Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jun 20 2025 15:58:23 FOUS30 KWBC 201557 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Fri Jun 20 2025 Day 1 Valid 16Z Fri Jun 20 2025 - 12Z Sat Jun 21 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE=20 UPPER MIDWEST... ....Northern Plains into Upper Midwest... Model guidance indicates a well-defined mid level shortwave will=20 eject east out of the Rockies and into the Northern Plains this=20 afternoon into tonight, with a surface low moving from southern MT=20 into the Dakotas. High res guidance show convection developing near the MT/ND border this afternoon to the north of a warm front. An=20 increasing low level jet by evening likely supports upscale growth=20 of this activity as it moves eastward across ND. This convection=20 will likely be quick-moving; however, 1"+ per hour rainfall appears probable (up to around 70% chance in the HREF neighborhood probs)=20 and a localized flash flood risk is possible. A Marginal risk was=20 expanded west into these areas. A Slight risk was maintained across portions of=20 northern/northeastern MN into far northern WI. An impressive low=20 level jet and moisture transport axis will quickly evolve this=20 evening into the overnight hours. CAMs forecast a pool of extreme=20 instability on the order of 4000-5000 J/kg and PWs increasing=20 towards 2". While the progressive MCS tracks east across ND, we=20 should see some downstream development over eastern ND into=20 northern MN on the nose of this intense low level jet and along the instability gradient. Some west-to-east training of this activity=20 is possible ahead of the upstream MCS that will also eventually=20 push across the area. The degree of training remains a bit unclear, but the 12Z CAMs=20 show max rainfall through 12Z of 3-5". Given the instability and=20 moisture in place, totals of this magnitude do seem plausible. Also some uncertainty on the axis of heaviest rainfall, which should=20 end up pretty narrow. The better instability will be south, however a strong mid level cap will be advecting northward likely putting=20 a limit on how far south organized convection will get. The current Slight risk area encompasses the highest HREF probabilities, and=20 while a bit broader than what will probably happen, accounts for=20 some latitudinal uncertainty. Overall think isolated to scattered=20 flash flooding could evolve from this setup, especially if there is overlap with the heavy rainfall axis from a couple days ago over=20 northeast MN. Just southeast of the southeastern side of the Marginal risk (into Lower Michigan), a few 12Z CAMs show an area of developing rain=20 this evening/overnight along a boundary (with potentially heavier=20 amounts >3"), but confidence in this scenario is quite low as some=20 other CAMs are basically dry. ....Iowa... Ongoing convection has been persistent this morning over=20 central/eastern Iowa, already producing 1-2"/hr rates and 2-3"=20 total rainfall. Cloud tops have maintained their intensity but may=20 be showing signs of warming. Will keep the Marginal Risk contour=20 which will cover the next few hours until this area finally=20 dissipates this afternoon. ....Northern Rockies... A strong mid-level low moving into the Pacific Northwest will=20 focus impressive synoptic ascent over MT today into tonight. Even=20 this morning, convection has blossomed over northwestern Montana=20 which will expand some this afternoon. PWs near the climatological=20 90th percentile supports some heavier rates/amounts. Instability is likely the main limiting factor for excessive rainfall, and the=20 probability of exceeding 1" per hour rain is pretty low (10-40%)=20 because of this. However do think we will get some weak=20 instability, which combined with the impressive dynamics, should=20 still allow for some embedded heavier convective cores with over=20 0.5" per hour rain (which was estimated per MRMS around 15Z). Most=20 areas will probably see closer to 1" of rain; however, localized=20 amounts over 2" are supported by the high res models which could=20 result in an isolated flash flood risk. Fracasso/Chenard Day 2 Valid 12Z Sat Jun 21 2025 - 12Z Sun Jun 22 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN GREAT LAKES, INTERIOR NORTHEAST, AND NORTHERN ROCKIES... ....Northern Great Lakes... Convection will likely be ongoing at 12z Sat across portions of northern MI into the UP. The overall convective complex should be propagating eastward pretty quickly by this time, although it may still