BBS: TELESC.NET.BR Assunto: HVYRAIN: Excessive Rainfall Discussion De: COD Weather Processor Data: Fri, 29 May 2026 20:08:40 +0000 ----------------------------------------------------------- FOUS30 KWBC 292008 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 408 PM EDT Fri May 29 2026 Day 1 Valid 16Z Fri May 29 2026 - 12Z Sat May 30 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST AND TENNESSEE VALLEY... 16z Update: Main change is an expansion of the Slight risk across TN, far southern KY, northern AR and southern MO. Overall would generally consider the flash flood risk isolated, but a few clusters of more concentrated impacts appear probable. FFG exceedance probabilities from both the 12z HREF and REFS show a broad corridor of 15-30% values approximately aligned with the adjusted Slight risk area, with 3" neighborhood exceedance probabilities of ~40-70%. So while coverage of these higher amounts should stay localized, weak flow and broad lower level convergence will support a few convective clusters capable of 2" per hour rainfall and totals exceeding 3". Made some tweaks to confine the Marginal risk over New England to just NH into western Maine. A widespread 1-2" of rain is expected as a strong mid/upper low dives southeast into the region, with both the HREF and REFS supporting localized totals over 3". Hourly rainfall rates will generally stay below 1", however could locally exceed 1" as the low drops south tonight and we get some enhanced convergence closer to the NH and ME coasts. A few spots could see event total rainfall around or just above 3", which combined with 1" per hour rates, could be enough to result in localized flash flood impacts. A Small Marginal risk was added for dryline convection across west TX. Storm motions should be quick enough to limit the risk, however, a few supercells could briefly anchor to the dryline and locally enhance rainfall totals. Chenard ...Previous Discussion... Largely maintained the previous Slight Risk area that was introduced in the Southeast US. A sheared 500mb vorticity maximum approaching from the west will be encountering a moist airmass with increasing amount of instability later during the day and evening. The latest numerical guidance kept expected precipitable water values in the 2 to 2.25 inch range along and south of a quasi-stationary front draped across the region from the TN Valley to the SC coast. Instability will be highest over AL on east into central and southern GA with MUCAPE between 1,000-2,000 J/kg most often. MUCAPE up to 1,000 J/kg will be available later tonight from the Savannah River to coastal SC. This combination should be enough to support and sustain localized rainfall rates in excess of 2 inches per hour where warm rain processes predominate from eastern AL eastward to the Savannah River this afternoon and tonight. ...Middle Mississippi Valley to the Southeast... The western portion of the front affecting the Southeast US will extend into portions of the Middle Mississippi Valley and provide the focus for showers and thunderstorms...some of which may produce locally heavy rainfall from the Tennessee Valley into the Middle Mississippi Valley into tonight. ...Eastern Washington to Northern Rockies... A marginal risk remained in place across far northeast Washington to northwest Montana due to an increase in the heavy rainfall signal for this period as an upper low over the West on Day 1 places this region in an area with better upper forcing and advection of moisture. The presence of anomalous moisture and some instability may present a threat for localized heavy rainfall and flash flooding, especially for burn scar locations within this region. Bann Day 2 Valid 12Z Sat May 30 2026 - 12Z Sun May 31 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF MONTANA... 20z Update: Upgraded to a Slight risk across portions of MT where=20 isolated to scattered flash flooding is possible. The eastern half of the Slight risk area should see the highest rainfall rates,=20 with HREF neighborhood probabilities of exceeding 1"/hr in the=20 30-50% range, and 2"/hr around 10%. This results in HREF FFG=20 exceedance probabilities broadly over 25%. There should be enough=20 coverage of higher rainfall rates to result in some flash flood=20 risk. Instability is less the farther west you go in MT, but=20 rainfall coverage will be greater, resulting in higher areal=20 averaged rainfall. The flood risk should gradually transition from flash flood to areal flood/river flood as you go west across the=20 state. Elsewhere, the Marginal risk was broadened across the Northern Plains into the MS Valley along an elongated moisture axis. Convective details are uncertain at this lead time, but that=20 entire corridor will have above average moisture and some=20 forcing.The most organized convective threat may end up over=20 portions of NE and SD Saturday night as stronger forcing ejects=20 into the area. However, there are also signals of organized=20 convective clusters within the moisture axis anywhere from AR to=20 IA. While convection is unlikely along this entire axis...where it does develop rainfall rate potential will be high enough for=20 localized flash flooding. Chenard ...Previous Discussion... ...Southeast... Latest numerical guidance has shifted a majority of the rainfall eastward enough compared with previous runs to preclude an upgrade to a Slight risk. In addition...the spread of model guidance both in terms of placement and amounts precluded an upgrade. As mentioned previously...some of the region, especially western parts of South Carolina and into the Appalachians, has been very wet as of late and is well primed, so may by more vulnerable to flooding concerns. ...Northern Rockies to High Plains... The upper level low over the West during the Day 1 period will weaken and shift northeastward, moving over the Northern Rockies to High Plains region on Saturday. Moisture anomalies will continue to be in the 90-90th percentile, supporting moderate to heavy rainfall, with widespread QPF greater than 0.5-1" likely across western and central Montana. There may not be a lot of instability to work with, so any flood or flash flood threats should be mainly localized with particular attention to burn scar locations. A fairly broad marginal risk remained in place across much of western and central Montana, into parts of northern Wyoming as well. Bann Day 3 Valid 12Z Sun May 31 2026 - 12Z Mon Jun 01 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL STRETCHING FROM MONTANA TO NORTHERN FLORIDA... 20Z Update: Expanded the Marginal risk to cover most of the anomalous moisture plume that will extend from MT into the=20 Southeast U.S. in between troughing over the West and Northeast.=20 Large scale forcing is more subtle outside of the Northern=20 Plains/Rockies...but there should be some weak shortwaves=20 embedded within the weak flow. Global deterministic models,=20 ensemble means and AI models show a broad convective threat along=20 this entire corridor, with no clear area of higher focus. Thus at=20 the moment a large Marginal seems to handle the localized flash=20 flood risk that should exist. Some fine tuning of the Marginal=20 and/or focused Slight risk upgrades will be possible as we get=20 closer and convective mode/evolution becomes clearer. Chenard ...Previous Discussion... ...Northern Rockies and Adjacent High Plains... The upper level low which has been supporting the prolonged wet period over the Northern Rockies will continue to move northward across Montana slowly on Sunday. This results in surface low pressure and associated surface front to makes its way out over the Plains and gradually weaken while the mid and upper level circulation keeps precipitation lingering over Montana and far northern Wyoming. Moisture remains anomalously high but instability still looks to be dependent upon cloud cover. As in the Day 2 period...any flood or flash flood threats should be mainly localized with particular attention to burn scar locations. ...Southeast... Until surface high pressure builds far enough southward into the Southeast states to decrease coverage of lingering convection on Sunday evening...the antecedent conditions keep a risk of flooding concerns over the area. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7q9UGCqtgog0Bl9yr0qtN1mXWiwnPCr5fHYjbfw-Cr5r= TuEBoNKevXGcaHyWgXzN_4fOwpnpXJyu4QWBZxRmBQxj8vo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7q9UGCqtgog0Bl9yr0qtN1mXWiwnPCr5fHYjbfw-Cr5r= TuEBoNKevXGcaHyWgXzN_4fOwpnpXJyu4QWBZxRmbfh3WaE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7q9UGCqtgog0Bl9yr0qtN1mXWiwnPCr5fHYjbfw-Cr5r= TuEBoNKevXGcaHyWgXzN_4fOwpnpXJyu4QWBZxRmHsc1SjQ$=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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