Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri May 26 2023 20:07:14 FOUS30 KWBC 262007 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 406 PM EDT Fri May 26 2023 Day 1 Valid 16Z Fri May 26 2023 - 12Z Sat May 27 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHERN ROCKIES, NORTHERN HIGH PLAINS, SOUTHERN HIGH PLAINS, AND THE CAROLINA COAST... ....Carolinas... Guidance in good agreement that a non-tropical low pressure will gradually strengthen off the SC coast today and tonight while gradually being pulled northwestward as an upper low consolidates across the Southeast. Regardless of having any tropical characteristics, this evolution will support a steady increase in shower activity shifting onshore, generally becoming heavy along the immediate coast after 06Z tonight when MUCAPE climbs above 500 J/kg. As this surface low strengthens, 850mb winds are progged to reach 30-45 kts, approaching twice the mean wind, and aligning to be anti-parallel to the Corfidi vectors. Together, this indicates expanding ascent with training of echoes from E/NE to W/SW, with the increased onshore flow driving PWs to 1.5 to 1.75 inches, above the 75th percentile according to the SPC sounding climatology, and more than +2 standard deviations on the NAEFS ensemble tables. The greatest PW anomalies are likely to just impinge upon the immediate coast from the Grand Strand through Cape Fear before the end of D1. While 12Z HREF probabilities for 24-hrs are modest for 1 and 3 inches, there has been a subtle expansion of both the neighborhood and EAS probabilities to the SW today, especially after 06Z, which has prompted a subtle shift in the SLGT risk. The MRGL risk was also expanded towards the Savannah River to account for the more saturated soils from 7-day rainfall that is as much as 600% of normal. Despite these changes, it still appears the greatest rain threat will occur across this area on D2. ....Great Basin through the High Plains... The amplified 500mb pattern will persist today with a large blocking ridge across the Upper Midwest permitting only slow translation of a longwave trough moving into the Intermountain West. Downstream of this trough, moisture advection will intensify as meridional transport from both the Pacific and Gulf merge into the High Plains and then shift northward to the Canadian border, reflected by PW anomalies on the NAEFS ensemble surging to +2 standard deviations from West Texas through Idaho, and reaching nearly +4 standard deviations in eastern Montana. This moisture advection will be fueled by 850mb inflow surging to 20-35 kts from the south, providing ample moisture for convection across a large area. Additionally, this flow will lift isentropically atop a front in the Northern high Plains, while then turning cyclonically around a wave of low pressure to drive upslope enhanced ascent across MT. This moisture and ascent will combine with a corridor of MUCAPE reaching 1000 J/kg, potentially exceeding 2000 J/kg in the southern High Plains and TX Panhandle. This will result in widespread convective development this aftn/eve from New Mexico northward into Montana and then westward to the Northern Great Basin. Storm motions across the High Plains may be relatively quick, noted by 15-20 kts 0-6 km mean winds, but this flow could be aligned to the Corfidi vectors, especially in MT, suggesting some training potential. Additionally, vorticity lobes rotating through the flow could help organize convection into clusters or even MCS in NM/TX, as well as northeast MT, with more scattered activity expected elsewhere, and this drives the rather large MRGL risk where briefly heavy rain rates could result in isolated impacts today. However, the inherited SLGT risks, which overlap where soils are most primed due to 7-day rainfall exceeding 400% of normal in many areas according to AHPS resulting in much lower 3-hr FFG, were adjusted only cosmetically for recent guidance including HREF exceedance probabilities. Additionally, considered a targeted SLGT risk for portions of western NE/northern KS where a morning MCS is producing torrential rain rates and flash flooding from 7-10" of rain overnight, but guidance is in good agreement this will wane later this morning, so the MRGL was maintained. Coordinated with WFOs LBF/GLD, however, as any additional rainfall this evening could quickly result in renewed runoff and additional flooding impacts which could necessitate a targeted SLGT risk later if confidence increases that this will occur. ....Florida... Farther to the southeast across the southern Peninsula of Florida, a surface front will waver in the vicinity before slowly pushing east tonight. This front will align into a region of above normal PWs overlapped with MUCAPE reaching 2000 J/kg southeast of the front. A wave of low pressure moving along this front combined with modest upper divergence should allow for scattered to widespread convective activity this aftn/eve as reflected by high-res simulated reflectivity. These storms will likely be slow movers noted by 0-6km mean winds progged to be just 5-10 kts, and with modest bulk shear