have a west to east orientation to it allowing for some training. The 00z HREF indicates 1" per hour exceedance neighborhood probabilities upwards of 60%, and 2" exceedance around 20%. HREF blended mean QPF is in the 2-3" range and these amounts seem plausible wherever the convective training axis is Saturday morning. Amounts higher than this will probable be hard to come by given the quick system movement, however even totals of this magnitude could result in a localized flash flood threat. Some latitudinal differences in the convective axis are noted, with both northern MI and the UP of MI potentially impacted. ....New England... The aforementioned convection over MI will try to move into portions of NY during the day Saturday. The maintenance of the activity is unclear as it will be moving into a more stable airmass. Generally think convection will weaken as it moves into NY...however with daytime heating some increase in instability over central NY could support at least some additional convective development during the day. By Saturday night steeper lapse rates advect into the Northeast, resulting in what should be a pretty rapid increase in MUCAPE over western NY into northern New England. A well defined mid level shortwave and increasing low level moisture transport will probably kick of organized convective development over southern Canada by Saturday evening. This activity should track into portions of NY and northern New England overnight into early Sunday. It seems likely that this activity will have a better chance of persisting into NY and New England given what should be a very strong axis of low level moisture transport over top steep mid level lapse rates. In fact we could very well have an impressive MCS diving into NY and northern New England later Saturday night. The specifics remain a bit unclear, but at least a localized flash flood risk seems probable even with a forward propagating system. Certainly a chance this setup keeps some backbuilding convection across the area, and possible we will eventually need a Slight risk. However, given the uncertainty regarding the details, think a Marginal risk will suffice for now. ....Northwest Montana... A Marginal risk was maintained across portions of northwest MT where rainfall will continue into Saturday. By this time this region will be within the comma head of the low, and looking at more stratiform rain with lower rates. Thus probably not really a flash flood risk by this point, but with storm total rainfall potentially into the 3-4" range, some possible flood impacts still justify a Marginal risk. Snow levels will drop towards 4000 feet by this time as well, so flood impacts will stay below that level. Chenard Day 3 Valid 12Z Sun Jun 22 2025 - 12Z Mon Jun 23 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF EASTERN NEW MEXICO AND SOUTHWEST TEXAS... Sunday is likely the beginning of what will be a multi day period of excessive rainfall potential over NM. The mid/upper level pattern features a near record to record ridge over the eastern U.S. and a well defined longwave trough over the west. Southerly flow in between these features will supply ample moisture to NM, with a connection all the way to the Caribbean Sea. Thus no surprise that forecast PWs are well over the 90th percentile, and possibly approaching near record levels for late June. The better forcing from the western trough should hold of until Monday and Tuesday (see the day 4 and 5 EROs), but by Sunday we should at least have enough moisture and instability around for isolated to scattered convective development. This appears to be a solid Marginal risk with localized flash flooding expected...and can not rule out eventually needing a Slight risk. Sometimes these first day of the event situations can overperform as the clean start can allow for more destabilization and greater convective coverage than expected. However with Monday and Tuesday currently expected to be the bigger days, will keep this day 3 as a Marginal for now and continue to monitor trends. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9NMj_3FwoEwIUj_BvbuyUfCI1g0aQQY8E34S1SY30BI0= rJO5vAKXpBtyM6mxc9jONunMjpvDw8bieqclRQfVSA9K-hQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9NMj_3FwoEwIUj_BvbuyUfCI1g0aQQY8E34S1SY30BI0= rJO5vAKXpBtyM6mxc9jONunMjpvDw8bieqclRQfVWFxger4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9NMj_3FwoEwIUj_BvbuyUfCI1g0aQQY8E34S1SY30BI0= rJO5vAKXpBtyM6mxc9jONunMjpvDw8bieqclRQfV5Zox7Fo$=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) .