Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Apr 29 2025 20:22:43 FOUS30 KWBC 292021 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 421 PM EDT Tue Apr 29 2025 Day 1 Valid 16Z Tue Apr 29 2025 - 12Z Wed Apr 30 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS... ....Southern Plains... 16Z UPDATE: No major change to the reasoning outlined below, but there was an overall trend to shift risk areas southward a bit. Although hi-res models (even the very recent 12Z runs) are struggling to capture ongoing convection in the region, they all do consistently show organized convection this afternoon and evening generating fairly strong cold pools, which in turn gradually pushes the effective front south into N TX by this evening and overnight. This is supported by RAP analysis showing higher DCAPE values across portions of N TX, as well as the 12Z FWD sounding which revealed a layer of fairly dry air just above the boundary layer. Of the available hi-res models, recent runs of the RRFS seem to best represent the forward propagating bow echo in S MO and additional development in SW OK. Therefore, the tweaks to the positioning of the risk areas leveraged the RRFS a bit more, although some deference was given to other guidance sources as well. Even models that aren't handling ongoing convection very well still indicate the development of cold pools, and the tendency for the axis of active convection to sink south with time. This lends some confidence to the overall expectations described here. The southward progression of the effective front seems to stall overnight, which may lead to a rainfall max in N TX, or at least very close to the Red River. PREVIOUS DISCUSSION: A repetitive scheme of successive convective outputs across portions of West TX through Oklahoma will result in significant flash flooding for a large portion of the above areas. The combination of several mid-level impulses ejecting northeast within a large scale diffluent pattern will interact with a quasi- stationary front aligned from the Western Rolling Plains of TX up towards the Red River Basin mainly west of I-35. Favorable mid- level ascent will allow for waves of convective initiation beginning late-morning and afternoon Tuesday with a secondary pulse around 00z Wednesday with a final pulse forecast after 06z as the mean trough finally starts pivoting east out of the Southwestern CONUS. Instability within the confines of West TX up to the Red River will allow for rapid development of convective clusters with some supercellular modes plausible prior to nightfall, eventually merging into a more multi-cellular scheme with backbuilding favored within the confines of the front denoted pretty well within the theta_E gradient output in each deterministic. All CAMs are well supportive of the 3-wave convective evolution with heavy rainfall situated across the Southern High Plains near the eastern Permian Basin with a general QPF maxima aligned southwest to northeast across the northern extension of the Rolling Plains in TX, up through the Red River and into the southern portion of the OKC Metro. This area has been the benefactor of multiple convective episodes already, leading to flash flooding within the corridor extending from Lawton down to the Red River. This area will be the most primed location for not only widespread flash flooding, but significant life-threatening flash flood prospects due to the well above normal soil moisture anomalies likely curbing any potential for rainfall to soak into the top soil layer. Latest 00z HREF probability fields are overwhelmingly indicative of the threat with one of the more robust indications occurring within the 6-hr FFG exceedance probability marker running between 70-90% across the area extending from Foard County in TX up to western Cleveland County in OK. Neighborhood probabilities of >5" total rainfall are running between 50-80% in the same location with >8" between 20-30% across Southwest OK between Lawton to the Red River. Neighborhood probs for >3" are significant across much of OK back into North TX with the southwest extension back into the southeast corner of the LUB CWA. Considering the NASA SPoRT soil moisture percentiles running between 70-99% for the 0-40cm layer across the entire area above, the propensity for flooding, even in rural zones will be higher than normal. Given expected rates between 1-2"/hr during peak storm cycles and the expectation for >3" of rainfall...the threat will be of the higher magnitude in terms of impact prospects. The previous MDT risk inherited was not only maintained, but expanded a bit further southwest to align with the higher probabilities with the neighborhood >5" potential and the elevated HREF EAS >2" probabilities that have been depicted. A high-end MDT risk is forecast across the Red River Basin west of Wichita Falls up into the southern fringes of the OKC metro, including the town of Norman. Lower end probabilities for flash flooding will exist across areas of the eastern Permian Basin out through the Concho Valley as well as across much of Oklahoma not included in the MDT. ....Mississippi and Ohio Valleys... 16Z UPDATE: No major changes were made to the risk areas here. PREVIOUS DISCUSSION: Mid-level impulses that eject out of the Southern Plains will continue to press northeast towards the Mississippi and Ohio Valleys with a progressive cold front moving southeast out of the Midwest during the same time frame. Afternoon destabilization coupled with the favorable addition of mid-level forcing and surface frontogenesis will lead to scattered convective pulses across the region with some organized elements focused within the confines of the front and under the shortwave propagation. Soils remain moist across much of the region extending from MO through the Ohio River Basin with an eastern extension to the Central Appalachians and Western PA Highlands. PWAT anomalies on the order of +1 to as high as +2.5 deviations cements a favorable deep moist environment conducive for heavy rainfall prospects within any convective scheme. The greatest concentration of heavy rain will likely be back over Southern and Central MO towards the 3 river confluence zone near KY/MO/IN/IL. HREF probs for >1" are very high (70-90+%) across much of the Mississippi and Ohio Valley areas with >2" probs relatively high (50-70%) in-of the Ohio River Basin over Southern IL through much of Eastern KY. The combination of convection over areas that are still in recovery from previous rainfall episodes and complex terrain will create a threat for isolated to perhaps scattered flash flood instances over a large area during the afternoon and evening hours Tuesday. A SLGT risk remains firmly in place across much of MO along and south of I-70 until the 3 river confluence zone referenced above. A MRGL risk encompasses all other areas east through Western PA and even the southwest corner of NY state. Lamers/Kleebauer Day 2 Valid 12Z Wed Apr 30 2025 - 12Z Thu May 01 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH-CENTRAL AND SOUTHEAST OKLAHOMA, NORTH TEXAS, AND WESTERN ARKANSAS... The primary change for this updated ERO for Wednesday was to recenter the Moderate Risk further south (approx. 50 mi). This was due to a corresponding shift in guidance, especially hi-res guidance, but also the UKMET, CMC, and ECMWF AIFS. It also fits with the updated expectations for the current Day 1 period, as guidance has signaled the potential for relatively strong cold pools reinforcing the effective front and gradually pushing it further south. This would center it more squarely near the Red River (OK-TX border region) or perhaps fully into N TX. There is still some uncertainty associated with the exact convective evolution over the 12-18 hours immediately prior to the start of the Day 2 period, so the Moderate Risk and Slight Risk were kept fairly broad to account for different scenarios. Nevertheless, there was a clear enough shift in the guidance to warrant an adjustment in the overall risk contours. A significant flash flooding event could be ongoing at the start of the period (~12Z Wednesday) over western North Texas and/or south- central and southwest Oklahoma. Late tonight the southward drift of the effective front and the associated zone of active convection may begin to slow or stall as the low-level jet (LLJ) begins to=20 ramp up considerably over Texas. Where the nose of the LLJ sets up relative to ongoing convection will go a long way to determining the primary area of concern for Wednesday. Most hi-res models show the convective cluster(s) propagating east through the morning toward AR and N LA, with renewed development upstream over N TX by the afternoon and another training and backbuilding configuration. This same guidance generally supports the idea of a 24-hour rainfall maximum in excess of 7 inches somewhere in the new Moderate Risk area, but there is lingering uncertainty as to precisely where that will occur. Although the probability is currently highest in the vicinity of the Red River near where SE OK borders N TX, it's conceivable it could occur closer to or inside the DFW metro area. The metro area is now fully in the Moderate Risk area with this latest update. Some significant flash flooding will be possible as much of the rain will likely fall in 6-12 hours and the high end amounts produced by some of the guidance would approach or exceed a 100 year ARI for those durations. Neighborhood probabilities of 100 year ARI exceedance are not particularly high, reflecting both the uncertainty of the exact placement of the heaviest amounts, and that amounts well in excess of those thresholds are not currently supported by guidance. Significant and life-threatening flash=20 flooding would be most likely in two scenarios:=20 (1) In western North Texas or south-central Oklahoma at the=20 beginning of the period, as a continuation from the previous night. These areas have been quite wet over the past week or so, so there is existing vulnerability to extreme rainfall. (This is more likely.) (2) In the Dallas-Ft. Worth metro area, as the rainfall in question would cause serious issues in a highly urbanized area. (This is also possible, but not certain, and highly dependent on the exact convective evolution.) Further north, a broad Marginal and Slight Risk area was maintained for the Mid Mississippi Valley and Central Plains. There is some signal for organized convection and localized heavy rainfall, including a scattering of low probabilities of FFG exceedance, but confidence in any specific scenario is lacking. Lamers Day 3 Valid 12Z Thu May 01 2025 - 12Z Fri May 02 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A BROAD AREA FROM THE GREAT LAKES TO MID MISSISSIPPI VALLEY TO WESTERN GULF COAST... An expansive plume of instability is expected to support scattered to numerous areas of thunderstorms during the day 3 period ahead of a surface low and trailing cold front. Machine learning probabilities from Colorado State support the idea of a broad Marginal Risk; this is what was inherited and it was generally maintained. The biggest change was to extend the area southwest through portions of Louisiana and south and southeast Texas. Confidence in specifics is low, and the overall flash flood threat should be limited by the fairly progressive nature of the convection. Precipitable water values are more anomalous in the Great Lakes region (above the 90th percentile; above 1.3 inches), but that is also where thunderstorms should be moving the fastest. Meanwhile, available moisture will be less anomalous further south, in TX and LA, but thunderstorms will be moving slower. Therefore, heavy rainfall ingredients will generally be offset across the whole region. Hi-res models that extend out beyond 48 hours show more significant localized rainfall maxima in TX and LA, which makes sense given stronger instability and slower overall storm motions. This may be where a targeted upgrade to a Slight Risk is eventually needed. Lamers Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt= __;!!DZ3fjg!634Zb9891iMIpx13G31Ih9Q6SwdAo3JuS6R8-hUgWsJbS_Mxerk-c_1Pqs-SOAH= hdH-pxBTRP-Ct8xew5hpAbZzAmlA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt= __;!!DZ3fjg!634Zb9891iMIpx13G31Ih9Q6SwdAo3JuS6R8-hUgWsJbS_Mxerk-c_1Pqs-SOAH= hdH-pxBTRP-Ct8xew5hpApviFtvA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt= __;!!DZ3fjg!634Zb9891iMIpx13G31Ih9Q6SwdAo3JuS6R8-hUgWsJbS_Mxerk-c_1Pqs-SOAH= hdH-pxBTRP-Ct8xew5hpAB3dZ2zM$=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) .