Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Aug 13 2023 00:00:12 FOUS30 KWBC 130000 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 759 PM EDT Sat Aug 12 2023 Day 1 Valid 01Z Sun Aug 13 2023 - 12Z Sun Aug 13 2023 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHEAST & CENTRAL PLAINS... ....South-Central Colorado east through the Oklahoma Panhandle and Southwest Kansas... A small MCS is trying to organize as it moves towards southwest KS. This is along a stalled out front draped west to east from Colorado all the through the Northeast. ML CAPE values are up to 3500 J/kg ahead of the complex across portions of southwest, central, and southeast KS and the environment has effective bulk shear of 30-50 kts. Increasing 850 mb winds between 03-06Z above 30 kt will likely coincide with expanding convection, out ahead of the MCS advancing eastward through the central Plains, supporting potential for repeating and training within its preceding warm advection pattern. Portions of KS been wet during the past week, with up to 600% of their average seven day rainfall. As the complex moves into increasing instability in the short term, the developing MCS may grow further and allow for cell mergers at its leading edge and backbuilding to locally enhance rainfall amounts over the area. Hourly rain totals up to 2.5" in possible in this environment. The excessive rainfall areas have been hemmed in from the west due to the heavy rain threat ending in some areas per radar reflectivity and 18z HREF trends. The 18Z HREF probabilities show the potential for local 5" amounts in KS, which could occur where cell training materializes, backbuilding occurs, cell mergers occur, or where mesocyclones align for a couple hours. ....Northeast... A fast-moving shortwave trough will cross northern NY through New England into tonight. North of the Slight Risk areas, the shortwave should be moving plenty fast enough to keep associated shower and thunderstorm activity also moving at a rapid pace. Thus, despite very favorable antecedent conditions in the form of wet soils across northern NY and northern New England, the Marginal was kept in place. In the Slight risk areas across southwest New England, southeast NY, and southwest PA, leftover energy on the southern periphery of the shortwave will linger back across the region. ML CAPE values maximize currently in the 1500-2000 J/kg range. Increasing moisture advecting in from the south bring PWATs towards 1.75". The influx of moisture and stalled out upper level energy could support backbuilding and training convection. The storms will be capable of hourly rain totals to 2" with local amounts to 4", and somewhat favorable antecedent conditions could result in more widely scattered flash flooding across this area. The Slight Risk areas were nudged based on 18z HREF and recent radar reflectivity trends. ....Mogollon Rim/Great Basin... Convection across this region should last for a few more hours. Storms in recent days have been successful at generating flash flooding in limited areas, and a repeat day as yesterday is likely once again as far as overall storm coverage is concerned. ....Central Sierra Nevada... No changes were made to the inherited Marginal Risk area. QPF values are underwhelming for 24-hour totals in this area, but the second straight day of convection in this area could support additional isolated flash flooding this afternoon and evening. Roth/Otto/Wegman Day 2 Valid 12Z Sun Aug 13 2023 - 12Z Mon Aug 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN EASTERN KANSAS, SOUTHERN AND CENTRAL MISSOURI, AND THE LOWER OHIO RIVER VALLEY... ....Eastern Kansas through the Ohio Valley... Soil moisture values have increased across the KS/MO region relative to a week ago given heavy rain on Wednesday and Saturday morning. An additional round of heavy rain is possible tonight (Saturday night) into Sunday morning, followed by yet another round Sunday night. There could be the need for an upgrade to Moderate Risk across southeastern KS into southwestern MO if there is enough overlap of Saturday night's rain with the Sunday night event. Given some shifts in the model guidance over the past 24 hours, and unknowns of QPF heading through Sunday morning, will hold off yet again on a potential upgrade to a Moderate Risk for portions of MO, possibly southeastern KS. This area can likely be revisited for the Saturday night forecast cycle with higher confidence given the first of the two expected rounds of heavy rain over the next 36 hours will be ongoing. Areas of heavy rain are expected to be ongoing across KS/MO at the start of the period, but moving eastward into the Lower Ohio Valley by noon. As a result of the morning rains, an outflow boundary may set up in the vicinity of the Ozarks and serve as a focus for additional thunderstorms with daytime heating. However, the approach of a potent shortwave trough from the Northern Plains will send a cold front southeastward into KS/MO Sunday evening with an increasing low level jet (20 to 30+ kt) out ahead of the front from OK/KS into AR/MO and a much better chance for thunderstorms. Upper level diffluence and right-entrance-region jet divergence are expected to enhance lift across the region as the upper trough approaches late Sunday. While the cold front itself will be progressive, concerns about west-east training exist out ahead of the front within an axis of forecast precipitable water values in excess of 2 inches from eastern KS into MO and the Lower Ohio Valley. Outside of the FV3, there is good support from the 12Z components of the 12Z HREF for 3-5 inches from southeastern KS into southwestern MO for the 24 hour period ending 12Z Monday. Subsequently, Therefore, the Slight Risk in place on the previous ERO issuance was shifted south and west. Farther east into the Ohio Valley, a Slight Risk remains were multiple rounds of convection will be possible, atop portions of fairly wet antecedent conditions. While heavy rainfall doesn't appear to be as widespread compared to locations farther west, there will remain the potential for some 2-4 inch totals through the 24 hour period. ....Upper Midwest... Trends in the 12Z deterministic models have shifted a mid-level shortwave trough southward over the Northern Plains/Midwest Saturday night. As a result, northern portions of North Dakota into the Upper Mississippi Valley are expected to have little or zero instability in place to support high rainfall rates with the latest model suite shifting the QPF axis southward, which prompted southward adjustments to the existing Marginal Risk. A strengthening surface low will develop out ahead of the north-central U.S. shortwave and track across the upper Midwest, with an east-west oriented front extending from the northern Mid-Atlantc into IA/IL and low level moisture transport across the frontal boundary. The heaviest rainfall is expected just north and east of the low center from SD across south-central MN and into WI. While localized rainfall totals exceeding 2 inches are likely, that rainfall should be both spread out over a somewhat longer duration with 1-2 inches in 3 hours time. Portions of MN into WI have been exceptionally dry recently, so much of the rainfall is likely to be beneficial to the area but wetter antecedent conditions exist over eastern SD. Impacts from rainfall are not expected to be widespread enough to warrant a Slight Risk, but the area will continue to be monitored. ....Maine... A Marginal Risk was introduced for this update in part due to 7-day rainfall anomalies of 300 to 600 percent over Maine. South of a closed mid-level low moving through Quebec, a 6-12 hour window for heavy rain will exist ahead of a frontal boundary with instability of 1000+ J/kg forecast for portions of southern/coastal Maine. Rainfall rates of 1-2 in/hr will be possible which could locally exceed area FFG. Beyond 00Z, the front and threat for heavy rain should shift offshore, ending any remaining threat for flash flooding. ....Central Sierra Nevada... No changes were made to the Marginal Risk area in the Sierras with this update. Little change in moisture anomalies are forecast for the region on Sunday compared to Saturday although storm motions may be a bit higher on Sunday. A localized risk of flash flooding will likely remain for the Sierra Nevada on Sunday but trends will have to be monitored for possible expansion into adjacent areas of the Southwest. Otto/Wegman Day 3 Valid 12Z Mon Aug 14 2023 - 12Z Tue Aug 15 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE GREAT LAKES, AS WELL AS FOR THE CENTRAL AND SOUTHERN APPALACHIANS THROUGH SOUTHERN PENNSYLVANIA AND NEW JERSEY... ....Upper Midwest... A low tracking from the Upper Midwest to western New York on Monday will produce locally heavy rainfall across the Slight Risk area as it tracks eastward. Similar to D2, the models have trended southward for Monday with the mid-level shortwave compared to previous days and there is better agreement in the ensemble spaghetti heights now compared to 24-48 hours ago. Thus, the inherited Marginal and Slight Risk areas were adjusted southward with portions of northern WI, the U.P. of MI and northern Lower MI removed from the Marginal Risk given sufficient instability is not expected to be present to support high rainfall rates. Farther south, low level flow overrunning a warm front extending through the Ohio Valley will support elevated thunderstorms across southern Lake Michigan into southern Lower Michigan within an axis of weak forecast instability (12Z NAM/GFS/ECMWF) but locally up to 1000 J/kg. Left exit upper level jet support will coincide with peak daytime heating to support areas of high rainfall with rates of 1-2 in/hr embedded within PWs over 1.5 inches. ....Appalachians to Southern New Jersey... An MCS tracking out of the Ohio and Tennessee Valleys will move northeastward across WV on Monday, as atmospheric moisture increases to 2.25 inches of PW. This will allow for very efficient rainfall rates, locally exceeding 2 inches per hour across central WV Monday afternoon. A 40 kt southwesterly LLJ will supply continued ample moisture to allow storms to backbuild and train across far southwestern VA and WV in advance of a cold front moving through from the west. The inherited Slight Risk is largely unchanged over this region. Further north and east, only minor tweaks were made to the existing Slight covering the MD/PA border into southeastern PA and portions of NJ. Confidence on placement of heavy rain is lower than average for Monday, east of the Appalachians, especially given uncertainly in instability near and north of a west-east oriented warm front, connecting into the deepening cyclone over the Midwest. Similarly anomalous moisture, with PWs over 2 inches, will track northeastward up the East Coast with strengthening low level flow into at least the 30-40 kt range. Veering low level flow over VA should support a notable dry signature downwind of the Appalachians where the Slight Risk ends. A warm front will be pushing north through the day and especially overnight Monday night. This front will help support lift of that moisture-laden air mass across southern PA and NJ along with increasing instability. Placement of a triple point low as the occluded cyclone approaches from the west will aid in determining the best instability, forcing and a nocturnal surge in heavy rain, but considerable uncertainty exists with these features reducing confidence in flash flood potential east of the Appalachians. There was a southward extension to the previous Marginal Risk made to include all of the DelMarVa Peninsula and much of south-central to southeastern VA given very favorable thermodynamic profiles for heavy rain in forecast soundings given high PWs and wet bulb zero heights near 15 kft. ....Arizona/New Mexico/southern Colorado... The 12Z model consensus shows mid-level ridging building over the southern Rockies from Monday into Tuesday with deeper-layer steering flow decreasing to ~10 kt or less. Strong easterly flow will advect higher than average moisture up the terrain and into AZ/NM on Monday. Standardized precipitable water anomalies from the 12Z NAM/GFS indicate 1 to 2+ over NM, shifting westward into eastern AZ for Monday night. Shortwave disturbances tracking along the western periphery of the ridge will provide additional support for afternoon and evening showers and thunderstorms across this area. There remains the possibility for portions of the Marginal Risk needing to be upgraded to a Slight with future updates. Antecedent conditions will play a big role in this, as wetter soils will support more widespread flash flooding by Monday. There is still some uncertainty on available instability and rainfall totals for Monday/Monday night, so the Marginal Risk was left largely unchanged with just a small expansion eastward to include the El Paso metro. Otto/Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_FVdLjcz1htI5j-wOkwlqxYNGEE_iskfFzPQhaQJ9ziB= ArULu6t9gwm-NUUvtsRi832kes6ReynoZC4Rb1IGoiuuHZg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_FVdLjcz1htI5j-wOkwlqxYNGEE_iskfFzPQhaQJ9ziB= ArULu6t9gwm-NUUvtsRi832kes6ReynoZC4Rb1IGHhHyQrE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_FVdLjcz1htI5j-wOkwlqxYNGEE_iskfFzPQhaQJ9ziB= ArULu6t9gwm-NUUvtsRi832kes6ReynoZC4Rb1IGBHPn41o$=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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