851 FXCA62 TJSJ 191524 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 1124 AM AST Tue Oct 19 2021 .UPDATE... Slow-moving showers are developing across the Virgin Islands due to local effects and sea breeze variations. Puerto Rico can expect today's convection developing along the Cordillera, slowly drifting southwestward across the southern slopes, and southwest quadrant. TJSJ 19/12z sounding observed a Total Precipitable Water of 1.72 inches. Organized convection due to the intense heating, sea breeze variations and local effects will increase the threat of flooding and mudslides across saturated soils and swollen rivers. && .AVIATION... Mainly VFR conditions will prevail through the forecast period. JPS can expect some SHRA/TSRA between 19/17-22z. TSRA will develop across the Cordillera Central producing frequent lightning and mountain obscuration, but should remain at a safe distance from JSJ/JBQ. The USVI will possibly observe a few -SHRA/SHRA but mainly VCSH. Winds will remain light and VRB but with sea breeze variations, and gusty in/near TSRA. && .MARINE... Seas continues up to 3 feet, and winds from the NE at 10 kt or less. However, thunderstorms will develop across portions of the local waters this afternoon into the evening; therefore, mariners should exercise caution around these storms. && .PREV DISCUSSION... /issued 505 AM AST Tue Oct 19 2021/ SYNOPSIS...Slow moving showers and thunderstorms are expected once again today in a light ENE steering wind flow. Winds will turn more from the east and increase slightly each day till the end of the week. Normal to above normal precipitable water (PWAT) content will continue over the region through at least Thursday. A mid/upper level ridge pattern is expected to promote drier air and more stable conditions aloft late in the week into the weekend. SHORT TERM...Today through Thursday... Moisture persists over the region, much as it has for the last several days, though there is a patch of slightly drier air over the region this morning. Based on the 0Z sounding and satellite estimates, PWAT values remain within seasonal normals, generally in excess of 1.7 inches, a marked decrease compared to previous days. And yet still more than sufficient to support shower activity. Winds are increasing over the area; cloud layer mean winds are likely to be above 5 knots for the first time since Friday. It's a very marginal improvement, but an improvement nonetheless. Showers are still going to be slow-moving, leading to locally high rainfall accumulations, much like has been seen in prior days, and has contributed to flooding. The steering flow is east-northeasterly to northeasterly today, and so rain is expected over the same general areas again today. Showers will develop during the early to mid afternoon over the interior of Puerto Rico/Cordillera, then lazily drift into southern/southwestern Puerto Rico. As such, there remains an enhanced risk for flooding, mostly urban and small stream flooding, but flash flooding is possible, as are rapid river rises and mudslides. Tomorrow, moisture increases somewhat across the area. As the frontal boundary to the north pulls away, replaced by a ridge pushing off of the US east coast and building into the central Atlantic, winds will continue to increase, moreso as we make our way through midweek into the second half of the week. That said, showers are still not going to be making any great haste tomorrow, either. The increase in steering flow will also contribute to moisture convergence expected over the area. Even with a "dry slot" ahead of an approaching tropical wave, PWAT values will be at least within the range of seasonal normals. Additionally, the ridging that has been in place aloft will erode, making conditions more favorable for convective activity. The steering flow will become more east- southeasterly, however, and so the peak in showers and thunderstorms tomorrow will be over the interior of Puerto Rico into the west and northwest. The threat for flooding remains, especially for the interior areas. A tropical wave is expected to make its way across the region late Wednesday into Thursday. Widespread shower activity is expected. It is quickly followed by another, though weaker, tropical wave. There is a small dry slot between the waves, which could lead to some of the afternoon convection being inhibited, as it is forecast to be over the islands in the afternoon. That being said, the dry slot is likely to be squeezed between these two waves, to the point where it effectively meets its demise if not over the CWA then not terribly far to the west. So the impacts of the dry air will be brief, and the decrease in moisture may not be particularly impressive. As such, another wet, active day is expected for Thursday. And therefore also the corresponding enhanced risk for flooding. LONG TERM...Friday through Tuesday... A drying trend is expected during the long term period, as drier mid-level air filters from the east on Friday and lingers through early next week. This is in response to a building mid to upper level ridge over the northeastern Caribbean. A weak Saharan Air Layer is expected to reach the area during this period. In addition, more stable conditions are expected, due to warmer 500 mb temperatures. This should inhibit widespread convective development across the islands, and decrease the potential for thunderstorm development. At the surface, a broad ridge across the central Atlantic should promote moderate to locally fresh trades, with a tropical wave currently forecast to pass well south of the area on Sunday, having little to no impact in moisture content across the local area. Another tropical wave, with better moisture content may reach the area on Tuesday. Having said that, expect showers to develop in a diurnal pattern over western PR each afternoon, and across the USVI and eastern sections of PR during the overnight/early morning hours due to passing trade wind showers with mostly light rainfall amounts. Brief isolated thunderstorms could form in the afternoon hours as the trade wind cap is not forecast to be strong enough at this moment. AVIATION...Generally VFR conditions expected across all terminals next 24 hours. Afternoon convection likely over interior into southern/southwestern PR. VCSH for TJBQ, and VCTS for TJPS. Cannot rule out tempo MVFR or worse, mostly for TJPS between 19/18Z and 19/22Z. Passing showers overnight causing VCSH, with SHRA possible, for TJSJ/TIST/TISX. Winds generally out of the E to near 10 knots after sunrise, subsiding after sunset. MARINE...A small northerly swell will move across the Atlantic waters later today. However, seas are forecast to remain at 3 feet or less, and the risk of rip currents will continue low across the islands today. Light winds will prevail once again from the east-northeast. Winds and seas are forecast to gradually increase during the second part of the workweek, and another long period northerly swell is forecast to move across the Atlantic waters and passages by late Friday into Saturday. The combination of wind driven seas and the swell will cause choppy seas up to 6 feet and a high risk of rip currents along the northern beaches of the islands through the weekend. && .PRELIMINARY POINT TEMPS/POPS... SJU 90 78 88 77 / 20 40 60 60 STT 89 78 88 78 / 50 50 50 50 && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...CAM LONG TERM....OM PUBLIC DESK...CVB