####018006516#### FXUS66 KSGX 040935 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 235 AM PDT Sat May 4 2024 .SYNOPSIS... Mostly cloudy cool weather will prevail this weekend with gusty winds in the mountains and deserts and periods of showers over and west of the mountains. Southwest to west winds will strengthen through the day today, peaking tonight and early Sunday. Patchy drizzle may occur this morning, but the best chances of accumulating rain will be tonight and early Sunday. Dry and warmer conditions will prevail Monday through most of next week. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... .SHORT TERM (Today-Monday)... Marine layer low clouds have filled in west of the mountains and have spread into the Cajon Pass. Elsewhere skies are clear. The marine layer cloud deck may be thick enough for some patchy light drizzle again this morning, though little in the way of accumulation is expected. A large upper level low currently positioned off the Oregon coast near 44.2N, 131.5W will progress east-southeast into the northern Great Basin this weekend. The trough axis and associated surface cold front will pass through So Cal tonight into Sunday. Both the GFS and EC show IVT of over 400 kg/m/s and a quick-hitting weak atmospheric river tonight into Sunday morning in mostly west- southwesterly flow. Despite the higher IVT, most of the moisture will be confined below about the 750-800 mb level. Given this and the short duration, rainfall amounts will be somewhat limited - generally less than 0.15 inch for the coasts/valleys with local amounts around 0.25 inch near the foothills. Orographics will enhance precipitation along the coastal slopes of the mountains with amounts of 0.25-0.50 inch, upwards of 0.75 inch on the southwest facing slopes of the San Bernardino Mountains. Amounts in the high desert, if any, will be a couple hundredths of an inch, with no precipitation expected in the low deserts. Precipitation will come to an end by early Sunday afternoon. Snow levels will begin near 7500-8000 ft tonight, falling to around 5500 ft behind the cold front as precipitation tapers off. Any snow accumulation will be limited to an inch or less. In addition to precipitation, this system will generate breezy southwest to west winds across the mountains and deserts this afternoon into Sunday, strongest this evening into tonight. Highest wind gusts will be around 50-60 mph along the desert mountain slopes and locally into the deserts with isolated gusts to around 70 mph through the San Gorgonio Pass. Winds slowly weaken late Sunday afternoon into Monday morning. Another round of weaker but still gusty west winds will occur Monday afternoon and evening. Significant cooling occurs through the weekend. Today, highs will be around 3-7 degrees below normal for most areas, near normal in the low deserts. Sunday is even colder with highs around 15-20 degrees below normal inland and 5-10 degrees below normal near the coast. This puts us at highs in the 60s for the coastal areas/valleys and high deserts, 40s in the mountains, and 70s to around 80 in the low desert. Temperatures rebound quickly for Monday, though highs will remain a few degrees below normal. && .LONG TERM (Tuesday-Friday)... The slow warming trend continues into Tuesday as the upper low moves out. Beyond Tuesday is a little less uncertain, however, as ensemble solutions begin to diverge with regards to a second low dropping out of Canada and setting somewhere into the Western US while a ridge sets up to our west. For Wednesday the ensembles are pretty much split half and half on warmer vs cooler scenarios. By Thursday, the cooler scenario with the low retrograding much further SW is the outlier containing only 7 percent of global ensemble members. Given the large uncertainty left NBM for temperatures which shows a very subtle warming trend through the week. This uncertainty also has implications on the depth of the marine layer and the strength of the marine inversion which would affect how far inland any clouds would extend and how much clearing we would have each day. && .AVIATION... 