Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jul 11 2023 01:12:12 FOUS30 KWBC 110112 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 911 PM EDT Mon Jul 10 2023 Day 1 Valid 01Z Tue Jul 11 2023 - 12Z Tue Jul 11 2023 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE CHAMPLAIN VALLEY AND VERMONT... ....WIDESPREAD SIGNIFICANT FLASH FLOODING LIKELY THROUGH TONIGHT... ....New England and Upstate New York... A closed upper-low (500 mb) over Upstate NY will continue to pivot very slowly eastward through overnight, driving additional totals of 2-6" across already hard hit areas of the Northeast. These totals are most likely across the HIGH risk area, which has been slightly reduced with the evening update (along the southern end) while still encompassing much of the Champlain Valley and Vermont. With PWs of 1.7-1.9" (around +2 sigma according to the NAEFS ensemble tables and above the 90th percentile from the SPC sounding climatology), modest 250-750 J/kg of MUCAPE through a deeply saturated column, and warm cloud depths above 11,000 ft, warm/efficient rainfall rates as high as 1-2"/hr will continue overnight. These rates in themselves will be enough to overwhelm soils to result in runoff and flash flooding, but pre-saturated grounds will result in extremely efficient runoff from any additional rainfall. 14-day rainfall from AHPS has been 200-300% of normal across much of central and northern New England, leading to USGS streamflow anomalies that are almost uniformly above the 90th percentile, and in some places are well above all-time record flows. This has also resulted in FFG that is as low as 0.5", further indicating the sensitivity of the local soils. The HREF ensemble probabilities also continue to be concerning on their own overnight, with HREF neighborhood probabilities reaching of 40-80% for the 3" threshold, and 20-40% for the 5" (concentrated over the HIGH risk area). The HREF EAS probabilities provide increased confidence in the area of greatest concern, with 30-60% values for the 1" threshold (centered over the HIGH risk area). The western edge of the SLGT and MOD risk areas will clear the quickest into the overnight hours, but the pivoting upper-low will allow for the extended residence time over the HIGH risk area that will be capable of generating the highest totals in association with with favorable topographic forcing. The areas to the east of the HIGH risk (New Hampshire into Maine) will also see rainfall gradually end from west to east in association with a developing dry slot, but the residence time will be a bit longer (which is reflected by the extension of the MOD risk to the east). ....Southeast... A broad but amplified longwave trough extends down into the Southeast this evening, with the base of the trough lingering along the Gulf Coast. This has resulted in more flattened, almost zonal, mid-level flow across the Gulf Coast/Southeast, within which weak vorticity impulses embedded within shortwaves will continue to traverse eastward through the night. At the surface, a quasi-stationary front ripples from Texas to South Carolina, interacting with the weak impulses aloft to produce waves of low pressure along it. Although convective coverage has already decreased substantially with the most organized activity moving offshore into the Atlantic and Gulf of Mexico, instability remains rather elevated (ML CAPE 1000-3000 J/kg) with PWs as high as 2.25 inches (more than 2 standard deviations above the climo mean according to NAEFS) and effective bulk shear of 20-40 kts. This will allow for continued isolated to widely scattered convection overnight (before enough building convective inhibition results in continued decreasing coverage) with rainfall rates of 1-2"/hr. ....Southern Plains... An anomalous mid-level trough encompassing much of the eastern third of the CONUS, and an amplifying ridge centered over the Desert Southwest has left pronounced NW flow across the Southern Plains this evening. Within this flow regime, weak shortwave impulses and associated vorticity maxima are rotating down across the region to drive bouts of locally enhanced ascent. As the ridge to the west intensifies this evening, and a potent vorticity max shifts out of Colorado, low-level flow will back in response, driving a warm front northeastward and producing impressive WAA into an elevated boundary late. This WAA will surge PWs to around 1.75 inches, coincident with a ribbon of MUCAPE reaching 2000-3000 J/kg, providing robust thermodynamics for heavy rain producing convection, with thunderstorms likely organizing into an MCS through effective bulk shear of 40-50 kts. The high-res guidance continues to be in agreement that an MCS will develop, but as is typical with these subtle shortwaves in summer, the placement still varies widely. This is noted in