Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Mar 15 2025 20:03:18 FOUS30 KWBC 152002 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 402 PM EDT Sat Mar 15 2025 Day 1 Valid 16Z Sat Mar 15 2025 - 12Z Sun Mar 16 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-SOUTH INTO THE TENNESSEE VALLEY... ....16Z Update... The shortwave trough over Texas and Oklahoma, which is south of the primary low circulation now over Wisconsin continues to drive much of the convection, severe, and flash flooding across the Lower Mississippi and Tennessee Valleys this morning. This shortwave is stronger than much of the previous guidance had suggested, which in turn has kept the primary area of storms that stretches from Louisiana to western Tennessee as of this writing further west. Thus, the heaviest rainfall totals this morning have been along the Mississippi River, with a secondary area of storms over northern Alabama into portions of eastern Tennessee. As the shortwave trough continues to get its act together, it will begin to shift eastward, resulting in the line of storms also moving east, with increasing forward speed. This in turn will gradually diminish the flash flooding threat as the line of storms will not have time to drop prodigious amounts of rain in these more eastward areas of Alabama, Georgia, and the Southern Appalachians. Given this, ERO upgrades were proposed along the Mississippi River where rainfall amounts over 3 inches in far southeastern Arkansas have already occurred, and the storms will continue to impact the area for another few hours before the significant eastward shift of the line begins. Conversely, because the storms have stayed further west than expected, there has been less rain into the southern Appalachians as the stronger storms with the eastward extent of the rain shield arm have stayed further west towards Chattanooga, and impacted the mountains far less. While all of these areas will see the main line of storms go through later today into tonight, the lack of prior rainfall should reduce the flooding impacts from the line significantly. For western North Carolina, there should still be an upslope component to the southerly flow along the line of storms as it moves through late tonight, which may locally enhance the rainfall as well as begin the rain sooner due to the added lift. The Slight Risk for this area has been maintained, while the Moderate for extreme southwest North Carolina and northern Georgia was downgraded with this update. The 12Z HREF guidance suite has also followed suit with the westward shift in the rainfall. A higher end Moderate has been extended southwestward to include northwestern Mississippi along the Mississippi River, Memphis, and extends northeastward into the Nashville area. Neighborhood probabilities in this area have increased to as high as 80% for 5 inches or more of rain. However, with the chances for 8 inches or more of rain only at 10-20% at most, we remain shy of criteria necessitating a High Risk upgrade. ....Pacific Northwest... No changes were made to the previous forecast. The heaviest rain will begin late this afternoon and persist through much of the night. With most of the expected rainfall capable of isolated flash flooding not having begun yet, there was no need determined for any changes. Wegman Day 2 Valid 12Z Sun Mar 16 2025 - 12Z Mon Mar 17 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE EAST COAST FROM NORTHEASTERN SOUTH CAROLINA THROUGH SOUTHERN NEW ENGLAND, AND FOR PORTIONS OF NORTHERN CALIFORNIA AND SOUTHWEST OREGON... ....2030Z Update... ....East Coast... A highly progressive cold front and negatively tilted longwave trough will traverse across much of the East Coast on Sunday. While the divergence associated with the trough will support storm longevity, instability will be lacking in most areas outside of the Southeast. Much of the Eastern Seaboard has also been in a dry period, where most rain will be beneficial. Given all of the above factors and fast forward speed of the front and associated rain=20 and storms, the Marginal Risk area was trimmed dramatically with=20 this update, largely confined to coastal areas, where some additional moisture from the Atlantic may locally help to spur on stronger storm development as the front approaches the coast. This remains a low-end Marginal Risk in all areas, but that is especially true in New England, where lack of instability will greatly diminish the flash flooding potential due to weak storms. The Marginal was maintained here due to the combination of urban and upslope/terrain factors, which should at least extend the duration of any heavier rainfall in this area. Further south into the Carolinas, the moisture availability will be greatly increased due to proximity to the Atlantic/Gulf Stream, so despite the storms moving through during the first half of the night, the heavier rates may lead to isolated instances of flash flooding. ....Pacific Northwest... A pair of lows moving off the Pacific will be forced into the=20 coast by the next longwave trough to move into the continental U.S. The positive tilt of the trough will aim a continuous stream of heavy rain into the southwest corner of Oregon and the northwest corner of California. This will result in several inches of rain impacting that area. The Marginal Risk is unchanged, as these coastal areas will be able to handle the heavy rain with relatively few impacts. The most likely areas to see some flash flooding will be the rapidly rising streams and creeks that drain the coastal mountains and Cascades in this region. The rain will push southward into northern California and the Sacramento Valley Sunday night, which could cause isolated instances of flash flooding in the bigger towns such as Redding. Should the area expecting the greatest rainfall (the CA/OR border region) push south into more prone areas of northern California, then a Slight Risk upgrade may be needed. Wegman Day 3 Valid 12Z Mon Mar 17 2025 - 12Z Tue Mar 18 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7FGdl3nPa83k4HvAgel5u9WRY6ZwwMsMk6DRBZ-ErDZ6= Z12cG6Qeu53AbtR9BQtOZMPg9aQMyHaQHbjk8job256hzj0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7FGdl3nPa83k4HvAgel5u9WRY6ZwwMsMk6DRBZ-ErDZ6= Z12cG6Qeu53AbtR9BQtOZMPg9aQMyHaQHbjk8job_Si6kMU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7FGdl3nPa83k4HvAgel5u9WRY6ZwwMsMk6DRBZ-ErDZ6= Z12cG6Qeu53AbtR9BQtOZMPg9aQMyHaQHbjk8jobi9pNhk4$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .