Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Mar 16 2025 01:00:54 FOUS30 KWBC 160100 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 900 PM EDT Sat Mar 15 2025 Day 1 Valid 01Z Sun Mar 16 2025 - 12Z Sun Mar 16 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE TENNESSEE VALLEY... ....Middle Tennessee, southern Appalachians to the east-central Gulf Coast... Heavy rain along with numerous areas of flash flooding were ongoing across Middle Tennessee into northern Alabama at 00Z with peak MRMS- derived hourly rainfall between 1 to 2 inches at times over the past 3 hours, and 3-hourly rainfall of 1-3 inches. This region=20 contained lower instability with MUCAPE estimates of generally less than 500 J/kg, except for a narrow area of Middle Tennessee. The ongoing flash flooding in the vicinity of Middle Tennessee was co-located with relatively lower flash flood guidance values and=20 was located on the northern edge of the better instability which=20 was situated over central Mississippi/Alabama with MLCAPE of 1000=20 to 2000+ J/kg via SPC mesoanalysis data. Strong low level moisture transport along with divergent and diffluent flow aloft, out ahead of a shortwave trough approaching the Lower Mississippi Valley, will continue to support areas of thunderstorms with embedded training through the night, as the precipitation axis shifts eastward through 12Z. The better instability to the south was co-located with higher=20 flash flood guidance values and area of flash flooding were more=20 scattered in nature. While the better forcing ahead of the shortwave will be north of the Gulf Coast region, the potential for higher rain rates (2-3 in/hr) due to the possibility of embedded=20 training will exist, driving the Marginal Risk. However, any flash flood impacts for southern locations are expected to remain isolated at best. ....Ohio Valley... The potential for 1-3 inches of rain will exist across the region with mostly weak instability of less than 500 J/kg. However, strong dynamics ahead of the aforementioned upper trough and lower flash flood guidance values support a Slight Risk, decreasing to Marginal with northern extent. Rainfall rates may briefly exceed 1 in/hr, but should generally stay below that through 12Z. ....Pacific Northwest... The approach of an longwave trough over the eastern Pacific tonight will allow for a surface low located ~200 miles west of the coast of Oregon to move into Washington by 12Z Sunday. A trailing cold front will extend southwestward from the low and should be slow moving as a return to low level southerly flow offshore accompanies the approach of a shortwave, located near the leading edge of the=20 offshore longwave trough. Within the anomalous moisture axis tied to the frontal boundary, precipitable water values are forecast by a consensus of guidance to peak between roughly 0.9 and 1.0 inches through 12Z Sunday along the coast of far northern California into southern Oregon.=20 Resulting IVT values of 700-800 kg/m/s (per recent RAP forecasts)=20 are expected to be long lasting while slowly edging southward=20 along the southern Oregon coast. 850 mb winds are forecast to peak=20 in the 60-70 kt range from the southwest overnight along the coast=20 and 850-700 mb mean layer winds of 50-70 kt will carry moisture=20 downstream to the Cascades. 12Z HREF and 18Z HRRR/NAM_nest guidance support the notion of hourly rainfall increasing overnight with=20 0.5+ in/hr becoming highly likely in the 06-12Z window. Given the=20 slow movement of the heavy rainfall axis, terrain influences are=20 expected to allow for 3 to 6 inch 12-hr totals ending 12Z Sunday=20 over the favored terrain of southwestern Oregon, and maxima of 2 to 3 inches for downstream locations in the Cascades below snow=20 levels (which will decrease with northward extent). No changes were made to the previous Marginal Risk area which covers much of western Oregon (west of the Cascade ridge) down into far northwestern California. Within this risk area will be the potential for excess runoff as rainfall totals steadily increase=20 through the night, containing embedded rates possibly exceeding 0.7 or 0.8 inches per hour toward the end of the period. Otto 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 and storms, the Marginal Risk area was trimmed dramatically with 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 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!-40VvAcSpsSrV4214_SGSLHdjK78SjpSN27RlnpG7rR8= h_fnbab69OZORe5ohTihBtip7DF0o7DFtCYqZA5N0DeR6Lw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-40VvAcSpsSrV4214_SGSLHdjK78SjpSN27RlnpG7rR8= h_fnbab69OZORe5ohTihBtip7DF0o7DFtCYqZA5NurSroGo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-40VvAcSpsSrV4214_SGSLHdjK78SjpSN27RlnpG7rR8= h_fnbab69OZORe5ohTihBtip7DF0o7DFtCYqZA5NfpLKWmU$=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) .