


Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
421 FOUS30 KWBC 020739 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 339 AM EDT Wed Apr 2 2025 Day 1 Valid 12Z Wed Apr 02 2025 - 12Z Thu Apr 03 2025 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM SOUTHERN ARKANSAS TO SOUTHERN INDIANA... ..A PROLONGED LIFE-THREATENING FLASH FLOOD EVENT WILL BEGIN TODAY WITH SEVERAL DAYS OF HEAVY RAINFALL IMPACTING A LARGE PORTION OF THE LOWER MISSISSIPPI AND OHIO VALLEY`S.. The setup for the D1 period is already underway with a broad upper trough pattern across the Central and Western CONUS taking shape and interacting with a building Western Atlantic ridge that`s already flexing its muscle in the overall longwave pattern. A deepening surface low over the Plains will migrate in-of the Upper Midwest with a cold front advancement towards the Mississippi Valley as we move into the first half of the period. The issue becomes two-fold by later this afternoon as the frontal boundary slows its forward momentum due to the surface reflection over the Midwest becoming vertically stacked and losing its vigor in progressing the pattern. Meanwhile, the ridge over the Atlantic will continue to strengthen leading to a stout western edge of the surface ridge pattern acting as a "wall" to prevent much more motion of the surface front migrating out of the Plains. By the evening, the front will likely be deemed quasi-stationary, meaning the overall advancement of the boundary will be limited and will only move based on subtle surface wave propagations that ride along the boundary, or from well-defined cold pools that originate from organized convective clusters that form within the vicinity. In this case, both of these outcomes are likely to play a significant role in location of the frontal positioning, as well as an inflection point for daily organized convection across the Lower Mississippi Valley up through the western half of the Ohio Valley for D1. Assessment of the 500mb vorticity field based off the past several deterministic outputs indicates the first in a set of 3 distinct surface waves that will originate out of the Southern Rockies and Plains that will eject northeastward out of the base of the mean trough and migrate right up the stationary boundary in place. This is a textbook signal for enhanced convective development with a sharp low to mid-level moisture advection regime along and ahead of the surface amplification, much of which is aided by a budding LLJ presence within the eastern flank of the surface wave. The latest CAMs and global deterministic are in agreement on a large areal extent of southwest to northeast oriented convection that will likely spawn between 18-00z this afternoon and evening, mainly once the primary mid-level perturbation enters the picture and provides sufficient ascent to trigger convection along the surface front. 00z HREF probabilities for >2" PWATs are generally modest between 25-50% across the Lower Mississippi Valley with the primary axis located across southern and eastern AR. >1.5" PWATs is very high, however across the same areas with a northern expansion all the way beyond the Ohio River near IL/IN/KY/OH by later in the evening. This is thanks to the strengthening nocturnal LLJ pattern that will transpire ~00z, maintaining a nearly steady state as we move through the overnight hours Wednesday into Thursday. Considering the variables above, a significant precip core is anticipated extending from the ArklaTex, north into the north- central Ohio Valley with a sharp eastern edge defined by the greater subsidence provided by the western fringes of the low to mid-level ridge anchored over and off the Southeast U.S. coast. This sharp delineation is typically an artifact of strong surface based convergence present just upstream, especially within a primed upper pattern fighting against the ridge itself. Across AR up through western TN and KY, a well-defined axis of heavy precip is progged across all major NWP, a testament to the agreement on the placement of the primary convective cell motions, but also where there is an expectation for the future cell mergers and forward propagation to occur leading to repeated rainfall threats during the course of Wednesday evening to Thursday morning. HREF neighborhood probabilities for >3" of rainfall indicate a large corridor of 70-90+% probs located across south-central and eastern AR, extending northeast up through western TN and the far southwestern KY area to the south of PAH. The >5" probs are also elevated within that same zone running 25-50% across that corridor leading to general continuity in what has been forecast the past several cycles. EAS probabilities are also very impressive for >2" over much of the above area with 60-80% located over almost all of western TN, including the