Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun May 25 2025 20:21:25 FOUS30 KWBC 252020 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 420 PM EDT Sun May 25 2025 Day 1 Valid 16Z Sun May 25 2025 - 12Z Mon May 26 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM CENTRAL OKLAHOMA TO EXTREME NORTHWESTERN ARKANSAS... 16Z Update: It is looking more likely that a bimodal distribution will develop with respect to the expected QPF across Oklahoma and the Red River valley of northern Texas later this evening and into the overnight hours. The northern portion of the Moderate Risk includes areas that were hammered with flooding rain earlier this morning, and additional rounds of convection expected through this afternoon will be an aggravating factor for additional flooding. Farther to the south, there will likely be a separate axis of enhanced QPF where a large MCS develops this evening into the early overnight hours in the generally vicinity of the Red River with repeated rounds of slow moving convection, so the Moderate Risk area was expanded south across more of southern Oklahoma to the Texas border to encompass this potential. It is important to note that some of the CAM guidance depicts this band of heavier rainfall farther south across north-central Texas, so this will bear close watching later today and a Slight Risk remains valid for this region. For the Southeast U.S., a Slight Risk has been added for portions of southern South Carolina and into eastern portions of Georgia, including the greater Charleston and Savannah metro areas. There has been a general upward trend in QPF magnitudes in the latest 12Z CAM guidance, and although the coverage will generally be scattered, there will likely be some slow moving cells with impressive rainfall rates that develop this afternoon as an MCV approaches the region. For additional mesoscale information pertaining to this, please reference MPD #313 that has been recently issued. The previous overnight discussion is appended below for reference. Hamrick ------------------------ ....Southern Plains to the Mid-South/Tennessee Valley... Convection now developing over the central High Plain is expected to gradually grow upscale through the remainder of the overnight as it moves further east into a region of deeper moisture associated with a low level warm front. Storms will likely be ongoing, with areas of heavy rain impacting portions of southeastern Kansas, northeastern Oklahoma, and the Ozarks as this system crosses the region through the remainder of the morning into afternoon hours. Some storms may persist and reintensify as they move east of the Ozarks, bringing a heavy rain threat and some potential for flash flooding into the Mid-South and Tennessee Valley by the afternoon. A lingering boundary and moisture will help support redeveloping convection back to the west across Oklahoma into the Texas Panhandle by the afternoon. These storms are also expected to grow upscale as the low level jet across Texas intensifies ahead of an upper trough moving across the Southwest. Moisture pooling on the nose of the low level jet (PWs ~1.75 in) within a region of strong ascent is expected to support the development of a convective complex, with heavy rains moving east near the Red River through the overnight. Forecast confidence is limited by differences shown by the CAMs regarding how this convection will evolve and their placement of heaviest amounts. Relied on the HREF to make adjustments to the previous outlook, which included shifting the Moderate Risk area further south across central/eastern Oklahoma and western Oklahoma. With possible contributions from both this morning's activity and the storms expected to follow this evening and overnight, the HREF is showing some of its highest neighborhood probabilities for amounts over 3 inches over this area. This also corresponds with an area of relatively lower flash guidance, reflecting some locally heavier totals that have fallen over the past 24-hrs. Maintained the Slight Risk as far west as the eastern Texas Panhandle, where this second round is expected to become better organized this evening. Also brought the Slight Risk further south across Northeast Texas, where some of the guidance shows slow-moving, heavy rain producing storms developing along a boundary ahead of the system. ....Front Range into the Central High Plains... Anomalous moisture supported by low level easterly flow is expected to fuel additional showers and thunderstorms developing along the terrain before moving east, with some threat for locally heavy amounts and isolated flash flooding concerns. Easterly winds along the northern extent of a low centered over the southern High Plains will continue to support standardized PW anomalies over 1.5 sigma, which along with daytime heating and ascent afforded by a mid- level shortwave and right-entrance region forcing is expected to support storm development. A Marginal Risk was maintained from the Front Range eastward into southwestern Nebraska, where the HREF is showing high neighborhood probabilities for accumulations over an inch, along with some notable probabilities for amounts over 2 inches along the high terrain. ....Florida and Georgia/South Carolina coasts... A stalled frontal boundary will remain across central Georgia and the SC Low Country. Southerly flow along the western extent of the subtropical ridge will support deepening moisture along and south of the boundary, reaching ~1.75 inches in some locations. This moisture along with daytime heating and enhanced convergence along land-sea interactions, will support showers and thunderstorms capable of producing locally heavy rates and perhaps isolated flash flooding concerns, especially in urbanized areas. A Marginal Risk was drawn for areas from the SC Low Country to South Florida, where the HREF shows high neighborhood probabilities for accumulations exceeding 2 inches. Pereira/Hamrick Day 2 Valid 12Z Mon May 26 2025 - 12Z Tue May 27 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL TEXAS TO THE TENNESSEE VALLEY... The shortwave associated with Sunday night's convection over the Southern Plains is expected to move out across the Mississippi Valley and the Mid-South. This along with trailing energy moving from the Southwest to the Plains will help shift the axis of deeper moisture and the greater potential for heavy rainfall farther southeast. The initial wave will carry with it some threat for heavy rain and flash flooding over the Mid-South and the Tennessee/lower Ohio Valleys as it helps direct deeper moisture over those regions late Monday into early Tuesday morning. However, perhaps the main threat for heavy rain and possible flash flooding will focus farther to the southwest along a boundary left in the wake of this system. Southerly flow ahead of a wave moving across the Southern Plains maintaining deep moisture, along with right-entrance region upper jet forcing providing large scale ascent, are expected to generate an environment favorable for heavy rainfall. The latest HREF 3-hourly flash flood guidance exceedance probabilities are up to about 20-25% across portions of central Texas and portions of the Tennessee River Valley, generally peaking in the 21Z-3Z time period. There has also been a higher signal in this same guidance near the Colorado Front Range, so the Marginal Risk area has been extended northwest to include that region. There is also better model consensus for higher QPF and localized HREF exceedance across portions of central/eastern Kansas into southeast Nebraska and adjacent states, so a Marginal Risk was also added here. Across the Southeast, the 12Z CAM guidance suite has come into better focus in depicting additional slow moving storms with high rainfall rates across much of Georgia and extending to the Low Country of South Carolina, so the Marginal Risk area was extended to the coast to account for this, especially since heavy rainfall has already developed across many of these same areas on Sunday. Pereira/Hamrick Day 3 Valid 12Z Tue May 27 2025 - 12Z Wed May 28 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL APPALACHIANS... An expansive Marginal Risk was maintained from areas along and east of the central/southern Appalachians back through the Ohio, Tennessee, and lower Mississippi River valleys and across Texas.=20 Similar to the Day 2 outlook, the Marginal Risk area was extended=20 eastward to include the Southeast U.S. coast where the potential=20 exists for additional scattered storms with heavy rainfall. A=20 Slight Risk area has been introduced for this update across=20 portions of the central Appalachians, where the 12Z models show=20 heavy rainfall totals associated with mid-upper level shortwave=20 energy moving out of the central U.S. and across the top of a=20 downstream ridge, with the potential for widespread 1-2 inch=20 rainfall totals with locally heavier amounts possible as deeper=20 moisture advects northward. Elsewhere across the U.S., the broad Marginal Risk remains valid with the potential for future Slight Risk areas as things come into better focus. One possible area is over the Tennessee Valley into portions of the Southeast, where shortwave energy moving out of=20 the lower Mississippi Valley, along with favorable upper jet=20 forcing, may contribute to some higher totals. Another area could=20 be associated with some of the deeper moisture and greater=20 instability back across south-central Texas. Some, but not all of=20 the guidance, show mid-level energy along with right-entrance=20 region upper jet dynamics supporting heavy amounts across the=20 region Tuesday afternoon into the evening hours. Pereira/Hamrick Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!859i8mzMKiyKDQ-vS-40jVQhxBH_WwU7pX0xM3wW-GN5= hI5ozUa_oEOaswT5Z58AoXPfnGnYQnQ1atoWDQeVH-Djzp0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!859i8mzMKiyKDQ-vS-40jVQhxBH_WwU7pX0xM3wW-GN5= hI5ozUa_oEOaswT5Z58AoXPfnGnYQnQ1atoWDQeV_2RbZVo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!859i8mzMKiyKDQ-vS-40jVQhxBH_WwU7pX0xM3wW-GN5= hI5ozUa_oEOaswT5Z58AoXPfnGnYQnQ1atoWDQeVdgb0_iU$=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. 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