040835Z...Coasts/Valleys/Foothills...Low clouds cover the region west of the mtns, with bases 1800-2200 FT MSL and tops to 3000 ft MSL. VIS reduced to 1-3SM where low clouds and terrain intersect, obscuring higher terrain. Expect clearing 17z-20z inland, with partial clearing near the coast and intermittent CIGs into the afternoon. Elsewhere...Clear skies with unrestricted VIS this morning, with increasing high clouds AOA 15,000 FL MSL later today. Mountains/Deserts...W-SW winds developing after 20z, with the strongest winds after 00z Sun. Expect W-SW winds 20-35 kt with gusts of 45-60 kt. Areas of MOD-STG up/downdrafts and LLWS over/east of mtns. && .MARINE... No hazardous marine conditions are expected through this morning. Gusty northwest wind gusts of 25 to 30 knots will occur this evening through Sunday evening, which will generate hazardous boating conditions, especially in the outer waters. Northwest wind gusts up to 20 knots will be possible over the outer waters each afternoon and evening for Monday through the middle of next week. && .SKYWARN... Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions. && .SGX WATCHES/WARNINGS/ADVISORIES... CA...Wind Advisory from noon today to 8 PM PDT Sunday for Apple and Lucerne Valleys-San Bernardino County Mountains-San Gorgonio Pass Near Banning. Wind Advisory from 3 PM this afternoon to 8 PM PDT Sunday for Coachella Valley-Riverside County Mountains-San Diego County Deserts-San Diego County Mountains. PZ...Small Craft Advisory from 3 AM to 10 PM PDT Sunday for Coastal Waters from San Mateo Point to the Mexican Border and out to 30 nm. Small Craft Advisory from 6 PM this evening to 3 AM PDT Monday for Waters from San Mateo Point to the Mexican Border Extending 30 to 60 nm out including San Clemente Island. && $$ PUBLIC...SS AVIATION/MARINE...PG ####018005963#### FXCA62 TJSJ 040936 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 536 AM AST Sat May 4 2024 .SYNOPSIS... A Flash Flood Watch remains in effect for all Puerto Rico and the U.S. Virgin Islands. Unstable conditions are expected to continue this afternoon influenced by an upper-level trough, an induced surface trough, and above-normal tropical moisture. In addition, lighter winds will continue to promote periods of heavy showers and strong thunderstorms across the local islands. The precipitable water content is forecast to remain above normal levels through most of the forecast period, with a drying trend anticipated late next week. && .SHORT TERM...Today through Monday... Thunderstorms and rain showers affected the US Virgin Islands overnight, where a Flood Advisory (St Thomas/St John) and a Flash Flood Warning (St Croix) were in effect overnight, saturating the soils. Additionally, yesterday's rain activity saturated Puerto Rico soils even more, and most of the rivers' streamflows continued much above normal. Minimum temperatures were between 70 and 75 F along the coast and between 60 and 65 near mountains and valleys. Winds were calm to light and variable, promoting the development of land breeze.   The weather pattern remains unstable and moist today, influenced by an upper-level trough with a jet stream, an induced surface trough, and above-normal tropical moisture pooling over the Northeast Caribbean. While we expect a brief respite in rain activity during the morning hours, the afternoon could see the formation of intense thunderstorms, particularly over and north of the Cordillera Central. Additionally, due to the dangerous river and soil status, these environmental and atmospheric conditions underscore the need to keep the Flood Watch in effect. Thus, we encourage everyone to stay updated and vigilant about the weather conditions. Be prepared for any changes. A surface high pressure will move into the Western Atlantic promoting that above normal moisture pools over Puerto Rico and the U.S. Virgin Islands Sunday and Monday. Regardless that we can anticipate periods without rain, the pattern will continue favorable for the formation of rain and afternoon thunderstorms. Given the significant amount of rain that we has received during the past week, any particular moderate to locally heavy rain will aggravate the potential to observe flooding and landslides (along steep terrains) across the islands. && .LONG TERM...Tuesday through Saturday... By the beginning of the long-term period, the latest models are suggesting a surface high pressure dominating over the western to central Atlantic. In our area, this will promote light to moderate winds from the east to northeast through the end of the workweek. At upper levels, another