Villalba to Las Maras. Today`s atmospheric sounding reveals a favorable setup conducive to convective development across Puerto Rico, particularly in the interior and northwestern sectors of the island due to the east- southeast wind flow. One of the most noteworthy features is the presence of steep lapse rates at both low and mid-levels of the atmosphere. This stability parameter, combined with the wind flow, the island`s topography, diurnal heating, and sea breeze, will act as a lifting mechanism enhancing convective initiation during the afternoon hours. Given the instability, moisture, and favorable wind flow, there is an elevated risk for localized flooding, especially in areas prone to poor drainage or where soils are already saturated from recent rainfall events. As the afternoon progresses into the evening hours, rain activity will slowly dissipate. By sunset, most of the shower and thunderstorm activity is expected to wane, allowing for clearing skies and relatively calm weather conditions overnight. These clear and calm conditions will promote nighttime radiational cooling, with low temperatures dropping into the low 70s along the coastal zones, while the mountainous interior could see temperatures dip into the mid-60s, providing a cooler and more comfortable night. Although the warming/hot trend is forecast to wane by Friday, there is a moderate to high confidence that we will observe another Heat Advisory due to above normal values tomorrow, Thursday, between 10 am and 4 pm AST. Generally, the typical May rain pattern will hold through the short-term period, with calm conditions during the mornings and overnight hours, without ruling out one or two passing showers. Then, afternoon convection will return mainly due to the diurnal heating, local effects, and sea breeze. .LONG TERM...Saturday through Wednesday... The weekend will begin with a typical seasonal weather pattern, as near-normal to slightly below-normal moisture levels combine with marginal instability aloft, associated with weak ridging over the northeastern Caribbean. Shower activity will follow the characteristic diurnal pattern, with passing showers affecting windward coastal areas of the islands during the night and early morning hours. This will be followed by afternoon convective activity, driven by surface heating, sea breeze convergence, and orographic lifting over the interior and western portions of Puerto Rico. A few isolated thunderstorms may develop at times during this period, but they are expected to be brief. Additionally, breezy conditions will result in fast-moving showers, limiting rainfall accumulations in any specific area. At most, a limited flooding risk is expected, potentially leading to ponding on roadways and poor drainage areas, with very localized urban and small stream flooding. A limited excessive heat risk will also persist through the weekend across urban and coastal areas, primarily affecting heat-sensitive individuals, especially those outdoors without effective cooling or adequate hydration. Uncertainty persists by early next week due to the interaction between a deepening upper-level trough to the west of the forecast area and an approaching tropical wave from the east, currently located near 28.5W. The timing and placement of these synoptic features remain fairly consistent across model guidance, with the latest runs positioning the trough axis over Hispaniola between Tuesday and Wednesday. This places our area on the favorable (eastern) side of the trough. At the same time, the tropical wave is expected to begin lifting northward, moving over or near our region. However, the most active portion of the wave now appears to be slightly farther west compared to previous model runs. Despite this shift, well above-normal moisture and increased instability aloft will be present across the forecast area, increasing the potential for numerous showers and thunderstorms. This could lead to an increased flooding threat for Puerto Rico and the U.S. Virgin Islands. However, the Caribbean may be under the influence of the suppressing phase of the MJO, which is indicative of reduced upper-level divergence and decreased favorability for deep convective activity. In fact, the latest CPC Global Tropics Hazards Outlook assigns a 50% to 65% probability of below-average rainfall across the northeastern Caribbean. This introduces additional complexity to the forecast. Nevertheless, it is important to continue monitoring the situation, as there remains a chance for increased moisture convergence and instability to result in a heightened flooding risk by early next week. && .AVIATION... VFR conditions will continue across all TAF sites. However, SHRA/TSRA will develop along the Cordillera Central through 21/23z, creating mountain obscuration and possibly affecting JBQ, where TEMPOs are in place. Winds will continue from the E-ESE at 10-15 kt but locally higher near TSRA/SHRA, and with sea breeze variations, winds will become calm to light and variable after 21/23z. Winds will turn more from the E at around 15 kt tomorrow after 22/13z. We expect a similar weather pattern tomorrow. && .MARINE... Light to gentle easterly winds continue today across most local waters. However, a building surface high pressure over the central Atlantic will bring moderate to locally fresh easterly trades tonight through the weekend. Isolated thunderstorms will continue to develop across the regional waters, particularly across the northern and western coastal waters of Puerto Rico each afternoon. && .BEACH FORECAST... Beachgoers, the risk for rip currents is low tonight and throughout the rest of the week. However, please exercise caution as life-threatening rip currents often occur in the vicinity of groins, jetties, reefs, and piers. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Heat Advisory until 4 PM AST this afternoon for PRZ001-003-005- 007-008-010-011. VI...None. AM...None. && $$ Day shift...CAM/GRS Evening crew...LIS/MRR
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