Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun May 18 2025 08:16:51 FOUS30 KWBC 180815 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 415 AM EDT Sun May 18 2025 Day 1 Valid 12Z Sun May 18 2025 - 12Z Mon May 19 2025 ....THERE IS SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE=20 CENTRAL PLAINS INTO THE LOWER AND MIDDLE MISSISSIPPI VALLEY... ....Plains, MS Valley and Southeast... A favorable pattern for widespread convection and heavy rainfall=20 today across the middle portion of the country. A longwave trough=20 and embedded shortwaves pushing eastward across the Rockies and into=20 the Plains will provide ascent over a broad region. The frontal=20 pattern will likely feature a developing low moving into the High=20 Plains of CO/NE, a warm or stationary front extending east of this=20 low across portions of OK/KS into MO/AR, and a dryline stretching=20 from central KS into TX. A large pool of instability is expected east=20 of the dryline and south of the stationary front, with values=20 upwards of 4000-5000 j/kg. Complicated convective evolution today into tonight, but the=20 greatest risk of flash flooding will likely be with any upscale=20 growth of convection that can occur this evening into tonight near=20 the warm/stationary front. There is a significant amount of spread=20 amongst the 00z HREF members and global guidance regarding=20 convective details today, supporting the idea of a lower confidence=20 forecast. However, while the confidence on the details may be lower=20 than normal for a day 1 forecast, there is decent confidence that=20 somewhere will see a more organized flash flood risk develop later=20 today into tonight. The overall expectation is that robust convective development occurs=20 along and east of the dryline this afternoon from central TX into OK=20 and KS. The activity over TX may try to grow into a few convective=20 clusters, and rainfall rates should be intense enough for a=20 localized flash flood risk. However it appears like the better=20 threat for more widespread organized development is over northern OK=20 into central KS, closer to the dryline/warm front intersection. This=20 activity will have greater lower level convergence to work with and=20 also be closer to the mid level vort energy swinging through the=20 High Plains. This activity should grow upscale Sunday evening into=20 the overnight, potentially into an MCS, at which point it should=20 feel the impact of the strong southwesterly low level jet and begin=20 turning easterly and then southeasterly aligned with the upwind=20 Corfidi Vectors and along the instability gradient. As this process=20 occurs some training/backbuilding on the southwest extent is=20 probable resulting in a scattered flash flood threat. The exact MCS location/track remains a bit uncertain. As already=20 mentioned, the HREF members are all over the place, and thus the 00z=20 HREF probabilities do not appear all that useful. Best guess at this=20 point is something close to the 00z GEM reg might be reasonable.=20 Also like the placement of the 00z AIFS and EC mean...with a=20 maximum QPF somewhere from northeast OK into southeast KS,=20 southwest MO and northwest AR. Do consider these areas to be a=20 higher end Slight risk today, as where any training MCS does=20 develop we could be looking at a swath of upwards of 3-5" of rain.=20 Main thing to watch will be how far north the warm front gets today over OK/KS and where convection first starts to grow upscale. If=20 this all happens earlier/farther south then OK into northern AR=20 become more in play for flash flooding...while if it happens later=20 and farther north then we are looking more at eastern KS into=20 central MO. Tend to think both the 00z and 06z HRRR are too far=20 north, but also can not completely rule that scenario out either. Another area of interest will be farther northwest into central and=20 western NE, closer to the low track. A multi-model signal exists for=20 a second QPF max over this area...which makes sense given what=20 should be impressive synoptic driven ascent and instability along=20 the stationary/occluded front. Considered expanding the Slight risk=20 into this area...but opted against that for now. This area remains=20 in severe drought and possible that a majority of the training=20 convection ends up over the high FFG sand hills. These factors=20 should keep the flash flooding localized in nature, and thus will=20 maintain the Marginal. The extension of the Marginal risk into Southeast accounts for=20 organized convection that should be ongoing this morning, and then=20 potential isolated redevelopment later today near the lingering=20 boundary where substantial instability will support heavy rainfall=20 with any storms that are able to develop. ....Northeast...=20 A Marginal risk continues over portions of northern New England on=20 Sunday as a deep layered low remains overhead. Instability and PWs=20 are lower today, however cold air aloft and some surface heating=20 should allow for showers and a few heavier convective cells to=20 develop this afternoon. Both the 00z HREF and 18z REFS indicates a=20 30-40% chance of exceeding 1" of rain in an hour, and cells will be=20 slow moving. Chenard Day 2 Valid 12Z Mon May 19 2025 - 12Z Tue May 20 2025 ....THERE IS SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE=20 CENTRAL PLAINS INTO THE LOWER AND MIDDLE MISSISSIPPI VALLEY... Another day of widespread heavy rainfall expected across the middle=20 portion of the country on Monday as the longwave trough and embedded=20 shortwave energy slowly moves eastward. It will likely be an active=20 day/night with a broad region where flash flooding will be possible.