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Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
659 FXCA62 TJSJ 161510 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 1110 AM AST Fri Aug 16 2024 .UPDATE...A few minor showers are being seen in the Caribbean south through west of Saint Croix, otherwise no shower activity has been noted as of 11 AM AST. The sounding came in considerable drier, as expected, with only 1.47 inches of precipitable water. Most of this is due to the ridging that is occurring over the area from the northeast at lower levels. To our east southeast a weak tropical wave enhanced by a TUTT low to our east northeast is throwing moisture northwest toward the area and this moisture is still expected to arrive on Saturday. The sounding does not appear particularly conducive to thunderstorms in the west northwest portion of Puerto Rico with a significant dry layer above the marine layer, but at this time it does not seem prudent to rule out local showers and possible thunderstorms there, so will continue with the inherited forecast slightly diminished. Rio Guanajibo and Rio Culebrinas have subsided below flood stage, therefore the flood warnings were allowed to expire. Some roads near these rivers may still be overtopped with water and closed today. && .AVIATION... (12Z TAFs) VFR conds thru 17/12Z although some mtn topping may occur btwn 17/08-17/12Z in the Luquillo range. SHRA/TSRA psbl VCTY TJBQ/TJMZ btwn 16/18-16/21Z. Winds SE 6-12kt and lcly higher in sea breezes especially alg the S coast of PR and the USVI. && .PREV DISCUSSION... /issued 655 AM AST Fri Aug 16 2024/ SYNOPSIS... Excessive heat hazards will be present today and during the next few days. There is a chance for afternoon showers and thunderstorms to develop each day, even during the driest periods, with increased frequency and intensity during the weekend and during the second half of the next workweek as tropical waves affect the region at those times. Pulses of Saharan dust will continue to reach the region. River flood warnings are in effect for the Culebrinas and Guanajibo rivers at least through early this morning. A moderate rip current risk will return tonight and persist through most of the weekend for several areas. A high risk of rip currents is forecast for some areas by Sunday night and into early next week as swells from Hurricane Ernesto and moderate to locally fresh winds impact the local waters, also creating choppy seas for small craft by Sunday into early next week. SHORT TERM...Today through Sunday... Satellite imagery and doppler weather radar depicted partly cloudy to variably cloudy skies overnight, with a mix of high level clouds spilling over the region from distant Hurricane Ernesto and patches of low to mid level clouds and few showers moving across the region while brushing parts of the islands. So far rainfall accumulations were little to none with these light passing showers. Rivers and streams along the northern half of Puerto Rico continued slightly higher than normal but were slowly declining to near normal. However, in the west section of Puerto Rico, both Culebrinas and the Hormigueros river remained at high level overnight although steadily subsiding. With flood waters still remaining over some major roadways, hazardous conditions for residents and drivers continue. For this reason a Flood warning remains in effect at least until later this morning. The early morning low temperatures were in the low 80s along the coastal areas and in the mid to upper 70s in higher elevations. Daytime highs today will range between 89 to low 90 degrees, with maximum heat indices expected to soar once again into the lower 110s over portions of the islands where Excessive Heat Warnings and advisories will be likely. Winds were calm to light and variable but will become east southeast 10 to 15 mph with sea breeze variations and occasionally higher gust during the afternoon hours. For the rest of the short-term period, a much drier airmass is expected at least into early Friday with winds becoming more east northeast in advance of a weak tropical wave and a mid to upper level trough expected to approach and cross the region from the east by Saturday afternoon through early Sunday. So far expect most of the moisture with this wave to pass south of the region, however some peripheral moisture may converge across the region bringing intervals of passing clouds and showers to the region. Recent model guidance and satellite derived layered precipitable water products both suggest a drier airmass with values dropping to between 1.35 and 1.65 inches at least until early Saturday, along with a low to moderate concentration of Saharan