Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Apr 15 2025 19:59:57 FOUS30 KWBC 151959 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 359 PM EDT Tue Apr 15 2025 Day 1 Valid 16Z Tue Apr 15 2025 - 12Z Wed Apr 16 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 2 Valid 12Z Wed Apr 16 2025 - 12Z Thu Apr 17 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN CALIFORNIA AND WESTERN NEVADA... 2030Z Update... Except for tightening up the eastern boundary of the Marginal Risk area...no changes made to the previously issued outlook area. Bann Previous Excessive Rainfall Discussion... A slow-moving upper level low embedded within a broader positively tilted trough will move towards the central California coast on=20 Wednesday. The associated upper level cold air will overspread=20 central California with sufficient surface heating to increase=20 instability along the Sierras to between 500 and 1,000 J/kg. The=20 upper low will also bring a bit of Pacific moisture into the region as PWATs increase to between 0.5 and 0.75 inches, which for=20 central California is as much as 3 sigma above normal and exceeds=20 the 95th percentile for climatology. Corfidi Vectors will generally be out of the north to northeast, which will favor south to=20 southwest moving convection developing on the Nevada side of the=20 Sierras then slowly moving over or along the range into the=20 foothills of the Central Valley. Given this setup, there is=20 potential for training convection since all of the factors that are coming together to support heavy rainfall from the stronger storms will be slow-moving and persist through the afternoon and evening=20 hours. Snow levels from the NBM Wednesday afternoon will be between 8,000 and 9,000 ft, allowing most of the precipitation that falls=20 to be in the form of rain except at the highest elevations of the=20 Sierras. Abundant snowpack also remains in place in the Sierras=20 below the melting level, which will allow for some contribution to=20 rising streams and creeks to be from snowmelt. Thus, an isolated=20 flash flooding risk remains in this area. The inherited Marginal=20 risk was trimmed on the edges, especially into western Nevada,=20 since the storms will generally track towards California. However,=20 no big changes were made with this update. Wegman Day 3 Valid 12Z Thu Apr 17 2025 - 12Z Fri Apr 18 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHWESTERN UTAH... 2030Z Update... Realigned the Marginal area just a bit based on the 12Z model and WPC deterministic QPF. Bann Previous Excessive Rainfall Discussion... The upper level low causing the marginal risk of excessive rainfall in northern California on Day 2/Wednesday will open up and become an extension of a longwave positively tilted trough over much of the West on Thursday. The low-turned-trough will accelerate eastward into southern California and eventually Arizona by late Thursday night. The energy associated with it will draw a more substantial upper level shortwave south into the Intermountain West Thursday and Thursday night. The advance of the associated cold air will increase the lift ahead of the front. Some of the Pacific moisture will draw northeastward into southwestern Utah ahead of the front, supporting more widespread rainfall across the region, especially Thursday evening into the overnight. While associated rainfall rates through here are unlikely to exceed a half inch per hour in any one area for an extended period of time, this rain falling on very dry and hydrophobic soils are likely to convert mostly to runoff. Typical flooding locations such as slot canyons will be at greatest risk for flash flooding. The rainfall should not be enough to cause widespread flash flooding, but is likely to cause isolated flash flooding. While higher elevations of Utah may see mostly snow from this front, once again most areas should remain as rain. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-VNv0w2w0HxdSnRjGQrntBmnl6UAb94VCAKwTHz78nHe= b7V_yujorMhfX2HKHaagmCOo0apZ1BRdJVoUTJM2TryE_NQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-VNv0w2w0HxdSnRjGQrntBmnl6UAb94VCAKwTHz78nHe= b7V_yujorMhfX2HKHaagmCOo0apZ1BRdJVoUTJM2wmva110$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-VNv0w2w0HxdSnRjGQrntBmnl6UAb94VCAKwTHz78nHe= b7V_yujorMhfX2HKHaagmCOo0apZ1BRdJVoUTJM23z6jBQk$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .