Subj : HVYRAIN: Excess Rain D1 To : All From : Mike Powell Date : Sat Aug 02 2025 18:20:47 FOUS30 KWBC 022000 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 PM EDT Sat Aug 2 2025 Day 1 Valid 16Z Sat Aug 02 2025 - 12Z Sun Aug 03 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST, SOUTHERN PLAINS, AND NORTHERN PLAINS... ....Southern Appalachians through the Gulf Coast... A wavering hybrid cold/stationary front will remain draped from eastern Texas through South Carolina today, with a slow southern translation of this feature expected into Sunday morning. This front will waver (with the gradual southern progression) in a region of broad troughing extending down from the Northeast, with weak flow across the Gulf Coast leading to the slow movement. However, the front will be entrenched within a plume of elevated tropical PWs (measured via 12Z U/A soundings of 2.2 to 2.4 inches, nearing daily records across the area.) This will support widespread showers and thunderstorms along the low-level convergent boundary of the front, with generally weak west-to-east 0-6km mean winds of just 5-10 kts supporting slow storm motions, and training thanks to this boundary parallel wind. Weak impulses within the flow will enhance ascent leading to locally clustered convection with rainfall rates above 2"/hr (briefly 4"/hr likely as reflected by HRRR 15-min rainfall above 1"), and there are likely to be two primary areas of higher excessive rain risk. The first area is across the Southern Appalachians from northern Georgia through eastern TN and far southwest NC. Here, a narrow ribbon of instability 750-1000 J/kg (MUCAPE) will extend northward and interact with increasing moist upslope flow as 925-850mb winds veer more E/SE late this aftn into the evening. Although PW anomalies are somewhat lower here compared to points south, increased bulk shear and weak mean winds will support heavy rainfall rates of 2-3"/hr within what could be nearly stationary cells at times. 0-10cm soils in this region are saturated above the 95th percentile according to NASA SPoRT, leading to compromised FFG and a 60-80% chance of exceedance from the HREF. Farther south from eastern AL though GA and onto the SC coast, training of cells which develop along the front and within the greatest plume of PW will support an increased excessive rainfall risk, especially through peak heating. Although soils here are a bit drier due to a lack of recent rainfall (7 day rainfall generally 25-50% of normal), a 60-80% chance of 2"/hr rain rates from the HREF and REFS could lead to stripes of rainfall exceeding 5 inches (40-70% chance from both HREF and REFS.) After consideration of the new probabilities and 12Z CAMs, the two slight risk areas were merged into one larger SLGT risk which encompasses more of GA/eastern AL as signals from both the HREF and REFS were similar. ....Northern Rockies through the Southern Plains... An elongated north-south stationary front will drape from Montana through Texas today, while multiple mid-level impulses embedded within generally zonal flow from the Rockies into the Plains interact with this surface feature as they emerge from the west. At the surface, flow downstream (east) of this stationary front will become enhanced as return flow persists out of the Gulf, driving an 850mb LLJ to 30 kts across the Central Plains. This surging LLJ will help draw impressive thermodynamics northward, characterized by PWs of 1 to 1.25 inches (locally higher) overlapped with 1000-2000 J/kg MUCAPE. This will support widespread convection developing beneath any of these shortwaves, with development most likely along and just east of the wavering stationary front into the more impressive thermodynamics. Although convection is expected to move briskly eastward within any clusters that develop thanks to 30-40 kts of bulk shear, rainfall rates of 1-2"/hr are anticipated (30-50% chance from the HREF) which could cause at least isolated excessive rainfall impacts, especially across any urban areas or more sensitive soils with lower FFG. The greatest risk appears to be across the TX panhandle where an MCS may form tonight on the periphery of the greater instability plume and then dive into the moisture beneath one of these shortwaves, with a subtly higher training risk along its SW quadrant, and both HREF and REFS 3"/24 hr probabilities peak above 40%, and the inherited SLGT risk was adjusted cosmetically. There may be a secondary area of heavier rainfall across eastern SD and into ND where some repeating or backbuilding convection may occur later tonight as westerly flow in the vicinity of an MCV/shortwave results in repeating development into the higher instability west, and convection moves into some more sensitive soils in eastern SD tonight. This has prompted a targeted SLGT risk for tonight as well, which is supported somewhat by the CSU UFVS first guess field which has a SLGT risk in the same general area, although displaced from the HREF/REFS probabilities which drove this new excessive area. Weiss $$ --- SBBSecho 3.28-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .