Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jun 29 2023 00:18:03 FOUS30 KWBC 290017 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 817 PM EDT Wed Jun 28 2023 Day 1 Valid 01Z Thu Jun 29 2023 - 12Z Thu Jun 29 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NEW ENGLAND... ....New England... An upper trough has lingered over the Northeast, channeling both deep moisture and mid-level energy across portions of the region. PWs are forecast to generally remain near or above 1.5" in the deep southerly flow near its surface front. Moderate to heavy rainfall has occurred within the Slight Risk area, and 18z HREF guidance indicates it has a few hours of life before CIN sets in.=20 Until it does, south to north training near the front due to unidirectional southerly flow remains possible. An approaching shortwave swinging into northern New England could shift activity eastward and also play a role in ending the flash flood/excessive rainfall threat. ....Upper Midwest... Mid-level energy emanating from the trough in the West, along with an amplifying trough approaching from the northwest over central Canada are expected to increase the coverage of showers and thunderstorms over northern WI and bordering sections of MN tonight into early Thursday. A warm advection pattern interacting with a ribbon of deeper moisture (PWs around 1.5 inches) helps lead to an environment conducive for heavy rainfall. While individual storms are likely to be progressive, some of the guidance does show some potential for west-east training, which may produce locally heavy amounts and an isolated flash flooding threat. Streamflows and recent precipitation anomalies are running on the dry side, supporting the flash flood risk as only an isolated threat. ....Central Plains through the Mid Mississippi Valley... To the north of a 594 dam upper-level ridge parked over the Lower Mississippi Valley, fast upper level flow with embedded subtle shortwaves will converge over the Midwest and towards a more stable atmosphere centered over the Ohio Valley. An instability gradient extending from eastern Iowa to the central Gulf Coast could provide focus for thunderstorms approaching from Nebraska to possibly anchor within the instability gradient overnight, once the low-level jet causes an increase in 850 hPa flow into the region. Uncertainty remains within the hires guidance on where thunderstorms will develop and if they will be able to grow upscale and train. The flash flood threat will be greatest should convection form and move along the better instability gradient near southeast IA and far northeast MO, very close to the effective mid-level cap edge indicated by the 700 hPa 12C isotherm. PWs of 1.5-1.75" within the region and a nearby warm front will allow for any rain rates to reach up to at least 2"/hr and linger over similar areas. This setup has high-end potential and not out of the realm of possibility for a corridor of 4-6" of rain -- reaching the Slight Risk threshold -- should conditions come together for a training band of storms along the western periphery of an MCS (where upwind Corfidi vectors become very weak by 12z Thursday per the 06z GFS). However, there remains far too much uncertainty on whether enough training, backbuilding, or enough mesocyclones can align for a higher risk level. The Marginal risk area highlights the area where HREF probabilities for heavy rainfall are relatively higher. Depending upon what unfolds, this may impact the new upcoming Day 1 ERO issued early Thursday morning. Roth/Snell/Pereira Day 2 Valid 12Z Thu Jun 29 2023 - 12Z Fri Jun 30 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL THROUGHOUT PORTIONS OF THE NORTHEAST AND SOUTHEASTERN FLORIDA, AS WELL AS ACROSS THE NORTHERN ROCKIES, CENTRAL PLAINS, AND INTO THE LOWER OHIO VALLEY... ....Northeast... One more day of scattered showers and storms, with locally heavy amounts possible, are expected for portions of northern New England as the slow moving upper trough finally exits to the northeast. While the threat will likely be on the downward trend as forcing weakens and drier air enters the region, additional localized 1-2" rainfall amounts are still possible over mostly saturated terrain. The 12z HREF highlights the White Mountains of New Hampshire and Maine as having the greatest chances for over 2" of rain within a 6-hour time period, with neighborhood probabilities between 40-60%. ....Florida... An axis of 850mb convergence off the southeast FL coast combined with upper level shortwave activity entering from the north while rounding a strong upper ridge over the Lower Mississippi Valley will mix with sea breeze activity to produce potentially heavy rain over the urban corridor