Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jul 05 2023 20:15:29 FOUS30 KWBC 052015 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 414 PM EDT Wed Jul 05 2023 Day 1 Valid 16Z Wed Jul 05 2023 - 12Z Thu Jul 06 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF CENTRAL AND NORTHEASTERN OKLAHOMA... ....16Z Update... Only minor changes were needed with this morning's update. A squall line tracking quickly southeastward across northeast OK has been rapidly weakening over the past hour or so. Expect this to continue and dissipate over the next couple hours. Meanwhile trailing convection has been developing over south-central KS. This seems to indicate that recovery time behind the squall line is rather small. This lends credence to the general agreement in the CAMs that a renewed round of showers and thunderstorms will develop this evening and track southeastward across the Slight Risk area in a similar fashion as the present convection is. Since none of the 12Z CAMs are handling the present squall line well, the thinking is the over-performance relative to the CAMs solutions will prime the soils ahead of this evening's storms more than any of the guidance is suggesting. Thus, despite the Slight still considered a low-end threat and is meant specifically for this evening's convection, rather than the dying storms ongoing at present, still think there is a threat for widely scattered flash flooding indicative of a Slight in this region. The inherited Slight Risk area was expanded north and west with this update following the latest trends in the guidance. Very few changes were needed to the surrounding Marginal Risk, including up into northern MI, where signals are stronger than further south, but high FFGs should keep any flash flooding to only isolated instances. No changes were made to the Marginal risk in the Carolinas/eastern TN and far eastern GA. The line of storms tracking across northern GA are progressive enough and are moving over a somewhat drier area of far northern GA and largely north of the Atlanta metro that these ongoing storms are unlikely to pose much of a flash flooding threat. Much of the rain in the eastern Carolinas is expected overnight tonight. Wegman ....Previous Discussion... ....Central/Southern Plains into the Upper Midwest/Mississippi Valley... The 00z HREF has come into better agreement with regard to the clustering for localized 3" exceedance today, and a Slight Risk area was introduced accordingly for portions of central and northeastern OK. The forecast is complicated by a mature MCS that is currently progressing south and east across KS/MO, which may itself bring localized excessive rainfall to portions of KS/OK (along the southwestern flank which may favor some limited training of 1-2"/hr rates). The resulting outflow from the MCS/MCV may well be the focus for afternoon convection initially, followed by the trailing cold front itself (along with forcing from a shortwave impulse ejecting off the Front Range by the late afternoon/evening). Should a larger-scale axis for training from this anticipated shortwave impulse become more evident later today (as the ARW and NAM-nest seem to suggest), the Slight Risk area may need to be expanded (perhaps most likely towards the northwest, per the HREF EAS 1" exceedance axis that stretches northwest along the KS/OK border). Farther northeast across the Upper Midwest/Mississippi Valley, highly anomalous moisture will continue to pool along the front (with PWATs reaching as high as 1.8-2.0") and support locally heavy rainfall. Since this convection appears to be much less organized, a Marginal Risk was maintained for this cycle. ....Carolinas into eastern Georgia and Tennessee... Maintained a Marginal Risk area across the eastern/coastal Carolinas, but also decided to expand this area farther west to encompass the majority of the Carolinas into eastern GA/TN. While convective organization on the mesoscale looks less likely across this region (with deep layer shear generally 20 kts or less), HREF 40-km neighborhood exceedance probabilities are still rather impressive.. depicting very high odds (50-80%) for 3" exceedance across much of the Carolinas, and decreasing to lower odds (20-40%) farther west into the Appalachians (where FFGs are also substantially lower due to the terrain/soil type). The tail end of a lingering frontal boundary (from a system that ejected into the Atlantic from the Northeast) will probably help to enhance afternoon sea breeze convection (as well as pulse storms farther inland), and overall tropospheric moisture is indeed quite impressive (1.7-2.1" PWATs) near the front. That said, the HREF ensemble agreement scale (10-100 km) neighborhood probabilities for 1" exceedance are fairly low (20-40% along across the eastern Carolinas and 10-20% farther inland), which speaks to the localized nature of any extreme 3-5"+ totals. This supports the continuation of a Marginal Risk, and the prospects for a Slight Risk upgrade appear low at this time. Churchill 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 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6-KncEezTvfQ6NhHIegJ6POcL6NKQSQQpkfEI9TmrbGd= EMbBPGonx29spb4r7CZZw5KAgTLYFvRbMhnM48K5XmW2wH8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6-KncEezTvfQ6NhHIegJ6POcL6NKQSQQpkfEI9TmrbGd= EMbBPGonx29spb4r7CZZw5KAgTLYFvRbMhnM48K5C_CJN18$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6-KncEezTvfQ6NhHIegJ6POcL6NKQSQQpkfEI9TmrbGd= EMbBPGonx29spb4r7CZZw5KAgTLYFvRbMhnM48K5G8qKvRI$=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) .