000 FXCA62 TJSJ 161532 AAA AFDSJU Area Forecast Discussion...UPDATED National Weather Service San Juan PR 1132 AM AST Thu Sep 16 2021 .UPDATE...Drier air is slowly entering the eastern side of the forecast area and has begun to affect Saint Croix. Also skies are mostly clear over the local waters. Some small showers have sprung up over Puerto Rico, just west of San Juan from a stream off of El Yunque and some other minor showers near Aguadilla and the central interior. At upper levels high pressure is now in control. Lingering moisture and moderately unstable air over Puerto Rico will promote thunderstorm development later this afternoon and our wind profile from the latest sounding out of San Juan at 16/12Z shows very little shear to blow apart convective columns that do form. Continue to expect the possibility of local urban and small stream flooding, but it should not be as widespread as yesterday. Temperatures in the east southeast surface flow are warming rapidly along the north coast with San Juan and Vega Baja already reporting 92 degrees as the maximum temperature so far. & .AVIATION...VFR conds to prevail, but MVFR and some mtn obscurations will occur in SHRA/TSRA to dvlp...mainly aft 16/16Z in central and nwrn PR. ESE sfc winds dominated by sea breezes of 10-15 kt. SHRA to dissipate aft 16/23Z ovr land, but will regenerate in ern PR aft 17/04Z. && .MARINE...Seas generally to continue less than 4 feet in gentle to moderate flow. Rip current risk remains low. No changes to marine situation from last discussion/forecast. && .PREV DISCUSSION... /issued 436 AM AST Thu Sep 16 2021/ SYNOPSIS... Drier and stable conditions aloft are expected during the short term period. However, a gradual erosion of low level moisture across the forecast area will promote shower and thunderstorm development over the interior and west to northwest sections of Puerto Rico, and also from El Yunque toward the San Juan metro area today. Long term period looks very uncertain as the local guidance continue to try solving a few low pressure systems that could become tropical cyclones. SHORT TERM...Today through Saturday... Tutt low now well west of the region , as a mid to upper level ridge will gradually build across the region today through Saturday. This will result in drier and stable conditions aloft during the period. The broad surface low over the western Atlantic will continue to lift farther northward while a surface high pressure ridge will build north and east of the region through Saturday. This will result in a gradual erosion of low level moisture across the forecast area. The prevailing southeasterly wind flow will become more easterly and increase Friday into Saturday as the high pressure ridge builds across the Northeastern Caribbean. A low to moderate concentration of Saharan dust particulates will spread across the region during the next few days. This will also aid in drying out out the low levels. However, for the rest of the morning hours, the environmental conditions will remain favorable enough to generate early showers and isolated thunderstorms over the coastal waters and parts of the north and east to southeast coastal sections of the islands. During the afternoon hours, the available moisture along with daytime heating and local effects will favor shower and thunderstorm development over the interior and west to northwest sections of Puerto Rico, and also from El Yunque toward the San Juan metro area. Isolated afternoon shower development will also remain possible in and around the USVI, but the activity should be of short duration. Urban and small stream flooding will be possible during the afternoon over Puerto Rico, with ponding of water on roadways and in poor drainage areas likely with the moderate to locally heavy rains during the morning hours. As mentioned , Saharan dust will filter in from the east, resulting in hazy skies. By Friday, mid-level ridge will strengthen across the region,while the surface high pressure will generate an easterly wind flow while slightly increasing. Somewhat drier conditions are expected for Friday, with a more seasonal weather pattern expected. Passing trade wind showers can therefore be expected during the morning hours followed by hazy conditions during the day along with locally and diurnally induced afternoon shower development. A similar weather pattern is so far expected for Saturday, however the Saharan air layer should diminish across the region by then. LONG TERM...Sunday through Friday... The National Hurricane Center, which provides the official forecast on tropical cyclone activity, continues to monitor various areas of interest across the Atlantic basin with low to high formation chance during the next 5 days. The western-most feature, or Invest 95L, located more than 500 miles south- southwest of the Cabo Verde Islands may pose a threat to the NE Caribbean. Since global models continue to disagree on the evolution of this system, the uncertainty for the long term period remains higher than normal. Please monitor the Tropical Weather Outlook and other products from the National Hurricane Center regarding the tropical waves int he Atlantic. However, current model guidance indicates that the best possible scenario for the long term period will be controlled by the typical seasonal weather pattern. Expect overnight and early morning isolated showers over parts of eastern portions of Puerto Rico and the U.S. Virgin Islands. Then, local effects, diurnal heating, ample low level moisture and favorable dynamics aloft will result in showers and thunderstorms across the areas of sea breeze convergence in Puerto Rico. The upper low is expected to move west and meander to the north of Hispaniola until Thursday. The overall moisture thereafter will start to depend on the development and trajectory of the tropical wave that is currently a few hundred miles to the south of the Cabo Verde Islands. AVIATION... Brief periods of MVFR conds in SHRA/Isold TSRA will continue across the regional waters and around PR and USVI at least til 16/14Z. SCT-BKN lyrs nr FL025...FL040...FL100. Prevailing VFR conds expected at all terminals with mainly VCSH psbl through the morning hours. Mtn TOP obscr psbl with passing SHRA ovr eastern PR due to low clds and SHRA. Sfc wnds light and variable to calm... bcmg fm east to southeast 10-15 kts aft 16/14Z with sea breeze variations. Aftn convection mainly ovr interior and W PR fm 16/17Z- 16/23Z with VCTS psbl at TJBQ/TJMZ and VCSH psbl at most of remaining terminals. MARINE... Moderate trade winds are expected to continue for the next several days. Generally tranquil marine conditions expected across the local waters due to seas of 4 feet or less with winds between 5 and 15 knots. These conditions are expected to prevail through the weekend. For the beaches, the rip current risk is low. && .PRELIMINARY POINT TEMPS/POPS... SJU 89 78 88 78 / 50 40 40 40 STT 91 76 91 76 / 50 40 40 30 && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...OM LONG TERM....WS PUBLIC DESK...CS