Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jul 04 2023 20:32:35 FOUS30 KWBC 042032 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 431 PM EDT Tue Jul 04 2023 Day 1 Valid 16Z Tue Jul 04 2023 - 12Z Wed Jul 05 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL PLAINS/HIGH PLAINS, THE MIDDLE/UPPER MISSISSIPPI VALLEY, AND THE NORTHEAST... ....16Z Update... ....New England... The biggest change from the inherited forecast was to expand the Slight Risk area well to the north to include nearly all of NH and much of southwestern ME. Recent rainfall from previous storms have already moistened up soils across this region. Widely scattered cellular activity is forming along the Canadian border from northern VT through the ME side of the NH/ME/QB junction area. These storms are initially slow moving, and may backbuild and train over these areas along the White Mountains before drifting to the southeast later today. Due to the expected high rainfall rates, including a 30% chance of 3" or more of rain and a 15% chance of 5 inches of rain and favorable antecedent conditions in this region, coordination with GYX/Gray, ME forecast office agreed with the expansion. No other significant changes were made to the Slight Risk area elsewhere, with NYC and points north also seeing potential for high rainfall rates in urbanized areas leading to a higher risk of flash flooding. ....Southeast... The only change of note was to expand the MRGL westward into southeastern OK, where nearly stationary and backbuilding convection has caused local rainfall rates to approach 3 inches per hour. Rainfall totals over the last 24 hours along the Red River in southeast OK/northeast TX are approaching 4 inches. A few flash flood warnings have also been issued earlier with these storms, which since they aren't moving, continue to pose a threat for additional flash flooding. ....Plains/Upper Midwest... An ongoing MCS stretching from the northeast corner of NE to the Twin Cities is fortunately moving along enough for now that there is a low risk of flash flooding, though the southeast corner of SD, the storms have been backbuilding, so the threat is still much higher than zero. Much of the threat will come later this evening into tonight as a renewed round of storms develops upscale into an MCS, which will primarily highlight the CO/NE/KS region. Meanwhile, once the current convection weakens, there will be a short break from NE through IA and MN before renewed convection develops again this afternoon. These storms will also be pretty fast-moving, but with a better chance of training, which in addition to the rain from the current storms may result in a more widely scattered flash flooding threat. Wegman ....Previous Discussion... ....Front Range into the Central Plains and Middle/Upper Mississippi Valley... A front draped across the Central Plains and Upper Midwest will help to focus multiple rounds of showers and thunderstorms today, and the inherited Slight Risk area was expanded a fair bit (both south and west into much of north-central KS, as well as north and east into more of MN and a bit of northwest WI). The Slight Risk area generally encompasses where the HREF probability-matched mean QPF depicts the potential for localized 3-5" totals (also corresponding well with where the HREF 40-km neighborhood exceedance probabilities for a 3" threshold are near or greater than 15%). An initial round of storms (from an approaching MCS over SD) this morning is expected to help set the stage for the main event this afternoon/evening, as precipitable water values are expected to pool to 1.5-2.0" (which ranges from the 90th percentile to the max moving average of surrounding upper-air locations, per SPC sounding climatology) with surface-based CAPE rising to 1000-3000 J/kg and deep layer (0-6 km) shear increasing to 20-40 kts (as an upper-trough digs southward over the Dakotas). Storm mode is expected to initially be semi-discrete, gradually organizing into clusters in the afternoon (with CAMs signaling the potential for one cluster propagating upwind/southward into NE/KS, while another may favor the mean wind towards the northeast into portions of IA/MN/WI). This may favor localized training across the west-east corridor from northeast CO through north-central KS (with supercells initially favored over the High Plains, propagating eastward and merging with the upwind propagating storms along the sagging portion of the front), as well as across the southwest-northeast corridor from eastern NE into portions of SD/IA/MN/WI (where the deep layer mean wind is initially more parallel to the front). Uncertainty in overall convective organization is greater outside of the Slight Risk, where a broader Marginal was also maintained (with some adjustments to the edges, based largely on the 00z HREF guidance). ....Northeast... A Slight Risk area has been maintained and expanded a bit for this outlook for portions of the Northeast, including much of MA/RI/CT southwestward into southern NY and far northern NJ. A lingering low pressure system along the coast will be the focus for renewed convective activity this morning into the afternoon, with the HREF honing in particularly on the 15z-00z period. While the HREF 40-km exceedance probabilities are not quite as high as they were 24 hours ago, the resulting QPE since then was rather impressive, with MRMS estimates indicating several localized swaths of 2-3" totals. These wet antecedent conditions will likely makeup for any reductions in QPF expectations today, though 2" and 3" exceedance probabilities (over 3-hr time intervals) are still as high as 20-40% and 10-20%, respectively. Given slow expected storm motions (10-20 kts) with highly anomalous tropospheric moisture (near the max moving average, per OKX and CHH sounding climatology), rainfall rates of 1-2"/hr are possible (with only limited training/repeating of these). Additional instances of flash flooding should remain scattered at best today, but the aforementioned wet antecedent conditions may allow for a significant event or two of flash flooding (with urbanized terrain and other poor drainage areas also of particular concern, should 2-4" short-term totals occur over