=20 Not much change was needed to the inherited risk area, with a Slight=20 risk stretching from northeast TX into southern IA. Expect=20 convection to again develop Monday afternoon along the dryline and=20 approaching cold front from TX into NE. The northern extent of the=20 Slight risk will be near the warm/stationary front extending east=20 across northern MO and southern IA. Strong forcing overrunning this=20 boundary will likely result in an expansive area of convection, with=20 some training/backbuilding possible. The southern portion of the=20 Slight risk from TX into eastern OK and AR covers initial dryline=20 development and possible upscale growth. While the low pressure=20 driven stronger convergence will be farther north over IA/MO, there=20 are indications that a corridor of enhanced low level moisture=20 transport could evolve from TX into AR aided by shortwave energy at=20 the base of the longwave trough. Several pieces of guidance thus=20 depict a secondary max QPF swath over these areas from possible=20 training convection.=20 Exactly where the focus of highest QPF ends up remains a bit of a=20 question, but generally anywhere within the Slight risk is in play.=20 There are meteorological reasons (described above) for two maximum=20 rainfall swaths, one farther north near the warm front over northern=20 MO and southern IA, and another from northeast TX into AR. However getting these details right at this lead time is difficult. So=20 whether or not we end up seeing two separate areas of training=20 convection or one more consolidated area, the main story is that=20 the ingredients will be in place for training/backbuilding=20 convection leading to excessive rainfall. Some of these areas will=20 also likely see heavy rain on day 1 as well, and so hydrologic=20 conditions could be more sensitive by this day 2 time frame. Thus=20 do consider most of the Slight area as a higher end Slight risk at this time, and think areas of flash flooding are likely. It seems=20 possible (maybe even probable) that an embedded MDT risk will=20 eventually be needed somewhere within the broad Slight area.=20 However uncertainty in both the convective details and hydrologic=20 conditions (pending what rainfall happens on day 1) leads to low=20 confidence in any MDT risk location at this time. Thus we will hold with a higher end Slight and continue to monitor trends. Chenard Day 3 Valid 12Z Tue May 20 2025 - 12Z Wed May 21 2025 The system described in the day 1 and 2 discussions begins moving=20 east on Tuesday. The overall setup for heavy rainfall amounts is=20 generally not as good by this time as it was on previous days=20 farther west. The large scale forcing is more progressive by Tuesday=20 and the degree of instability is not as robust. Thus not surprising=20 that the global model QPFs are generally lower than what is forecast=20 farther west on day 2. With that said, there is likely still an=20 excessive rainfall risk Tuesday. While the system is becoming more=20 progressive by this time, there will be a stationary front extending=20 from IL into KY through, with enough moisture transport and synoptic=20 ascent to support a broad area of convection along this front. This=20 setup thus could allow for either multiple convective rounds near=20 the front, or even a more prolonged period of repeat/training=20 convection. The main question will be whether we are able to=20 maintain enough instability to support prolonged intense convection=20 and also whether the mid/upper level forcing is enough to allow for=20 a persistence of stronger convection. It is also worth noting that=20 portions of KY and WV are more sensitive to additional rain, with=20 higher soil saturation and streamflows in place. Overall think this is a solid Slight risk, with isolated to=20 scattered flash flooding possible anywhere from IA to=20 KY/TN/WV...generally along and just south of wherever the=20 stationary/warm front ends up by this time. The potential for=20 greater flash flood coverage and higher impacts is=20 uncertain...noting the aforementioned uncertainties with=20 forcing/instability and the general downtick in model QPFs by this=20 time. However the orientation of the forcing and stationary front=20 does suggest a potential for training and more significant rainfall=20 totals, which could open the door for a corridor of more widespread=20 and/or higher impacts. The 00z RRFS depicts a swath of strong=20 convection Tuesday, but generally shows things progressive, keeping=20 most areas in the 1-2" range...likely not enough for widespread=20 flash flooding. However it does show some swaths over 3", especially=20 over portions of KY and TN where a bit more training occurs. So we=20 will continue to monitor trends, and pay close attention to areas=20 that may be more sensitive to additional rainfall, such as KY and WV. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5mym00IOxD5G259UgVi0V8mc109k4G0mnKb-eZKuxEz3= iaq7LBqPJySIxWXbQjLpFIKuXuuEMsQ4lI1e_khBYVcPkdI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5mym00IOxD5G259UgVi0V8mc109k4G0mnKb-eZKuxEz3= iaq7LBqPJySIxWXbQjLpFIKuXuuEMsQ4lI1e_khBGT5mKWQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5mym00IOxD5G259UgVi0V8mc109k4G0mnKb-eZKuxEz3= iaq7LBqPJySIxWXbQjLpFIKuXuuEMsQ4lI1e_khBkZFo0pg$=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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