Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jul 06 2023 00:57:02 FOUS30 KWBC 060056 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 856 PM EDT Wed Jul 05 2023 Day 1 Valid 01Z Thu Jul 06 2023 - 12Z Thu Jul 06 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN/CENTRAL PLAINS... ....Upper Midwest... A slowly advancing frontal boundary, currently draped across eastern Wisconsin through central Illinois, is providing the necessary focus for a few lines of slower moving thunderstorms this evening. Southwesterly flow ahead of that feature continues to pull northward anomalous moisture, currently characterized by precipitable water values exceeding 1.5-1.75" from Illinois to Michigan. Current radar imagery and rain rate estimates along with the 18Z HREF neighborhood probabilities suggest potential for localized 1-1.5" hourly totals will continue through the evening hours before somewhat diminishing overnight. Localized 2"+ totals will be possible, impacting some sensitive urban locations which could lead to some instances of flash flooding. ....Carolinas... Lingering frontal boundary draped across the Carolinas will provide the necessary focus for continued convection well into the evening and overnight hours, particularly across portions of coastal South Carolina into eastern/central North Carolina. A weak shortwave noted in water vapor may provide the extra necessary lift to spark the additional convection despite the waning daytime heating/instability and with precipitable water values remaining well above normal (upwards of 2"+ in SC) and slow storm motions, this will be more than enough ingredients worthy of continuing the Marginal Risk for isolated/localized flash flooding through the rest of the period for teh potential of isolated 1-2"+ totals. ....Southern/Central Plains... Relatively few changes made to the ERO areas for this region as the 18Z HREF and recent HRRR runs still suggest convection will organize across portions of southeast Colorado then track east/southeast through southern Kansas and northern Oklahoma later this evening through tonight. The HREF EAS 1" probability maxes out near 70 percent along the KS/OK border and reaches nearly 25 percent for 2" threshold for the 12-hour period ending 12Z, so the Slight Risk area looks to be on track based on the latest guidance for the potential for localized 2-3". It's a little uncertain how the ongoing convection across the eastern Texas Panhandle into southwest Oklahoma will potentially disrupt the expected convection later tonight but the most recent guidance still leans toward convection developing after 03Z across southeast CO then tracking southeast through KS/OK for the rest of the period. Taylor Day 2 Valid 12Z Thu Jul 06 2023 - 12Z Fri Jul 07 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS... ....2030Z Update... No huge changes were made to the inherited risk areas. The Slight Risk area was refined a bit with the strongest signal for heavier rain shifting back towards the northwest, thus removed much of central OK from the Slight Risk, but left northern OK in the Slight risk as a buffer for storms moving into OK from KS. The highest likelihood of heavier rains is across west central and south central KS. The best chances of those heavier rains will be overnight Thursday night/Friday morning, but in similar fashion to today, leftover rains are likely to be ongoing at the start of the period as well, with a long break in the organized convection for much of the afternoon. Elsewhere, the Marginal Risk area for the coastal Carolinas on down into northeast Florida was removed with this update. While scattered showers and thunderstorms remain likely, they are expected to be pulse in nature, with next to no signal for organization of those storms. A bit more rainfall is expected into FL, but high FFGs in this area should limit any flash flooding to very isolated instances, if any at all. The Marginal Risk area from the western Gulf Coast up through the TN and OH Valleys and into eastern Lower MI is largely unchanged with this update. The guidance is in poor agreement on excessive rainfall chances, especially in the TN Valley, though the higher chances of multiple-inch-producing storms will be down into LA, where the FFGs are higher. Storms are likely to develop along the TX Gulf Coast, but it's unlikely those storms will cause too many problems unless they move over a more urbanized area. Wegman ....Previous Discussion... ....North/Central High Plains into the Central/Southern Plains... The inherited Slight Risk area was maintained (with no adjustments) across portions of OK/KS/CO, as the latest model guidance remains in rather good agreement with regard to the potential for 2-3" totals (both deterministic solutions of the GFS/ECMWF/UKMET/Creme and with regard to exceedance probabilities of the GEFS/ECENS). Given the expected strong low-level return flow behind the departing system (discussed on Day 1), would expect robust diurnal convection to develop along the Front Range and North/Central High Plains in the afternoon (which seems depicted well by the convective allowing FV3), likely propagating upwind into KS/OK. While the ensemble guidance in particular is more enthusiastic about western KS than north-central OK, this is likely due mostly to the parameterized nature of the convection (as downscaled deterministic solutions and available CAM guidance depict a more realistic southward propagation). The Slight Risk will likely be further refined with the inclusion of more members of the HREF later today. ....Ohio Valley into the