trough will pass mainly to our north from Tuesday through Thursday, maintaining unstable conditions across the islands. A moist airmass should linger over the local as moisture from the tropics will be pulled and join the remnants of the previous surface trough. The Precipitable Water (PWAT) model guidance shows values above normal climatological levels through at least next Thursday; values around 2.00-2.25 inches. Consequently, the wet pattern is expected to continue with daily showers and possible isolated thunderstorms development. Shower activity should develop during the morning hours over portions of eastern PR and USVI, followed by afternoon convection over central and western PR resulting in greater accumulations. We encourage residents and visitors to remain weather-aware because any additional shower activity over saturated soils will further enhance the potential for flash flooding and mudslides. On the bright side, more stable conditions are forecast by the end of the week into the weekend as a mid-level ridge over the western Caribbean extends into the local area, bringing a drier air mass into the region resulting in more seasonal PWAT values and decreasing the potential for showers development. && .AVIATION... (06Z TAFS) Expect an unstable weather pattern, promoting the formation of SHRA/TSRA, especially in the afternoon and evening. VCTS/TSRA may continue to promote tempo MVFR to brief IFR conditions btwn 04/16- 23z near JSJ/JBQ/IST. Expect SSE/SE winds at 5 to 15 kt with sea breeze variations (aft 04/13z) and stronger gusts near heaviest SHRA/TSRA activity. && .MARINE... An induced surface trough moving across our area will yield lighter winds, while promoting shower and thunderstorm development through early next week. A surface high pressure extending from the western to central Atlantic will promote light to moderate trade winds across the regional waters through the forecast period. && .BEACH FORECAST... A fading northerly swell and light 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 through Sunday afternoon. Estimated rainfall totals around 3 to 5 inches in portions of Puerto Rico and 2 to 4 inches in Saint Croix, US Virgin Islands. Additional rainfall accumulations are expected to range from 2 to 4 inches. 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/AVIATION...CAM LONG TERM/MARINE....YZR ####018006914#### FXUS65 KBYZ 040939 AFDBYZ Area Forecast Discussion National Weather Service Billings MT 339 AM MDT Sat May 4 2024 .DISCUSSION... Today through Sunday night... The short term will start out pretty quiet as a shortwave ridge moves into the region bringing warm temperatures and mostly clear skies to the region. Max temperatures will be in the 60s F for most. Today will be breezy for some today as southerly winds funnel through the Bighorn Basin leading to elevated winds gusting into the 20s mph from Carbon County up to Wheatland County. This corridor has a 20-40% chance of seeing a wind gust over 30mph. Toward the evening and overnight the strong winds will shift to locations north and east of Billings. Winds will also shift from being more southerly to more easterly in the evening as we start to feel the effects of the 500mb low making its way across western CONUS. NBM shows a 40-70% chance of getting a gust over 30mph. Sunday will see winds pick up out of the south ahead of the upper low as the shortwave ridging moves off to the east. Winds will be stronger than Saturday, especially for eastern areas as Sheridan, Fallon, and Carter Counties have 70-90% chance for wind gusts over 40mph. These strong winds will bring significant warm air advection to the region with temperatures in the 70s and 80s F. Increased temperatures across the region to NBM 50th percentile as the NBM deterministic had temperatures below the 25th percentile in some locations. This also had the effect of lowering RH values into the high teens to low 20s for Sunday. The combination of hot, dry, and windy conditions will lead to elevated fire concerns. This is reflected in fire indices with the hot dry windy index showing 95th percentile conditions for locations from Sheridan County up through Rosebud County where RH values are lowest. The Fosberg index is also showing values in the 50-60 range indicating fire concerns. Sunday will see the 500mb low move closer bringing precipitation chances (30-60%) to southeast Montana. Lowered Pops across the region Sunday evening as models like the GFS are delivering a dry slot across much of the area. Saturday night into Monday morning could see precipitation chances become more widespread although it is hard to say given model uncertainty. By 12z Monday morning, ensembles are generally keeping precipitation values below 0.1 inches with the only locations having a >50% chance being west of Billings. Torgerson Monday through Friday... Models remain in general agreement that a strong negative tilt upper low will be pushing across Wyoming Monday to settle in a position over western North Dakota by Tuesday. While details on the exact track and strength of this weather system remain in question (EC vs GFS solutions), we remain quite confident based on cluster analysis that our region will see below normal temps, quite windy conditions at times and possibly significant precipitation. Lower elevations will see primarily rain but elevations above 6500 ft could see substantial wet snowfall. Monday/Monday night...Model solutions have significant differences here with some dry slotting over our central zones by the GFS while the EC is quite wet for our central zones and basically most of our CWA. Our precipitation forecasts reflect a strong lean toward the much wetter EC (stronger closed low) due to a more consistent message among its ensemble members. The negative tilt trough hits our eastern zones the hardest Monday, but if the EC solution is most accurate...the central could get hit hard to (think >0.25 liquid). W/NW winds will increase Monday into Monday night making for a blustery and uncomfortable time to be outdoors. High temps will be in the 50s west to lower 60s east. Tuesday/Tuesday night...Ensembles suggest upper low will generally settle over western North Dakota. Strong W/NW winds are generated across the region mainly from the Snowies to Carter County. We are muting model output of wind speeds at this time a bit, but there is a signal from the ensembles that gusts to 50 kts are possible (40-70% probability) in this corridor Tuesday and we will monitor this for a possible High Wind event across our eastern plains. Look for bands of precipitation to affect the CWA during this time. Even worse conditions than Monday for outdoor activities. This dynamic low is likely to continue generating strong winds and bands of precipitation Wednesday/Wed Night and into Thursday before a secondary lobe of energy swings around the cyclonic flow and sends the upper low gradually to the southeast by the end of the work week. Highs will be in the 40s and 50s at midweek before some warming returns us to the 50s and lower 60s Friday with just some lingering PoPs of 20-30%. Impacts: While our high country may see heavy wet snow at times (30-50% chance of over a foot in the high country), the main impacts we are concerned about are the harsh combination of wind and rain for people caught outside and especially for young livestock. Make sure to stay up to date with the latest forecast for your location as details emerge. BT && .AVIATION... The next 24 hours will be dry with minimal cloud cover. The biggest threat to aviation will be winds that start to pick up across the region for areas west of Billings in the afternoon before the strongest winds shift to locations north and east of Billings in the evening and overnight. These winds will be southerly/southeasterly and gust to near 25kts. Most impacted airports will include K1KM and KBHK. Torgerson && .PRELIMINARY POINT TEMP/POPS... Tdy Sun Mon Tue Wed Thu Fri ------------------------------------------------------- BIL 065 039/076 046/057 043/058 042/053 041/056 041/065 0/U 02/R 66/R 34/R 57/R 77/R 43/R LVM 063 041/071 040/051 038/052 036/049 036/052 037/062 0/B 06/T 77/R 35/O 68/O 66/R 22/R HDN 067 038/081 046/059 042/059 040/052 039/057 040/066 0/U 02/R 56/R 54/R 68/R 88/R 43/R MLS 062 040/084 050/062 041/054 038/050 040/059 041/064 0/U 01/N 48/T 76/R 78/R 77/R 32/R 4BQ 064 041/083 049/061 040/057 039/049 039/056 040/062 0/U 01/N 57/T 64/R 57/R 77/T 43/R BHK 059 037/078 047/062 037/052 034/049 036/057 037/061 0/U 00/N 68/T 86/R 68/R 76/R 32/R SHR 065 036/082 042/056 037/059 036/052 035/052 036/061 0/U 01/N 66/R 55/R 57/R 78/R 54/R && .BYZ WATCHES/WARNINGS/ADVISORIES... MT...None. WY...None. && $$ weather.gov/billings