Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
376 FXCA62 TJSJ 171620 AAA AFDSJU Area Forecast Discussion...UPDATED National Weather Service San Juan PR 1220 PM AST Tue Sep 17 2024 .UPDATE...The height of the top of the marine layer rose to around 4500 feet and this was enough for the formation of some overnight showers in a zone of convergence extending out of the remnants of Tropical Depression Gordon, which was recently downgraded to a trough. The moisture from this band will slowly sink southward over the area today through Thursday and some additional showers and possibly thunderstorms will be seen during the period. Temperatures have been a little more moderate today, but heat indices have still reached from 108 to 116 degrees in the usually hotter locations, therefore we will continue our warnings and advisories for the area. The morning sounding from San Juan, Puerto Rico was somewhat wetter due to the intrusion of moisture from the band out of the remnants of Tropical Depression Gordon, but most of the additional moisture was seen between 10 and 17 thousand feet. The radar was nearly devoid of significant shower activity in the forecast area and this is likely due to the limited instability found in the sounding. This morning`s CAPE came in at only 1571 J/Kg and the sounding was nearly capped at 11 kft or 700 mb. This is expected to improve. && .SHORT TERM... .LONG TERM... && .AVIATION... && .PREV DISCUSSION... /issued 500 AM AST Tue Sep 17 2024/ SYNOPSIS... A dry air mass continues to dominate the region and will allow mostly stable conditions today. Hot conditions will prevail; an Excessive Heat Warning has been issued for the urban and low- elevation areas of northern and western Puerto Rico, as well as for St. Croix. For the rest of the coastal areas, including Vieques, Culebra, St. Thomas, and St. John, a Heat Advisory is in effect, both from 10 AM to 5 PM AST. Easterly winds will dominate from today through tomorrow, then winds will become light and more variable in the latter half of the week. An upper level trough is expected to bring favorable conditions for the development of showers and isolated thunderstorms by the end of the week. SHORT TERM...Today through Thursday... A line of showers developed along the coast of northeast Puerto Rico, reaching portions of the San Juan metro area during the overnight hours. Other than that, only a few showers were observed in the offshore Atlantic and Caribbean waters. The islands are still covered by a pocket of dry air today, as is evident in satellite imagery. There is a patch of moisture just to the northeast of the region, but the moisture should stay away for today. A surface to upper level ridge continues to linger today, with a trade wind cap inversion around 850 mb. Above these levels, the air mass is dry, and the available moisture is trapped closer to the surface. In fact, the dewpoint depression for today is around 20 degree Celsius, an indicative of dry conditions. Rainfall activity should be very limited, but showers should manage to form over western Puerto Rico in the afternoon. This activity will be short- lived, and accumulations are not expected to be too significant. The main story for today, again, is the scorching heat across the U.S. Caribbean territories. Temperatures at the 925 mb are above normal, and with lows only dropping to the low 80s, and lots of sunshine, it will feet very hot during noon and in the afternoon hours. On Wednesday, a small area of moisture will cross the islands from north to south. Most of the activity should arrive in the overnight or in the early morning hours, but the increase in moisture should lead to stronger and more widespread showers over the interior and western Puerto Rico in the afternoon. Isolated to scattered showers are also expected to reach the Virgin Islands and eastern Puerto Rico during this period. On Thursday, the surface high weakens, and so will the winds. In fact, the low level steering flow is expected to be from the east at 5 knots or less. Moisture levels are not going to be too high, but with enough heating, showers are again expected for the interior and west, drifting toward the south and southwestern Puerto Rico. LONG TERM...Friday through Monday... Recent model guidance indicates that an upper-level trough will deepen and move southward between Puerto Rico and the northern Leeward Islands by Friday. This will help destabilize the upper levels and increase the likelihood of early morning and afternoon convection across the interior and western sectors of Puerto Rico. Additionally, the latest satellite-derived precipitable water values show an increase from mostly below normal to normal levels. Therefore, while there wont be enough moisture for widespread rain, there will be enough to develop showers. Throughout this period, winds are expected to remain very light and somewhat variable. As a result, urban and small stream flooding may occur due to locally and diurnally induced slow-moving showers and thunderstorms with locally heavy rainfall, especially in the central and western sections of Puerto Rico. By the weekend, surface winds are expected to shift to a more south- southeasterly direction and remain very light. The latest models suggest that the area of convergence associated with the upper- level trough will be over the region, which may help to limit rain chances. However, daytime heating and local effects will continue to enhance afternoon convection in some areas, particularly in the interior, northwestern, and north-central Puerto Rico. In contrast, the U.S. Virgin Islands is expected to experience mostly isolated showers. Hot conditions are likely to persist throughout this period, with temperatures potentially reaching the upper 90s and heat indices remaining elevated. Temperatures at 925 mb will remain well above normal through the weekend. The combination of high temperatures and moisture could create hazardous heat conditions, particularly in urban and low-elevation areas. Therefore, it is very likely that Heat Advisories and possibly Excessive Heat Warnings will be issued daily for some areas. Residents and visitors should take precautions to stay hydrated by drinking plenty of water, wearing light and breathable clothing, and avoiding prolonged sun exposure, especially during peak heat hours. AVIATION... (06Z TAFs) Mainly VFR conditions expected to prevail through the forecast period. Isolated SHRA will be possible in the vicinity of USVI and TJSJ, but without significant impacts to operations. Winds will be from the E at 12-16 kts, with stronger gusts. MARINE... A weak surface high-pressure system will promote gentle to moderate easterly winds. By midweek, the winds are expected to become lighter and more northeasterly. Today, very isolated showers are anticipated over the local waters as drier air continues to settle in. A few thunderstorms are possible off the western coast of Puerto Rico each afternoon. Overall, seas will range from 3 to 5 feet, occasionally reaching up to 6 feet across most local waters, with seas between 1 to 3 feet across most coastal waters over the next few days. BEACH FORECAST... && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Excessive Heat Warning until 5 PM AST this afternoon for PRZ001- 005-008-010. Heat Advisory until 5 PM AST this afternoon for PRZ002-003-007- 011>013. VI...Excessive Heat Warning until 5 PM AST this afternoon for VIZ002. Heat Advisory until 5 PM AST this afternoon for VIZ001. AM...None. && $$ SHORT TERM...WS LONG TERM....MRR AVIATION...99