Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Apr 01 2025 20:21:58 FOUS30 KWBC 012020 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 420 PM EDT Tue Apr 1 2025 Day 1 Valid 16Z Tue Apr 1 2025 - 12Z Wed Apr 2 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL PLAINS... 16Z Update: Upon examination of the 12Z CAM guidance suite and in consultation with the affected WFOs, a Slight Risk is now in effect across extreme northeast Kansas, extreme southeast Nebraska, and portions of northwest Missouri. A nocturnal MCS is likely to develop to the northeast of the main surface low across central Kansas, with the CAM guidance in agreement on indicating a band of heavy convection with short term training potential, mainly during the 3Z to 12Z time period. Three-hourly HREF neighborhood probabilities of flash flood guidance exceedance are on the order of 15-30 percent across portions of this region, with rainfall totals up to 3 inches possible by 12Z and 1-2 inch per hour rates possible. The MCS should weaken and become more progressive after 12Z Wednesday. Otherwise, the forecast remains on track and the previous discussion is appended below for reference. /Hamrick -------------------- Consensus continues to be strong for the overall evolution of a broad surface cyclone developing in-of the High Plains to Midwest over the next 24-36 hrs. A strong mid- level trough has ejected into central CA and will maneuver eastward out of the western CONUS, aiding in the initiation of a deep surface cyclone across the Colorado Front Range by later this morning, intensifying during its life cycle through Wednesday morning and beyond. The process is manifested through a well- defined axis of difluence downstream of a robust mean trough/closed-low reflection over the Rockies, kicking eastward into the High Plains of the Dakotas. Current deterministic output indicates sub-990mb pressures by later this afternoon with significant low-level moisture advection within the lead side of the circulation when assessing the forecast 850-700mb RH field. A textbook warm conveyor belt signature will mature with a connection based from the Gulf driving warm, moist air poleward and enhancing a blossoming QPF shield by the time we reach this evening. The primary focus for heavy rainfall will ensue after sunset as the initiation of a powerful LLJ across the Central Plains on the order of 65-75kts will create an axis of growing low to mid-level shear juxtaposed within a budding warm sector downstream of the primary circulation. As the surface cyclone matures, a more defined cold front will develop along the tail end of the low as it moves into the Plains creating a focus for convective development as it maneuvers towards the warm sector. All major deterministic, including the latest 00Z CAMs output indicate a rapid development of convection across portions of northern KS into southeast NE advancing east- northeast within the bounds of the cold front and warm sector of the cyclone. There is also some growing consensus of a further south and southwest extension of the convective field within eastern KS down into OK due to the ejection of a weak mid-level perturbation caught up in the mean flow that intersects the prolific LLJ bisecting the areas above. This correlates well with the current ML output that has been steady with a small sector of convection displaced from the main line that will ultimately develop over KS into NE and IA. HREF neighborhood and EAS probs for the primary axis of convection over northeast KS and points northeast are very much elevated with a core of 50-70%+ for at least 2" total via neighborhood probabilities and 40-60% for at least 1" total via a the more conservative EAS, a testament to a solid overlapping signal for heavy rainfall within the above zone. A wide swath of 30-50% probs for at least 2"/3-hrs is also situated within the primary zone of interest, alluding to modest FFG exceedance probabilities for the 3-hr time frames between 00-12z Wednesday. Further south, the probabilities are lower, but still within the lower bounds to at least consider the potential. Latest First Guess Fields have been steady to include that area across OK and eastern KS and a few CAMs are worthy of the consideration. These types of synoptic and mesoscale setups being depicted could offer some surprises due to the increasingly favorable thermodynamic and kinematic evolutions forecast within a rapidly improving environment, so to refrain from missing out on potential, was sure to maintain the previous MRGL risk with only minor adjustments within the southern and northern bounds of the risk area attributing to current probabilities and CAMs signals. Kleebauer Day 2 Valid 12Z Wed Apr 2 2025 - 12Z Thu Apr 3 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM EASTERN ARKANSAS TO SOUTHERN INDIANA... 