high-res simulated reflectivity, as well as HREF EAS probabilities that are minimal for 1 inch. However, the HREF neighborhood probabilities for 1"/hr and 2"/hr rates peak along and north of the Red River Valley of the South into Oklahoma, which is also an area that has received heavy rainfall the past 7-day noted by AHPS departures that are 300-600% of normal. Storm motions may generally be progressive within this MCS, but some increasingly right-angled Corfidi vectors to the mean flow suggest at least short-duration training is possible, and the HREF 3-hr maximum probability for 3" exceeds 30% tonight. Locally, this event could produce 3-5" of rainfall, which if it falls atop pre-conditioned soils from recent MCS events could lead to instances of flash flooding. Confidence in the position of the SLGT risk has increased a bit, but the exact track of the resulting MCS is still fairly uncertain. The SLGT was expanded modestly northwestward, owning to recent trends in the HRRR since 18z. ....Upper Midwest and Great Lakes into the Central Plains... A cold front will continue to dig southward from Canada and the Upper Midwest this evening, having become more elongated west to east across the northern tier of the CONUS as lobes of vorticity rotate around the anomalous closed low near Hudson Bay. This front is serving as a focus for showers and thunderstorms as ascent maximizes through low-level convergence, modest height falls/PVA, and weak RRQ diffluence as a jet streak pivots southeast during the evening. This ascent overlaps favorable thermodynamics for heavy rain characterized by MUCAPE of 1000-2000 J/kg and impressive moisture flux as 20-30 kts of southwest 850mb inflow has surged PWs to 1.3-1.7 inches along and ahead of the boundary. The elongation of the front from west to east has allowed the mean 0-6 km flow to become aligned parallel to it, suggesting an increased training threat from Wisconsin through the U.P. of MI (whereas farther south into the Central Plains this orientation is more perpendicular with less risk of training). At the same time, Corfidi vectors turning more into the intensifying inflow are becoming nearly anti-parallel to the mean wind and collapse to just around 5 kts allowing for at least some backbuilding potential into the higher instability. With rain rates exceeding 1"/hr at times, this training/backbuilding scenario could produce additional areas of 1-3" of rainfall. Despite modest recent rainfall leading to FFG that is around 2"/3hrs across the region, there are some pockets of more sensitive soils, especially in the U.P. and into western Kansas.=20 ....Northeastern Washington through Montana... An upper-low and associated cold front digging southward out of Canada will continue to drive widely scattered convective activity through the evening and into the early overnight hours. Ingredients for convection include PWs of 0.8-1.2" (more than 1 standard deviation above the climo mean according to NAEFS), ML CAPE of 500-1500 J/kg, and deep layer shear of 20-40 kts. Rainfall rates will continue to occasionally reach and exceed 1"/hr, and storm motions will generally be quick to the east, but repeated rounds could produce additional areas of 1-2" of rainfall. While the FFG exceedance probabilities are modest, parts of MT have had excessive rainfall noted by AHPS departures that are 150-300% of normal leading to above normal USGS streamflows, while recent burn scars are also more vulnerable than depicted by FFG. Any repeating rounds of heavy rain could produce runoff and isolated instances of flash flooding. Churchill/Weiss Day 2 Valid 12Z Tue Jul 11 2023 - 12Z Wed Jul 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-MISSOURI BASIN... ....1930Z Update... ....Mid-Missouri Basin... Changes were overall fairly minimal for the Day 2/Tuesday period. An MCS tracking southeastward along a front will result in numerous showers and thunderstorms across much of western and central IA and small portions of the surrounding states developing around sunset and moving across the Slight Risk region largely overnight Tuesday night. The abundance of moisture available for the storms will be primary component driving the threat for flash flooding, as PWATs may exceed 2 inches, which is over 3 sigma above normal in southeastern NE. Fast storm motion will be the primary factor working against flash flooding, along with average soil moisture conditions in most of the Slight Risk area, and of course much of IA being farmland that is typically eager for rainfall. Thus, while this is considered a lower-end Slight risk, localized rainfall totals to 3 inches are possible. This is most likely to occur where cell mergers prolong the heavy rainfall rates over a given area. There is some hint, particularly in the 12Z HRRR guidance that some rain may get started as far east as Chicago in the predawn hours Wednesday morning, however at the moment that Hi-Res model is the only one hinting as such, so for now the Marginal Risk area maintained through northern IL will remain as a Marginal. Much of the rainfall event here will occur into Day 3/Wednesday. The Marginal Risk along the Gulf Coast remains largely as inherited as well. The primary concerns are for New Orleans and Baton Rouge for flash flooding, with much of the rest of LA and the Gulf Coast not expected to pick up enough rainfall to induce flash flooding, due to high FFGs. Afternoon convection occurring into eastern MI around and north of the Detroit area will occur over an area that has seen around 200% of climatological normal rainfall over the past 2 weeks. Thus, a small Marginal Risk was added to account for the isolated flash flooding potential. The Marginal risk area across northern New England was trimmed hesitantly given the ongoing major flooding situation, as the rain may conclude in this area a bit sooner than previous forecasts. Always a good thing. Wegman ....Previous Discussion... ....Northern Plains into the Upper Midwest... Squeezed mid-level flow across the Northern Rockies will dive into NW flow over the Upper Midwest as a broad trough persists across the eastern CONUS. Shortwaves and accompanying vorticity maxima rotating through the flow will advect eastward and then drop southeast, joining the LFQ of an upper jet streaking from the Pacific to produce robust ascent across the area. This synoptic lift will interact with a wavering frontal boundary draped northwest to southeast from MT through MO, with increasing low-level S/SE flow impinging into this boundary. This low-level inflow will reach 15-25 kts, surging PWs to 1.25-1.75 inches, more than +1 standard deviation above the climo mean according to NAEFS, to provide substantial available moisture for heavy rainfall. Additionally, instability will climb to 1000-2000 J/kg within a ribbon lifting north along the front, with a steep CAPE gradient developing from SW to NE during this time. This overlap of forcing and moisture/instability should result in scattered to widespread thunderstorms along the front, with the greatest excessive rain threat likely developing through MCS development late in the forecast period. Thunderstorms that develop along the front from eastern MT through IA will likely have rainfall rates of 1"/hr or slightly higher within the favorable thermodynamics. The available high-res keeps activity generally scattered along the front, and storm motions using the 0-6km mean winds as proxy should be relatively quick, but along the front to support at least short-term training. This could result in isolated runoff responses, especially over more sensitive soils from 14-day rainfall that is 150-300% of normal in some areas. However, the greater flash flood risk exists from far eastern SD through northern MO where the guidance suggests an MCS will develop and potentially train along the instability gradient. There continues to be spread in the timing and placement of this MCS, but it is likely one will develop. At this time, the SLGT risk was tailored to best match the 12-hr HREF and 24-hr GEFS probabilities for 1 inch and 3 inches, but additional adjustments are likely. ....Northern New England... The excessive rain event from D1 will wane during D2, but guidance indicates the deformation will persist across northern VT/NH/ME during the first few hours of this forecast period. Additional scattered showers are possible through the day across central/northern New England as well before much drier air advects eastward into New England to shut off the rain. The ECENS/SREF/GEFS probabilities all suggest at least modest exceedance for 1" of rainfall, with briefly heavy rates likely within this pivoting deformation axis thanks to MUCAPE of 500-1000 J/kg. The rainfall in itself on D2 is not likely enough in any location to produce flash flooding. However, the soils will likely be extremely saturated from heavy rain and possibly significant flash flooding during D1, so any additional rainfall could result in further instances of flash flooding. The MRGL risk was trimmed to match updated guidance, but at least an isolated threat still exists before the system exits by late D2. ....Southern Plains through the Gulf Coast... A stalled cold front aligned along the Gulf Coast will weaken and dissipate into a trough Tuesday, but still provide a focus for ascent and thunderstorms during peak heating. A broad trough still encompassing much of the eastern CONUS will maintain an axis down to the Gulf of Mexico, producing subtle height falls and steepened lapse rates to help drive MUCAPE to 2000-3000 J/kg during the aftn/eve. This impressive instability combined with PWs still progged to reach around 2 inches will produce a sufficient environment for heavy rain rates exceeding 1"/hr. The overlap of highest PWs and greatest MUCAPE does not look ideal on D2 except near Louisiana and the northern Florida peninsula, so the excessive rain risk appears modest. However, rain rates of 1+"/hr within these favorable thermodynamics slowly training along the front through parallel 0-6km mean winds of just 10 kts, could produce locally 1-3" of rainfall as progged by HREF, SREF, and GEFS probabilities. The MRGL area is quite broad as determining a focus for greatest threat is challenging, but a subtle focus may exist along the sea breeze/collisions from eastern Louisiana through the western Florida Panhandle where isolated higher rainfall amounts are possible. Weiss Day 3 Valid 12Z Wed Jul 12 2023 - 12Z Thu Jul 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE MID-MISSISSIPPI VALLEY AND SOUTHERN GREAT LAKES... ....1930Z Update... No significant changes were needed to the Risk area across the Midwest for the Day 3/Wednesday period either. Southern portions of the Slight for MO and southern IL are looking to get a 1-2 punch from 2 separate MCS's. The first MCS will be ongoing from the overnight period of Day 2/Tuesday night and will move down the Mississippi River. Meanwhile a stalled out front will keep a portion of that shower and thunderstorm activity ongoing off to the east from the Chicago area east through Detroit and into the Finger Lakes of western NY by Wednesday night. Meanwhile a second MCS will develop Wednesday evening along the IA/MO border and intensify as it tracks southeastward across eastern MO and southern IL overnight Wednesday night. Most indications are that this second round of storms will be more intense than the first, and the heavy rain expected during the overnight period will increase the chances for impacts resulting from any flash flooding, particularly in the St. Louis area. Wegman ....Previous Discussion... ....Mid-Mississippi Valley through the Great Lakes... Elongating ridge over the Desert Southwest and continued expansive but broad troughing across the east will drive persistent NW mid-level flow over from the Northern Plains through the Ohio Valley. Within this regime, subtle shortwave impulses and associated vorticity maxima will rotate E/SE, combining with the diffluent LFQ of a zonally oriented jet streak to produce waves of synoptic ascent. At the same time, a surface stationary boundary will waver nearly in place, aligned from Nebraska to Indiana, such that favorable overlap of the synoptic lift will occur with low-level convergence along the front. This setup will support scattered to widespread convection on Wednesday. During the day, return flow around a high pressure over the Appalachians will surge moisture northward out of the Gulf of Mexico. 850mb winds will steadily increase to 20-30 kts, possibly higher Wednesday night, impinging favorably into the front to produce additional isentropic upglide. This inflow will also advect robust thermodynamics northward, with PWs rising above 2 inches and MUCAPE possibly exceeding 3000 J/kg. This will support widespread thunderstorms during the day with intense rainfall rates above 1"/hr extending from Missouri through Michigan along the front. These storms should be progressive along the front, but mean winds aligned to the boundary could support training to produce flash flooding. More concerning is the potential for a potent MCS to dive along the front late D3. Uncertainty is high in the placement of this feature, but most available guidance suggests the shortwave aloft will interact favorably with the CAPE gradient and the baroclinic zone to produce an MCS diving to the southeast in the vicinity of MO/IL. Some of the guidance is quite aggressive with rainfall as effective bulk shear of 40-50 kts and Corfidi vectors pointing from the higher instability pool allows for storms to impressively organize and backbuild to the west. With rain rates likely 1-2"/hr or more, this could result in an axis of very heavy rainfall, and GEFS/ECENS/SREF probabilities all support an area of 2"+ rainfall despite a lot of spread in position. This will need to be monitored with later forecast packages as it is possible a targeted MDT risk area may be needed with later updates as the event comes into focus. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4LFk3rSe8R4cjY7sEELFkOW5K5s-dPruJbOkrYN8yzrb= JM0feg2A1FADcFfo6BAwZFUy4SVgxVBTHYnbX5XH4PD1vpg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4LFk3rSe8R4cjY7sEELFkOW5K5s-dPruJbOkrYN8yzrb= JM0feg2A1FADcFfo6BAwZFUy4SVgxVBTHYnbX5XH4iJcfzg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4LFk3rSe8R4cjY7sEELFkOW5K5s-dPruJbOkrYN8yzrb= JM0feg2A1FADcFfo6BAwZFUy4SVgxVBTHYnbX5XHESe0m8Y$=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) .