of southeast FL. PWs near 2" will provide enough moisture for the slow moving storms to possibly contain up to 3"/hr rainfall rates, but storm organization should become outflow dominant rather quickly and may contain cell mergers over southern FL. This leads to the potential for up to 3-5" near sensitive urban areas along the southern half of the Gold Coast, prompting the Marginal Risk. 12z HREF neighborhood probabilities highlight a 15-25% chance for at least 5" of rain within 12-hours ending 06z Friday. ....Northern Rockies and Central Plains... A potent shortwave ejecting out of the larger scale western U.S. trough north of the south-central U.S. upper ridge will help spark numerous showers and storms across the Northern Rockies and central High Plains. Locally heavy rainfall amounts impacting areas of WY and western NE already affected by above average rainfall over the past month may result in isolated flash flooding concerns. Meanwhile, fast westerly flow in the mid-to-upper levels will support showers and storms racing east out into the central Plains. This will limit the flash flood threat somewhat as storm motions could approach 30-40kts, but brief instances of training storms could lead to localized rainfall amounts up to 3-4". This greatest rainfall threat primarily exists over southeast WY and western NE late Thursday into the overnight hours. The higher FFG within the Sandhills limits the threat throughout much of north-central NE, but locations elsewhere can expect isolated chances for flash flooding. 12z HREF guidance highlights scattered 20-40% neighborhood probabilities for 2"/6-hr during the day 2 time period from WY to eastern NE. ....Mid-Mississippi to Lower Ohio Valley... Ongoing activity during the Thursday morning time period over parts of southern IL, IN and/or central KY may continue into the Day 2 time period with the threat of isolated flash flooding. This activity remains highly uncertain and dependent on when storms begin across IA tonight. Regardless, a highly unstable atmosphere with SBCAPE greater than 3000 J/kg is expected to surge back into the region as the upper ridge over the south-central U.S. flexes northeastward. This will allow for additional thunderstorm development into Thursday night that should progress southeastward across the Ohio Valley and maybe towards the Tennessee Valley along the tight instability gradient. Localized rainfall totals of 2-4" are possible as PWs reach between 1.75-2" and are depicted within latest hires guidance, but placement uncertainty is high enough to leave out a Slight Risk at the moment. Snell/Pereira Day 3 Valid 12Z Fri Jun 30 2023 - 12Z Sat Jul 01 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL ROCKIES TO THE CENTRAL/SOUTHERN APPALACHIANS AND ACROSS PORTIONS OF THE NORTHEAST... ....Central High Plains to the Appalachians... Mid-level energy will continue to move across the top of the ridge centered over the lower Mississippi Valley and the Southeast, likely to help initiate additional rounds of showers and storms from the Ohio Valley eastward as they interact with an axis of deeper moisture. Meanwhile, a more defined upper trough moving out of the West is likely to generate additional storms back to the west from the foothills of the central Rockies into the Plains. Given the typical uncertainty and fast flow aloft to the north of the strong upper ridge, a broad Marginal Risk was drawn for now. However, embedded upgrades to a Slight Risk may be forthcoming, especially if the models begin to agree on successive days of heavy rain across the central Plains or Midwest. ....Northeast... A Marginal Risk was maintained to highlight an increasing threat for heavy rains to spread back into portions of the Northeast that were recently impacted by heavy amounts. Mid-level energy and deeper moisture will be directed northeast into the region ahead of a low centered north of the Great Lakes and usher in renewed PWs near 1.75". Although confidence in the details is limited, there is at least some potential for moderate to heavy amounts to begin to develop across some parts of Pennsylvania and New York that were recently impacted with heavy amounts and where FFGs remain relatively low. Snell/Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!82Vpie4JBmF_QyoEA_951biUMDGbmzvIkV6pyYpwpVC5= 1YL0yx04E2V_IVBOSaxlt-yNrMJQvI-UkmDjaER8pmOfCt0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!82Vpie4JBmF_QyoEA_951biUMDGbmzvIkV6pyYpwpVC5= 1YL0yx04E2V_IVBOSaxlt-yNrMJQvI-UkmDjaER8fhw0v9M$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!82Vpie4JBmF_QyoEA_951biUMDGbmzvIkV6pyYpwpVC5= 1YL0yx04E2V_IVBOSaxlt-yNrMJQvI-UkmDjaER85bdyxBU$=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) .