Mid-South, Deep South, and Gulf Coast... The Marginal Risk area was maintained for this cycle, as the timing of the front remains fairly consistent among the guidance for this cycle. Expect this front to locally enhance what would otherwise be typical pulse convection summertime storm mode, with anomalous moisture pooled along the front supporting fairly disorganized convection capable of locally heavy downpours. While there is a fair bit of clustering particularly along the Gulf Coast (enhanced by sea breeze convection), FFGs are rather high here (to as much as 4-5" over 3-hr). Expect any instances of flash flooding to remain rather localized (and confined mainly to poor drainage areas). ....Coastal Carolinas southward into Coastal Georgia and Northeast Florida... A Marginal Risk area was maintained once again across this region, largely continuing the meteorological reasoning from Day 1 (as the tail end of a lingering coastal front may help to enhance diurnal sea breeze convection in the afternoon). Fairly high FFGs will likely keep any instances of flash flooding localized, but refinement to the risk area (and any potential upgrades) will become more clear with the additional CAM guidance later today. Churchill Day 3 Valid 12Z Fri Jul 07 2023 - 12Z Sat Jul 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL/SOUTHERN PLAINS INTO THE MIDDLE MISSISSIPPI VALLEY... ....2030Z Update... Some bigger changes were made to the Day 3/Friday forecast as compared with Day 2. The high-end Slight Risk area over KS/MO/OK was adjusted per the latest guidance, shifting it south out of the Kansas City metro, but expanded a bit east into southwestern MO. The region from the MO/OK border northwest to central Kansas as the best chance of seeing rainfall totals over 3 inches. Since the primary storms are highly likely to remain well south and west of Kansas City, the metro itself should see much lighter rainfall rates, that should locally exceed 1 inch, but shouldn't fall fast enough to cause widespread flash flooding issues. The Marginal Risk area from southeast AR through much of the Southeast and through the Delmarva was dropped with this update. There is very little signal for organized thunderstorm activity capable of producing flash flooding in the mostly high FFG environment, so while pulse convection in this region will still be plentiful, think the flash flooding risk falls in the sub-Marginal 0-5% category. Conditions are only very marginally more favorable into the Northeast, where recent rains combined with a very slightly better signal for more organized convection prompted the Marginal Risk area to remain in place. There is some uncertainty as to how far east off the mountains the storms will get, so for now the NYC Metro is just outside the Marginal Risk area, other than the western suburbs in NJ. There is some potential a much smaller Marginal Risk area may need to be reintroduced somewhere in the Southeast with future updates, but very high uncertainty as to where that should be persists. Wegman ....Previous Discussion... ....Northern/Central High Plains into the Central/Southern Plains and Middle/Lower Mississippi Valley... The inherited Slight Risk has been maintained and expanded a bit across portions of MO/KS/AR/OK, owning to a rather impressive signal for areal average QPF of 2-3"+ at this juncture (as we are still lacking any CAM guidance). The 00z GFS/ECMWF/UKMET/CMCreg are all depicting totals approaching or exceeding 3" across some portion of the Slight Risk area, and the highest confidence exists across southwest portions (KS/OK) where the favorable southwest flank of the MCS for training heavy rainfall tends to setup (as best depicted by the downscaled GFS/ECMWF). That said, the setup is highly dependent how things evolve on the mesoscale through Days 1-2, and confidence in identifying a potential Moderate Risk area is challenging given this (though the prospects of eventual introduction seem decent, given the expected rainfall that has yet to occur on Wednesday/Thursday as well). The synoptic pattern certainly remains favorable for additional MCS development, with northwest flow aloft and shortwave impulses ejecting off the Front Range (with anomalous tropospheric moisture still in place). A much broader Marginal Risk surrounds the Slight Risk area, generally encompassing areas where instability will likely be sufficient for convection. ....Gulf Coast and Deep South into the Mid-Atlantic and Northeast... A relatively slow moving front will begin to approach the eastern seaboard by Friday, allowing for increased convective coverage once again for a large portion of the eastern CONUS. It is difficult to discern any significant synoptic forcing at this point, though the RER of a modest jet streak could assist with uplift across portions of the Northeast, in particular. But currently downscaled global guidance depicts very spotty 1-3" localized totals with a lack of any significant clustering or organization to pinpoint any Slight Risk areas. The inherited Marginal Risk has been maintained and seems appropriate for now. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6qsF3WON5nHFQFxbx057YZUn1zqN0UKjsMtD5XPfwldG= PjGxezht7N0CpTl5XZcpBTcRyGWDped3Ctky0OfPjC-HEwg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6qsF3WON5nHFQFxbx057YZUn1zqN0UKjsMtD5XPfwldG= PjGxezht7N0CpTl5XZcpBTcRyGWDped3Ctky0OfPz7LFevo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6qsF3WON5nHFQFxbx057YZUn1zqN0UKjsMtD5XPfwldG= PjGxezht7N0CpTl5XZcpBTcRyGWDped3Ctky0OfPwtXoiF0$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .