Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 06 2025 19:29:17 FOUS30 KWBC 061929 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 329 PM EDT Sun Apr 6 2025 Day 1 Valid 16Z Sun Apr 06 2025 - 12Z Mon Apr 07 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN UNITED STATES... ....16Z Outlook Update... Main changes to the ongoing outlook were dictated by latest convective trends. Biggest change is to confine the Moderate Risk area to locations of central and southern Alabama that will experience the greatest risk of training deep convection. MCSs across that area are likely to slow and/or stall eastward progression as mid-level forcing/ascent remain well west of the region. Additionally, instability/moisture and deep convergence will be maximized across the Moderate Risk area for much of the afternoon and evening. At least one recent CAM suggests potential for 5-7+ inches of rainfall near/south of Montgomery through the early evening, which could cause locally significant flash flood potential despite relatively dry antecedent conditions. Outside of the Moderate Risk area, widespread clouds/showers and the presence of a weak front across southeastern Kentucky and vicinity has limited instability across Kentucky and West Virginia. Rainfall is expected to occur in these areas and soils are wet/susceptible to supporting efficient runoff. Limited instability appears to be a mitigating factor for a larger flash flood risk. The Slight Risk area has been reduced in size based on these factors. Lastly, a broad Marginal Risk area has been retained across portions of the Mid-South into western/central Kentucky for this outlook. While deeper convection and heavy rainfall potential has shifted well to the east of the region, critical/sensitive ground conditions exist as a result of the 8-14 inches of rainfall that have occurred over the past 3-4 days. Widespread impacts from excessive runoff continue currently. In environments like these, even light rainfall (i.e., a quick 0.25 inch) can result in exacerbation of flood impacts, and this could occur on at least an isolated basis as forcing associated with an upper low centered over western Oklahoma gradually approaches the region. See the previous discussion below for more details. Cook ....Previous Discussion... Introduced a Moderate risk mainly across central and parts of southwest Alabama today. Even though the synoptic-scale forcing has finally dislodged the focus mechanism for heavy rainfall...expectation is for storms initially in Mississippi and far northern Alabama at the start of the period to slow in forward motion as they pushes across parts of Alabama later today...leading to some 3 to 5 inch totals with isolated higher totals. Confidence in these amounts was boosted as more high- resolution CAMS in the 06/00Z model production cycle continued to depict focused, training bands of convection from central Alabama with a few of them showing localized amounts exceeding 6-8 inches. Of note was that the 06/00Z guidance tended to favor higher amounts extending southward rather than northeastward toward Georgia. The 00Z HREF neighborhood probabilities tended to show a risk of 1 inch and 2 inch per hour rates at the start of the outlook period gradually slow down later this morning with occasional upticks/downturns in the probabilities across into the afternoon. This is still conditional on when/where cloud cover allows for best instability to form and on any lingering convective outflows that can serve as a focus for renewed convection. Bann Day 2 Valid 12Z Mon Apr 07 2025 - 12Z Tue Apr 08 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST US ON MONDAY... 19Z Update: Upon examination of the 12Z model and CAM guidance suite, the inherited Marginal Risk area was expanded a little to the north to include the greater Richmond, VA metro area and all of Hampton Roads, and a modest extension to the west to include the foothills of the southern Appalachians. The ARW has a slower exit=20 of some of the heavier rainfall through midday Monday, with the=20 potential for some 1-2 inch totals along and just northwest of the=20 Interstate 85 corridor. There may also be a second area of enhanced rainfall across the coastal plain of North Carolina and extending into southeast Virginia where higher PWs and instability will exist ahead of the cold front, mainly south of Interstate 64. Depending=20 on how much convective training occurs, some instances of 2-3+ inch totals will be possible on Monday before things taper off Monday=20 evening, with some lingering lighter rains after sunset and ending=20 overnight. One of the mitigating factors here is recent dry weather and higher flash flood guidance, so the threshold of flooding is=20 higher and therefore this precludes the need for any Slight Risk=20 areas at this time.=20 Hamrick ....Previous Discussion... Overall...the ongoing forecast remained on track and only some minor adjustments were needed to account for a few pieces of guidance which still favored a slower solution and, consequently, higher rainfall amounts over a somewhat larger territory than in the previous outlook. Still think that any lingering moderate to heavy rainfall will continue to shift eastward on Monday as the large scale flow pattern remains progressive. Current model QPF generally remained in the 1.5 inch to 2.5 inch range with isolated higher totals possible from far northern Florida northward into the eastern portions of the Carolinas. The plume of 1.75+ inch precipitable water values slowly decreases with time before being shunted entirely off the Carolina coast Monday evening (and across the central Florida peninsula late Monday night/early Tuesday morning. Until that happens...moisture transport of deeper moisture remains favorable to support a low- end risk of excessive rainfall (especially in regions of poor drainage), Bann Day 3 Valid 12Z Tue Apr 08 2025 - 12Z Wed Apr 09 2025 The probability of rainfall exceeding Flash Flood Guidance is less than 5 percent. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6WDUwRgLcPaA2UyZrXKvAXir3riNy3Spll01TkeR405R= -jWjQPWPNsBqNvAUm4mRD27fzO1059Ms4jLHnQn7W375WeQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6WDUwRgLcPaA2UyZrXKvAXir3riNy3Spll01TkeR405R= -jWjQPWPNsBqNvAUm4mRD27fzO1059Ms4jLHnQn7evLQ6CI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6WDUwRgLcPaA2UyZrXKvAXir3riNy3Spll01TkeR405R= -jWjQPWPNsBqNvAUm4mRD27fzO1059Ms4jLHnQn7TmHlwsg$=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) .