dust to linger across the region through the period. Expect generally fair weather skies but still a 40-60% chance for afternoon showers with isolated thunderstorms over northwestern Puerto Rico driven by significant diurnal heating and sea-breeze convergence. By Saturday afternoon, more favorable conditions are expected due to the nearby mid-to-upper level trough and increasing PWAT values nearing 2.0 inches as the tropical wave cross the eastern Caribbean and approaches just south of the region. This will likely aid in increasing the potential for showers and isolated thunderstorms, with up to a 70% chance of precipitation by Saturday afternoon and into early Sunday. The forecast then calls for a limited to elevated risk of excessive rainfall and lightning hazards for Saturday through early Sunday. Thereafter by Sunday afternoon onwards, more typical weather conditions are forecast with east to southeast winds and diminishing moisture content once again. Overall warmer-than-normal conditions will persist in the coming days and coastal and urban areas can expect nighttime lows in the lower 80s, while higher elevations will see temperatures drop to around low to mid 70s. Daytime highs will range from the upper 80s to lower 90s in lower elevations to around 80 degrees in higher elevations. Maximum heat indices in many coastal and urban regions may soar above 108 degrees Fahrenheit. These conditions will present significant health risks and may require the issuance of Excessive Heat Advisories and/or warnings. It is crucial for residents and visitors, especially those participating in outdoor activities, to take necessary precautions to stay safe. LONG TERM...Monday through Friday... No major changes to the long term forecast. Model guidance indicates precipitable water (PWAT) values decreasing to below normal and normal values to start the workweek, coinciding with a pulse of saharan dust that will also serve to promote hazy skies. PWAT values are forecast to further decrease on Tuesday before gradually increasing to above normal values (above 2 inches) through the second half of the workweek. This last increase will be due to a tropical wave that, in addition to increasing moisture, will increase the potential for enhanced shower activity across the area. Surface high pressure across the Atlantic will spread westward on Monday as Ernesto moves farther north. This will promote more east to east northeast winds through midweek, becoming more easterly by the second half of the workweek. Heat risks will persist through the workweek in addition to a generally typical weather pattern. This pattern will include periods of passing overnight and early morning showers across windward sectors of the islands (especially windward sectors of Puerto Rico), followed by afternoon showers and thunderstorms mainly over the interior to western Puerto Rico each day. This activity will be somewhat enhanced, Wednesday onwards. AVIATION... (06Z TAFs) VFR conds at all terminals thru prd. Few passing SHRA ovr regional waters and en route btw islands. Some -SHRA reaching windward side of islands but no sig impacts to flight VIS. Low level steering wnds generally fm ESE 5-15 kts BLO FL120. Sfc wnds calm to light and variable bcmg fm east to northeast 10-15 kts aft 16/14Z with sea breeze variations and hir gusts from time to time durg aftn on the north coast today but some will appear aft 16/15Z. SHRA activity with chc of isold TSRA in NW and interior PR btwn 16/17-16/22Z. MARINE... Hurricane Ernesto, currently around 640 miles north northwest of San Juan, will continue to generate moderate southeasterly winds despite its distance. Easterly winds are expected to return over the weekend. Swells from Ernesto and moderate to locally fresh winds from a tropical wave will create choppy seas for small craft early next week. BEACH FORECAST... The rip current risk will remain low for all beaches during the day. However, a moderate risk will return tonight for northwestern to north-central Puerto Rico. The moderate risk will spread to other areas during the weekend. A high risk of rip currents is forecast for some areas by Sunday night and into early next week as swells from Hurricane Ernesto and moderate to locally fresh winds from a tropical wave impact the local waters. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Excessive Heat Warning until 5 PM AST this afternoon for PRZ001- 005-008-010-011. Heat Advisory until 5 PM AST this afternoon for PRZ002>004-007- 012-013. VI...Excessive Heat Warning until 5 PM AST this afternoon for VIZ002. Heat Advisory until 5 PM AST this afternoon for VIZ001. AM...None. && $$ SHORT TERM/AVIATION/MARINE...WS PUBLIC...CVB UPPER AIR...RVT