Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat May 17 2025 08:35:56 FOUS30 KWBC 170832 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 432 AM EDT Sat May 17 2025 Day 1 Valid 12Z Sat May 17 2025 - 12Z Sun May 18 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHERN NEW ENGLAND... ....Texas to Alabama... Convection will likely develop across portions of central TX this=20 afternoon as increasing forcing overruns the dryline. Pretty=20 impressive thermodynamic environment, with upwards of 5000 j/kg of=20 CAPE forecast along and east of the dryline, along with substantial deep layer shear. This will likely result in rather explosive=20 convective development this afternoon, and while not the strongest=20 large scale forcing, there is enough mid level vort energy and=20 upper jet dynamics to support scattered to perhaps numerous=20 convective coverage. Supercells are likely, with some upscale=20 growth into convective clusters probable. Given the CAPE/shear=20 profile, large hail is the main threat, although even with such a=20 robust CAPE profile the deep layer moisture is actually still=20 pretty good. Thus these cells will likely be capable of producing=20 some heavy rainfall rates over 2"/hr as well.=20 Limiting factor for flash flooding will be quick cell motions=20 (30-40 kt mean westerly flow) and the high FFG over the area.=20 However right moving supercell motions are closer to 15-20 kts)=20 and the various cell movement deviations we see in a supercell=20 environment will likely lead to some cell mergers as well. Thus do=20 think we will see isolated flash flooding, with a focus over=20 central to northeast TX. Not sure we will get the FFG exceedance=20 coverage needed for a Slight risk given the generally quick cell=20 motions and high FFG, so we will stick with a Marginal risk. Even=20 the 00z RRFS, which tends to run hot with QPF rates/totals, only=20 depicts isolated hourly FFG exceedance. Convection will likely be ongoing this morning across the eastern=20 portion of the Marginal risk (MS/AL), but should be in a decaying=20 state. However we could see at least isolated redevelopment along=20 the leftover outflow/front this afternoon, and anything that does=20 develop would likely be slow moving and capable of resulting in a=20 localized flash flood threat. The Marginal risk was also expanded to the north into more of OK/AR,=20 southeast KS and southern MO. There are indications that an=20 overnight cluster of storms should develop here later tonight into=20 Sunday morning. This is where the stronger moisture convergence and=20 instability gradient should be and thus it makes sense that we=20 would get some elevated convection develop somewhere along this=20 axis. The 00z HREF is not that aggressive with development here,=20 but the HREF members can often be slow in catching on to training=20 elevated convective development. There is enough of a signal in the 00z global guidance to suggest we may see the HREF trend wetter on=20 future cycles, and there is at least some threat of an isolated=20 flash flood risk. ....Northeast...=20 Multiple rounds of convection are expected across the Northeast=20 today into this evening. PWs and instability in place support high=20 rainfall rates, and both the 00z HREF and 18z REFS depict a 30-50%=20 chance of hourly rainfall over 1" and the 06z HRRR has a swath of=20 1- 2" per hour rainfall. In general, quick cell motions will help=20 limit the duration of these heavy rates, but the combination of=20 multiple convective rounds and high rates indicates at least some=20 flash flood risk is in place. Recent high res model runs indicate a stationary/warm front in place over portions of VT/NH this=20 afternoon, which may act as a focus for some short duration=20 training of cells. Expect the best chance for isolated to scattered flash flooding to focus near and south of this boundary, which=20 ends up encompassing much of central/northern VT into central NH.=20 This is generally a bit south and west of the inherited Slight=20 risk area. Thus we will expand the Slight risk into this region,=20 while maintaining it in northern NH and western ME (less confidence on higher rates in these latter areas, but heavy rainfall over the past 24 hours may keep the area a bit more sensitive to additional rainfall today). Chenard Day 2 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 on Sunday across the middle portion of the country. A longwave=20 trough and embedded shortwaves pushing eastward across the Rockies=20 and into the Plains will provide ascent over a broad region. The=20 frontal pattern will likely feature a developing low moving into=20 the High Plains of CO/NE, a warm or stationary front extending east of this low across portions of OK/KS into MO/AR, and a dryline=20 across OK/TX. A large pool of instability is expected east of the=20 dryline and south of the stationary front, with values upwards of=20 4000-5000 j/kg. Some convection may be ongoing near the=20 warm/stationary front Sunday morning, but more robust development=20 is expected by Sunday afternoon into the overnight hours as=20 stronger forcing ejects into the Plains.