####018005496#### FXCA62 TJSJ 032037 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 437 PM AST Fri May 3 2024 .SYNOPSIS... A Flash Flood Watch is in effect for all Puerto Rico and the U.S. Virgin Islands. Unstable conditions due to the prescence of an upper- level trough approaching the area from the west and an induced surface trough, increasing moisture levels. In adittion,lighter winds will continue to promote periods of heavy showers and strong thunderstorms across the local islands. && .SHORT TERM...Tonight through Sunday... An upper-level trough and an induced surface trough will promote wet and unstable conditions throughout the weekend, particularly tonight and Saturday. Tropical moisture from the Caribbean moving into the area is increasing the chances for widespread rainfall and flooding through this weekend. Showers and thunderstorms continue to be the dominating weather features through at least Sunday, particularly along the eastern half of Puerto Rico and the U.S. Virgin Islands. Afternoon convection due to diurnal heating and local effects with the enhancing effects of the upper/surface trough will most likely bring the heaviest rainfall and strongest thunderstorms. An upper- level maxima will move across the islands during this time, with winds around 75 knots on Saturday which will enhance ventilation aloft to increase the possibility of storms. The Precipitable Water (PWAT) model guidance shows values in the 75th percentile or above normal climatological levels through the weekend. The 500 mb temperatures also suggest values around -7 degrees Celsius, meaning that there is an increase in the potential for thunderstorm activity across Puerto Rico and the U.S. Virgin Islands. As saturated soils prevail, any brief period of heavy showers or persistent light rainfall could lead to urban and small- stream flooding,  localized flash flooding, and mudslides, particularly across the interior, the eastern half of Puerto Rico, and the U.S. Virgin Islands. A Flood Watch was issued through Sunday afternoon. We encourage citizens and visitors to be aware of any additional updates in the forecast. .LONG TERM...Monday through Friday... //From previous discussion issued at 439 AM AST Fri May 3 2024// Wet and unstable conditions should continue through midweek next week. If the rainfall forecast materializes during the short- term period, any additional shower activity over saturated soils will further enhance the potential for flash flooding and mudslides. Residents and visitors are encouraged to monitor the evolution of the weather conditions over the long-term period. A surface high pressure is expected to build over the western Atlantic during the first part of the week. This will promote moderate winds from the east to northeast through the end of the workweek. In addition, the remnants of the previous surface trough to our north or just across the northeastern Caribbean are expected to merge with a frontal boundary across the central Atlantic and linger over the local area through at least Thursday. At upper levels, another trough will pass mainly to our north from Tuesday through Thursday, maintaining unstable conditions across the islands. The highest precipitable water (PWAT) values are expected during this period, with global guidance suggesting 2.00-2.25 inches of PWAT. Daily rainfall amounts could range between 1 and 3 inches, with the highest accumulations in diurnal activity over portions of eastern, central, and western PR. More stable conditions are forecast for Friday as a mid-to-upper level ridge over the western Caribbean extends into the local area, bringing drier air aloft and causing the PWAT to drop to around 1.50 inches. && .AVIATION... (18Z TAFS) VRB weather conditions will persist through the period. Developing VCTS/TSRA will continue to promote tempo MVFR to brief IFR conditions. ESE winds up to 15 knots through 04/00z, then increasing again after 04/12Z. Stronger gusts near heaviest SHRA/TSRA activity. && .MARINE... A surface high pressure extending from the western to central Atlantic will promote light to moderate trade winds across the regional waters. An induced surface trough to our north will yield lighter east to east-southeast winds promoting shower and thunderstorm development across the area through early next week. && .BEACH FORECAST... A fading northerly swell and ligh winds will promote low to moderate risk of rip currents during the next several days. However, flooding rains and thunderstorms are expected through the weekend across the islands. && .HYDROLOGY... A Flash Flood Watch remains in effect for all of PR and the USVI from this morning through Sunday afternoon. Rainfall totals between 3 and 6 inches and locally higher are possible through the weekend. Please refer to the latest Flash Flood Watch (FFASJU) for more information. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Flood Watch through Sunday afternoon for PRZ001>013. VI...Flood Watch through Sunday afternoon for VIZ001-002. AM...None. && $$ SHORT TERM...ICP/MRR/RC LONG TERM....DSR/LIS AVIATION...MRR PUBLIC DESK/UPPER AIR/MARINE...GRS ####018005869#### FXUS65 KBYZ 032038 AFDBYZ Area Forecast Discussion National Weather Service Billings MT 238 PM MDT Fri May 3 2024 .DISCUSSION... Tonight through Sunday night... The Northern Rockies region is under cyclonic, northwesterly flow at 500 mb this afternoon, with considerable cloud cover and a few isolated rain/snow showers. Changes are in store though, beginning tonight. Tonight into Saturday night, the 500 mb trough that has been affecting our weather the past few days will move eastward enough to allow shortwave ridging to move into our area from the west. This ridge will bring dry weather and partly to mostly sunny skies to the area. We will also experience some warming as surface high pressure moves into the northern/central Plains and low-level southeasterly winds on the western side of the surface high facilitate compressional warming and warm air advection. A corridor of wind gusts to 30-35 mph will exist from Wheatland County to southern Carbon County on Saturday. Sunday, a 500 mb low will move from the OR/CA/NV area to southern ID/northern NV. This system will bring clouds to the area, and an increasing chance of showers from west to east. For Sunday afternoon, PoP values are 20-90% over south central MT and western Sheridan County, WY. Isolated thunderstorms are possible as well (20% chance). The greatest PoP values are in the Crazy and Beartooth/Absaroka Mountains. Furthermore, continued low-level southeasterly flow will cause additional warming on Sunday. Over southeast MT and Sheridan County, WY, southeasterly winds will gust to 35-45 mph. Minimum relative humidity values should drop to around 20% Sunday afternoon. So, the combination of warm temperatures, gusty winds and low relative humidity will create elevated fire weather concerns Sunday in southeast MT and the lower elevations of Sheridan County, WY, wherever a greenup has not yet occurred. Sunday night, the 500 mb low will move into western WY and bring a 40-90% chance of showers (with a 20% chance of thunderstorms) to the area with the highest PoP values in the mountains and areas west of Billings. Models generally agree up to this point in the forecast (through Sunday night) that the greater precipitation amounts, generally 0.10 to 0.50 inches, will be west of southeast MT. In addition, a cold front will cross the area Sunday evening, bringing cooler air to the area but still warm enough to support rain in the lower elevations. Snow levels will fall from 9,000-10,000 feet Sunday evening to 6,000-8,000 feet by Monday morning. Low temperatures will generally be in the 20s to lower 30s tonight, then 30s-40s Saturday night and Sunday night, with Sunday night the warmer night. High temperatures will mainly be in the 60s Saturday and 60s-lower 80s Sunday (warmest in southeast MT and Sheridan County, WY). RMS Monday through Friday... By Monday, ensembles place an upper level low will over western Wyoming tracking east into eastern MT and western Dakotas by Monday night. This low will linger over the western Dakotas through Wednesday bringing multiple rounds of precipitation as well as strong northwest winds. There is still much spread amongst models on total QPF as well as the timing of the heaviest precipitation. Mon through Wed night, the region has about a 60-85% chance of getting at least a half an inch with about a 50-70% chance of at least an inch for southeast MT (areas east of Billings). For this same time period, the Absaroka/Beartooths and Bighorns have about a 40-60% chance of getting at least a foot of snow. Snow levels will generally be around 5000-6000 ft. With that, locations along the foothills could see a few inches of snow with Red Lodge at about a 40% chance of getting at least 3 inches. With regards to winds Monday, the strongest gusts will be off the western foothills along 1-90 as well as the high hills in southern Big Horn Co. These areas have about a 40-60% chance of getting wind gusts of at least 50 mph. Windy conditions will persist through Wednesday with gusts into the 30s and 40s. Precipitation chances start to diminish Thursday into Friday. Keep an eye on the forecast as placement of timing and amounts are likely to chance. Also be aware of changing cool and wet conditions for young livestock. High temperatures will be in the 50s through the week, possibly reaching the 60s by Friday. TS && .AVIATION... Isolated showers will be possible through this afternoon. VFR conditions will prevail through the period. TS && .PRELIMINARY POINT TEMP/POPS... Sat Sun Mon Tue Wed Thu Fri ----------------------------------------------------------- BIL 030/065 040/074 047/058 043/055 041/051 040/057 042/067 00/U 02/W 78/R 45/R 67/R 66/R 33/R LVM 028/063 042/066 040/053 037/050 036/048 036/052 036/062 00/B 17/T 98/T 46/O 67/O 55/O 23/R HDN 028/068 039/080 046/059 041/056 039/051 039/058 040/067 00/U 02/W 69/R 56/R 77/R 76/R 43/R MLS 028/063 041/081 050/060 042/052 038/052 041/061 042/067 10/U 01/N 59/T 76/R 78/R 75/R 32/R 4BQ 028/065 043/080 049/059 040/053 038/049 039/056 041/063 00/U 01/N 59/T 65/R 67/R 76/T 33/R BHK 025/060 038/074 048/060 038/050 034/051 037/058 037/063 10/U 01/N 59/T 87/R 77/R 65/R 33/R SHR 026/066 039/077 042/055 038/056 036/050 035/052 037/061 10/U 02/W 69/T 66/R 77/R 77/R 44/T && .BYZ WATCHES/WARNINGS/ADVISORIES... MT...None. WY...None. && $$ weather.gov/billings