Area Forecast Discussion
Issued by NWS San Juan, PR

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000 FXCA62 TJSJ 021525 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 1125 AM AST Tue Mar 2 2021 .UPDATE...Drier than normal air is dominating the local area, and the relatively dry air will continue through the rest of today. The 12Z sounding indicated that there is a fairly stable atmosphere and the precipitable water is 1.05 inches, which is below than the 25th percentile for this time of year. The ENE winds will continue today at around 15 mph with occasional gusts, but these are expected to turn easterly and slightly lighter on Wednesday, then becoming ESE and even lighter by Thursday and Friday. There is a quick patch of moisture moving in early Wednesday, which could once again bring brief showers, but accumulations are expected to be minimal. && .AVIATION...VFR conds expected for the next 24 hours. Brief -SHRA may be possible in the vicinity of the local terminals, but no significant hazards are expected. The local winds will be from the E to ENE at 15 to 20 knots with occasional gusts through 02/22Z. The winds will gradually decrease overnight as they become more easterly. && .MARINE...Small Craft Advisory continues in effect for the local waters until 2 PM. The latest observations and model guidance indicates that we will be able to let the advisory expire early this afternoon. The winds across the local waters will be between 10 and 20 knots for the rest of today and tonight. There is also a high risk of rip currents for many of the local beaches. && .PREV DISCUSSION... /issued 352 AM AST Tue Mar 2 2021/ SYNOPSIS...Overall fair weather conditions are expected to prevail through the end of the workweek. However, areas of low- level clouds and light showers will move at times across the islands. An induced surface trough is forecast to increase moisture content and shower development between Friday into early in the weekend. Trade winds are forecast decrease and turn more from the southeast during the second part of the week. SHORT TERM...Today through Thursday... As the surface high pressure to the north of the islands continues to weaken, the pressure gradient is gradually slacking off, resulting in a decrease in sustained wind speed over the area. At the mid levels, high pressure also dominates the western Caribbean, which is resulting in a trade wind cap around 850 mb. This will maintain moisture trapped in the lower levels of the atmosphere. According to Total Precipitable Water from GOES-16, a relatively dry air mass, with values around 0.9 inches, is currently located over Puerto Rico and the U.S. Virgin Islands. In fact, the forecast soundings and the latest model guidance are not suggesting any significant increase in moisture over the islands, meaning that rainfall should be very limited through the next few days. Even with the low moisture content expected, trade wind showers may still form and affect the eastern half of Puerto Rico and the U.S. Virgin Islands, with additional showers developing in the afternoon over western Puerto Rico, but none of these showers are expected to be significant. As the surface high pressure migrates toward the eastern Atlantic and a cold front exits the eastern coast of the United States, winds will begin to shift more from the east-southeast on Wednesday and from the south-southeast on Thursday. In fact, the southeasterly wind flow is expected to extend into the mid-layers of the atmosphere. As a result, warmer temperatures are expected by the end of the workweek, with possible high temperatures reaching the upper 80s, especially across the coastal areas. LONG TERM...Friday through Wednesday... The mid-level ridge over the region is forecast to weaken by Friday, but it is expected to build and strengthen once again over the central Atlantic. Meanwhile, an amplifying mid to upper level trough over the western Atlantic will extend into the northeastern Caribbean. A surface front is expected to remain well north of the region, while an induced pre-frontal trough is expected to develop just west of the local area, promoting a weak and moist southerly wind flow on Friday and Saturday. Although global models are not as aggressive as last night solutions, precipitable water (PWAT) content is forecast to remain just above climatology around 1.40 inches. Therefore, expect diurnally induced afternoon showers to develop over the interior and northern sections of PR, due to the light steering winds, slow moving showers could lead to minor flooding. On Sunday, trade winds are forecast to return to around 15-20 kts and overall fair weather conditions are expected with afternoon showers developing over western PR. For early next week, another induced surface trough is forecast to develop over the local area and increase shower development across the islands. PWAT content is expected to continue above climatology at 1.40-1.50 inches through midweek, favoring afternoon shower development over western PR. AVIATION...Mainly VFR conditions expected to prevail through the forecast period. Surface Winds will be out of the ENE at 10 to 15 knots with stronger gusts. VCSH may occur over the Leeward and U.S. Virgin Islands, as well as TJSJ early in the period, which could cause brief periods of low ceilings. MARINE...Choppy wind driven seas up to 8 feet will continue through at least today across the regional waters. Small Craft Advisories will expire later this afternoon. Seas will gradually improve during the next few days across most of the local waters, however, seas up to 6 feet are expected to continue mainly across the Atlantic waters and passages. East to northeast winds up to 20 kts will prevail through Wednesday, thereafter, winds turn more from the east to southeast at 10-15 kts. A 3-5 ft northerly swell will move briefly across the Atlantic waters and passages between Thursday afternoon and Friday. There is high risk of rip currents today for the northern and eastern beaches of PR, Culebra, St. Croix and northwestern beaches of St. Thomas. && .PRELIMINARY POINT TEMPS/POPS... SJU 82 72 85 72 / 30 40 40 30 STT 84 73 83 74 / 20 40 40 30 && .SJU WATCHES/WARNINGS/ADVISORIES... PR...High Rip Current Risk until 6 PM AST this evening for Culebra- Northeast. High Rip Current Risk through late tonight for North Central- Northwest-San Juan and Vicinity. VI...High Rip Current Risk until 6 PM AST this evening for St Croix- St.Thomas...St. John...and Adjacent Islands. AM...Small Craft Advisory until 2 PM AST this afternoon for Caribbean Waters of Puerto Rico from 10 NM to 17N-Anegada Passage Southward to 17N-Atlantic Waters of Puerto Rico and USVI from 10 NM to 19.5N-Coastal Waters of Northern Puerto Rico out 10 NM-Coastal Waters of Northern USVI and Culebra out 10 NM-Coastal Waters of Southern USVI Vieques and Eastern Puerto Rico out 10 NM-Mona Passage Southward to 17N. && $$ SHORT TERM...JA LONG TERM....CS PUBLIC DESK...FRG

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