Memphis metro. These types of outputs from the prob fields are historically aligned with higher-end risk days, especially over regions that have already primed soils or exhibit large scale urban footprints privy to runoff potential. This setup is shifting towards the significant category in terms of expectation for the first day of a long-lasting period of enhanced rainfall concerns thanks to the pattern hitting a "longwave roadblock". Totals for the D1 period are likely to reach 2-4" across a large domain with localized totals of 5-8" plausible in areas that see greater training and focused cell mergers once the storm mode shifts from supercellular to more of a multi-cell cluster after cold pool mergers late in the evening. The previous Moderate Risk was relatively maintained, but did extend the risk a bit further east due to CAMs output signaling a greater risk of outflow dominant regimes pushing the eastern extent of the heavier precip further into northern MS and western TN. A high-end Moderate is most likely from the Pine Bluff, AR up through the southwestern KY, northwest TN border south of Paducah. The Memphis, TN metro and the eastern side of Little Rock, AR is the most likely large metro corridors under the greatest threat for significant flash flood prospects when assessing the latest probabilities and general QPF output. Further north into KY and IN, a Moderate Risk still exists for most of western KY up into southern IN as guidance indicates a secondary maxima of QPF due to cell initiation late Wednesday into early Thursday along the leading edge of the surface wave as it migrates northeast. Some guidance is very aggressive with precip outputs >2" over a short time scale, enough to cause flash flood concerns within any urban corridors and valleys located within the river flood plane. Considering the signals for >1" at the very least, the Moderate Risk was drawn north to account for the potential with a general expectation this setup will evolve into that area as we work into the D2. This is an increasingly significant setup approaching with potential for high impacts and life-threatening flash flooding spanning the course of several days. Be sure to prepare if you live in a flood zone anywhere from Arkansas, northeast through the Ohio/Tennessee Valley`s. Anyone surrounding will want to monitor this setup closely as small changes could have heightened impacts given the forecasted setup. Kleebauer Day 2 Valid 12Z Thu Apr 03 2025 - 12Z Fri Apr 04 2025 ...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR EXTREME SOUTHEAST MISSOURI TO WESTERN KENTUCKY WITH MAJOR FLOODING EXPECTED... ..A PROLONGED LIFE-THREATENING FLASH FLOOD EVENT WILL BEGIN TODAY WITH SEVERAL DAYS OF HEAVY RAINFALL IMPACTING A LARGE PORTION OF THE LOWER MISSISSIPPI AND OHIO VALLEY`S.. The D2 period will be an ongoing atmospheric evolution with the first half of the time frame being a continuation of the significant rainfall from the D1 as the succession of cold pool mergers and convective clusters propagate across the Ohio and Tennessee Valleys before finally fading as they lose steam moving into the wall of subsidence to the east. Scattered showers and storms will still be ongoing within the confines of the quasi- stationary front just due to continued low-level convergence and smaller mid-level perturbations ripping through the "atmospheric speedway" setup from southwest to northeast across the Southern Plains up through the Ohio Valley. Despite the expectation for lower rates during the early to mid-afternoon period on Thursday, any additional rainfall will act as a priming mechanism for what will transpire later in the evening as the second of 3 major surface waves takes shape across the Lower Mississippi to Ohio Valleys. The introduction of yet another stout LLJ pattern and attendant low-level moisture flux will yield another round of widespread convection within proxy to the surface front that will modestly undulate north to south within the confines of the aforementioned areas. The positioning of the front will make all the difference in the inevitable convective cell motions and subsequent QPF distribution, something that will be important when assessing where the worst flooding will occur as the grounds around much of the Low to Mid-Mississippi Valley and western Ohio Valley will be flood prone from the previous convective impacts. As of this time, there is ample agreement from global deterministic, as well as the relevant ensemble packages for an axis of heavy precipitation to impact a zone spanning central and northeastern Arkansas up through western Tennessee and Kentucky with the northern edge of the heavier precip aligned with the KY/OH border near the Ohio River basin. Additional amounts of 2-4" with locally up to 7" will impact those above zones, overlapping several areas