Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri May 16 2025 16:03:46 FOUS30 KWBC 161602 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1202 PM EDT Fri May 16 2025 Day 1 Valid 12Z Fri May 16 2025 - 12Z Sat May 17 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OHIO AND TENNESSEE VALLEYS AS WELL AS WESTERN MAINE... ....Arklatex into the OH/TN Valleys... An active morning of convection will continue across portions of the TN and OH Valleys (generally western TN through KY and into WV), with a secondary round of convection likely this evening, maybe just a bit displaced south from the current activity. This convection is blossoming within an impressive warm sector behind a warm front and ahead of an approaching cold front. Forcing for=20 ascent is maximizing in this region due to an overlap of weak=20 height falls downstream of an impressive upper low positioned in=20 the Northern Plains, into which progressive shortwave impulses are=20 rotating E/NE, and the LFQ of a generally zonal jet streak is=20 impinging. The southerly flow ahead of the cold front will=20 gradually veer more to the SW with time, but still maintain=20 efficient moisture transport, surging PWs to above 1.75 inches on=20 850mb inflow that may reach 40-50 kts. This will lead to intense=20 moisture confluence into the approaching front, and as 0-6km mean=20 winds veer to become more boundary-parallel, this will likely setup a situation to encourage WSW to ENE training of heavy rain rates. The inherited MRGL and SLGT risks across this region have been modified generally just for cosmetic purposes, but some subtle S/E trends in the probabilities and axis of greatest instability drove most of the changes. With MUCAPE likely reaching 2000-3000 J/kg thanks to the presence of an EML, the resultant enhanced thermodynamics will support rainfall rates of 1-2"/hr as progged by the HREF, and short-term rates of 3"/hr are also possible as suggested by the 15-min HRRR precipitation accumulations. Despite storm motions that will remain generally quick at 30-40 kts (using 0-6km mean winds as proxy), shorter Corfidi vectors aligned to the mean flow additionally supports the risk for short-term training or multiple rounds of convection. Where rainfall repeats most significantly, both the HREF and REFS probabilities exceed 60% for 3"/24 hrs, although FFG exceedance probabilities are far more modest. This suggests that the inherited SLGT risk is warranted, with subtle modifications to account for the newer guidance, but no additional upgrade is needed. The MRGL risk was also extended a bit SW into far NE TX to account for some potential modest backbuilding/regenerative activity this evening. ....Mid-Atlantic/Northeast... The impressive closed low moving from the Northern Plains into the Upper Midwest will force downstream WAA and mid-level divergence across the Mid-Atlantic and into the Northeast. This will additionally result in scattered to widespread thunderstorms today, with clusters of organized thunderstorms likely thanks to 20-40 kts of bulk shear. Ascent will likely maximize late this aftn and through the evening, especially across northern New England as a modest mid-level impulse/vorticity maxima lifts northeast and interacts with robust thermodynamics characterized by PWs of around 1.25 inches (above the 90th percentile according to the SPC sounding climatology) overlapping MUCAPE as high as 2000 J/kg. Locally backed 850mb flow angling more from the SE will provide some upslope ascent as well, becoming an impetus for additional convective development into the terrain of Maine, some of which could become tied to features to slow overall motion until the shortwave swings through this evening.=20 Convection that develops ahead of the shortwave will be slow moving as reflected by Corfidi vectors that are just around 5-10 kts, and both HREF and REFS probabilities indicate rain rates have a high chance (>70%) of exceeding 1"/hr. Where storms move slowly, and then are swept out by more organized thunderstorm activity developing beneath the shortwave this evening, rainfall exceeding 3" is likely in some areas, especially western Maine. Although soils are generally dry here as reflected by NASA SPoRT due to a lack of recent rainfall, this still poses an increased flash flood risk and the SLGT risk inherited was modified only cosmetically. To the SW of this slight risk, and as far south as the Mid-Atlantic, periodic clusters of convection, potentially as small mesoscale convective systems (MCS) may generate and track rapidly eastward across portions of the region. While these will likely contain heavy rain rates exceeding 1"/hr, fast forward motions should limit the flash flood risk even atop some areas that are more sensitive due to recent rainfall, so the MRGL risk remains. Weiss Day 2 Valid 12Z Sat May 17 2025 - 12Z Sun May 18 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM CENTRAL TEXAS THROUGH CENTRAL ALABAMA AS WELL AS FOR PORTIONS OF THE NORTHEAST... ....Texas to Alabama... Convection will likely develop across portions of central TX Saturday afternoon as increasing forcing overruns the dryline. Enough mid level vort energy, upper jet support and robust instability to support likely upscale growth of convection as it moves into northeast TX. The extent of the flash flood risk will likely come down to convective longevity at any one location as cells should be pretty fast moving. This fast movement may limit the areal coverage of flash