Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri May 26 2023 15:52:09 FOUS30 KWBC 261552 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1151 AM 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 ACROSS COASTAL AREAS IN NORTHERN SOUTH CAROLINA AND SOUTHERN NORTH CAROLINA... ....Southeast and Lower Mid Atlantic... Based on the latest guidance trends and collaboration with WFOs ILM, MHX, and CHS, have made minor tweaks to yesterday's Day 3 Slight Risk area, while hoisting a Moderate Risk over coastal locations in northern SC-southern NC (including Myrtle Beach and Wilmington). As was alluded to in the Day 1 discussion, the more appreciable deep-layer instability is expected to expand into coastal locations after 12Z Sat (i.e. MUCAPEs 500-1000 J/Kg). Meanwhile, continued robust low-level east to southeast inflow (50-60 kts), will remain between 5-6 standard deviations above normal per the GEFS and SREF through 00Z Sunday. This will maintain robust (anomalous) low-level moisture transport/flux. Since this is expected to be a cyclone with some depth of a warm core per cyclone phase space diagrams and conceptual models, the low-level inflow slightly exceeds the magnitude of the 850-300 hPa flow, which when combined with a period of instability would favor cell training, repetitive banding, and/or cell mergers as the surface low slowly migrates inland. Much of the guidance advertises local amounts of 5+ inches, particularly within the NC Coastal Plain (including the Wilmington area) which has had above normal rainfall over the past week. ....Northern Rockies and High Plains... An upper level trough across the region broadens and shows a little bit of forward progression when compared to Friday. A localized or isolated flash flood threat possible anywhere within the broad risk area as CAPE rises above 2000 J/kg within a broad region of 0.75" (at elevation) to 1.25-1.5" closer to the 100th meridian. Montana will be aided by upslope flow for portions of the time frame, while a more enhanced flash flood threat (i.e. upgrade to Slight Risk was noted across portions of central-north central and northeastern MT, given the recent heavy rains and latest model deterministic and probabilistic QPFs. There's a somewhat coherent signal for heavy rainfall across western TX, but for the moment, enough spread to preclude the addition of a Slight Risk area. While there's better agreement on heavy rainfall near the NE/WY border, magnitudes in the guidance do not appear high enough at the moment for the inclusion of a Slight Risk area. ....In and near Northern California, southern Oregon and northwest Nevada... A closed low drops into the region from the northwest. This feature will lure precipitable water values up to ~0.75" and increase CAPE to 500-1000 J/kg. The concern would be for convection tied or backbuilding back to the higher terrain. Local amounts in the 1-2" are anticipated, which would be problematic, particularly in areas with snowmelt. Added a Marginal Risk area to cover this scenario. Roth/Hurley Day 3 Valid 12Z Sun May 28 2023 - 12Z Mon May 29 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE PLAINS AND LOWER MID ATLANTIC STATES... ....Northern Rockies and High Plains... The blocky, stagnant upper level pattern leads to a fairly uniform excessive rainfall risk on Day 3 (Sun-Sun night), though compared to Days 1-2, shifting ever-so-slightly eastward across the Plains. Once again, the guidance shows spotty/isolated areas of higher QPF given the convective mode, however at this point given the spread in the models with these convective bulls-eyes, at this point prefer to maintain the Marginal Risk area from yesterday's experimental Day 4 ERO. ....Southeast and Lower Mid Atlantic... The surface low center moves inland across the Carolinas on Day 3 (Sun-Sun night), then weakens considerably as it becomes vertically-stacked with the mid and upper lows. Therefore deep-layer kinematics and advection diminish considerably, as does the moisture transport/flux. Model areal-average QPFs therefore are substantially lower compared to Day 2, however as per the ECMWF and GFS, greater available deep-layer instability on Day 3 (MUCAPEs 1000-2000 J/Kg), owing largely to the steepening mid level lapse rates (7 to 7.5 C/KM), will allow for localized higher rainfall totals to the tune of 2-3+ inches within a couple hours. Therefore expect a more isolated flash flood threat on Day 3, and as such have maintained the Marginal Risk area that was inherited from yesterday's experimental Day 4 ERO (though based on the guidance trends, have moved the outlook area north a little). Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5S4lNZCylUZcAXTUA9mf6-9pGE6d5snr-HSMFrnnxWZC= E0zqyIVLSCZj25XXjJX5qWD2KZaYC0tiDKL5scaIWqcHHHo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5S4lNZCylUZcAXTUA9mf6-9pGE6d5snr-HSMFrnnxWZC= E0zqyIVLSCZj25XXjJX5qWD2KZaYC0tiDKL5scaIHBQuL48$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5S4lNZCylUZcAXTUA9mf6-9pGE6d5snr-HSMFrnnxWZC= E0zqyIVLSCZj25XXjJX5qWD2KZaYC0tiDKL5scaI7tHAvQY$=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) .