Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jun 28 2023 15:50:25 FOUS30 KWBC 281550 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1149 AM EDT Wed Jun 28 2023 Day 1 Valid 16Z Wed Jun 28 2023 - 12Z Thu Jun 29 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NEW ENGLAND... ....New England... An upper trough will continue to linger 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 ahead of the associated front. This along with daytime heating has already helped to generate areas of moderate to heavy rainfall. Supported by a period of south to north training ahead of the slow-moving front due to uniform 850-300mb flow parallel to the frontal boundary. The overnight HREF guidance continued to show the threat for localized amounts of 2-3", maximized along an axis extending from central Massachusetts into New Hampshire, and northeastern Vermont. Much of New Hampshire, Vermont, and central Maine (centered on the White Mountains) are experiencing elevated to daily-max streamflows per USGS and widespread 14-day precip anomalies of 100-300%. Therefore, terrain is likely more susceptible should training bands of heavy rain capable of dropping 2-4" of rain occur. The Slight Risk was introduced with the 16z update to highlight the increased threat near and surrounding the White Mountains for scattered flash flooding. An approaching shortwave nearing northern New York State should swing into northern New England by this evening and allow for shower activity to shift eastward and end the flash flood threat by tonight. ....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 support an increasing chance for rain across portions of the northern WI and bordering sections of MN tonight into early Thursday. The general consensus of the HREF guidance shows heavier precipitation spreading initially into the Minnesota Arrowhead region in the warm advection pattern and along a ribbon of deeper moisture (PWs around 1.5 inches) nosing through the region. This will likely be followed by the evening development of storms in the more unstable air farther south across northwestern Wisconsin. 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. The previous Marginal Risk was shrunk to eliminated much of MN due to the lack of sufficient instability needed to produce excessive rainfall rates. ....Northern Rockies... With an upper trough remaining positioned to the west and a relatively moist airmass still in place, isolated to scattered, slow-moving showers and storms are expected to develop once again across the region. These may produce locally heavy amounts and generate isolated flash flooding concerns, especially given the wet antecedent conditions. The 12Z HREF continues to show some appreciable probabilities for rain totals of 1-2" and probabilities of exceeding 2, 5, and 10-year ARI values, maximized near the the track of low-to-mid level center moving across southwestern Montana. ....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 developing thunderstorms to anchor along should timing of shortwaves allow for overnight development. Vast uncertainty remains within the hires guidance on where thunderstorms will develop and if they will be able to grow upscale and train as the low-level jet kicks in tonight over the Mid-Mississippi Valley. The flash flood threat will be greatest should convection form and move along the better instability gradient near southeast IA and far eastern MO. If thunderstorms form farther to the north this evening over northeast IA, then they will likely have to feed to elevated instability and have a lesser chance for training as they slide southeastward. Additionally, 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 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 in placement for a higher risk level. Thus, the Marginal Risk was maintained for the 16z update. This risk area also highlights the area where HREF probabilities for amounts exceeding 3 inches in 6 hours and 5-year ARI values are relatively higher (centered over west-central IL and along the MO-IL border). However given the uncertainty, this area may be adjusted with updates later today and may impact the Day 2 ERO updates as well. Snell/Pereira Day 2 Valid 12Z Thu Jun 29 2023 - 12Z Fri Jun 30 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHEAST AND FROM THE NORTHERN ROCKIES INTO THE CENTRAL PLAINS... ....Northeast... One more day of showers and storms, with locally heavy amounts possible, is expected for portions of northern New England.=20 However the threat will likely be on the downward trend as the upper trough continues to weaken and lift to the northeast, followed by drier air spreading across the region. ....Northern Rockies to the central Plains... Afternoon and evening thunderstorms will remain possible across portions of southwestern Montana and western Wyoming as an upper trough continues to slowly approach the region. Locally heavy rainfall amounts impacting areas already affected by above average rainfall over the past month may result in isolated flash flooding concerns. Meanwhile, mid-level energy ejecting east of out of the trough is expected to support showers and storms moving farther east out into the central Plains. Models continue to present a pretty good signal for training storms and heavy accumulations developing late Thursday into the overnight near a slow-moving boundary and axis of deeper moisture from southeastern Wyoming/northeastern Colorado into central Nebraska. However, they continued to differ on the location of heavier amounts across the Plains, with some painting an axis centered across the less flash flooding prone Sandhills, while other are focused farther to south -- nearer the Kansas border. Pereira=20 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, 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. ....Northeast... A Marginal Risk was drawn 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. 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. Pereira=20 Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_T0HUjbCCTTVkn-TYZX7FDW3Ob8-Msdbcb2YQF4sva7E= I5-H9wm9XcDSaV2O-xqIMNnxiDrYlV-2i9sCJfMDchSyzDA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_T0HUjbCCTTVkn-TYZX7FDW3Ob8-Msdbcb2YQF4sva7E= I5-H9wm9XcDSaV2O-xqIMNnxiDrYlV-2i9sCJfMDsd_unWQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_T0HUjbCCTTVkn-TYZX7FDW3Ob8-Msdbcb2YQF4sva7E= I5-H9wm9XcDSaV2O-xqIMNnxiDrYlV-2i9sCJfMD5wp8Kqs$=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) .