Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
880 FXCA62 TJSJ 112033 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 433 PM AST Sat May 11 2024 .SYNOPSIS... A distant mid to upper-level trough will remain close enough to keep marginally unstable conditions through the rest of the weekend. With southeasterly winds and available moisture, heat indices could exceed 102 degrees Fahrenheit, potentially reaching around 108-110 degrees in localized coastal areas of northern Puerto Rico over the weekend. Lingering saharan dust will persist through at least early next week. && SHORT TERM...Tonight through Monday... Afternoon convection began after 2PM today and by this time, 4:27PM, a line of thunderstorms stretches from Utuado to Catano with up to 2 inches of rainfall detected according to radar estimates. Rainfall will continue through the rest of the afternoon into the early evening hours for these areas, however conditions should improve into the night. A distant mid-to-upper level through will remain close enough to keep marginally unstable conditions during the weekend. In the meantime, an associated upper-level jet streak (70-90 knots) will continue to spread across the northeastern Caribbean, further enabling the development of deep convection through late tonight, with 500 mbar temperatures falling to around -7 degrees. Consequently, there is a potential for thunderstorm development, as indicated by the Galvez-Davison Index. Stormy conditions are possible late tonight over the local waters, with some activity affecting Puerto Rico`s eastern half, Vieques, Culebra, and the U.S. Virgin Islands. With the lingering moist airmass, strong winds aloft associated with a upper-level low northeast of the region, and a weak lower level steering flow from the south-east all combines to allow unstable weather to prevail this evening through Monday, especially during the afternoon hours. Conditions are expected to improve over the next few days however afternoon showers with isolated thunderstorms on Sunday and Monday are still likely. Wind speeds will persist at 5-15 mph from the southeast, shifting to an easterly flow on Monday. This shift will relocate the areas with the potential for the highest rainfall totals from northern sections of Puerto Rico on Sunday to western sections early next week. Temperatures may soar along the coast and urban areas into the upper 80s to mid-90s, while higher elevations could see highs in the upper 70s to mid-80s. With abundant moisture across the region, heat indices could exceed 102 degrees Fahrenheit, potentially reaching around 108-110 degrees in localized coastal areas of northern Puerto Rico over the weekend mainly during the hours between 10am and 3pm && LONG TERM...Tuesday through Saturday... /from previous discussion/ The NASA aerosol optical thickness (AOT) guidance continues to suggest lingering Saharan dust particles to start the period. Model guidance to suggest a mid- to upper-level ridge affecting the region by the start of the period, promoting more typical weather with afternoon convection due to diurnal and local effects. However, model guidance suggests an upper trough closing in on the islands from the northwest by Friday and moving across through Saturday. Precipitable water (PWAT) values are forecast to generally stay at normal values, between 1.5 to near 2.0 inches, through the workweek as patches of moisture advect into the region, Friday and Saturday look to be the wettest days with high end normal values to possibly above normal values. Light to moderate easterlies are forecast through the period as a surface high moves into the western Atlantic. Most available moisture should reach up to 850 to 800 mb for the period, with model guidance suggesting additional available moisture through the mid levels by Friday and Saturday. Limited Heat Risk (heat indices from 102 to 107 degrees Fahrenheit) or even higher, especially during the daily maximum heating, will persist across several lower elevation sectors of the islands where no significant rain is observed. This diurnal heating, local effects and sea breeze variations will continue to promote up to strong afternoon convection, steered by easterly winds and possibly enhanced by the upper through on Friday and Saturday. This will promote convective activity mainly over sectors of the interior to western half of the Puerto Rico and downwind of the local islands. Above- normal sea surface temperatures can also promote nighttime showers across windward sectors of the islands, patchy fog is also forecast for areas of the interior during the early morning hours. && .AVIATION... (18Z TAFS) SHRA/TSRA expected to develop north of Cordillera Central and near TJBQ, dissipating around 11/23Z. Periods of reduced VIS and lower ceilings expected. SHRA will increase across the Caribbean waters and near the USVI terminals after 00Z. Winds are light, out of the SW at 5-7 kts, switching from the SE around 06z. After 12Z, speeds will pick up to 9-11 kts. SHRA/TSRA will develop again after 12/17Z for NW PR, impacting TJBQ. && .MARINE... A surface high pressure across the central Atlantic and a building low in the western Atlantic will allow southeasterly winds to prevail through the weekend. A weak northerly swell will continue to fade across the Atlantic waters and passages. Strong showers and thunderstorms over the waters can result in locally higher seas and gusty winds. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ CAM/MMC/RC