=20 The main area of focus for flash flooding appears to be near the=20 aforementioned warm/stationary front where some training of convection=20 is probable. The overall expectation is that robust convective=20 development occurs along/ahead of the dryline over central OK/KS=20 during the afternoon hours. Strong deep layer southwesterly flow=20 will likely quickly push this activity northeastward towards the=20 warm/stationary front. Activity will likely grow upscale Sunday=20 evening into the overnight, potentially into an MCS, at which point=20 it should feel the impact of the strong southwesterly low level jet=20 and begin turning easterly and then southeasterly aligned with the=20 upwind Corfidi Vectors. As this process occurs some=20 training/backbuilding on the south/southwest extent is probable=20 resulting in a scattered flash flood threat. The exact MCS location/track remains a bit uncertain. All but=20 certain that the 00z NAM is too far north, as its QPF evolution is=20 an outlier and does not align with pattern recognition. The GFS=20 and GEFS mean are also probably too far to the northeast, with the=20 aforementioned expected MCS propagation taking it on a farther=20 south track. The 00z Gem Reg and RRFS are very far south, tracking=20 across eastern OK into central/northern AR. This may be too far=20 south, but can not be ruled out as easily as the farther north NAM=20 can. The AIFS has a track record of pinpointing organized=20 convection location quite well, and the 00z AIFS favors northeast=20 OK into southeast KS and southwest MO. This also happens to=20 generally be the middle ground of the other available models, and=20 so does appear to represent the most likely outcome at this time.=20 Thus a higher end Slight risk will remain in place across this=20 corridor, where scattered flash flooding is expected. The Slight=20 risk area was cut back a bit on its northern extent and expanded=20 to the southwest into more of OK and AR to account for the southern trend. Can not rule out additional southward shifts/expansion..but for now think the Slight risk area represents the general region=20 of most concern well. Another area of interest will be farther northwest into central and=20 western NE, closer to the low track. A multi-model signal exists=20 for 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 into this area...but opted against that for now. This area remains in severe to extreme drought and possible the rainfall max ends up over the high FFG sand hills. But we will continue to monitor=20 trends and an additional focused Slight risk may eventually be=20 needed. A localized flash flood risk also exists both along the dryline from=20 central TX into OK...and over the Southeast where a lingering=20 boundary and substantial instability will support heavy rainfall=20 with any storms that area able to develop. ....Northeast...=20 A Marginal risk continues over portions of northern New England on Sunday as a deep layered low remains overhead. This will be the=20 3rd straight day of localized heavy rainfall, and thus some areas=20 may be more sensitive by this time. Generally thinking both=20 instability and moisture will be lower by Sunday, however cold air=20 aloft under the low will likely still support some heavier=20 convective cells and an isolated flash flood risk. Chenard Day 3 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 A Slight risk stretches from northeast TX northward into southern IA.=20 The northern extent of the Slight risk will be close to or just=20 north of the stationary front extending east of the central Plains low pressure. Strong forcing overrunning this boundary will likely result in an expansive area of convection, with some=20 training/backbuilding possible. The southern portion of the Slight=20 risk is driven by initial dryline convective development, likely=20 enhanced by the eventual eastward acceleration of the cold front=20 south of the low. The convergence from the dryline, cold front and=20 lifting warm front will all aid in convective development. The=20 general stagnant nature of the frontal feature through much of the day (both the warm/stationary front and dryline) support a=20 training/backbuilding convective threat. 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. There is some signal that the axis from northeast TX into southern MO could actually see the most intense rates and greatest flash=20 flood potential, with both the 00z CMC and AIFS highlighting this=20 corridor. However ingredients will also be quite favorable farther=20 north into MO and southern IA as well (pending exact frontal=20 positioning) and thus both of these areas could end up seeing=20 training/backbuilding at some point Monday and/or Monday night.=20 Some of these areas will also likely see heavy rain on day 2 as=20 well, and so hydrologic conditions could be more sensitive by this=20 day 3 time frame. Thus do consider just about all of the Slight=20 risk as a higher end Slight at this time, and think areas of flash flooding are likely. It seems possible (maybe even probable) that=20 an embedded MDT risk will eventually be needed somewhere within=20 the broad Slight risk area. However uncertainty in both the=20 convective details and hydrologic conditions (pending what rainfall happens on day 1 and 2) leads to low confidence in any MDT risk=20 location at this time. Thus we will hold with a higher end Slight=20 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!9iFPnULk4ZwdSUGtEiWMPXxtFSOA4vP6ts3SfqY3ilo6= evx-R71WO9SF-wBhta2X62J3SAN0vuiicn1Xp1c6LodVlCs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9iFPnULk4ZwdSUGtEiWMPXxtFSOA4vP6ts3SfqY3ilo6= evx-R71WO9SF-wBhta2X62J3SAN0vuiicn1Xp1c6TT_zgBg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9iFPnULk4ZwdSUGtEiWMPXxtFSOA4vP6ts3SfqY3ilo6= evx-R71WO9SF-wBhta2X62J3SAN0vuiicn1Xp1c60fsr1hI$=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) .