AVIATION
Moisture ahead of a tropical wave passage will increase overnight and Monday for increasing showers tonight and increasing thunderstorms on Sunday. Generally VFR conds to prevail except nr SHRA in PR and the USVI. Mtn obscurations. Wind flow is east around 10 knots with land breeze variations. Aft 08/14Z SHRA/TSRA activity to increase area wide, with sct-nmrs TSRA aft 08/17Z in wrn and interior PR.

PREV DISCUSSION
/issued 402 PM AST Sat Sep 7 2024/

SYNOPSIS...

The upper-level trough will continue to promote shower and thunderstorm activity through Sunday, particularly across the waters, interior, and western sectors of Puerto Rico, along with an approaching tropical wave. The axis of this wave will move across the local region tomorrow and Monday, further enhancing instability and increasing rainfall activity. Above-normal heat indices are still expected to persist across some sectors throughout the next several days.

SHORT TERM
Today through Sunday

A variable weather pattern prevailed from the early morning into the afternoon hours. Initially, isolated passing showers were observed across most of the local waters, streaming over the eastern sections of Puerto Rico and some coastal areas of the U.S. Virgin Islands.
Around noon, shower and thunderstorm activity intensified across the mountains, particularly along the interior and the west, resulting in us issuing Special Weather Statements and Flood Advisories for those areas. Daytime temperatures remained in the upper 80s to lower 90s, with heat indices reaching 108-110°F, based in unofficial weather stations.

An upper-level low just northeast of the region will continue to provide an unsettled weather pattern for the rest of the day and into tomorrow. This instability will combine with the tropical moisture of an approaching tropical wave, with the axis over the Lesser Antilles. This combination will favor thunderstorm development over the islands through the short-term period.
Consequently, the risk for excessive rainfall will remain elevated, mainly for Puerto Rico. Saturated soils could lead to localized flash flooding and mudslides, particularly across small streams, creeks, and areas of steep terrain. Residents and visitors should exercise caution or avoid outdoor activities during the afternoon. Across the USVI, minor flooding may occur on roads and in poor drainage areas as showers increase.

The islands can expect warmer conditions through the short-term period, with maximum temperatures near the upper 80s to lower 90s in most urban and coastal areas. Heat indices could exacerbate health issues in vulnerable populations. Given the forecast conditions, heat advisories and warnings will likely be issued.

LONG TERM
Tuesday through Saturday
//from previous discussion//

The upper trough that was at or near the islands during the short term period is forecast to persist through most of the long term period increasing instability, although weakening past midweek. By the end of the week, a weak mid-level ridge will somewhat reduce instability aloft. Precipitable water (PWAT) values will continue mainly above 2.1 inches for the period, above normal values for this time of the year. East to east-southeast winds will aid in providing moisture to the area. Expect variable weather conditions as local, orographic and diurnal effects generates afternoon convective activity, boosted by the disturbance aloft and available moisture.
At least through early next week, this convective activity will increase the risk of flooding during the afternoon for sectors of interior and western Puerto Rico. Lines of convection (showers and thunderstorms) can also develop from the USVI, Vieques, Culebra, and from mountains of Puerto Rico. Periods of passing overnight and early morning showers are also forecast across windward sectors of the islands. The Climate Prediction Center's (CPC) Global Tropics Hazards Outlook indicates a greater than 50% probability of above- average rainfall across the northeastern Caribbean during this period. Model guidance indicates a slight increase in Saharan Dust on Tuesday. With available moisture and east to east-southeast winds, heat indices in coastal, urban and lower elevation areas can reach above 108 degrees Fahrenheit. The National Hurricane Center (NHC) is monitoring an area, currently, in the eastern to central tropical Atlantic, with a low cyclonic formation chance through the next seven days. While it's too early to assess any potential impact on our forecast area, please continue to monitor future updates from the NHC and form WFO San Juan.

AVIATION...
(18z)

VFR conditions will persist for most of the period, with brief MVFR conditions for TJBQ and TJPS due to afternoon TSRA and SHRA. Winds will continue from the E at up to 15 knots, with gusty winds expected. TSRA and SHRA may result in lower ceilings across the mountains and reduced visibility.

MARINE...

A surface high pressure across the Atlantic will promote gentle to moderate easterly winds over the next few days. An upper-level trough moving from the northeast will promote shower and thunderstorm activity across the regional waters along with an approaching tropical wave. This wave will continue to approach the US Virgin Islands and Puerto Rico tonight. Otherwise, thunderstorms will affect the coastal waters of western Puerto Rico each afternoon.

BEACH FORECAST...

A moderate rip current risk is present along the northern and eastern-facing beaches of Puerto Rico. Low risk elsewhere. It is important to remember that life- threatening rip currents are still possible in the surf zone of these beaches.

SJU WATCHES/WARNINGS/ADVISORIES
PR...None.
VI...None.
AM...None.