Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Mar 25 2023 20:07:51 FOUS30 KWBC 252007 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 406 PM EDT Sat Mar 25 2023 Day 1 Valid 16Z Sat Mar 25 2023 - 12Z Sun Mar 26 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Snell Day 2 Valid 12Z Sun Mar 26 2023 - 12Z Mon Mar 27 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF THE SOUTHEAST... Multiple rounds of heavy rain are possible along and just to the north of a lingering quasi-stationary front extending from the Southeast to the central Gulf Coast States, which may prompt isolated instances of flash flooding. Initial convection across central AL and into east-central GA should be ongoing by 12z Sun. These storms by themselves will be capable of 1-1.5"/hr rainfall rates until modest 850 mb southwesterly flow weakens to below 30 kt by about 15z. The resulting cold pool should aid in suppressing the aforementioned frontal boundary closer to the Gulf Coast and setting the stage for the next round Sunday evening. Tropospheric precipitable water content per the 12z HREF is forecast to peak in the 1.5-1.8" range by Sunday evening in response to a surge of 850 mb moisture flux and overarching shortwave swinging from the southern Plains to the Ohio Valley. While not extreme, this moisture source would equate to around +2 standard deviations above climatology (per both the ECENS and GEFS) and into the 75th-90th percentiles. There will also be no shortage of instability, with a corridor of 2000-2500 J/kg MUCAPE Sunday evening along and just inland from the central Gulf Coast. Resulting convection is anticipated to initially develop to the southwest across LA and expand with the increasing 850 mb flow along the stationary front through southern/central MS and AL. Sufficient instability and southwest 850 mb flow running into the frontal boundary will support areas of organized convection (within a larger area of scattered shower/thunderstorm activity) and the potential for supercells, which in turn are likely to contain intense rainfall rates. The slow-moving frontal boundary will also allow for training cells or multiple cells to overlap areas that received previous rainfall. Rain Sunday morning across central AL could also prime soils before the evening convection rolls through and increase the potential flash flooding threat. 12z HREF neighborhood probabilities depicts the upper-bound of potential rainfall during this period by highlighting 15-20% chances for 12-hour QPF to exceed 5" across parts of central/southern AL. This is mainly due to FV3 and HRRR members, which are likely overdone spatially but not out of the realm of possibilities in a localized sense given the overall setup. A Slight Risk was considered during this update centered around parts of central AL, but opted for only an upgrade to a Marginal Risk given recent dry conditions (less than 2% 0-40 cm soil moisture percentiles per NASA SPoRT-LIS) and some lingering uncertainty on where exactly the overlapping heavy rain could set up. WPC QPF is in line with the latest HREF guidance in highlighting the potential for a swath of 2-4" total rainfall. Localized amounts over 4" are possible and would likely overcome even some of the more drier soils in the region. Will continue to monitor the need for an expanded risk area to the west across southern MS and the possibility of a targeted Slight Risk if CAM guidance shows more agreement. Heavy rain to the east into the Carolinas is expected remain fast-moving and limit any flooding impacts. Snell Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7O0TXIxqyM---GddD7z-Rmapxe4p6s2vd4T9ONRA0U8s= vVyZ2aMlcRtE22fCKp90fyJPdm6xizbOX3AZ16iZ0iFhGgo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7O0TXIxqyM---GddD7z-Rmapxe4p6s2vd4T9ONRA0U8s= vVyZ2aMlcRtE22fCKp90fyJPdm6xizbOX3AZ16iZiMQqLD0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7O0TXIxqyM---GddD7z-Rmapxe4p6s2vd4T9ONRA0U8s= vVyZ2aMlcRtE22fCKp90fyJPdm6xizbOX3AZ16iZnmJ2IGA$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .