Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
594 FXCA62 TJSJ 200811 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 411 AM AST Mon May 20 2024 .SYNOPSIS... Hot conditions will prevail across the local islands through Tuesday. Some Saharan dust will linger around too, resulting in hazy skies. Then, it will become unsettled and wet, as a trough develops west of the islands. The risk for flooding and mudslides will increase for the second half of the workweek. Conditions will gradually improve during the weekend. && .SHORT TERM...Today through Wednesday... Showers lingered across northwestern Puerto Rico during the early night hours and then across the local waters onward through the morning with some light showers brushing the eastern coastal region of Puerto Rico. Another hot day is expected with a Heat Advisory in place for all coastal areas of Puerto Rico and the USVI today from 10 AM to 6 PM local time. An area of weak high pressure across the central Atlantic and low pressure at the surface in the western Atlantic will allow light to moderate southeasterly flow to prevail through Wednesday. This will bring warmer than usual temperatures across the islands, especially for coastal areas that are expected to have heat indices between 100 and 110, some isolated areas higher. Due to this, heat advisories are anticipated through at least tomorrow. Relatively normal precipitable water values through tomorrow will allow a seasonal passing showers and afternoon convection to prevail, with north western Puerto rico having the best chance of rain during the afternoon hours. Conditions will begin to shift to a wetter pattern by Wednesday morning as as a deep layered trough moves through the Caribbean basin, raising a moist airmass into the region. Models hint that the strongest segment of the jet aloft will remain well to the northwest of the area. However, 500 mb temperatures and precipitable water values are forecast to reach values that bring the expectation of numerous showers and scattered thunderstorms. Surface winds will also strengthen over Wednesday night due to the development of a surface trough west of the region. Due to this squally weather, the risk of flooding becomes present for areas that receive heavy rainfall. This is the beginning of a wet pattern as Wednesday appears to be the first day of this rain event which is expected to last through the weekend. .LONG TERM...Thursday through Monday 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... (06Z TAFS) Mainly VFR conditions expected to prevail. SHRA will move across the local waters, and could reach the USVI and PR terminals at times. After 16z an increase of rain is expected across the Cordillera Central and around TJBQ, where periods of reduced VIS and low ceilings are possible. Winds will from the SE at 8-13 kts, with stronger wind gusts and sea breezes variations. && .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. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Heat Advisory from 10 AM this morning to 6 PM AST this evening for PRZ001>003-005-007-008-010>013. VI...Heat Advisory from 10 AM this morning to 6 PM AST this evening for VIZ001-002. AM...None. && $$ SHORT TERM...RC LONG TERM....ERG