flooding, however certainly an opportunity for at least some training as convection grows upscale. The 00z CAMS and AIFS suggest northeast TX towards the AR/LA border has the best chance of seeing convective training potentially leading to a flash flood threat. Overall still not enough model support to suggest Slight risk coverage, but localized flash flooding appears probable with this setup, especially over more sensitive urban areas. Areal averaged rainfall may only average around an inch, but would expect to see localized totals around 3", much of which would fall in just a couple hour period. Convection will likely be ongoing across the eastern portion of the Marginal risk (MS/AL) at 12z Saturday, but likely in a decaying state. However we could see at least isolated to scattered redevelopment along the leftover outflow during the day Saturday, and anything that does develop would likely be slow moving and capable of resulting in a localized flash flood threat. ....Northeast... A couple rounds of convection appear likely across the Northeast on Saturday...one in the morning and another by afternoon/evening. Both rounds are expected to feature quick moving cells with deep layer mean flow over 30 kts. This will likely limit the extent of any flash flood risk, however localized 1" in an hour amounts are still probable given the moisture and instability forecast. These higher rates combined with the potential for multiple convective rounds supports an isolated flash flood threat and a continued Marginal risk area. Chenard Day 3 Valid 12Z Sun May 18 2025 - 12Z Mon May 19 2025 ....THERE IS SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL PLAINS INTO THE LOWER AND MIDDLE MISSISSIPPI VALLEY... ....Plains, MS Valley and Southeast... A favorable pattern for widespread convection and heavy rainfall on Sunday across the middle portion of the country. A longwave trough and embedded shortwaves pushing eastward across the Rockies and into the Plains will provide ascent over a broad region. The frontal pattern will likely feature a developing low moving into the High Plains of CO/NE, a warm or stationary front extending east of this low across portions of KS into MO/AR, and a dryline across OK/TX. A large pool of instability is expected east of the dryline and south of the stationary front, with values upwards of 4000-5000 j/kg. Some convection may be ongoing near the warm/stationary front Sunday morning, but more robust development is expected by Sunday afternoon into the overnight hours as stronger forcing ejects into the Plains. The main area of focus for flash flooding appears to be across portions of eastern KS into MO and/or AR near the aforementioned stationary/warm front. Convection should first develop over central KS with upscale growth into an MCS likely. Mean flow is off to the northeast, however with a strong southwesterly low level jet, Corfidi vectors are pointed more towards the southeast. Thus would expect convection to turn easterly and then southeasterly as it organizes Sunday evening. As this process occurs some training/backbuilding on the south/southwest extent is probable resulting in a scattered flash flood threat. The exact MCS location/track remains uncertain, but think the 00z GFS is likely too far to the north and east...with the aforementioned expected MCS propagation taking it on a farther southern track. Thus tend to think something closer to the 00z ECMWF and UKMET is more likely. The experimental 00z RRFS (the first CAM to go out into day 3) also seems to show a plausible evolution and placement over eastern KS into western MO. The setup does have the potential for a swath of over 5" of rainfall where training/backbuilding ends up being maximized. Thus this is trending towards a higher end Slight risk, with at least scattered flash flooding probable. Another area of interest will be farther northwest into western NE and southwest SD, closer to the low track. A multi-model signal exists for a second QPF max over this area...which makes sense given what should be impressive synoptic driven ascent and instability along 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 was not ready to go with a large Slight risk area yet. But we will continue to monitor trends and an expansion of the Slight risk may eventually be necessary. A localized flash flood risk also exists both along the dryline from central TX into OK...and over the Southeast where a lingering boundary and substantial instability will support heavy rainfall with any storms that area able to develop. ....Northeast... A Marginal risk continues over portions of northern New England on Sunday as a deep layered low remains overhead. This will be the 3rd straight day of localized heavy rainfall, and thus some areas may be more sensitive by this time. Generally thinking both instability and moisture will be lower by Sunday, however cold air aloft under the low will likely still support some heavier convective cells and an isolated flash flood risk. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9VH6e0EcTfjkDkVtcVN89J7GzgRzMEBtidWquzwxnJBb= yUMICCfZuJ7g4uZuaHqW_7ZS__u_8O3Q-jGbrhD_HchgteI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9VH6e0EcTfjkDkVtcVN89J7GzgRzMEBtidWquzwxnJBb= yUMICCfZuJ7g4uZuaHqW_7ZS__u_8O3Q-jGbrhD_gbMd5EU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9VH6e0EcTfjkDkVtcVN89J7GzgRzMEBtidWquzwxnJBb= yUMICCfZuJ7g4uZuaHqW_7ZS__u_8O3Q-jGbrhD_Q4RR5TU$=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) .