Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Wed Jun 28 2023 15:51:00 FOUS30 KWBC 282000 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 359 PM 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 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 $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .