Subj : HVYRAIN: Excessive Ra 2/2 To : All From : Mike Powell Date : Mon Jul 10 2023 16:09:00 ....Upper Midwest and Great Lakes... A cold front will dig southward from Canada today, but gradually become more elongated west to east across the northern tier of the CONUS. This evolution will be driven by a potent vorticity lobe rotating around the anomalous closed low near the Hudson Bay, with a secondary lobe swinging southward into MN late in the day help to stretch the front. By the end of D1 this front should finally eject southeast towards the Ohio Valley. This front will serve as a focus for showers and thunderstorms today 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 will overlap favorable thermodynamics for heavy rain characterized by MUCAPE of 1000-2000 J/kg and impressive moisture flux as 30kt SW 850mb inflow surges PWs of 1.25 inches into the boundary. As the front becomes stretched west to east, this will allow the mean 0-6km flow to become aligned parallel to it, suggesting an increased training threat from Wisconsin through the U.P. of MI. At the same time, Corfidi vectors turning more into the intensifying inflow become nearly anti-parallel to the mean wind and collapse to just around 5 kts suggesting at least some backbuilding potential into the higher instability. With rain rates progged by the HREF to exceed 1"/hr at times, this training/backbuilding scenario could produce areas of 1-3" of rainfall. Despite modest recent rainfall leading to FFG that is around 2"/3hrs, there are some pockets of more sensitive soils, especially in the U.P., but HREF exceedance probabilities peak only around 10-20%. The MRGL risk was maintained and adjusted cosmetically, as some isolated flash flood responses are possible during the most prolonged training today. ....Northeastern Washington through Montana... A shortwave ejecting out of the Pacific Northwest this morning will amplify into a closed low as it moves into Alberta, Canada, leaving increasing downstream mid-level divergence, which will overlap with the favorable LFQ of an upper jet streak to provide pronounced ascent across the Northern Rockies and Northern High Plains. This will intersect a cold front digging southward out of Canada, with increased moist flow advecting from the southeast along the front. This will push PWs to 1-1.25", more than 1 standard deviation above the climo mean according to NAEFS. The atmosphere will marginally destabilize ahead of this front, with MUCAPE possibly exceeding 500 J/kg in places. The overlap of this deep layer ascent into this narrow ribbon of favorable thermodynamics should result in scattered thunderstorms, which will have rainfall rates that may reach 1"/hr at times as progged by the HREF neighborhood probabilities. Storm motions will generally be quick to the east, but repeated rounds could produce some 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. 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. $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .