Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Mar 27 2023 07:39:24 FOUS30 KWBC 270739 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 338 AM EDT Mon Mar 27 2023 Day 1 Valid 12Z Mon Mar 27 2023 - 12Z Tue Mar 28 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF CENTRAL ALABAMA, GEORGIA, AND WEST-CENTRAL SOUTH CAROLINA... ....Southeast... The main frontal boundary responsible for the active weather across much of the Deep South and Southeast will press southward towards the Gulf and Atlantic coasts today and tonight, as scattered showers and thunderstorms along and ahead of the front lead to additional chances for isolated to scattered flash flooding. Convection early in the time period is forecast to be ongoing across portions of central AL/GA, in some areas overlapping across yesterday's High/Moderate Risk area where significant rainfall had already occurred. Given the potential severity of the flooding in these highly saturated areas, a Slight Risk was introduced where rainfall (and scattered instances of flash flooding) could still be ongoing in the morning. PWATs will remain around 1.5" during this Day 2 timeframe and 850 mb flow turning more westerly through midday will remain around 40 kts. This boundary is forecast to continue to be a focus for scattered thunderstorms from southern AL, southern GA, and the Florida Panhandle through the evening hours. 00Z HREF neighborhood probabilities depict 20-40% chances for 3-hour QPF to exceed 2" across this part of the Southeast through 06Z Tuesday. This signal along with current rainfall trends aided in the decision to maintain a Marginal Risk here. The next in a parade of shortwaves to eject out of the western U.S. trough will aid in a separate area of developing convection across eastern TX that should push into LA and southern MS after 06Z Tuesday. The shortwave tracking through the Mid-Mississippi Valley (similar to the one yesterday) will allow for additional upper divergence and an uptick in southerly low-level flow/moisture transport out of the Gulf of Mexico. However, upper-level ridging retrograding westward underneath this shortwave across the far southern Gulf of Mexico will allow for convection to develop farther west with this round. Between 06-12Z Tuesday, the 00Z HREF indicates 30-50% probabilities along the southern TX-LA border for over 2" of rain in 3 hours, which could lead to isolated flooding concerns, even for areas with relatively high FFGs. ....Northern California... Rapidly deepening upper-level low and surface cyclone off the Pacific Northwest late Monday night will usher in the next (modest/compact) Atmospheric River to California towards the tail end of the Day 1 ERO period. IVT values after 06Z Tuesday increase to 500-600 kg/m*s north of the San Francisco Bay Area and will spread southward along the California coast on Day 2. Areas along the northern California coast will have the best chances to achieve 0.5+ inch/hr rainfall rates per the 00Z CAMs, and thus the potential for isolated flooding or rapid runoff through 12Z Tuesday. Snow levels will rapidly drop to around 3000 feet, limiting any snowmelt and runoff from the higher terrain. Hurley/Snell Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4AhdbNM2FepU6SQDHkCmSp5GeKRO9ONxqj9jb9r5d4tF= KQDkKyc2FLyS2a_qy6pr5WvSLtW2uwDeegin9AgAf-_zuz4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4AhdbNM2FepU6SQDHkCmSp5GeKRO9ONxqj9jb9r5d4tF= KQDkKyc2FLyS2a_qy6pr5WvSLtW2uwDeegin9AgAYezwqu4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4AhdbNM2FepU6SQDHkCmSp5GeKRO9ONxqj9jb9r5d4tF= KQDkKyc2FLyS2a_qy6pr5WvSLtW2uwDeegin9AgAl1Ra-KI$=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) .