FOUS30 KWBC 040019 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 819 PM EDT Thu Apr 3 2025 Day 1 Valid 01Z Fri Apr 04 2025 - 12Z Fri Apr 04 2025 ...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER TO MID-MISSISSIPPI VALLEYS WITH MAJOR FLOODING EXPECTED... ..A PROLONGED LIFE-THREATENING FLASH FLOOD EVENT WILL CONTINUE TODAY WITH HEAVY RAINFALL IMPACTING A LARGE PORTION OF THE LOWER MISSISSIPPI AND OHIO VALLEY'S ONCE AGAIN... ...16Z Outlook Update... The overall forecast philosophy is on track and only minor spatial adjustments have been made to categorical areas. The High Risk areas remains in place across portions of Arkansas, Tennessee, and Kentucky, where a nearly stationary frontal boundary should linger throughout the day and foster multile rounds of deep convection. 12Z model QPFs suggest another 2-4 inches of rainfall generally from Little Rock northeastward through south-central Kentucky, with much of that rain falling on sensitive ground conditions (low to near-zero FFGs) from prior rainfall and flood impacts last night and this morning. Significant impacts remain possible, with the only lingering uncertainty for convective coverage tied to gradually rising geopotential heights from this afternoon onward. Little Rock metro was added to the High Risk area after consultation with WFO LZK, with ongoing high-water impacts continuing as of this discussion issuance. The Slight Risk area was expanded eastward into more of West Virginia, southeastern Kentucky, and Middle Tennessee for this outlook. Portions of West Virginia have experienced impacts from heavy rainfall already this morning, and an extensive fetch of moderate to heavy rainfall is expected to persist much of the day given renewed convective development across the Mid-South per latest radar mosaic imagery. Lastly, Marginal risk areas were maintained across 1) west Texas for overnight redevelopment of convection in areas that experienced heavy rainfall this morning and 2) expanded into the Philadelphia/New York City and adjacent areas for banded/training convective potential in the 08-12Z timeframe. A quick 0.5-1.25 inch of rainfall is possible across urban areas of the I-95 corridor, which could pose a few runoff issues. See the previous discussion below for more information. Cook ...Previous Discussion... Significant rainfall over the past 12-24 hrs will only add to the favor of widespread flash flooding later this afternoon and evening as the next round of heavy rainfall occurs as the next surface wave rides up the quasi-stationary boundary in place across the Lower to Mid-Mississippi and Ohio Valleys. Widespread 2-4" of rain with maxima between 5-6" has fallen this evening across a zone spanning from southern Arkansas up through western Tennessee and Kentucky leading to several flash flood warnings as the first surface wave lifts north. Some overturning of the atmosphere is anticipated by morning with areal theta_e's decreasing across areas north of the Ohio River and a tightening instability gradient positioned along and south of the quasi-stationary front nestled from southwest to northeast over the Mississippi Valley. Despite some limiting factors early on, small mid-level perturbations embedded within the mean flow will still cause some scattered light to moderate convective development through the morning and afternoon leading to a continuation of priming from the prior evening. The nocturnal period will once again become the main period of interest as the nocturnal LLJ kicks back in across the Deep South and banks into the stationary front bisecting the Mississippi and southern Ohio Valley. The same areas that saw the significant rain and flash flooding this evening will be hit once again with another wave of convection that will initiate across the ArklaTex and southern Arkansas and eventually move northeast as the flow aligns parallel to the stationary front. Additional precip totals of 2-4" with local to 6" will once again be in the cards for much of Arkansas, western Tennessee, and southwestern Kentucky which will ultimately put 48-hr totals between 4-8" with locally up to 10" during the time frame. Local FFG's are already very low after today's precip with much of the corridor spanning eastern AR up through north-central KY exhibiting 1 and 3-hr FFG intervals running below 1/1.5" respectively. 00z HREF is very much indicating that rainfall rates during the peak of diurnal convection will reside within that 1-2"/hr marker with FFG exceedance probabilities above 50% for the 1/3/6-hr intervals in place. Neighborhood probabilities for >3" total are between 40-60% across eastern AR with as high as 60-80% over northwest TN and southwestern KY. >5" neighborhood probs are running between 25-40% within the same corridor, backed up by EAS probs >2" sufficiently above 50% along the Mississippi north of Memphis to points northeast. Despite the overall output being slightly lower than what occurred this evening, the prescience has been set for significant flood potential given the already saturated soils, expected rates, and regional rivers running high from the previous period of rainfall. The previous High Risk inherited was generally maintained with small adjustments accounting for recent rainfall trends, updated FFG's, and encompassing elevated probabilities from both the hi-res and national ensemble blends. A Moderate Risk still in effect for a large area surrounding the High Risk in place across the Lower to Mid-Mississippi Valley with a span that covers areas of southwest Arkansas up to as far northeast as Cincinnati. This is quickly becoming a high-end flash flood scenario with another day or two of rainfall expected on top of what has occurred and what will occur. If you lie within a flood plane or any area that is prone to flash flooding across Arkansas, western Tennessee, and western Kentucky, you will want to pay close attention and have a plan to seek higher ground, if possible. Kleebauer Day 2 Valid 12Z Fri Apr 04 2025 - 12Z Sat Apr 05 2025 ...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OZARKS AND MID-SOUTH... ...LIFE-THREATENING FLASH FLOODING EXPECTED FRIDAY AFTERNOON THROUGH EARLY SATURDAY... ...21Z Outlook Update... Both global models and CAMs have come into agreement with development of at least one north-northeast to south-southwest oriented band of convection located generally from the ArkLaTex into southeastern Missouri. Convection should be oriented generally parallel to fast, meridionally oriented flow aloft, with abundant moisture, low- level shear, and instability profiles all favoring abundant rainfall (potentially exceeding 10 inches on a localized basis). These totals will fall on areas that will have experienced 2-6 inches of rainfall leading up to the forecast period, with wet soils/terrain variations all suggestive of a potentially widespread and significant flash flood event. Expansion of Moderate Risk (and attendant probabilities) was considered across more of Illinois and central Indiana, but pre- existing dry conditions and uncertainty about duration of precipitation were mitigating factors for a higher flash flood threat. This region will be evaluated for a possible upgrade in later outlooks. Outside of these changes, the overall outlook was on track with an eventual broad axis of heavy precipitation expected from north/central Texas northeastward to Ohio/Pennsylvania. See the previous forecast discussion below for more details. Cook ...Previous Discussion... The previous synoptic scale forecast was relatively unchanged within the run to run assessment over the course of Friday into Saturday morning. 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 48 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 +3 to +4 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 activity, 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 east 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. The heaviest precipitation axis based off the latest NBM probabilities still has a significant footprint of at least 3" located near the Dallas metro and points just east (40-60%) to as high as 60-80% in a large area spanning the Red River of northeast TX up through much of western and northern AR, through the Ozarks of MO, finally reaching the Mississippi River south of St.Louis. Ensemble mean QPF output signals a massive area of at least 2" with urban centers like Dallas, Little Rock, St. Louis, and Springfield, MO all within a core of the heavy precip footprint. Considering the probabilistic output of the NBM, the consistency within guidance for a significant corridor of very heavy rainfall, and a well-defined axis of training precip potential, a broad Moderate Risk was maintained and even expanded to account for the expanse of likely flash flooding. A high-end Moderate is a likely tag for places within southeast OK up through western and northern AR through the Ozarks of Missouri as these areas will have the greatest threat of enhanced rates over top of either complex terrain or primed soils. An upgrade is plausible in future updates pending additional convective probability details. Kleebauer Day 3 Valid 12Z Sat Apr 05 2025 - 12Z Sun Apr 06 2025 ...