Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
397 FXCA62 TJSJ 162022 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 422 PM AST Sun Jun 16 2024 .SYNOPSIS... Warm days with heat indices near 108 to 112F for most coastal areas and urban sections will continue for the beginning of the week. Afternoon convection is expected across the western and central interior to northern sections of Puerto Rico. Given the expected conditions, an excessive heat watch is in effect for Monday. Marine conditions remain calm with seas around 1 to 3 feet. && .SHORT TERM...Tonight 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. A moist airmass situated across the region will allow showers and isolated thunderstorms to linger into the evening hours, before clearing out overnight. The mid to upper ridge will begin to erode tomorrow as an upper trough is forecast to cross the west and southwest Atlantic and establish just west of the region by Tuesday afternoon. It appears that another surface trough will pass though the region tomorrow, continuing a wet pattern, especially during the afternoon hours. A pattern of clouds and plenty of sunshine will be seen tomorrow, allowing afternoon showers and isolated thunderstorm development over portions of the central interior and west sections of Puerto Rico, lingering around and moving to the northern coastlines meanwhile providing beneficial rain and possibly urban flooding. More Saharan dust is projected to arrive on late Tuesday after the passage of this wave, however will be short lived as a thick tropical airmass moves into the region afterwards. Expect an increase of low level moisture convergence and lower level instability 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 and southeasterly winds will again favor maximum heat indices to exceed 100 degrees tomorrow. 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... /from previous discussion/ 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... (18Z TAFS) sOME SHRA/TSRA dvlpd just bfr 16/16Z and are lingering just east of TSJU, +SHRA with isold TSRA were also seen just NW of TISX. SHRA/TSRA are still expected to dvlp wrn PR and interior aft 16/18Z with lcl MVFR/IFR conds across TJMZ aft 16/1730Z and TJBQ aft 16/1830Z. Conds should return to VFR by 16/23Z. Winds SE less than 10 kt with strong sea breeze influences. Gusts up to 22 kt in sea breezes and nr TSRA. Winds bcmg calm or land breezes aft 16/23Z. Higher cloud coverage north of 19.5 degrees north with SHRA/TSRA activity. Similar prognosis on Monday. && .MARINE... Surface high pressure over the Atlantic and an induced trough north of the area will promote light to moderate east to southeast winds. Moist air will allow showers and isolated thunderstorms to develop over portions of the local waters resulting in localized hazardous marine conditions. Winds will continue to diminish and veer, remaining southeasterly through the beginning of the week. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Excessive Heat Watch from Monday morning through Monday afternoon for PRZ001-002-005-008-010-012-013. VI...Excessive Heat Watch from Monday morning through Monday afternoon for VIZ001-002. AM...None. && $$ ERG/RC/YZR