19Z Update: The overall forecast remains in good shape compared to the overnight issuance with only some minor adjustments needed. The 12Z CAM guidance generally has the heaviest QPF signal on the southern edge of the MCS events that will develop along the stalled out frontal boundary, and combining this fact along with historical trends for max QPF to occur slightly south of the multi-model consensus in many cases, the updated Marginal Risk area was adjusted to the southeast by about a row of counties across western Tennessee, western Kentucky, and eastern Arkansas. The HREF flash flood guidance exceedance probabilities are generally highest during the 00Z to 9Z time period Thursday, when multiple rounds of training convection are expected to produce widespread 2 to 4 inch rainfall totals, with isolated 4 to 6 inch totals possible. It appears more likely the 3-hourly guidance values will be exceeded compared to the 1-hourly guidance, so this will likely be more of a prolonged duration event. This will also saturate the soils and elevate stream and river flows ahead of the additional heavy rainfall expected during the Day 3 period 12Z Thursday to 12Z Friday, with a major flooding event looking more likely. The previous forecast discussion from overnight is appended below for reference. /Hamrick ------------------- The D2 period will represent the initial stages of a prolonged heavy rain event that will reside over the Lower Mississippi and Ohio Valleys with a multitude of frontal waves riding along a persistent quasi-stationary front that will undulate at times when interacting with the aforementioned surface waves. The general longwave pattern will become increasingly favorable for a "highway" of moisture and mid-level impulses to eject northeastward out of the southern Rockies/Plains and up through the Mississippi and Ohio Valleys beginning Wednesday afternoon, onward. The consensus among all ensembles and much of the deterministic fields are the first wave of the pattern to maneuver over the mid-Mississippi Valley around the base of a broad trough located over the Plains/Midwest creating ample surface-based ascent to couple with the powerful upper jet component situated over the Central CONUS in response to the evolving pattern the day prior. At the surface, a cold front will run into a stout western ridge extension from a surface high located over the Western Atlantic into the Canadian Maritimes. The pattern essentially comes to a stalemate thanks to an expected +2 to +3 standard deviation height anomaly centered over the Western Atlantic. This ridge will counter the deepening trough over the Central U.S. creating a funneling affect for deep moisture advection through the Mississippi and Ohio Valley's, along with ample mid-level impulses that will be preside over the same areas. The gridlocked setup will create a formidable moderate to heavy rain event across the above areas with depictions from guidance indicating a widespread 2-4" with some areas exceeding 5" increasingly likely within the corridor impacted by the multitude of convective elements spawning southwest to northeast in proxy to the quasi- stationary front. Latest prob fields based within the NBM blend and relevant EC/GEFS ensembles signal a high likelihood of totals >3" over the span of 18z Wed to 12z Thu with more rain continuing beyond the D2 period. The maxima is focused within a narrow corridor from central and northeast AR up through the mid-Mississippi Valley encompassing far southeastern MO, western KY, and southern IN with the probability for >2" running between 60-80% and >3" settling between 40-60% without a full implementation of the CAMs into the blend. Rainfall rates will be generally between 1-2"/hr at peak intensity when assessing the current HREF/NBM outputs, however, there's a chance for higher rates located within the southwestern flank of the stationary boundary (Much of Arkansas up into western KY) as assessment of the PWAT advection regime signals potential for localized PWATs exceeding 2" at times during organized convection. Historically, this tends to lead to locally higher rates with prospects of 2-3"/hr possible, especially when accounting for intra-hour rates during maturing convection. In all, the setup is very much conducive for substantial rainfall totals in a span of 12-hrs that will allow for high flash flood concerns within urban zones and an increasing potential for significant flash and river flooding outside those areas just due to the overall QPF footprint anticipated. 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 3 Valid 12Z Thu Apr 3 2025 - 12Z Fri Apr 4 2025 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR EXTREME SOUTHEAST MISSOURI TO WESTERN KENTUCKY WITH MAJOR FLOODING EXPECTED... 20Z Update: Major and potentially catastrophic flooding is=20 becoming more likely across portions of the Mid-South from=20 northeastern Arkansas to western Kentucky, including portions of=20 northwestern Tennessee and southeastern Missouri. Very heavy=20 rainfall in the 12 hours preceding this time period will greatly=20 saturate the soils over the region and thus dramatically reduce=20 flash flood guidance, which will easily be exceeded Thursday into=20 Thursday night. There is strong model consensus for several inches=20 of additional rainfall during the Day 3 period over many of the=20 same areas. Additional shortwave energy tracking along the stalled=20 out frontal boundary will serve as forcing mechanisms for more MCS=20 activity with multiple rounds of convective training likely, with=20 rainfall rates exceeding an inch per hour for potentially multiple=20 consecutive hours. Major rises on area creeks and streams are=20 expected. After consultation with the affected field offices, river forecast centers, and the National Water Center, a High Risk area=20 has been introduced for this update. The previous forecast=20 discussion is appended below for reference. /Hamrick ------------------ The D3 period is a general extension of the D2 time frame with the synoptic scale pattern yielding very little difference in what is driving the setup. The biggest change from day to day with the positioning of the quasi-stationary front will be the northern extent becoming a bit more elongated from west-southwest to east- northeast as we move through KY leading to a better push of moisture and surface based ascent focused across more of the western 1/3rd of KY up through the Ohio River basin. This will encompass areas like Louisville to Cincinnati that might be on the edge of the heavier QPF footprint on D2, but will be within the northern periphery of the heavier precip come D3. The key to the extent of the significant flood prospects will be where the D2 and D3 overlap in the overall QPF distribution. There is a considerable probabilistic output within the 01z NBM for D3 totals to exceed 2" with a wide swath of 60-80% probs for the threshold situated from southwest AR all the way up through southern OH, an area that will experience significant rainfall the period prior. The 72-hr probability for >6" of total rainfall is between 30-60% across northeast AR up through northwest TN and western KY with the max prob field positioned between Paducah, KY down to Jonesboro, AR. This corridor is located within a notorious flood plane thanks to a litany of streams and river channels with the Mississippi River the main stem that bisects the region. The forecasted surface wave in question is well-defined when assessing the global deterministic 500mb vorticity pattern. A developing 250mb jet coupling is also being orchestrated by some of the global deterministic with, at minimum a substantial right- entrance region jet dynamic well- positioned over much of the Mississippi Valley. This setup is textbook for a continuation of the "meteorological highway" of moisture and mid-level vorticity advection that historically has caused significant flash flood issues over the Lower Mississippi and Ohio Valley's. This is only day 2 of the forecasted 4 day event, so the priming of soils will have been underway and only getting worse as time moves on. The previous MDT risk inherited from the D4 was relatively maintained, however there was some expansion of the MDT to the northwest to account for a small trend within the forecasted alignment of the quasi-stationary front and amplitude of expected surface wave ejecting out of the southern plains. This period is considered to be a higher-end Moderate, meaning the prospects for a targeted high risk are within reason pending the previous period's QPF outcome and the convective expectation as we get closer to the D3 time frame. The area of vested interest for a potential upgrade includes that corridor of northeast Arkansas up through western Tennessee, far southeast Missouri, western Kentucky, and slivers of both southern Illinois and Indiana. This setup is shaping up to be a more extreme flooding scenario considering the multi-day prospects and forecasted 72 to 96 hr rainfall totals reaching 10-15+ inches in the hardest hit locations. 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 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4HMnVsW7CVKCRFVdwLD_VwYn04jAq3NoyWNVnVz5bCVe= 8suFInM_yAKIV8JgtzfonxYZxUuEXoO-ZIOQxV8RulkIvoc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4HMnVsW7CVKCRFVdwLD_VwYn04jAq3NoyWNVnVz5bCVe= 8suFInM_yAKIV8JgtzfonxYZxUuEXoO-ZIOQxV8RQ-jinvE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4HMnVsW7CVKCRFVdwLD_VwYn04jAq3NoyWNVnVz5bCVe= 8suFInM_yAKIV8JgtzfonxYZxUuEXoO-ZIOQxV8RYmXiu2w$=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) .