Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jul 03 2023 08:34:07 FOUS30 KWBC 030834 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 433 AM EDT Mon Jul 03 2023 Day 1 Valid 12Z Mon Jul 03 2023 - 12Z Tue Jul 04 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST AND MID-ATLANTIC, THE UPPER MIDWEST/NORTHERN PLAINS, THE MID-SOUTH, AND SOUTH FLORIDA... ....Appalachians and Mid-Atlantic into New England... Models still linger sufficient moisture to keep modest precipitation amounts across the region as a system slowly pulls away to the East Coast. Additional rainfall amounts after the start of the outlook period at this morning are not expected to be overly impressive, but wet antecedent conditions leading up to this time period could still yield areas with isolated flooding or problems in regions of poor drainage. Therefore, the Marginal risk area in this part of the country was maintained (with expansion southward into both the Appalachians, to account for lingering morning precipitation, and farther south into the Mid-Atlantic, due to a small window of opportunity for afternoon storms to locally exceed 3"). ....Montana/Wyoming Eastward to Northern Minnesota... A cold front will drop south across the Northern Plains today and tonight as the parent surface low pressure remains well north of the US/Canadian border. This front should provide a focus for showers and thunderstorms from south-central Montana and northern Wyoming eastward across South Dakota/Nebraska and into the Upper Midwest. Moisture pooling along and ahead of the front combined with divergence aloft from an upper level jet will create an environment favorable for storms to produce locally heavy rainfall, although highest precipitable water values only look to be modestly above 1.25 inches which may limit rates and any associated excessive rainfall potential. No major changes were needed with this forecast update. ....Mid-South... A lingering frontal boundary extending from the Mid-Atlantic to the Lower Tennessee Valley, pooling 1.75-2.00 inch PWs combined with a subtle shortwave ejecting across the central/southern Plains should allow for slow-moving storms to develop across the ArkLaMiss and extending into central/northern Alabama. Minimal low-to-mid level flow is likely to limit storm motion and could result in localized flash flooding concerns and rainfall amounts up to 2-3" within a relatively short period of time. The inherited Marginal Risk area was expanded a bit, based on the 00z HREF QPF footprint (and associated HREF 3" neighborhood exceedance probabilities as high as 20-40%). ....South Florida... A Marginal Risk area was maintained for portions of South Florida to account for slow moving cells that develop in response to sea breeze convergence, with the potential for 1-2 inches of rainfall to fall within a 30-minute time period for affected areas (with localized totals in excess of 3" possible in only an hour or so). The best coverage for convection is likely to be within the typical 18Z-00Z time period, and poor drainage/metropolitan areas are most at risk for localized flash flooding. Churchill/Bann/Hamrick Day 2 Valid 12Z Tue Jul 04 2023 - 12Z Wed Jul 05 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL PLAINS, MIDDLE/UPPER MISSISSIPPI VALLEY, AND NORTHEAST... ....Upper Midwest Across the Plains into Portions of Wyoming and Montana... The front which helps focus showers and thunderstorms today will continue to be the focus for additional showers and thunderstorms on Tuesday. Models continue to have pretty good agreement with clustering over southeast NE into western IA and southern MN, where a Slight Risk area was maintained. The GEFS continues to be the heavier of the global ensemble guidance, suggesting the potential for some 3" exceedance (which agrees well with the machine learning CSU First Guess ERO, which depicts a Slight Risk across much of the same area). The GFS ramps up precipitable water values to nearly 1.75 inches from eastern Nebraska into southwest Minnesota by 05/00Z immediately ahead of a maximum in moisture flux convergence values along/ahead of the front.. so the placement of the GEFS maxima (and resulting CSU First Guess ERO) makes good sense. While we do not have the entirety of the HREF suite within range just yet, the FV3 and NAM-nest extend fully out through Day 2 and are also in good agreement with the placement of the maxima. Uncertainty in coverage and/or intensity is greater outside of the Slight Risk, where a broader Marginal was also maintained (with some adjustments to the edges). ....Northeast... A Slight Risk area has been introduced with this outlook for portions of the Northeast, mainly from much of MA southwestward to southern NY and far northern NJ. There is increasing confidence that the low pressure system that is slowly moving offshore will linger through at least the early afternoon, as the CAMs are particularly intense with convection from 18z-21z on Tuesday (with HREF 3" neighborhood exceedance probabilities over 3-hr as high as 20-50%). Think this initial CAM guidance may be a bit overdone in this region, but given the prior days occurred and expected rainfall decided it was prudent to introduce a Slight Risk area. While the confidence in the location is fairly high, there may be some needed expansion or contraction to the Slight Risk area in subsequent updates (largely based on where the heavy rainfall occurs on Day 1, as well as any significant trends in the CAM guidance). ....Southern Mississippi Valley/Tennessee Valley Eastward to Carolina Coast... The Marginal Risk area has been maintained (and expanded a bit) along/near a lingering frontal boundary extending from the southern Mid-Atlantic to the Lower Tennessee Valley with 1.75-2.00 inch PWs and subtle shortwave energy ejecting into the central/southern Plains to allow for another round of slow-moving storms to develop, which could result in isolated instances of flooding. The latest area reflects where the best QPF footprint exists (and incorporating CAM guidance where available). Churchill/Bann Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4V3lhZXNzj-utZemziNIl1Df2tbLBFTkEKcWn9ywvTfQ= aZR6pQETJG055baPr8gSvBcD3MduBgdjXaJbljM9b_DnAsA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4V3lhZXNzj-utZemziNIl1Df2tbLBFTkEKcWn9ywvTfQ= aZR6pQETJG055baPr8gSvBcD3MduBgdjXaJbljM9Mb1ea6Q$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4V3lhZXNzj-utZemziNIl1Df2tbLBFTkEKcWn9ywvTfQ= aZR6pQETJG055baPr8gSvBcD3MduBgdjXaJbljM9jQAj6o8$=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. 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