Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jul 03 2023 19:04:44 FOUS30 KWBC 031904 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 303 PM EDT Mon Jul 03 2023 Day 1 Valid 16Z Mon Jul 03 2023 - 12Z Tue Jul 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER SOUTHERN NEW YORK STATE, NORTHEASTERN PENNSYLVANIA, SOUTHERN NEW ENGLAND, MUCH OF SOUTH DAKOTA, NORTHEASTERN WYOMING, AND SOUTHEASTERN MONTANA... A Slight risk was added to the outlook across portions of southern New England, southern New York, and northern Pennsylvania. Models are persistent in developing areas of east-northeastward moving convection initially across New York and Pennsylvania early in the afternoon. These storms will migrate toward the Slight risk area through the overnight hours, with localized areas of 2-3 inch rainfall amounts possible especially where multiple rounds of locally heavy rainfall can materialize. Additionally, portions of the Slight risk area have experienced between 2-5 inches of rainfall over the past week, with low FFGs (perhaps around or below 1 inch/hr) also expected to support localized excessive runoff. A second Slight risk area was added to the outlook from southeastern Montana into much of western South Dakota. Surface analyses indicated a nearly stationary, east-west oriented boundary extended across the area that will serve as a focus for scattered convective development later this afternoon. Flow aloft was parallel to this boundary, and indications are that localized training of cells are probable that should bolster local rain rates into the 1+ inch/hr range at times. Additionally, increased low-level flow and convergence along the surface boundary will continue to focus convective development through the evening and overnight hours, continuing the flash flood threat through at least 09Z tonight across the Dakotas. Lastly, a small Slight was considered across northern Mississippi and northern/central Alabama today. While a relatively focused axis of low-level convergence along the Tennessee River Valley should aid in continued thunderstorm development in those areas today, storms should be both diurnally and outflow driven, with slow propagation suggesting that spots of heavier rainfall may be too isolated for an upgrade. Nevertheless, some concern exists that heavier rainfall could eventually propagate into the Birmingham, AL metro and surrounding areas where FFGs are locally low (around 1.5 inch/hr). For more information, see the previous discussion below. Cook ....Previous discussion... ....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, NORTHEAST, AND COLORADO... ....21z Outlook Update... Much of the ongoing forecast is on track. Several slow-moving clusters of storms are expected to traverse areas from Nebraska through southern Minnesota during the afternoon, evening, and early overnight hours on Day 2/Thursday. Additionally, slow-moving clusters are expected to persist from 12Z-21Z from northern New Jersey through southern New England, and recent rainfall has left ground conditions wet and sensitive to additional runoff there. Marginal areas cover much of the Deep South and interior Southeast for afternoon/evening storms, and a few areas of convection are expected especially across sensitive ground conditions in southern Montana that could pose isolated flash flooding. The biggest change to the ongoing forecast is to introduce a Slight risk for portions of central/northeastern Colorado.=20 Low-level easterlies/upslope flow against higher terrain and favorable instability parameters will result in scattered thunderstorm development during the afternoon and early evening.=20 Additionally, ground conditions are susceptible for runoff due to the recent 2-8 inches of rainfall over the past week and burn scars (especially north of Denver). Convective coverage is a bit in question in this area, but given the signal for an additional 1-2 inches of rainfall (likely falling in a short amount of time) and sensitive ground conditions justify a Slight risk upgrade for this outlook update. See the previous discussion below for additional details. Cook ....Previous Discussion... ....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 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5k83uA3ROaLVGPhIyFA9QyJhK-v7Gt7_27LKdoeL2yCk= YncuwR7JJ5Qyt9x0wFyuwGvsOyrzrZo7wa-uFN-k5mJihts$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5k83uA3ROaLVGPhIyFA9QyJhK-v7Gt7_27LKdoeL2yCk= YncuwR7JJ5Qyt9x0wFyuwGvsOyrzrZo7wa-uFN-kPwvfpcM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5k83uA3ROaLVGPhIyFA9QyJhK-v7Gt7_27LKdoeL2yCk= YncuwR7JJ5Qyt9x0wFyuwGvsOyrzrZo7wa-uFN-kVzm_iss$=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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