Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jun 19 2025 00:58:04 FOUS30 KWBC 190057 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 857 PM EDT Wed Jun 18 2025 Day 1 Valid 01Z Thu Jun 19 2025 - 12Z Thu Jun 19 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE GREAT LAKES AND THE MID-SOUTH STATES... 01Z Excessive Rainfall Discussion... The area extending from parts of the southern and eastern Great Lakes into the Mid-South remain the primary concern for excessive rainfall this evening and into the overnight hours. The HRRR and HREF keep pushing showers and thunderstorms southward across parts of the Ohio and Tennessee Valley with spotty 15 percent neighborhood probabilities of rainfall exceeding 1-hour flash=20 flood guidance in parts of Tennessee. Farther north...the threat of heavy to potentially excessive rainfall lingers as mid-level=20 height falls continue tracking northeastward and support from a=20 departing upper jet streak. In some cases..the concern arises from=20 repeat/training convection. Bann Previous Excessive Rainfall Discussion... ....Great Lakes through the Mid-Mississippi Valley... Strung out shortwave embedded within amplifying mid-level flow will swing northeastward today as the primary trough axis shifts from the Central Plains to the Ohio Valley. The collocated height falls/PVA accompanying this energy will interact with the modest RRQ of a departing upper jet streak to help deepen a surface low moving into the Great Lakes. This low will strengthen along the front, leading to scattered/widespread showers and thunderstorms, but with two focused regions of excessive rainfall -- northern Illinois across Lower Michigan and from near Memphis into Kentucky. The greatest risk for heavy rain capable of producing flash flooding appears to be centered across the lower peninsula (L.P.) of Michigan. Heavy rain is favored along a warm front ahead of the low track, and along a deformation axis in the same region with embedded convection (some occurring already this morning/noontime). 850mb flow ahead of the low will surge to 20-30kts, drawing PWs as high as 1.75 inches into MI, coincident with MUCAPE nearing 1000 J/kg. This will support rainfall rates that have a moderate chance (40-60%) of exceeding 1"/hr at times. The heaviest rain rates are expected along the leading warm front thanks to stronger instability this evening, and during that time the combination of 20-30 kts of bulk shear and 0-6km mean winds aligned to the front, despite progressive speeds near 30 kts, will support rainfall that will likely (70% chance) reach 3", and may reach as much as 5" in a few isolated locations. This could result in instances of flash flooding (15-30% chance of 3-hr FFG exceedance) despite generally dry antecedent conditions, and the SLGT risk was maintained here. Farther to the south, the cold front will be the primary mechanism for ascent/convective development as it tracks east from the Central Plains to the Tennessee Valley. This front will impinge upon intense thermodynamics characterized by MUCAPE above 3000 J/kg and PWs approaching 2 inches (>90th percentile). Thunderstorms will likely develop along the length of this front during peak instability, and where bulk shear exceeds 25 kts, generally from IN through AR, clusters of more organized convection are expected. While cells will generally move rapidly along the front, Corfidi vectors progged to collapse to just 5-10 kts suggests some backbuilding/short term training, which could enhance rainfall to more than 3 inches in some areas (max ensemble rainfall is 5-7"). There is still some latitudinal spread in the axis of highest rainfall potential between the HREF and REFS members, leading to lower confidence in placement of heaviest rain. However, the EAS probabilities from both ensemble clusters, while varying greatly in magnitude, are focused generally in the same area, and atop a region of more sensitive soils and lower FFG due to 7-day rainfall more than 300% of normal. The SLGT risk was maintained in this area due to the potential for this heavy rain falling atop vulnerable soil conditions. ....Mid-Atlantic into New England... Southwest return flow around a Bermuda-type ridge off the Southeast coast will continue today, pumping elevated moisture northward into the Mid-Atlantic States. PWs are forecast to reach around 2 inches (12Z IAD sounding was 2.15" / 99th percentile / near-record for this time of year per the ESAT). This PW will be spread through the entire column as reflected by long-skinny CAPE profiles among the regional soundings, indicating efficient rainfall processes within thunderstorms that develop along a weak surface trough skirting across the area. Both the HREF and REFS probabilities indicate a moderate potential (30-50%) for 1"/hr rates, which, despite generally progressive motion, could cause axes of 2-3" of rain where multiple rounds occur (especially over the Mid-Atlantic DC-PHL-TTN). Recent HREF/RRFS members show potential for >4" rainfall in this urban corridor. If any of this heavy rain falls atop these urban areas or more sensitive soils from recent rain (where FFG is as low as 1-1.5"/3hrs), instances of flash flooding could result. Maintained the Marginal risk here due to the expected coverage (more isolated) though local impacts may be significant depending on the exact placement. Fracasso/Weiss Day 2 Valid 12Z Thu Jun 19 2025 - 12Z Fri Jun 20 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE MID- SOUTH THROUGH INTERIOR NEW ENGLAND... 