those areas). ....Southern Mississippi Valley/Tennessee Valley Eastward to Carolina Coast... The Marginal Risk area has once again been maintained with little adjustment for this cycle. Diurnal convection is expected along/near a lingering frontal boundary extending from the southern Mid-Atlantic to the Lower Tennessee Valley with 1.75-2.00 inch PWs and subtle shortwave energy ejecting into the central/southern Plains to allow for another round of slow-moving storms to develop, which could result in isolated instances of flooding. The latest area reflects where the best QPF footprint exists (and incorporating CAM guidance where available). Churchill Day 2 Valid 12Z Wed Jul 05 2023 - 12Z Thu Jul 06 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MIDWEST, CENTRAL MISSISSIPPI VALLEY, CENTRAL/SOUTHERN PLAINS, AND EASTERN/COASTAL PORTIONS OF THE CAROLINAS... ....2030Z Update... No major changes were made with this afternoon's update. Uncertainty remains high regarding where the core of the heaviest rain is forecast for Wednesday. Just since last night the primary area of the highest probabilities has shifted from central MO to near the junction of I-35 and the KS/OK border. Due to that uncertainty and wild shifts in where the heaviest rainfall totals are expected, the Marginal Risk has been left in place for the Plains through MI. Likewise, only cosmetic changes were made to the Marginal Risk area for the eastern Carolinas, where isolated flash flooding is possible with the most persistent heavy rains with near stationary cells or cell mergers. There is some possibility the southern Appalachians of NC through GA may need to be added to the Marginal Risk with a future update, but the signal there is weak enough that it was left out of the western Carolinas for now. Wegman ....Previous Discussion... ....Upper Midwest/Mississippi Valley into the Central/Southern Plains... The unseasonably strong frontal system that has been discussed in the Day 1 ERO will gradually push southeastward, with the bulk of the forcing for convection following suit. Anomalously high moisture pooling along the front will present the risk for localized flash flooding across a rather large area, but there remains insufficient clustering/agreement to introduce any Slight Risk areas at this time (though central MO is situated near the highest probabilities). As the remainder of the CAM guidance covers the entirety of the Day 2 period later today, this will be further evaluated. Right now, the Marginal Risk area was maintained from the prior Day 3, and generally encompasses areas with the potential for localized 2" exceedance (per the available CAM guidance). ....Eastern/Coastal Carolinas... Maintained a Marginal Risk area again across the Carolinas as well, with the potential for short-term localized 2" exceedance (per the available CAM guidance). The tail end of a lingering frontal boundary (from the system ejecting into the Atlantic from the Northeast) will likely help to enhance afternoon sea breeze convection, and given prior days rainfall a Marginal Risk seems appropriate (despite the sandier soils and relatively high FFGs). Churchill Day 3 Valid 12Z Thu Jul 06 2023 - 12Z Fri Jul 07 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL PLAINS... ....2030Z Update... The only change worth noting was an expansion of the previous Slight Risk area southeastward into northwestern and north-central OK. With the D2/Wednesday area of heaviest rain now shifted into OK, the local maximum of forecasted rainfall expected for D3/Thursday in many of the same areas of OK was enough to increase the confidence that similar flooding impacts as expected in KS will also spread into OK. Elsewhere, typical summertime convection is likely from the lower MS Valley to the Mid-Atlantic, so few changes were needed to the inherited Marginal Risk area. Wegman ....Previous Discussion... ....North/Central High Plains into the Central Plains... The inherited Slight Risk area was maintained across much of western and central KS, as the global guidance continues to remain in rather good agreement with regard to the potential for 2-3" totals (with the deterministic ECMWF, GFS, UKMET, CMCreg, and the ECENS being the strongest contributors). Given the expected strong low-level return flow behind the departing system that has been discussed during Days 1-2, would expect robust diurnal convection to develop along the Front Range and North/Central High Plains, propagating upwind into KS/OK. While the exact location may need tweaking for future updates, the overall model agreement provides higher than usual confidence at this range. ....Ohio Valley into the Mid-South, Deep South, and Gulf Coast... The inherited Marginal Risk area was broadened a bit for this cycle, owing to some timing differences with the progression of the front across the global model suite. The unseasonably strong nature of the front is expected to locally enhance what would otherwise be typical pulse convection summertime storm mode, and thought an expansion of the Marginal was appropriate given this. That said, any meaningfully organized signal from the global models is currently lacking, and this area will need to be refined as we get into the range of the CAMs. ....Coastal Carolinas southward into Coastal Georgia and Northeast Florida... A Marginal Risk area was maintained and expanded a bit southward into Day 3, largely continuing the meteorological reasoning from Day 2 (as the tail end of a lingering coastal front may help to enhance diurnal sea breeze convection in the afternoon). The global guidance does a poor job resolving these largely mesoscale features at this range, and the risk area may need to be adjusted with future updates. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7mUXSLiuPCJessFIOvMY0P5SldTqf9S9_YdrAqcB_zhN= L3W7oI4I06KxfnAq7AnFeM8zDE-icOofmz2VM4DdA2dgLkk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7mUXSLiuPCJessFIOvMY0P5SldTqf9S9_YdrAqcB_zhN= L3W7oI4I06KxfnAq7AnFeM8zDE-icOofmz2VM4Dd7wmKit0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7mUXSLiuPCJessFIOvMY0P5SldTqf9S9_YdrAqcB_zhN= L3W7oI4I06KxfnAq7AnFeM8zDE-icOofmz2VM4Ddzp2z3xE$=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) .