Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Mar 31 2025 07:40:55 FOUS30 KWBC 310740 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 340 AM EDT Mon Mar 31 2025 Day 1 Valid 12Z Mon Mar 31 2025 - 12Z Tue Apr 01 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL GULF COAST... ...Central Gulf Coast.. A fairly amplified longwave pattern over CONUS with a deep low=20 over Ontario will allow for a trailing cold front to slowly propagate east-southeastward through the Deep South with a defined convective footprint along and ahead of the frontal boundary. Recent trends have shifted the previous QPF max closer to southeastern LA within the axis of the stronger LLJ providing the best regional shear and low-level convergence progs near the southwestern edge of the front. 00z HREF neighborhood probs for >2" are running between 60-90% within the confines of the southeastern parishes, including the urban corridor between Baton Rouge to New Orleans. Considering the urban factors and the previous heavy rain impacts over the past 36 hrs, especially within Baton Rouge, the prospects for flash flooding are relatively elevated. Local PWATs will trend between 1.7-2.0" when assessing the HREF mean, a suitable environment for increased rates approaching up to 2"/hr. This falls within the lower threshold for flash flood potential according to the recent FFG's in place.=20 The threat remains focused within the zone of southeastern LA and southern MS where the best frontal convergence and instability is co-located within the bounds of the front. Heavier QPE footprint=20 lingering over towards MOB from the past 12 hrs will allow for a heightened concern for flash flooding as the convective cluster/line progresses eastward. Thankfully, models have backed off a bit on the heavier precip being located over the areas that were heavily impacted this afternoon/evening. There's still an opportunity for a few heavier cells to impact that portion of the I-10 corridor from Biloxi to Mobile, so wanted to maintain some continuity from the previous forecast SLGT in the area to cover for the sensitivity factors. The main change was the broad SLGT coverage was shifted westward into the southeastern LA parishes and southern MS to encompass the highest probs for 2-4" of rainfall, most of which is forecast to fall within a 3-6 hr time frame between 12-18z. A broad MRGL exists over the Deep South into the southern Appalachians for isolated threats of heavier precip within the confines of the terrain of eastern TN, north GA, and the western edge of Upstate SC.=20 ...Mid Atlantic.. The deep longwave pattern across the eastern half of the CONUS will translate into a bout of heavier rain across the Mid Atlantic=20 later this afternoon and evening thanks to strengthening right- entrance region jet dynamics that will develop over the course of=20 the period. Recent guidance has been keen on an increasingly potent upper jet motioning across the eastern Great Lakes with forecast speeds approaching 140-150kts as it traverses north of the northeastern CONUS during the middle of the period. Considering the deep layer ascent provided by the jet being coupled with sufficient low to mid-level moisture advection occurring along and ahead of the front/mean trough progression, the pattern will yield a blossoming of convection beginning over the northern Mid Atlantic, eventually building southward towards a weak frontal wave approaching from the southwest as it exits out of the Tennessee Valley. A line of heavy rain is likely over parts of northern VA up through the Delmarva into eastern PA and NJ with the northern extent reaching the lower Hudson into NYC and eastern LI as the low rides northeastward. The max potential for this convective signature is capped due to the progressive nature of the precip field with most of the rain falling in a 3 hr window as it advances eastward. Neighborhood probs are elevated for >1" totals (60-90%) within a corridor from northern VA up through NYC, however the probabilities drop off considerably when moving closer to 2" (10-30%), a testament to the progressive nature of the pattern. The areal footprint of urbanization is one of the main reasons why the threat of flash flooding exists in this scenario leading to a MRGL risk maintained over the above corridor. There was an expansion further southwest into northern VA and the DC metro to account for recent trends within the probability fields and the bias for convection to develop further southwest within a frontal dynamic.=20 Kleebauer Day 2 Valid 12Z Tue Apr 01 2025 - 12Z Wed Apr 02 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL PLAINS & MIDWEST... A strong mid-level trough ejecting out of the western CONUS will aid in initiating a deep surface cyclone across the High Plains of the Central U.S. by late-Tuesday through Wednesday morning. The process is manifested through a well-defined axis of difluence downstream of the mean trough/closed-low reflection over the Rockies, kicking eastward into the plains. Current deterministic output indicates sub-990mb pressures with continued intensification through the end of the period generating 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 Tuesday 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. Most guidance now 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.=20 The normally conservative NBM probs for >1" have grown to 50-60%=20 within a area focused over northeastern KS, southeast NE, into=20 western IA with some 30-50% probs extending southwest within the=20 southern edge of the convective flare up along the cold front. Most deterministic output signals 1-2" with locally higher totals=20 embedded within those above zones. ML output has been fairly=20 consistent with that area being the primary focus for the heaviest=20 QPF core which makes sense synoptically considering the expected=20 rapid cyclone intensification. Warm front over the Mississippi=20 Valley will have to be monitored for some enhanced convective=20 concerns as it motions to the north with some guidance dropping=20 appreciable precip in an area that has some relatively lower FFG=20 signals. The previous MRGL was maintained with some expansion south and east to address the extension of the forecast heavier=20 convection across the maturing warm sector and within the axis of=20 strongest isentropic ascent over western IL.=20 Kleebauer Day 3 Valid 12Z Wed Apr 02 2025 - 12Z Thu Apr 03 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS=20 OF THE LOWER MISSISSIPPI AND OHIO VALLEYS... The D3 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.=20 At the surface, a cold front will run into a stout western ridge=20 extension from a surface high located over the Western Atlantic=20 into the Canadian Maritimes. The pattern essentially comes to a=20 stalemate thanks to an expected +2 to +3 standard deviation height=20 anomaly centered over the Western Atlantic. This ridge will counter the deepening trough over the Central U.S. creating a funneling=20 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=20 depictions from guidance indicating a widespread 2-4" with some=20 areas exceeding 5" increasingly likely within the corridor impacted by the multitude of convective elements spawning southwest to=20 northeast in proxy to the quasi- stationary front. A Moderate Risk=20 remains over an area situated from northeast AR up through the mid- Mississippi Valley of western TN/KY into southern IN. A broad=20 Slight Risk encompasses the MDT with a footprint extending as far=20 north as Lower Michigan.=20 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=20 this setup closely as small changes could have heightened impacts given the forecasted setup. Kleebauer=20 Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4mqiTrg1LxmTpB-JaN_V9zGEapMVAk-y7bKNlSfBvm2W= R-eGGKrVKabe8RHZMLSwE1fE6RbGxoZHLHL5SQGE-qw4X1s$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4mqiTrg1LxmTpB-JaN_V9zGEapMVAk-y7bKNlSfBvm2W= R-eGGKrVKabe8RHZMLSwE1fE6RbGxoZHLHL5SQGE-PBgFnk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4mqiTrg1LxmTpB-JaN_V9zGEapMVAk-y7bKNlSfBvm2W= R-eGGKrVKabe8RHZMLSwE1fE6RbGxoZHLHL5SQGEcBZ6kxw$=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. 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