Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri May 26 2023 08:56:33 FOUS30 KWBC 260856 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 455 AM EDT Fri May 26 2023 Day 1 Valid 12Z 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... A quasi-stationary upper low upstream with rather robust upper difluence and along-stream upper divergence east of the low will favor the formation of a surface low over a favorable genesis region over the Gulf Stream waters off the concave SE US coastline. The relatively stagnant upper pattern (Rex Block with the closed upper dropping slowly through the Mid South) will make for a prolonged period of favorable deep-layer forcing and robust moisture transport, particularly across the SC and southern NC coast on Day 1 and over much of the Carolinas by Day 2. There is some spread in the guidance with respect to the QPF, though again the models confine the heaviest rainfall offshore (over the Gulf Stream) along with the more robust deep-layer instability. By the same token, the guidance brings 500+ J/Kg of MUCAPE and PWs ~1.75" closer to the coast after 00Z Sat, along with highly anomalous low-level easterly flow (50-60 kts at 850 mb by 09-12Z Sat, which is 5-6 standard deviations above normal). While the bulk of the heavy rainfall is expected to occur on Day 2 (after 12Z Sat), the rapid ramp-up in favorable deep-layer forcing and thermodynamic profiles Friday night supports a targeted Slight Risk area across coastal locations in northeast SC and southern NC. ....Great Basin-Northern Rockies-High Plains... A slightly progressive and broadening upper trough is forecast for the Western States. The deterministic and probabilistic QPF from the guidance support a localized or isolated flash flood threat possible anywhere within the broad risk area. Sufficient moisture combined with 2000+ J/kg of CAPE, and ample low-level inflow in the Plains will help aid the development of convection with heavy rainfall. A more enhanced risk lies across eastern MT, northeast WY, and western ND along/near the surface front, and over parts of eastern NM and the TX Panhandle/Cap Rock/Rolling Plains with the favorable upslope low-level flow. Both of the Slight Risk areas were expanded to account for recent heavy rainfall (including what has fallen more recently Thu night). The areas NM east into TX in particular have broad areas of 300-600% of average rainfall during this past week per the latest AHPS analysis. Hurley 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!8PAaymYlmnxKk_LL1r2rqY85LleDIq97EPx4WNKB1_Er= lzIEH7s7s2AxhHwKXA80eBTQ7lVDXKgGvaHf8LlNUIEuuUg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8PAaymYlmnxKk_LL1r2rqY85LleDIq97EPx4WNKB1_Er= lzIEH7s7s2AxhHwKXA80eBTQ7lVDXKgGvaHf8LlNfblHbUo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8PAaymYlmnxKk_LL1r2rqY85LleDIq97EPx4WNKB1_Er= lzIEH7s7s2AxhHwKXA80eBTQ7lVDXKgGvaHf8LlNgtNIYKI$=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) .