Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Nov 21 2025 18:58:23 FOUS30 KWBC 211858 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 158 PM EST Fri Nov 21 2025 Day 1 Valid 16Z Fri Nov 21 2025 - 12Z Sat Nov 22 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHEAST CALIFORNIA AND SOUTHWEST ARIZONA... 16z update: ....Southern California/Lower Colorado River Valley... GOES-W shows solid Atmospheric River has turned westward into the TROWAL north of the initial closed low/occluded surface low across southern California. This has directed enhanced (95th-99th percentile) moisture and moisture flux across the Imperial Valley, Mojave Desert and Lower Colorado River Valley to maintain enhanced rainfall potential. Some filtered insolation may allow for increased instability later this afternoon especially within the Lower Colorado, for some scattered thunderstorm development capable of .5-1"/hr rates and localized 1-1.5" totals. The strong upstream digging shortwave on the western side of the upper-level closed low will elongate the pattern from NNE to SSW over the California Bight which will stall the frontal zone and moisture plume. While most of the flux will be directed toward NW Baja California, this will also maintain the placement of the AR across the areas that longer duration light to moderate showers could result in longer duration FFG exceedance (6+hr) as well. As such, the the current Slight Risk in place remains solid. Have expanded the Marginal Risk a bit further northwest into the southern San Joaquin Valley to account for the longer term trends of persistent TROWAL moisture flux across the area. ....Lower Missouri, Ohio and Tennessee River Valleys... 12z CAMs along with 06z/12z HREF probabilities continue a show a downward trend with only a few locations of 1"/hr or 1"/3hrs reaching above 25% across Middle TN and S KY. Additionally, observational trends depict the deeper moisture and more unstable/warmer air has been trending south and eastward with a broadening of the drier EML extending through the Delta Region of the Lower MS Valley, further severing needed moisture connection to the the Gulf. In coordination with local forecast offices and river forecast centers, it was determined to remove the Marginal Risk area from the eastern third of the U.S. Please note, this does not remove the potential for an isolated incident or two of flash flooding/FFG exceedance across the area, but the coverage potential is considered less than 5%. Gallina ~~~~Prior Discussion~~~~ Showers and thunderstorms are expected across southern California, southern Nevada and southwest Arizona as lobes of vorticity rotate to the east of the low offshore. Instability appears to be on the weak side which will likely be a limiting factor, however some brief heavier rates are still possible from short training segments within the deep layer southerly flow. A Slight Risk for excessive rainfall remains in effect for southern California and southwest Arizona and a Marginal Risk spans from the California coast to south-central Arizona. For the Missouri/Ohio/Tennessee Valleys there will be an elevated threat for locally excessive rainfall. During this period a steady stream of above average moisture will be transported northward and encounter an east-west boundary. Isolated areas of moderate to heavy rainfall that leads to spotty problems from run-off can be expected. However...the flow aloft becomes increasingly flat and confluent during the day which will reduce the risk of any excessive rainfall. A Marginal Risk area was maintained from eastern Kansas to West Virginia/western Virginia/western North Carolina. Campbell Day 2 Valid 12Z Sat Nov 22 2025 - 12Z Sun Nov 23 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Churchill/Campbell Day 3 Valid 12Z Sun Nov 23 2025 - 12Z Mon Nov 24 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN PLAINS... ....20z Update... Maintained the inherited Slight Risk with only minor adjustments made, primarily to shift the axis slightly south to southwest and=20 tighten the gradient along the north and south. This reflects the=20 latest trends in QPF, which are fairly tightly clustered for this=20 time range (relatively high spatial confidence). This is somewhat concerning, as this brings the forecast axis of QPF even more into alignment with areas that realized heavy rainfall in prior days.=20 However, the categorical outlook is still held at a Slight Risk=20 due to a combination of meaningful time between the prior event and the upcoming event (three full days inbetween should allow for soil moisture and streamflow recovery) and uncertainty with regard to the amount of training and resultant QPF (with most of the new=20 guidance suggesting localized 2-3" totals, while some solutions, including the downscaled 12z ECMWF, depict localized totals of up=20 to 4-5"). Churchill ....Previous Discussion... The low pressure system that brought heavy rainfall for the West Coast during the Day 1 period will be exiting into the Southern Plains and the warm, moist air will support scattered to widespread heavy rainfall. There is an increasing signal that this setup could be a significant event, especially for north-central portions of Texas and into southern Oklahoma where a couple of solutions are suggesting a narrow swath of 4 to 6 inches possible. A Slight Risk for excessive rainfall is in effect for for much of north- central/northeast Texas and south/east Oklahoma. A broader Marginal Risk spans from the Big Bend vicinity to southwest Missouri and western Arkansas. This part of the country will continue to be monitored for a possible upgrade to a Moderate with future updates. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4_ZzqUAF4sMff32qvYRAQ-G3VBOxR_fbSkn64pNv0Jvw= RWBg1QlPeokKvGcX2e5d1_ITwfBxTGrulV5T7YbdQVaGgS0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4_ZzqUAF4sMff32qvYRAQ-G3VBOxR_fbSkn64pNv0Jvw= RWBg1QlPeokKvGcX2e5d1_ITwfBxTGrulV5T7YbdLiPPQ5A$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4_ZzqUAF4sMff32qvYRAQ-G3VBOxR_fbSkn64pNv0Jvw= RWBg1QlPeokKvGcX2e5d1_ITwfBxTGrulV5T7Ybd9IWYM8U$=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 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .