Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat May 27 2023 20:13:03 FOUS30 KWBC 272012 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 412 PM EDT Sat May 27 2023 Day 1 Valid 16Z 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 AS WELL AS PARTS OF THE CAPROCK OF TEXAS... ....Carolinas... Compact and newly occluded low pressure off the SC coast will slowly maintain strength and lift onshore the SC coast later this evening and overnight 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 exists across the Great Lakes and Northeast, producing a tightening pressure gradient across the area. This results in intensifying easterly winds through a deep layer of the atmosphere, leading to strong moist advection and instances of heavy rainfall. NAEFS ensemble tables indicate the negative U-wind or easterly component will become as strong as 4 to 5 standard deviations above normal across much of the eastern Carolinas on Saturday, surging integrated water vapor transport or IVT anomalies between +3 and +4 and PW anomalies to +1.5 to +2. Aiding in this robust moisture transport will be 850mb winds reaching 35-50 kts, with the resultant WAA driving warm cloud depths to around 13,000-14,000 ft, sufficient for optimal warm rain processes. This evolution will release 500-1000 J/Kg of MUCAPE across along the coast from coastal SC into NC. Meanwhile, with 850mb winds peaking around 2x the mean of the 0-6km winds will allow convective bands to back-build across the Gulf Stream, leading to cell training. Together, these parameters produce an environment extremely supportive for heavy rain along and just inland from the Carolina coasts, supported by HREF exceedance probabilities. The 12Z HREF in fact shows several hours of elevated probabilities (40-60+ percent) of >1"/hr rainfall rates migrating slowly northeast across the Moderate Risk (especially the eastern half) area during much of the day 1 period (through ~10Z Sun). Meanwhile, probabilities of >2"/hr rates peak between 25-35 percent across the southern NC Coast (around Cape Fear) between 13-18Z Sat. Probabilities of the 24hr QPF exceeding 3" peaks above 50 percent over much of the NC coast within the Moderate Risk area, while probs of >5"/24hr peak between 60-70% over coastal southern NC along and north of Cape Fear. 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. The greatest southerly push in 850 flow will be to the east of the surface low and across parts of the Crystal Coast of NC, which will allow for MUCAPE to potentially increase to 1000-1500 J/kg, supporting more intense rainfall rates while also overlapping with the highest training potential. Where these heavy rates can train, rainfall will likely quickly eclipse 2-4" through this tonight, with locally higher amounts of more than 8 inches possible per the latest CAM output, highest near Cape Fear and the eastern half of the Moderate Risk, where the aforementioned HREF probabilities are greatest and frictional convergence onshore could also enhance ascent. 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 20-25% probs for 5 inches per the 00Z HREF. The 16z update included the removal of the Slight Risk across many inland areas of central NC and SC due to lack of instability and expected rainfall rates under an inch per hour while showers move quite progressively until reaching the higher elevations of the southern Appalachians. There was also a small southward shift in the risk areas near the eastern SC/GA border due to recent radar trends. ....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. Farther to the south across the TX Panhandle south into the Caprock, Rolling Plains, and northern TX Hill Country, a secondary area of greater flash flood risk may exist as reflected by 12Z HREF probabilities for more than 3 inches of rain reaching 50%. Numerous areas of convection developing off the Front Range of the southern Rockies will progress eastward this evening, with a few to numerous MCSs, some of which could exhibit training on the southwest flank overnight. Depicting where exactly this training develops is nearly impossible, but the Slight Risk encompasses the area of greatest risk, while the Moderate Risk includes where the potential overlaps with saturated ground conditions. If higher rainfall rates and training cells overlap with the sensitive terrain, significant instances of flash flooding are possible. Within the broader risk of excessive rainfall spanning from the TX Panhandle to the Big Bend, a region centered over Lubbock has experienced rainfall amounts of 4-10"+ over the last two days. This leads to FFGs well within reach and supports an upgrade to a Moderate Risk. The 12z HREF highlights this area as having 30-50% chances for 3-hr QPF to exceed 3-hr FFG. Then across Montana and northwest ND where rainfall over the past 24hrs along with the 7-day rainfall anomalies of 200-300% of normal per AHPS produce leads to more favorable antecedent conditions for flash flooding. Additionally, low-level flow will back to the east around an area of low pressure, presenting 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 Slight Risk was expanded a bit to the east to include more of eastern and northeast MT and a small portion of northwest ND, accounting for recent heavy rainfall (ongoing this morning) along with trends int the newer guidance. ....Northern California, southern Oregon and northwest Nevada... 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. ....Northern Mississippi to southwest Tennessee... A Marginal Risk was introduced with the 16z update across parts of northern MS, southwest TN, and extreme eastern AR due to ongoing slow-moving convection combined with another round possible this evening. Storms this morning in the area have contained radar estimated instantaneous rainfall rates up to 2-3"/hr and totals as high as 3-4" in very localized spots (not verified by observations as of 1530z). 12z HREF neighborhood probabilities depict 40-60% chances for an additional 2" of rain by 03z tonight over similar areas, prompting the upgrade. Snell/Hurley Day 2 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 AS WELL AS PARTS OF THE EDWARDS PLATEAU AND SOUTH-CENTRAL TEXAS.. ....Central Appalachians into parts of the Mid Atlantic... The surface low off the Carolina coastline moves onshore near the northern SC shores Sunday morning and then weakens, almost in place while drifting slowly northward, as it becomes vertically stacked across NC. Even though the surface cyclone and 850 mb inflow will weaken substantially, periods of heavy rain producing showers are likely, especially on the western edge of the mid-level circulation from the higher elevations if western NC to nearby southwest VA. For areas farther east, additional organized north-south bands of convection are possible in eastern NC Sunday morning within an environment of 1.5" PWATs. Despite some of this rain falling atop soils that are almost guaranteed to be fully saturated from the previous day's rainfall, the coverage and intensity suggest a MRGL risk is sufficient to cover the excessive rain risk for most of the region. Other isolated instances of localized heavy rain are possible across central NC and southern VA into the far eastern Ohio Valley and southern WV associated with low level fgen and ~500 J/kg of MUCAPE, supporting a Marginal Risk. More organized convection falling atop sensitive terrain is possible across the Blue Ridge of NC and into southern VA, where 1.5-3" of additional rainfall is likely per the latest guidance, due in large part to the additional upslope enhancement. This heavy rain falling atop soils saturated from Day 1 and into the inherently flashy terrain could result in more scattered instances of flash flooding. 12z HREF neighborhood probabilities depict 20-30% chances for 2" of rain in 6 hours into Saturday evening, which provides enough confidence to maintain the previous risk area. ....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. Localized rainfall totals of 1-3" are possible throughout much of the MRGL risk area, which was trimmed on the western border from CO to MT in order to account for a more progressive trend in latest guidance. An elevated risk for even heavier totals has emerged across parts of the Edwards Plateau and south-central TX, thus a SLGT Risk being introduced across the region. Here, PWATs are expected to nose as high as 2" along the Rio Grande River Valley while MUCAPE also increases to 2000-2500 J/kg per the 12z HREF. Shortwave approaching out of northern Mexico and potential lingering surface boundaries from previous day's MCS activity could provide focus for development and brief training of higher rain rates in this area. FFGs across the Edwards Plateau and surrounding regions are very low (1-2"/hr and 1.5-2.5"/3-hr). 12z HREF neighborhood probabilities of 20-40% for 3" QPF within 6 hours are scattered throughout the new Slight Risk area. Snell/Hurley Day 3 Valid 12Z Mon May 29 2023 - 12Z Tue May 30 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE SOUTHERN PLAINS... ....Parts of south-central Oklahoma southward to the Rio Grande Valley and the Texas Gulf Coast... Weak, broad mid-upper level trough with embedded convectively-enhanced shortwave energy will make for a favorable dynamical and thermodynamical environment for more numerous thunderstorms with embedded localized areas of more intense rainfall. There is a somewhat more enhanced signal in the guidance for more organized heavy rain farther south (Rio Grande Valley and Lower-Mid Texas Coast), however at this point given the weak kinematic forcing, not enough to support a Slight Risk. Prior MCS activity across south-central TX will also likely influence where additional rounds of thunderstorms set up by the D3 period, but those details remain uncertain at the moment. The inherited Marginal Risk was tightened on both sides to account for latest guidance honing in on the areas most likely to see the potential for 1-2"+ totals within an environment containing 1.5-2" PWATs. Snell/Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9ywg9m8nHrg8n3FrV6Xei6rUsBOkCiLdnIYBC5CenRGH= VCrLDw7p1_w9vj3lUBp7Tz9tNSRKs1D0_UhdUTrFiGFFI6c$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9ywg9m8nHrg8n3FrV6Xei6rUsBOkCiLdnIYBC5CenRGH= VCrLDw7p1_w9vj3lUBp7Tz9tNSRKs1D0_UhdUTrFkARH4AU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9ywg9m8nHrg8n3FrV6Xei6rUsBOkCiLdnIYBC5CenRGH= VCrLDw7p1_w9vj3lUBp7Tz9tNSRKs1D0_UhdUTrFSeoKsc0$=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) .