2030Z update... no significant changes to the forecast. Fracasso Mid-level trough axis initially positioned across the Mississippi Valley will become more progressive on Thursday as it weakens in response to an expanding ridge over the Southern Plains. As this trough weakens and moves quickly towards the Atlantic coast, a surface cold front beneath it will additionally track eastward, likely exiting the Mid-Atlantic and New England coasts by Friday morning, while lingering into the Southeast. This front will provide a focus for convective development Thursday, especially during peak heating as downstream SW 850mb flow of 25-35 kts surges PWs to 1.75+ inches, reflective of an environment that will support extremely efficient rainfall within thunderstorms. This indicates that despite the rapid frontal progression, and accompanying forward motion of thunderstorms (0-6km mean winds 20-25 kts), intense rainfall rates of 1-2"/hr could still produce isolated impacts, especially where any repeating storms can occur, or if heavier rain falls atop urban areas or more sensitive soils. At this time no SLGT risks have been introduced as storm motions may be more perpendicular to the front, and thus, mostly progressive. However, there is at least a subtle signal for some heavier rainfall across the Central Appalachians. This area is extremely sensitive due to recent heavy rainfall (7-day rainfall 150-300 percent of normal) which has primed soils and reduced FFG to 1-2"/3hrs. If this signal increases, a targeted SLGT risk may be needed across these most vulnerable areas. Weiss Day 3 Valid 12Z Fri Jun 20 2025 - 12Z Sat Jun 21 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE NORTHERN GREAT LAKES AND PORTIONS OF THE NORTHERN ROCKIES... 2030Z update... expanded the Marginal risk outline across the northern Plains due to spread in the 12Z guidance. Otherwise, no significant changes. Fracasso ....Northern Great Lakes... Low pressure emerging from the Northern High Plains will extend a warm front downstream and into the Upper Midwest/Great Lakes. As this warm front lifts steadily northeast, especially the latter half of D3 /Friday night/, it will serve as a focus for blossoming convection. The downstream amplifying 850mb flow lifting out of the Gulf will reach as high as 40-50 kts into the Upper Midwest, and with the presence of the low to the west, these winds will back steadily to the SE to surge higher PWs, approaching 1.75 inches, into the warm front and into MN/WI. The combination of moisture convergence and isentropic ascent invof this front will help expand clusters of thunderstorms Friday night, leading to an axis of heavy rain that has some potential (10% or less from the NBM/GEFS/CMCE/ECENS/SREF) of exceeding 3 inches. Despite high FFG across this region, isolated flash flooding could result from this rainfall. ....Northern Rockies... Extremely anomalous mid-level low will dig into the Pacific Northwest Friday, pushing 500mb heights to below the 00Z minimum of the climatological database from NAEFS. Where intense height falls/PVA combine with the increasing diffluence along the LFQ of a Pacific jet streak, surface pressure falls will result, leading to a deepening low pressure moving from ID into MT. The increasingly backed low-level flow ahead of this system will cause 850mb winds to surge out of the E at 15-25 kts, lifting favorably to upslope into the terrain while additionally transporting a ribbon of PWs above 1 inch into the High Plains and banking into the eastern side of the Rockies. This will cause expanding precipitation, with embedded rainfall rates above 0.5"/hr where a narrow corridor of instability exists over northern MT. Locally heavy rainfall of 1-2", with locally higher amounts, will result, leading to the potential for isolated flash flooding atop FFG that is only 1-1.5"/3hrs. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!95lkhXbPCR8RTVF8O03kVvXV9Md2ubSsBFVSSqBLxEbS= 9AZDHLwEUuY83H4Fq13DfO6-l3pBSYpOth0X7KHdJbHSMzY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!95lkhXbPCR8RTVF8O03kVvXV9Md2ubSsBFVSSqBLxEbS= 9AZDHLwEUuY83H4Fq13DfO6-l3pBSYpOth0X7KHd-K5aPSc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!95lkhXbPCR8RTVF8O03kVvXV9Md2ubSsBFVSSqBLxEbS= 9AZDHLwEUuY83H4Fq13DfO6-l3pBSYpOth0X7KHd7Uubfss$=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) .