Area Forecast Discussion
Issued by NWS San Juan, PR

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960 FXCA62 TJSJ 231552 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 1152 AM AST Fri Apr 23 2021 .UPDATE... An easterly perturbation will continue to move across the region today bringing a slight increase in low level moisture. However recent TJSJ 23/12Z upper air sounding suggests a fairly strong cap inversion 4-6 degs around 700 millibars with precipitable water at 1.37 inches and overall dry and stable conditions aloft. Low level steering winds were from the southeast around 15 knots. That said, still expect some locally and diurnally induced afternoon showers over and around the islands, but widespread and significant accumulations are not anticipated. No change to the inherited short term forecast package and reasoning at this time. && .AVIATION UPDATE...VFR at all terminals durg prd.SCT-BKN lyrs nr FL025..FL50...FL090. Quick passing SHRA ovr the regional waters between PR and the Northern Leeward islands, accompanying the weak low level easterly disturbance crossing the area. No other changes to the previous AVN discussion, locally and diurnally induced afternoon showers are still forecast to develop over parts of the islands durg the rest of the aftn, followed by clearing skies and diminishing SHRA ovr lanD areas overnight anD into the early morning hours. No significant operational wx impacts at this time. && .MARINE...No change to the previous marine discussion. Overall fairly tranquil seas are expected through the weekend. However a light to moderate chop can be expected over the offshore waters and local passages due to winds between 10 to 15 knots. && .PREV DISCUSSION... /issued 657 AM AST Fri Apr 23 2021/ SYNOPSIS... An easterly perturbation will move across the islands today, providing enough low-level moisture to promote passing showers across the windward portions of PR/USVI in the morning, followed by afternoon convection across the western sections. A dry air mass will arrive on Saturday, and a ridge will build aloft, resulting in fair weather conditions. In the long term, a moist and unstable weather pattern could be the weather story between Tuesday and Thursday next week. SHORT TERM...Today through Sunday... An easterly perturbation will promote somewhat unstable weather conditions for today. Passing showers will continue to affect portions of eastern Puerto Rico and the U.S. Virgin Islands during the morning hours. This activity will be followed by afternoon convection over parts of the interior and northwestern quadrant of Puerto Rico, as well over the San Juan metro area. This activity could lead to at least ponding of water on roadways and in poor drainage areas. Showers will diminish by sunset. For tomorrow, mainly fair weather conditions, with limited rainfall activity, are expected across the forecast area. Model guidance now suggest a drier air mass coupled with a surface high pressure that will yield less available moisture content at the low levels. Aloft, stable conditions are expected, due to a building upper-level ridge. Precipitable water values are forecast to be below normal. Hence, Saturday seems to be the driest day of the short term period. Incursion of African dust particles is suggested by observations, thus somewhat hazy skies could be noticed. Patches of moisture will reach the islands by Sunday afternoon due to the influence of a polar trough, located in the western Atlantic. However, these showers are not expected to be impactful, as they will not lead to significant rainfall accumulations. The most active part of the day will be in the afternoon, as local effects and diurnal heating can spawn showers over parts of northwestern Puerto Rico. LONG TERM...Monday through Friday... A polar trough will move from the western to the eastern Caribbean Monday through Tuesday. This mid- to upper-level trough will then amplify over PR/USVI Tuesday through Thursday, to be then displaced by a ridge building from the west on Friday. Meanwhile, a surface high pressure will push a disturbance in the easterly winds over the islands early next week. Then, a cold front will approach the islands from the northwest Tuesday through Thursday, enhancing tropical moisture over us. Another surface high will build over the Western Atlantic Thursday onward. That said, expect the advective pattern to continue Monday and early Tuesday, which could bring passing showers across the windward sections of PR/USVI during the overnight and morning hours, followed by afternoon convection across the northwest quadrant and interior portions of Puerto Rico. The weather pattern should shift to a more unstable one Tuesday through Thursday, as the polar trough amplifies and tropical moisture increase across the islands. Therefore, the potential to have inclement weather due to thunderstorms, and including urban and small stream flooding, could increase, especially Tuesday through Thursday, as model guidance continues to suggest. A more stable pattern should return by Friday. AVIATION... Mostly VFR conditions expected to prevail over the next 24 hours across all terminals. VCSH and brief gusty winds with and around SHRA expected for TJSJ and TJPS through 23/10. Afternoon convection will yield SHRA for TJBQ/TJSJ til 23/22. Winds out of east- southeast at 10 to 15 knots, with sea breeze variations with occasionally higher gusts during the afternoon. MARINE... Mariners can expect tranquil marine conditions during the next several days. Seas will range between 3 and 5 feet most of the time. Light to moderate east to southeast winds will prevail through early next week. Beachgoers, a moderate risk of rip currents is expected to continue across some of the east, northern and southern beaches of the islands. && .PRELIMINARY POINT TEMPS/POPS... SJU 75 88 74 89 / 30 10 20 30 STT 75 87 75 86 / 20 20 30 30 && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...RAM LONG TERM....GL PUBLIC DESK...CS

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