Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Aug 11 2024 00:23:12 FOUS30 KWBC 110022 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 822 PM EDT Sat Aug 10 2024 Day 1 Valid 01Z Sun Aug 11 2024 - 12Z Sun Aug 11 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR EASTERN=20 PORTIONS OF THE CAROLINAS... ....Eastern Carolinas... Ongoing afternoon-driven convection along and east of the stalled=20 frontal boundary will slowly subside over the next few hours, but=20 has still maintained some heavier rainfall rates of 1-2"/hr per=20 MRMS. Trimmed the Slight Risk area to areas near and east of I-95=20 where lower FFG values may be exceeded and localized flooding was=20 ongoing.=20 ....Rockies into the Plains... Upper ridge axis slices across northern AZ, where some cell=20 movement has been fairly slow. To the south, late afternoon=20 convection continues westward through the CO River Valley while to=20 the north the movement is generally to the east. Instability=20 remains fairly abundant where it hasn't rained already, so the=20 broad Marginal Risk outline was maintained for the Southwest. Over WY/CO, activity continues this afternoon/evening but mostly higher FFG values suggest any flooding threat will be more isolated. The eastward extension of the Marginal Risk onto the=20 central/southern Plains remains in place in anticipation of=20 overnight activity into the very early morning hours on Sunday over Oklahoma. The prospects for significant flooding are low, but non- zero as the 18Z HREF still shows ~20% probabilities of at least=20 2"/hr, though FFG values increase to the east to near 3"/hr. The=20 progressive nature of the storms combined with very dry antecedent=20 conditions will mitigate a greater threat of flash flooding but=20 still the Marginal Risk was warranted. Fracasso Day 2 Valid 12Z Sun Aug 11 2024 - 12Z Mon Aug 12 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE CAROLINAS, SOUTHWEST UTAH, AND FOR PORTIONS OF THE OZARKS AND CENTRAL PLAINS... ....20Z Update Summary... General continuity across the Carolinas with the SLGT risk over the region as another day of scattered to widespread convection capable of heavy rainfall will develop in-of the stationary front bisecting the region. The SLGT risk across the Central Plains into the Ozarks was maintained with some adjustments on the northeastern edge of the risk area based on the latest probabilistic trends and with coordination from the local Kansas City WFO. A new SLGT risk was added across Southwestern and Central UT for the threat of heavy rainfall with a greater heavy rain footprint compared to previous periods. More on each region in the respective sub- headings below.... Kleebauer ....Carolinas... 20Z Update: Minor adjustments were made across the Carolinas with the biggest change coming across the northwest side of the SLGT risk where guidance has scaled back on the westward extent of heavy rainfall between Raleigh to Morehead City. There is still a formidable signature for locally >3" of rainfall within the neighborhood probability fields off the 12z HREF (60-90%) across far Eastern NC to the west of Cape Hatteras. This is in correlation to the strongest instability located within the deep layer moisture lingering along the NC coastal plain. The heavy rain threat extends down the coast of SC as well with the best potential for flash flooding occurring close to Charleston and areas adjacent. This is almost a repeat of what is occurring in D1, and with the antecedent well-above normal soil moisture forecast, there was no reason to deviate from the previous forecast. Kleebauer ...Previous Discussion.. The stalled out front over the Carolinas will continue to draw deep tropical moisture northward up the Southeast coast. The front will have made enough progress to likely keep the rainfall down into southeastern Virginia, but there will likely be more storms into South Carolina. Thus, the Slight risk is just a bit south of the Day 1/Saturday Slight Risk. Nonetheless with ongoing flooding from Debby, additional rainfall, even if more widely scattered, will have an outsized potential for impacts from additional flooding. 00Z HREF guidance through 00Z Monday suggests additional elevated potential for exceeding FFGs in NC, and plentiful moisture and instability with a stalled out front providing the forcing will likely help organize the storms, increasing their flooding potential. Wegman ....Central Plains/Ozarks... 20Z Update: The area across Northwest OK will see an initial round of convection with heavy rainfall capable of 1-3" within the morning before the threat dwindles with the loss of the nocturnal LLJ. A break will ensue for most of the afternoon and early evening before the forecasted MCS initiating over KS moves into the targeted region after nightfall. The SLGT risk from previous forecast was maintained with general continuity with some shift southward based on a shift in the heaviest QPF placement, a trend that may continue just based on the recent ML output. The setup is contingent on the axis of highest theta-E in conjunction with the northeastern extension of the ridge across the Central and Southern Plains. The event remains on the lower end of SLGT due to the drier soil moisture residing over the expected area of impact. The environment is conducive for heavy rain however, and the probability fields are relatively favorable for pockets of 2-4" of rainfall within a few hours of impact as the complex shifts southeast out of KS into the MO and Northwestern OK. Considering the consistency in the placement of the forecasted complexes during the period and the continued favorable probabilistic output from the CAMs, the SLGT was justified, but certainly on the lower end of the risk threshold. Kleebauer ...Previous Discussion.. MCS development on the leading edge of a low level jet (LLJ) is expected from southeast Kansas through northwest Arkansas. Despite recent dry weather, training convection with PWATs increasing to around 2 inches will support storms capable of rates to 3 inches per hour at times. This very heavy rainfall combined with terrain interactions in the Ozarks will favor widely scattered instances of flash flooding. In coordination with the impacted offices, a Slight Risk upgrade was introduced with this update. Given CAMs frequent trouble with correctly simulating storms with MCSs, there are likely to be both temporal and intensity adjustments with the expected rainfall. Despite this potential, there was good enough agreement in the guidance to draw in the Slight risk, which for now remains low-end given the recent dry weather and dry soils. Wegman ....Great Basin and Southwest... A slightly stronger mid-level ridge is forecast across the Southwestern US with a zone of elevated low to mid-level moisture advecting northward into the Great Basin and portions of the Inter Mountain West during the day on Monday. The combination of +1 to +2 deviation PWATs, ample diurnal destabilization, and a weak mid- level perturbation ejecting out of NV into UT will allow for a bit more robust convective development in-of portions of Southwest and Central UT leading a low-end SLGT risk placement across the aforementioned area. Ensemble based QPF output is respectably high compared to recent periods with signals for 1-2" across the terrain Southwest UT, extending eastward thanks to the progression of the disturbance moving overhead. 12z HREF neighborhood probabilities up around 50-70% for at least 2" exist within the corridor from Gunnison down towards the AZ border in Western UT. 10-year ARI exceedance probs are also running very high in the same region with signals of 50-70% showing up in the recent HREF output. Modest 100-year ARI probs are also located within West-Central UT, so the threat is unlike the recent convective pattern when those signals were either muted or extremely marginal (Likely driven by one CAM member). In coordination with the Salt Lake City WFO, a SLGT risk was introduced to cover for the threat with emphasis on areas around the I-70/15 split where the highest probabilities and QPF are positioned. The threat may also extend further south into Northwest AZ, but the setup is not as aggressive comparatively and lies a bit closer to the MRGL risk based on the recent precip forecast. This will be an area to monitor in future updates. Kleebauer Day 3 Valid 12Z Mon Aug 12 2024 - 12Z Tue Aug 13 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE FOUR CORNERS REGION, CENTRAL PLAINS, OZARKS, AND THE CAROLINAS... ....20Z Update... The premise behind each MRGL risk area remains valid with a general run-to-run consistency on the placement of the heaviest QPF axis' over the course of the period. The area with the best opportunity for a SLGT risk upgrade remains over the Ozarks thanks to the opportunity for back-to-back days of complexes that could move through the area. There's still some uncertainty on how the overall mesoscale pattern plays out as remnant outflow boundaries from convection the previous period could shift the threat north or south pending how it evolves. The ensemble bias corrected QPF and NBM 75th percentile QPF signatures align, more or less across that span of Southern MO and Northern AR with some overlap from the previous D2 SLGT risk located across Southwestern MO. Decided to maintain continuity in the area to account for some shift in the coming model cycles, but the prospects are still relevant for an upgrade in the near future. Elsewhere, the threat across The Colorado Front Range into Northwest KS took a slight step forward in the prospects for any upgrade with some global deterministic output yielding formidable QPF maxima located near and along I-70 out by Goodland and points west. Shortwave ejecting out of the Central Rockies could be sufficient for primed large scale forcing within an environment that favors above normal precip with PWAT deviations running between +1 to +2 across the Central High Plains. Close proximity of a stationary boundary will also allow for an axis of convergence that would solidify a suitable initiation and storm motion inflection point for the setup. For now, a MRGL risk remains in effect with a point of interest for a future SLGT if the precip signature maintains itself or increases in intensity. Carolinas will be within one last day of potential flash flood concerns as the stationary front lingers over the far eastern sections of the area. The signal for excessive rainfall is less than prior days, but still warrants some attention given the antecedent conditions expected through the D3 period. Only minor adjustments were made to reflect the mean QPF footprint within the ensembles. Kleebauer ....Previous Discussion... Ongoing rainfall from Sunday night may continue into Monday morning across southern Missouri. With only a few hours of rain in the morning, the Marginal across Missouri was maintained. However, should rainfall from Day 2/Sunday overperform, then a future Slight may be needed. Once again another round of storms Monday afternoon across the Four Corners region is likely to result in isolated flash flooding. However, there is greater uncertainty with coverage by Monday in this region with lesser forcing than on Sunday. For the Carolinas, lingering moisture with the same front may cause yet another round of storms Monday afternoon into the evening. With greater potential that the bulk of the activity will be offshore by Monday afternoon, the previously inherited Marginal risk remains largely unchanged. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6OT_86zAvo0_IfFA_S44OIR6ywsWt0C04491HSNoBkhD= GOSzxTXSx7aWecbgBdW8cLXDQE2tgk-CJatyv3SfJmNr0Tc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6OT_86zAvo0_IfFA_S44OIR6ywsWt0C04491HSNoBkhD= GOSzxTXSx7aWecbgBdW8cLXDQE2tgk-CJatyv3SfTOjarno$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6OT_86zAvo0_IfFA_S44OIR6ywsWt0C04491HSNoBkhD= GOSzxTXSx7aWecbgBdW8cLXDQE2tgk-CJatyv3SfxJsqIFs$=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. 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