Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
225 FXCA62 TJSJ 160843 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 443 AM AST Sun Jun 16 2024 .SYNOPSIS... A more east-southeasterly wind flow will relieve the heat indices today. Although conditions will be warmer, the heat indices will remain near 108 to 112F for all coastal areas and urban sections of the vicinity of Caguas. Given the expected conditions, a heat advisory is in effect for the abovementioned areas, and an excessive heat watch is in effect for the same areas for Monday. The available moisture will induce showers across the mountains and the western interior during the afternoon hours. && .SHORT TERM...Today through Tuesday... A broad surface trough with axis extending northward across Hispaniola and the Mona passage and strong High pressure over the central and northeast Atlantic will maintain a light east southeast wind flow across the region. Fragments of trailing low level moisture will continue to affect the region bringing periods of passing showers to the coastal areas of the islands during the rest of the morning hours. Recent Doppler weather radar VAD (Velocity Azimuth Display) wind profile and previous ASCAT Satellite derived winds both suggest light low level wind from east southeast below 700 millibars. Meanwhile the upper ridge will hold across the region today into early Monday before eroding, as an upper trough is forecast to cross the west and southwest Atlantic and settle in just west of the region by Tuesday afternoon. This overall local pattern suggest another day of good surface heating with light winds and local sea breezes. Therefore good breaks in clouds and plenty of sunshine will give way to afternoon showers and isolated thunderstorm development over portions of the central interior and west sections of Puerto Rico. A few late afternoon showers will be possible in and around the San Juan metro as well some of the USVI but mostly hot and sunny conditions should prevail. Only traces of Saharan dust will linger across the region today through Monday but yet another plume of dust is expected to filter in and reach the Southeast Caribbean by late Tuesday accompanying and trailing the next tropical wave. This tropical wave is so far forecast to enter and cross the southeast Caribbean late Monday through Tuesday. By then and increase in low level moisture convergence and the erosion of the aforementioned upper ridge should aid in enhancing early morning and afternoon convection across the islands during the latter part of the period. Daytime high temperatures are forecast to be in the upper 80s to low 90s especially along the north and west coastal areas. This along with the humid conditions ans southeasterly winds will again favor maximum heat indices to exceed 100 degrees, resulting in excessive heat impacts especially along the west, north and east coastal and urban areas of Puerto Rico as well as the adjacent islands including the USVI. Please refer to the latest Non-Precipitable Weather Message(NPWSJU) issued by WFO SJU PR for additional information on the expected heat impacts. .LONG TERM...Wednesday through Sunday... More unstable weather conditions are expected at the beginning of the long term due to an upper-level trough just west of the region, with the axis over Hispaniola, leaving the CWA on the divergent side of the TUTT. Although there are some discrepancies in the intensity of the unstable conditions between global model guidance, the islands should expect a deteriorating weather trend from Wednesday into Thursday. At the surface, moisture from a tropical wave will combine with this instability, increasing the potential for shower and thunderstorm development. Isolated thunderstorms are possible in the afternoon across the mountains and over the local waters due to colder temperatures at 500 MB, ranging from -7 to -8 degrees. According to the Glvez-Davison Index (GDI), there is a potential for isolated to scattered thunderstorms from Wednesday through Friday. Therefore, the forecast reflects afternoon showers with a limited flood threat. Hazy skies might be possible as a big pulse of Saharan Dust Air Layer invades the local islands. From Saturday into Sunday, a broad surface high pressure over the Central Atlantic extending to the Eastern Atlantic will result in variable weather conditions. Under the east-southeasterly wind flow, plenty of surface moisture will continue to stream across the local islands, keeping the potential for convection in the afternoon, especially across the mountains and the western interior. Although plenty of moisture will remain in place, more stable conditions will be present at the upper level, provided by an upper-level ridge. A similar temperature pattern is expected across the islands by the first part of the long term. According to model guidance for 925 MB temperature, the trend calls for warmer conditions, resulting in heat indices around 108 to 112F. Residents and visitors are urged to stay informed of possible advisories or warnings due to excessive heat. && .AVIATION... (06Z) Wdly SCT SHRA ovr regional waters and passages between PR and the USVI til 16/12Z with Isold TSRA ovr Offshore ATL waters. SCT ocnl BKN lyrs nr FL025...FL050..FL080. VFR conds at all terminals but VCSH psbl at TJSJ/TIST/TISX til 15/12Z. Brief MVFR with Mtn top Obscr psbl ovr Eastern PR due to low clds and passing SHRA. SFC calm to lgt/vrb bcmg mainly fm the SE between 8-12 kts aft 16/14Z. No sig operational wx impacts attm. && .MARINE... A broad surface high pressure over the Central Atlantic extending into the Eastern Atlantic, interacting with an induced surface trough just north of the CWA, will continue to promote light to moderate east-southeasterly winds. Under this pattern, the local waters will experience seas up to 4 feet and winds up to 10 knots. Isolated to moderate showers with thunderstorm activity could result in localized hazardous marine conditions. There is a low risk of rip currents for all the exposed coastal beaches in the region. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Heat Advisory from 10 AM this morning to 6 PM AST this evening for PRZ001-002-005-008-010-012-013. Excessive Heat Watch from Monday morning through Monday afternoon for PRZ001-002-005-008-010-012-013. VI...Heat Advisory from 10 AM this morning to 6 PM AST this evening for VIZ001-002. Excessive Heat Watch from Monday morning through Monday afternoon for VIZ001-002. AM...None. && $$ SHORT TERM...RAM LONG TERM....LIS AVIATION...RAM