Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Mar 27 2023 01:59:24 FOUS30 KWBC 270159 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 958 PM EDT Sun Mar 26 2023 Day 1 Valid 01Z Mon Mar 27 2023 - 12Z Mon Mar 27 2023 ....THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF CENTRAL ALABAMA AND GEORGIA... ....Southeast... A mix of discrete to semi-discrete supercell storms continue to organize linearly from WSW to ESE along a outflow-modified frontal zone this evening, generally extending from south-central MS into central AL/GA. A low amplitude shortwave is evident just upstream near the Ark-La-Tex via GOES-East water vapor imagery, providing large scale lift and ascent for additional convective organization going into the overnight hours. The environment will also remain very supportive of heavy rainfall, given ample deep layer shear of 50-70 kts, ML CAPE of 1000-2000 J/kg, and precipitable water values of 1.4-1.7 inches (near the max moving average per BMX/FFC sounding climatology). In addition, a fairly strong (25-35 kts) southwesterly low-level jet (LLJ) is feeding additional moisture transport into the region. This LLJ is expected to strengthen rather significantly tonight (while hardly veering), with a bulls-eye of 35-50 kts forecast over southern AL. This has increased the probabilities for excessive rainfall notably across central AL/GA, where a High Risk has been introduced (encompassing much of the prior Moderate Risk area). In addition, the Moderate Risk has been expanded east and west to include the remaining portions of central AL/GA (while extending a bit westward into south-central MS as well). HRRR runs since 18z have been rather consistent in depicting additional localized totals of 3-5 inches across the High Risk area (with the most recent 23z run depicting a concerning stripe of 5-8+ inches). These rainfall maxima are very concerningly close to the same areas that experienced flash flooding earlier this morning (where 2-4+ inches have already fallen, and where FFGs are 1-2 inches or less). Widespread instances of flash flooding are expected in the High Risk area, and flash flood impacts locally may be very significant to extreme through the morning hours. Please see ongoing and subsequent MPDs for more information. ....Southeast Missouri to Southern Indiana... A compact shortwave crossing the Midwest this evening is associated with a slow-moving frontal boundary from southern MO to IN, which has an area of up to 100-250 J/kg of SBCAPE and 200-400 m^2/s^s of 0-3 km SRH to work with. A line of showers and thunderstorms is slowly developing near the Marginal Risk area and could move in a linear fashion over similar areas given the frontal boundary and mean-layer wind profile appear nearly parallel. Rainfall rates will not be that impressive and likely remain under 1"/hr, but rainfall totals up to an inch could pose an isolated flooding/flash flooding risk, especially given this region experienced over 4" of rain not too long ago. Streamflows are running high and soil moisture percentiles are very high per NASA SPoRT. The antecedent conditions were the main driver for the upgraded Marginal Risk. Additionally, 12z HREF neighborhood probabilities do show up to 25% chances for 0.5"/hr rainfall rates between 00z and 06z Mon. Churchill/Snell Day 2 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... By Monday morning, the same frontal boundary impacting much of the Southeast and Gulf Coast with scattered showers and thunderstorms will linger throughout the day and lead to additional chances for isolated to scattered flash flooding. Initial convection early in the time period is forecast to be ongoing across portions of central/southern AL/GA and could overlap with some areas of the Day 1 High/Moderate Risk area (where scattered instances of flash flooding may continue through midday). 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 early evening hours. 12z HREF neighborhood probabilities depict 20-40% chances for 6-hour QPF to exceed 2" across this part of the Southeast through 00z Monday night. This signal along with current rainfall trends aided in the decision to introduce 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 Monday night. The shortwave tracking through the Mid-Mississippi Valley (similar to today) will allow for additional upper divergence and an increase in southerly flow 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. By 12z Tuesday, 12z HREF highlights very high probabilities along the southern TX-LA border for over 2" of rain in 6 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 atmospheric river to California towards the tail end of the Day 2 ERO. 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 3. Areas along the northern California coast will have the best chances to achieve higher than 0.5"/hr rainfall rates and the potential for isolated flooding or rapid runoff through 12z Tuesday, hence the upgraded Marginal Risk area. Snow levels will rapidly drop to around 3000 feet, limiting any snowmelt and runoff from the higher terrain. Snell/Churchill 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!50ySTvzUCb6nWOMjDSbE1sDLJ6Mat9320EMWi5h_ZDwM= wjV63AFnzBa2qpjws0cUwXCZQPSqzpgRFfmdjr98oUoLBZk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!50ySTvzUCb6nWOMjDSbE1sDLJ6Mat9320EMWi5h_ZDwM= wjV63AFnzBa2qpjws0cUwXCZQPSqzpgRFfmdjr98zRUIN7A$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!50ySTvzUCb6nWOMjDSbE1sDLJ6Mat9320EMWi5h_ZDwM= wjV63AFnzBa2qpjws0cUwXCZQPSqzpgRFfmdjr986wHEcZY$=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) .