that will have seen significant rainfall the previous 24 hrs. 01z NBM probabilities for rainfall amounts >6" over the D1-2 period are now upwards of 40-60% located from Pine Bluff, AR up through western KY between Paducah and Bowling Green. The area with the highest probabilities is across western TN, just north of the Memphis metro, but very close by the major metro center. Considering the elevated theta_e alignment and +2 to +3 deviation PWATs present, rainfall rates within matured convective cores will likely breach 1"/hr with 2-3"/hr within reason, especially in the early evening when instability is highest and the LLJ is at peak intensity. The deep moist convergence regime will be well-established and cell motions will remain generally parallel to the frontal boundary thanks to the stagnant synoptic scale pattern in place. This will lead to significant, life-threatening flash flooding to impact a large area where FFG`s will likely lean critically low. Considering all the above variables, there was no reason to remove the High Risk. Instead, the risk area was expanded to account for the latest probabilities and relevant run to run continuity from the pertinent NWP. A Moderate Risk spans from southwestern Arkansas to the northeast, just outside Cincinnati on the Kentucky side of the Ohio River. Memphis, Little Rock, Pine Bluff, Bowling Green, Louisville, and Evansville are all major urban corridors within the Moderate Risk with areas like Paducah and Union City the larger towns/metros embedded within the High Risk. The travel routes most susceptible to flash flooding will be along the I-69 and I-55 interstates within western KY, western TN, and northeastern AR. Kleebauer Day 3 Valid 12Z Fri Apr 04 2025 - 12Z Sat Apr 05 2025 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR A LARGE PORTION OF THE LOWER AND MID-MISSISSIPPI VALLEYS... The final surface wave within the atmospheric parade across the Southern Plains to Mississippi Valley will show itself by late- Friday morning through the remainder of the period. This wave will likely be the strongest of the 3 allowing a much more amplified surface evolution that will shift the focus of heavier precip further northwest while also providing some vigor in the overall QPF distribution. The trend the past 24 hrs is for a zone of significant (3-6+") totals to be recognized across portions of the Southern Plains up through the Ozarks and Mid-Mississippi Valley of Missouri, spanning as far north as south-central Illinois. When assessing the theta_e environment, there is a noticeable push north of more modestly unstable air with the most pertinent area of available instability situated across southern MO down through AR, southeastern OK, and even northeastern TX. A sharp uptick in regional PWATs with deviations between +2.5-3.5 anomaly-wise are forecast within that zone correlating with a substantial rise in area convection blossoming late Friday afternoon, onward. This setup is fortunately further west compared to the previous periods, so the overlap of the heaviest rainfall is not anticipated to be as problematic as the previous period. Having said that, the pattern will yield high precip totals due to widespread convective schema, falling over some areas that will have seen appreciable rain, and over other areas that are more prone to flooding due to complex terrain situated over the Ozarks. The pattern is such that several areas from northeast of DFW up through IL will see rainfall amounts of 3-6" with locally higher beginning Friday afternoon with the heaviest rainfall likely to occur by the evening with the implementation of yet another LLJ structure thanks to the surface wave mentioned previously. Further to the west, another closed upper reflection across the Southwest U.S. will only entice greater upper forcing within a broad axis of difluence situated over the Plains. This setup is more privy to convection forming as far west as the Western Rolling Plains of TX to points east and northeast. Even there, modest 1-2" with locally higher totals are forecast in-of those zones. As a result, there was a general expansion of the Slight Risk further southwest into Central TX, an expansion of the Moderate Risk to include the eastern Red River Basin up through the southeast half of Missouri into southern Illinois, and a high-end Moderate impacting southeastern OK up through western AR and much of southern and southeast MO. The threat of widespread flash flooding is increasing for areas within the Moderate risk and aforementioned higher-end Moderate designation. Be sure to stay up to date for the latest changes as we move through the remainder of the week as this pattern remains very active and prone to significant life- threatening flash flood concerns. Kleebauer Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt