An upper-level trough north of the region combined with sufficient low-level moisture to result in the development of showers and thunderstorms, affecting portions of eastern PR during the late morning into the early afternoon hours.
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The episode as NWS SJU wrote it up.
An upper-level trough north of the region combined with sufficient low-level moisture to result in the development of showers and thunderstorms, affecting portions of eastern PR during the late morning into the early afternoon hours.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS San Juan, PR wrote its last area forecast discussion before the first event at Jul 26, 10:54 AM AST, 21 minutes earlier.
Area Forecast Discussion National Weather Service San Juan PR 1054 AM AST Thu Jul 26 2018 .UPDATE...Will introduce a few changes in the current forecast package to include isolated thunderstorms over the extreme northwest portions of Puerto Rico this afternoon. 12z sounding indicated lower temperatures at the mid/upper levels and more unstable conditions than 24 hours ago. Satellite images also indicate a low level feature moving west across the Caribbean waters. The combination of this low level feature (convergence zone) with the instability at mid/upper level resulted in a higher shower activity than previously expected. Will expect the low level weather feature to continue to move west and be across the western portions of Puerto Rico during afternoon hours. Diurnal heating will further destabilize the local atmosphere and so isolated thunderstorms are a possibility. && .PREV DISCUSSION... /issued 836 AM AST Thu Jul 26 2018/ UPDATE...Made a quick update to the morning zone forecast to increase probability of showers over eastern half of the big island of Puerto Rico...Vieques and Culebra. Doppler radar indicates a large area of moderate to locally heavy showers moving west over the areas mentioned above. Will expect this to continue to affect eastern sections at least through mid morning with convection then transfered to the western interior and the northwest portions of the big island during the afternoon. PREV DISCUSSION... /issued 416 AM AST Thu Jul 26 2018/ SYNOPSIS...Lingering moisture has been replaced by a drier air mass with Saharan dust this morning. This air will remain over the region through Friday. Showers are possible due to local and diurnal effects through Sunday. Saturday low level moisture embedded in the trade winds will increase the chance for showers across the U.S. Virgin Islands and portions of eastern Puerto Rico during the morning, then showers move to the western areas later in the afternoon. Drier air will filter into the area on Sunday just ahead of an approaching tropical wave. The wave is forecasted to reach the area late Sunday and increase rain chances on Sunday and Monday before moving away on Tuesday. SHORT TERM...Today through Saturday... Surface and mid- upper level ridge will remain the dominant feature across the region through Saturday. The strong surface ridge centered across the north central Atlantic will continue to promote moderate to fairly strong east to southeast trade winds through the short term period. A Tutt low now north of the region is to sink southwest across Hispaniola by Saturday. Recent Naaps model guidance and Satellite derived aerosol products suggest the Saharan air layer will continue to spread across the region today through Friday with the highest concentration of suspended dust particulates expected today then gradually diminishing on Saturday before increasing once again the latter part of the weekend ahead and trailing the next tropical wave. Locally and diurnally induced showers with slight or no chance of isolated thunderstorms today due to lack of upper air support and the spreading of the dry air layer and saharan dust across the region. Similar conditions expected on Friday with mostly fair weather but hazy conditions and chance of early morning passing showers and isolated to scattered afternoon showers mainly over portions of western Puerto Rico. Fewer showers in the form of streamers can be expected elsewhere including in and around the USVI. On Saturday guidance suggest a gradual diminishing of the Saharan dust concentration but still still expect another surge of dust to return ahead of and trailing the next tropical wave by late Sunday through early next week. The prevailing easterly trade winds on Saturday should support and steer afternoon convection over portion of central and west PR, with also better chance for isolated thunderstorms on Friday and Saturday due a slight increase in low level moisture convergence, daytime heating and forcing. Otherwise mostly fair weather but hazy conditions for most of the short term period. LONG TERM..Sunday through Thursday...The long-term period will remain tranquil with patches of low level moisture embedded in the trades will move across the region with locally and diurnally induced afternoon convection mainly across portions of northwestern and western Puerto Rico. A surface ridge of high pressure in the north Atlantic will continue to promote easterly winds of 10 to 15 mph across the Caribbean and north Atlantic waters through Thursday. Sunday relative humidity values at 1000 to 850 mb and 700 to 500 mb will range between 70 to 80 percent by 29/18Z; and precipitable water values will range from 1.4 to 1.7 inches. As a result of moisture ahead of an approaching tropical wave. Sunday or early Monday a weak tropical wave is forecasted to move