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FXCA62 TJSJ 282155 AFDSJU

Area Forecast Discussion National Weather Service San Juan PR 555 PM AST Sun Apr 28 2024


SYNOPSIS

A surface high pressure over the western Atlantic will promote breezy northeasterly winds through at least Wednesday. Drier airmass between a surface low over the north central Atlantic and the surface high should reach the local area under the northeasterly fetch, but moisture content will oscillate between normal to above-normal values. As a consequence, light to moderate showers with isolated thunderstorms cannot be ruled out of the forecast.
By mid-week onwards, instability and moisture levels increase once again as an upper level trough lingers over the western Atlantic and lighter east to southeast winds return. Life-threatening rip currents are expected during the first part of the week.

SHORT TERM
Tonight through Tuesday...

Today, partly cloudy skies were observed across the islands. Isolated showers developed in the morning hours in areas from the central north to the interior of Puerto Rico, the local islands, and surrounding waters. Then, a line of showers developed northeast of Vieques moving toward the southeast coast of Puerto Rico. Moving to the afternoon hours, rainfall development was seen through portions of the interior and southwest Puerto Rico due to lingering moisture, daytime heating, and local effects. Based on the TJUA radar rainfall estimates, accumulations today were around 1 to 1.50 inches across southwestern municipalities such as San Germán, Sabana Grande, Guayanilla, and Peñuelas. Values between 0.01 to 0.25 inches were recorded in the rest of the forecast area, while northwest PR remained mostly dry.

For tonight, expect additional showers to move across north- central and eastern Puerto Rico. Winds will continue from the northeast between 10 to 15 mph with gusts of approximately 25 mph or higher through mid-week. Then, shifting from the east on Wednesday and becoming lighter.

Variable weather conditions are anticipated for the short-term period, as a broad surface high pressure moves offshore the eastern coast of the U.S. towards the Central Atlantic. This weather feature will promote a slightly drier airmass to spread into the region. Nonetheless, models are now showing that precipitable water content values will not drop significantly, but instead will oscillate between seasonal to above-normal values. As a consequence, light to moderate showers with isolated thunderstorms cannot be ruled out of the forecast every afternoon across portions of the interior and southwest Puerto Rico.

LONG TERM
Wednesday through Sunday...

FROM PREV DISCUSSION
/issued 455 AM AST Sun Apr 28 2024/

A more unstable and wetter pattern is expected to evolve during the long term period as a mid-to upper-level trough develops and remains nearly stationary over the west/southwestern Atlantic. By Wednesday, the surface high pressure north of the area is expected to weaken and in response winds are expected to veer from the southeast, and become lighter through the rest of the period.
Meanwhile, old frontal boundaries and a restrengthening of the surface low over the north central Atlantic will induce weak troughs or perturbations across the northeastern Caribbean through the end of the workweek. Steering winds will acquire a westerly component and convective development with heavy showers are possible over portions of the eastern interior, San Juan and vicinity and over the USVI, particularly on Thursday and Friday.

Having said that and based on the latest guidance, all afternoons through the long term period should be active with possible flooding impacts and mudslides in areas of steep terrain across Puerto Rico. For the USVI, an increase in the frequency of showers and isolated thunderstorms is possible from Thursday onwards. Heat indices across the lower elevations of the islands could likely range between 102-106 degrees on Friday and Saturday before the onset of afternoon convection.

AVIATION...
(18Z TAFs)

Expect -SHRA to continue for the next hour or so across TJSJ and TJPS followed by VCSH possible. Winds around 6 to 10 knots overnight, but breezy conds are expected to return tomorrow morning with winds out of the NE at 13-17 kts, and stronger gusts throughout the day along with VCSH.

MARINE...

Northeasterly moderate to fresh winds should prevail through at least late Tuesday as a surface high pressure moves off the U.S.
East Coast. Then, becoming easterly and slightly diminishing by Wednesday. These increasing winds and a northerly swell spreading across the local waters on Monday will further deteriorate seas through at least mid-week. Small Craft Advisories are in effect for the Atlantic waters, the Mona Passage, waters north of Culebra and St. Thomas, and the Anegada Passage.

BEACH FORECAST...

Short-period waves and onshore flow along the northern and eastern beaches of the islands will promote a moderate risk of rip currents to continue this afternoon and tonight. A northerly swell arriving tomorrow (Monday) and stronger winds will increase the risk of rip currents and promote life-threatening rip currents through at least late Wednesday. For more information and details about the latest forecast please refer to the Surf Zone Forecast (SRFSJU) and the Coastal Hazard Message product (CFWSJU).


SJU WATCHES/WARNINGS/ADVISORIES
PR...High Rip Current Risk from Monday morning through Wednesday afternoon for PRZ001-002-005-008.

VI...None.

AM...Small Craft Advisory until 6 PM AST Wednesday for AMZ711.

Small Craft Advisory from noon Monday to 6 PM AST Wednesday for AMZ712.

Small Craft Advisory from 6 PM Monday to noon AST Wednesday for AMZ716-723.

Small Craft Advisory from noon Monday to noon AST Wednesday for AMZ741.

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