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FXCA62 TJSJ 030852 AFDSJU

Area Forecast Discussion National Weather Service San Juan PR 452 AM AST Sun Dec 3 2023

SYNOPSIS

A seasonal weather pattern will persist for most of the short term with the influence of shallow patches of moisture and a high surface pressure over the central Atlantic. Therefore, residents can expect brief passing showers in the morning, followed by localized afternoon showers over the interior and southwestern quadrant. By Thursday into Friday, the islands can expect more moisture associated with fragments of a frontal boundary moving in pushed by the east-northeasterly winds. Seasonal temperatures are forecast for the islands.

SHORT TERM
Today through Tuesday...

A fragment of moisture pushed by the trade winds brought showers across the windward sections along the northern and eastern sections of Puerto Rico and the U.S. Virgin Islands overnight. Rainfall did not produce flooding along these areas, while the rest of the region observed mostly clear skies. The thermometers reported the coastal low temperatures in the mid-70s degrees Fahrenheit and around the low 60s near mountains and valleys. Surface winds were mainly from the east-northeast at ten mph or less.

Under a trade wind pattern, the advection of fragmented moisture will result in occasional rain showers, mainly across the windward sections. At mid-levels, high pressure will create hostile dynamic conditions for deep convection. Therefore, we expect the formation of shallow afternoon convection due to local effects and sea breeze limited to the southwest quadrant of Puerto Rico today. Regardless of St Croix, where the maximum temperatures have been around the low 90s, the rest of the islands may observe the typical maximum temperatures for December.

A tropical upper tropospheric trough (TUTT) amplifying east of the Lesser Antilles will induce a series of surface perturbations that the easterly winds will advect into the region, creating the typical December advective weather pattern across the U.S. Virgin Islands and Puerto Rico. The amount of rain will be tied to the arrival of patches embedded in the winds that may move inland. We forecast that temperatures will range around the December-typical values for our region.

LONG TERM
Wednesday through Sunday...

The long-term remains on track. Model guidance continues to suggest a surface high pressure dominating the local pattern resulting in an easterly trade wind by Wednesday into early Thursday. This wind flow will push a drier airmass across the area on Wednesday, followed by a patch of moisture on Thursday. A pre- frontal trough from a frontal boundary moving southeastward into the Caribbean will enhance a relaxation in the pressure gradient, turning winds more light and variables Thursday at least until Friday. Light and variable surface winds are not forecast to last long, as a broad building surface high pressure exiting the Eastern Seaboard of the United States over the western Atlantic and a frontal boundary tighten the pressure gradient late Friday.
The east-northeast wind and the moisture from the frontal boundary will create an aggressive pattern across the areas. Therefore, residents and visitors can expect periods of quick passing showers across northern regions in the early morning and evening hours.
Although an increase in moisture will, a mid-level ridge will provide stable and drier conditions aloft, limiting the life of the showers.

AVIATION
VFR conditions will prevail today. Winds will be mainly from the E- ENE at 10 kt or less, after 03/13z will range mainly at 10-15 kt with stronger gusts due to sea breeze variations. Clouds and mountain obscuration could develop along the interior and southwest PR by the afternoon (03/16-22z). The frequency of SHRA will increase near JSJ/IST/ISX after 03/22z.

MARINE

A broad surface high-pressure over the western into the central Atlantic will continue to promote moderate easterly trade winds across the regional waters for the rest of the weekend and into the upcoming workweek. The mild winds and the northerly swells will maintain choppy to hazardous seas for small craft, mainly across the local offshore waters and passages through the weekend into Monday evening. Therefore, there is a Small Craft Advisory until Monday for the offshore Atlantic waters and the Anegada Passage. Improving seas are forecast for Tuesday onwards. For beachgoers, there is a high risk of rip currents across the northern sections of Puerto Rico and the USVI today, diminishing to moderate in the evening.

SJU WATCHES/WARNINGS/ADVISORIES
PR...High Rip Current Risk through this afternoon for PRZ001-002-005- 008.

High Rip Current Risk until 6 AM AST early this morning for PRZ012.

VI...High Rip Current Risk until 6 AM AST early this morning for VIZ001.

AM...Small Craft Advisory until 8 PM AST Monday for AMZ711.

Small Craft Advisory from 2 AM to 2 PM AST Monday for AMZ723.

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