into the caribbean waters south of Puerto Rico. The increase in moisture will cause showers during the morning across the U.S. Virgin islands and eastern areas of Puerto Rico, then the showers will move to the western areas of Puerto during the afternoon. After the passages of the wave, dry Saharan air will filter in from the east and remain across the region through Thursday. Diurnal and locally induced showers are possible during the afternoon across portions of western Puerto Rico. A retrograding TUTT/ trough positioned north of Puerto Rico will be monitored during the long term period. The position of the TUTT/trough could increase rain chances and cause isolated thunderstorms, as a result of upper level forcing. This upper level forcing combined with low level instability provided by the tropical wave could cause showers and isolated thunderstorms, and brief heavy rainfall. AVIATION...VFR, however expect increasing suspended Saharan dust particulates across the flying area mstly btw FL050-FL100 but Flight vis >6sm thru prd. Few low clds -SHRA/SHRA w/mtn obscr ovr E PR til arnd 26/14Z. SHRA dvlpg ovr W-NW PR 26/17Z-26/22z with brief MVFR due to low clds and SHRA. Sfc winds E-ESE 5-10 kt bcmg 15-20 kt aft 26/14z. L/lvl wnds E-SE 15-25 kts blo FL150, then backing and decreasing w/ht abv to nr FL350. MARINE...Seas will range from 4 to 6 feet across the regional and local water ways. Small craft operators are urged to exercise caution as seas will be up to 6 feet and winds up to 20 knots. Winds will be out of the east to southeast at 15 to 20 kts. There is moderate to low rip current risk for all the local beaches. && .PRELIMINARY POINT TEMPS/POPS... SJU 88 78 89 78 / 40 30 30 30 STT 83 81 83 80 / 20 20 20 30 && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && SHORT TERM...RGH LONG TERM....RGH
Hazardous Weather Outlook...UPDATED National Weather Service San Juan PR 1110 AM AST Thu Jul 26 2018 AMZ710>745-PRZ001>013-271515- San Juan and Vicinity-Northeast-Southeast-Eastern Interior- North Central-Central Interior-Ponce and Vicinity-Nortwest- Western Interior-Mayaguez and Vicinity-Southwest-Culebra-Vieques- The nearshore and off shore Atlantic and Caribbean Coastal Waters- 1110 AM AST Thu Jul 26 2018 This Hazardous Weather Outlook is for Puerto Rico and the adjacent Atlantic Coastal Waters. .Day One...This Afternoon and Tonight .Thunderstorms...There is a chance of thunderstorms with heavy rainfall and brief gusty winds this afternoon across portions of northwestern Puerto Rico. .Flooding...Moderate to locally heavy rains will lead to ponding of water on roadways and in poor drainage areas mainly over central and western Puerto Rico. .Wind...Sustained winds in excess of 20 MPH/17 kts expected across the outer Atlantic waters and local passages. .Waves...Choppy seas up to 6 feet is expected mainly across the Atlantic waters and local passages. .Rip Currents...There is a moderate risk of rip currents for most of the local beaches of Puerto Rico. .Heat...East to southeast trade winds will result near or slightly above normal temperature and heat indices of 102 to 107 along the north coast and southern coast of Puerto Rico. .Days Two through Seven...Friday through Wednesday A moderate risk of rip currents will continue over the next several days across the north and south beaches. Saharan dust will continue to hold over the region through at least Friday. A TUTT low will meander north and west of Puerto Rico through the long term period therefore isolated thunderstorms will be possible over portions of western Puerto Rico during the afternoons. .Spotter information statement... Widespread spotter activation is not anticipated, however individual spotter are encouraged to report flooding and rip currents to the National Weather Service in San Juan. AMZ715-725-VIZ001-002-271515- St. Thomas St. John adjacent Islands-St Croix- Nearshore Atlantic and adjacent Caribbean Coastal Waters- 1110 AM AST Thu Jul 26 2018 This Hazardous Weather Outlook is for the U.S. Virgin Islands and the ADJACENT Coastal Waters. .Day One...This Afternoon and Tonight .Wind...Sustained winds in excess of 20 MPH/17 kts expected across the outer Atlantic waters and local passages. .Waves...Choppy seas up to 6 feet is expected mainly across the Atlantic waters and local passages. .Rip Currents...There is a moderate risk of rip currents for the northern and eastern beach of St. Croix. .Days Two through Seven...Friday through Wednesday A moderate risk of rip currents will continue over the next several days for some of the north and east beaches of St. Croix. .Spotter information statement... Spotter activation will not be needed.
PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN JUAN PR
1058 AM AST THU JUL 26 2018
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
1053 AM TSTM WND DMG AGUADA 18.38N 67.19W
07/26/2018 AGUADA PR EMERGENCY MNGR
AFTER 2 PM AST, A THUNDERSTORM CAUSED DAMAGE TO A ZINC
ROOF IN A HOUSE IN BARRIO CARRIZALES, SECTOR TABLONALES,
115 STREET, KM 25.1. THE FAMILY REPORTED DAMAGE TO THEIR
FURNITURE.
&&
ERODThe national radar mosaic for the moment the first event began.
1 heavy rain, 1 flood. Local standard time, as filed.
Landslide reported at Road PR-31 KM 11.8 in Barrio Pena Pobre sector Cuesta del Pilon.
Flooding reported at Road PR-31 in Barrio Rio Blanco in sector La Pitina.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Puerto Rico in 2018Every state in 2018This day in weather historyWarnings in Puerto Rico, July 2018
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 776713, 776715.