forecast, could result in storm mergers and clusters, especially where some enhanced convergence occurs near the surface wave. Total coverage of convection may be limited to scattered, but with rainfall rates likely reaching 2"/hr at times according to the HREF probabilities, atop soils that are saturated from 7-day rainfall that is more than 300% of normal in many areas resulting in high streamflows according to USGS, could result in isolated flash flooding today, especially if any slow moving storms impact the more urban areas. Weiss Day 2 Valid 12Z Sat May 27 2023 - 12Z Sun May 28 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE COASTAL PLAIN OF NORTHEAST SOUTH CAROLINA THROUGH THE CRYSTAL COAST OF NORTH CAROLINA... ....South Carolina through Virginia... Consolidating area of low pressure off the SC coast will slowly strengthen and lift onshore the SC coast late Saturday as it gets pulled northwestward by a closed mid-level low over the Southeast. To the north of this feature, an impressive surface high pressure will expand southward from New England the Great Lakes, producing a tightening pressure gradient across the area. This will result in intensifying easterly winds through a deep layer of the atmosphere, leading to strong moist advection and widespread heavy rainfall. NAEFS ensemble tables indicate the V-wind component will become as strong as -4 to -5 sigma across NC on Saturday, surging PWs to more than +2 sigma onshore. Aiding in this robust moisture transport will be 850mb winds reaching 30-50 kts, with the resultant WAA driving warm cloud depths to around 13,000-14,000 ft, sufficient for warm rain processes. This evolution will release more than 500 J/kg of MUCAPE across the area, highest near the coast, with the 850mb winds being 2x the mean 0-6km winds leading to enhanced convergence for ascent. Together, these parameters produce an environment extremely supportive for heavy rain, and the HREF neighborhood probabilities indicate increasing potential for 2"/hr rain rates through the day. Individual cell motions are expected to be relatively quick at 20-25 kts to the west/southwest. However, Corfidi vectors anti-parallel to the mean wind indicate a likelihood for backbuilding into the greater instability just offshore, which will yield training of echoes from east to west. Where these heavy rates can train, rainfall will likely quickly eclipse 2-4" through Saturday, with locally higher amounts of more than 8 inches possible, highest near Cape Fear where the HREF probabilities are greatest and frictional convergence onshore could enhance ascent. The MDT risk was expanded as far south as Winyah Bay, SC and as far north as Cape Lookout, NC to account for the recent HREF guidance, and to best overlap more sensitive antecedent soils from 7-day rainfall that has been 300-600% of normal. Despite the sandy soils inherent to this area, this rainfall will likely produce instances of flash flooding, with urban areas being most sensitive to this rainfall. Farther to the west, a secondary maxima of rainfall is likely into the Blue Ridge mountains of NC as the impressive and moist easterly wind upslopes efficiently into the terrain, driving robust confluence of the moisture transport vectors. This is reflected both by deterministic guidance as well as a relative peak in probabilistic rainfall data noted by 20-30% chance for 5 inches in the HREF. Considered a targeted MDT risk here, but antecedent soils are quite dry noted by USGS data indicating streamflows are below the 25th percentile. Despite heavy rain rates and amounts, the SLGT risk still seems appropriate and this was collaborated with WFO GSP and RNK. ....High Plains through the Northern Rockies... Another day of widespread convection is likely downstream of a longwave trough ejecting slowly across the Intermountain West. Significant mid-level ridging elongated across the Great Lakes will gradually shift east allowing for at least some eastward translation of this trough, but at the same time it will be slowed by a reinforcing shortwave deepening and dropping across the Pacific Northwest. Downstream of the longwave trough and pinched between it and the high pressure, low-level meridional flow will intensify noted by 850mb winds surging to 20-30 kts across the Southern Plains, and 40+ kts farther north into the Dakotas and Montana. This air will originate from the Gulf of Mexico as return flow develops around the high to the east, helping to drive PW anomalies of +1 to +2 standard deviations northward as far as the Canadian border. This helps to lengthen the moisture transport vectors northward, coincident with increasing MUCAPE during the aftn/eve which is progged to reach 2000-3000 J/kg along a ribbon draped across much of the High Plains. While this setup should support scattered convection across a large area once again, there may be two areas of focus for a greater chance of flash flooding. The first is across Montana where rainfall on D1 plus 7-day rainfall anomalies of 200-300% of normal produce more favorable antecedent conditions for flash flooding. Additionally, low-level flow will back to the east around an area of low pressure, leading to some enhanced isentropic ascent atop a stalled front, while also increasing the upslope ascent into higher terrain. This moist ascent combined with reduced storm motions in an area of greater storm coverage could result in locally 2-3" of rainfall as forecast by the HREF probabilities reaching 10-20% for more than 3 inches. The inherited SLGT risk was adjusted cosmetically to account for this new guidance. Farther to the south across the TX Panhandle and Rolling Plains, a secondary area of greater flash flood risk may exist as reflected by HREF probabilities for more than 3 inches of rain reaching 50%. This area is also pre-saturated noted by higher USGS streamflow anomalies. While convection is likely here on D2 (and also on D1 to further pre-condition the area), there is uncertainty into both coverage and speed of storms Saturday, with scattered forward propagating cells likely. After coordination with WFO LUB, held off on any upgrade at this time, but a SLGT risk may be needed with future updates. ....In and near Northern California, southern Oregon and northwest Nevada... The inherited small MRGL risk was adjusted slightly, but overall minimally. A shortwave dropping south out of the Pacific Northwest will result in some subtly enhanced ascent and moisture centered near the Shasta/Trinity region of northern CA. PW anomalies exceeding +1 standard deviation are likely, and 0-6km mean winds are weak suggesting slow storm motions. However, bulk shear is quite low to prevent much organization, HREF probabilities for even 0.5"/hr are modest, and antecedent conditions are quite dry. Still, a non-zero threat exists for isolated flash flooding, especially where any storms can tie to the terrain in the 500+ J/kg of SBCAPE, which would reduce motion even further. For this reason the MRGL risk was maintained. Weiss Day 3 Valid 12Z Sun May 28 2023 - 12Z Mon May 29 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE BLUE RIDGE OF NORTH CAROLINA AND VIRGINIA.. ....Carolinas and Virginia... The surface low moves onshore Saturday evening and then weakens, almost in place, Sunday, as it becomes vertically stacked across NC. The entire system is likely to decay by the end of D3. However, at least during the first half of D3, Sunday and Sunday evening, periods of heavy rain producing showers are likely, especially in NC and VA. While the onshore flow will weaken, noted by 850mb flow falling to 15-25 kts, NAEFS ensemble tables still suggest v-wind anomalies of -2 to -3 standard deviations driving PWs of around 1.25" inland coincident with up to 500 J/kg of MUCAPE. This should still support rain rates of 1"/hr or more streaming westward and training at times. Despite some of this rain falling atop soils that are almost guaranteed to be fully saturated from D1-2 rainfall, the coverage and intensity suggest a MRGL risk is sufficient to cover the excessive rain risk for most of the region. The exception to this is across the Blue Ridge of NC and into southern VA where an additional 1-3" of rainfall is possible, noted by GEFS and ECENS probabilities, thanks to additional upslope enhancement. This heavy rain falling atop soils saturated from D2 and into the more sensitive terrain could result in more scattered instances of flash flooding, and after coordination with RNK/GSP, a targeted SLGT risk was added for the eastern slopes of the region. ....Rio Grande Valley northward through the Northern High Plains... A persistent and slow moving pattern driven by a trough to the west and ridge to the east will promote another day of scattered to widespread showers and thunderstorms from Texas to the Dakotas. Once again, the guidance is struggling to point out any specific area that has a greater chance for convection, resulting in another broad MRGL risk for excessive rainfall. Return flow out of the Gulf of Mexico will continue to surge above normal PWs within this narrow north-to-south corridor, supporting heavy rainfall in any convection that develops in response to a ribbon of elevated SBCAPE reaching 2000 J/kg. The flash flood risk will be somewhat dependent on where rainfall is most intense on D1 and D2 as well, limiting confidence despite some signals for more than 3" of rain D3 on the GEFS and ECENS ensemble probabilities. For this reason the MRGL risk was generally unchanged, but an expansion across the RGV was added after coordination with WFO BRO as some convection coming off the Sierra Madre may dive southeastward into the better moisture producing a heavy rain threat there late in the forecast period. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4dOkw3fQCLqX5Up2cnVX1oENEEfOkeMswFGA52xyFD9W= PSjELImEnl1C4-ZDu3soahWl5tBI2gxTutPptIVMwrEM-iM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4dOkw3fQCLqX5Up2cnVX1oENEEfOkeMswFGA52xyFD9W= PSjELImEnl1C4-ZDu3soahWl5tBI2gxTutPptIVMAmNucGg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4dOkw3fQCLqX5Up2cnVX1oENEEfOkeMswFGA52xyFD9W= PSjELImEnl1C4-ZDu3soahWl5tBI2gxTutPptIVMI74MPvo$=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) .