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OZARKS AND LOWER TO MID-MISSISSIPPI VALLEYS WITH MAJOR FLOODING EXPECTED... ..LIFE-THREATENING FLASH FLOODING IS EXPECTED ON SATURDAY AFTERNOON THROUGH SUNDAY MORNING.. ...21Z Outlook Update... Substantial changes were made to the ongoing outlook with the expectation that deep, slow-moving and intense convection would still be ongoing across Arkansas at 12Z Saturday. The High Risk expansion in Arkansas is essentially a continuation of the significant flash flood threat from the D2/Fri forecast period. South-soutwesterly flow aloft at the start of the period is expected to continue fostering a very slow eastward progression of convection, while 1.75+ PW values,, 1500+ J/kg MLCAPE, and convective training/mergers should continue to foster high to locally extreme rain rates and FFG exceedence on a widespread basis. At least 3-5 inch rainfall totals are expected, with locally higher amounts possible. Eventually, veering flow aloft will result in a faster forward speed of any convective complex(es) toward the Ohio and Tennessee River Valleys during the latter half of the forecast period. As this occurs, low FFGs are expected to remain from abundant antecedent rainfall, and it is very likely that ground conditions will not have completely recovered from abundant antecedent rainfall. In this scenario, widespread significant impacts are expected with both ongoing and newer instances of flash flooding. Additional details are available in the previous discussion below. Cook ...Previous Discussion... The combination of intense right-entrance region jet dynamics, highly anomalous atmospheric moisture, and very saturated antecedent soil moisture will lead to another high impact flash flood forecast for Saturday afternoon through Sunday morning across the Lower to Mid-Mississippi Valley. This threat will extend further east as a cold front helps finally sweep the stagnant pattern out of the region, but not before another round of heavy rainfall inundates areas that will be sensitive to any appreciable rainfall. A stout 190-200kt upper jet located over the Great Lakes will lend to a pronounced upper ascent pattern situated over the Mississippi Valley during the period. At the surface, our quasi-stationary front that will linger through the course of several days will still be confined to similar areas that have been impacted by its presence in the days prior. Upper low located over the Southwest U.S. will begin to open up and eject eastward, becoming sheared on the northern edge due to an amplifying trough digging out of Canada. To the east, our Western Atlantic ridge that has provided a stalemate to any frontal progression moving east of the Mississippi will provide one last period of favorable low to mid-level convergence as the pattern evolves with the large trough to the west countering the ridge across the Southeast. In the middle of the atmospheric squeeze-play, elevated moisture presence and favorable instability will remain present to be activated with the addition of diurnal destabilization and increasingly favorable ascent to match. The combination will allow for a rapid development of widespread convection across the Missouri Valley with slow advancement eastward towards the Low to Mid-Mississippi Valley convergence point. This setup has trended prolific in the QPF realm of the forecast with very little deviation in all major deterministic at this juncture. An expanse of 3-6" with locally up to 8" of rainfall is forecast across a large portion of Arkansas extending east- northeast through far southeastern Missouri, all of western and central Tennessee and Kentucky, to as far north as the Ohio/Kentucky border along the Ohio River. Northwest Mississippi will also lie within the axis of heaviest precipitation leading to quite a large area of not only heavy precip, but significant overlap of areas that will have super saturated grounds that will struggle to maintain the ability to absorb more rainfall. This setup will yield a very dangerous scenario where widespread flash flooding with considerable and potentially catastrophic impacts would be favored. The agreement among all major NWP for substantial rainfall totals and projected hourly rates >1-2"/hr is a testament to the overall gravity of what is to come. In coordination with the local WFO's across Arkansas, Tennessee, and Kentucky...a High Risk area for excessive rainfall has been added over central and eastern AR, all of western TN and KY, far southeastern MO, and the southern edge of IN. Additional storm totals will bring multi-day amounts between 8-12" with some areas potentially reaching as high as 14" over the span of 5 days. A broad Moderate Risk spans the outer perimeter of the High Risk leading to several more areas prone to significant flash flood concerns due to the setup. This is becoming an increasingly life-threatening situation for areas of the Lower and Mid- Mississippi Valley region. 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