000 FXCA62 TJSJ 201933 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 333 PM AST Mon May 20 2024 .SYNOPSIS... The warm-to-hot spell will persist through Tuesday afternoon under an east-southeasterly wind flow. The mid-level ridge will restrict widespread afternoon activity across the islands through Wednesday. However, afternoon convection due to excessive heating and local effects will impact portions of PR and the USVI daily. From Wednesday afternoon onward, we anticipate a moist and unstable weather pattern as above-normal moisture pools over the islands, interacting with a deep layer trough aloft. Consequently, a Hydrologic Outlook (ESFSJU) has been issued for Puerto Rico and the Virgin Islands from Wednesday through Friday due to an increased risk of flooding. && .SHORT TERM...Tonight through Wednesday... Calm but warm to hot weather conditions prevailed through this morning, with above-normal heat indices recorded, particularly across the urban and coastal areas from the San Juan Metropolitan Area to Northwest Puerto Rico. This is where an Excessive Heat Warning has been issued. The excessive heating has led to strong afternoon convection across isolated areas. The rest of PR and the US Virgin Islands have observed mostly clear skies under an east- southeasterly wind flow. Sea breeze-driven afternoon and evening convection will produce downpours across the northwest quadrant of Puerto Rico for the rest of this afternoon into the evening. A 60 to 80 percent chance of rain is expected for those locations. Excessive heating due to the combination of warm air temperatures and available moisture will result in warmer-to-hot heat indices for the urban and coastal sections in Puerto Rico and the US Virgin Islands tomorrow afternoon. Therefore, we may issue Heat Advisories or Excessive Heat Warnings for that period tomorrow early morning. Once again, afternoon convection may develop near the San Juan Metropolitan Area, the interior, and the northwest quadrant under the east-southeasterly wind pattern. Downpours may result in ponding of water or flooding of poorly drained areas in these locations. An approaching deep-layer trough aloft will increase instability across the region. Additionally, moisture from a trade wind perturbation and plenty of tropical moisture lifted from the southern Caribbean will pool above-normal moisture content over the islands. Therefore, a Hydrologic Outlook (ESFSJU) was issued from Wednesday through Friday for Puerto Rico and the Virgin Islands due to an increased risk of flooding. .LONG TERM...Thursday through Monday...PREV DISCUSSION.../issued 416 AM AST Mon May 20 2024/ On Thursday, the islands will be squeezed between a surface high pressure over the central Atlantic and a surface trough/low over Hispaniola. Plenty of moisture will stream from the Caribbean Sea into the islands, with precipitable water values above 2.5 inches possible (above climatology). Additionally, pockets of vorticity at the mid levels will be advected into the islands, hence conditions will favor the continuation of showers and thunderstorms over the region into Friday. It is not that it will rain all the time, but these showers could be heavy from time to time, maintaining elevated the risk for flooding, mudslides and rapid river rises. Also, early on Thursday, as the low lifts northward, a swath of winds will move over the region, bringing gusts of 20-25 knots with the heavy showers and thunderstorms. By the weekend, trailing moisture from the low, now expected to be well north of Puerto Rico will continue to impact the region. Although the rain is not expected to be as widespread, it could aid in the development of convection for the interior, north and west of Puerto Rico in the afternoon. Some showers will also move in the vicinity of the Virgin Islands. On Sunday and Monday, some improvement is anticipated, mostly for the Virgin Islands and eastern Puerto Rico, but conditions will remain very favorable for afternoon activity, again for the interior, northern and western Puerto Rico. The heat risk was maintained below the limited criteria for Wednesday and Thursday, mostly because of the cloud coverage, but it is expected to be back again Friday onward, when the sun peeks once more across the northeastern Caribbean archipelagos. && .AVIATION... (18Z TAFS) VFR conds expected during the next 24 hours. However, SHRA in and around TJSJ/TJBQ may cause brief MVFR conds thru 20/22z. Sfc winds ESE arnd 14 kt with stronger sea breeze influences til 20/22Z followed by winds ovr land less than 6 kt in land breezes. VCSH ovr the USVI and ern PR aft 20/22Z. Aft 21/14Z sea breezes will return with winds up to 15 kt in ESE flow. Trace HZ due to Saharan dust, but vsby should remain P6SM. && .MARINE... A surface high pressure across the central and northeast Atlantic will drive the trade winds from the southeast at moderate speed. A trough will develop south of Hispaniola, increasing shower and thunderstorm activity across the local waters Wednesday onward. This trough may increase winds across the Mona Passage and Atlantic waters Wednesday night into Thursday. && .BEACH FORECAST... The risk for rip currents is low, however, life-threatening rip currents often occur in the vicinity of groins, jetties, reefs, and piers. && .HYDROLOGY... A significant increase in moisture caused by an approaching mid to upper level trough, and a developing surface low pressure system will result in an enhanced risk for flooding later this week, especially between Wednesday afternoon through Friday evening, across Puerto Rico and the Virgin Islands. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Excessive Heat Warning until 6 PM AST this evening for PRZ001-005-008. PR...Heat Advisory until 6 PM AST this evening for PRZ002>003-007-010>013. VI...Heat Advisory until 6 PM AST this evening for VIZ001-002. AM...None. && $$ ICP/CAM/MRR