Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jul 04 2023 00:07:50 FOUS30 KWBC 040007 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 805 PM EDT Mon Jul 03 2023 Day 1 Valid 01Z Tue Jul 04 2023 - 12Z Tue Jul 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST AND NORTHERN PLAINS... ....Appalachians and Mid-Atlantic into New England... Sufficient moisture remains to keep modest precipitation amounts, locally heavy, across the region as a system slowly pulls away to the East Coast. Wet antecedent conditions could still yield areas with isolated flooding or problems in regions of poor drainage with ongoing thunderstorm activity, which should be fading from here on out. The Marginal and Slight Risk areas remain, but the southern portion of the Marginal Risk area was pared back given radar reflectivity and available 18z HREF guidance. ....Montana/Wyoming Eastward to Northern Minnesota... A cold front will drop south across the Northern Plains as the parent surface low pressure remains well north of the US/Canadian border. This front, along with the northern Rockies themselves, have been providing 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, except across the High Plains (west of the 100th meridian) were a Slight Risk area remains, which at the moment is somewhat south of the signal in the 18z HREF guidance -- normally not a bad place to be due to mesoscale model biases which tend to be too far northward. Changes were small 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 continue to allow for slow-moving storms to develop across the ArkLaMiss and extending into central/northern Alabama over the next few hours, with the most recent mesoscale guidance suggesting that portions of OK may get back in the game late in the period due to the nocturnal low-level jet. 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 shifted somewhat southward, using radar reflectivity and available 18z HREF guidance as a guide. Roth 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 Valid 12Z Wed Jul 05 2023 - 12Z Thu Jul 06 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MIDWEST, MISSISSIPPI VALLEY, CENTRAL/SOUTHERN PLAINS, AND EASTERN/COASTAL PORTIONS OF THE CAROLINAS... ....21Z Outlook Update... The 12Z suite of model guidance continues to depict areas of locally heavy rainfall ahead of a seasonably strong surface boundary across the central U.S., within a low-level upslope regime in Colorado, and also across the Southeast. Uncertainty regarding convective coverage in Colorado precludes any upgrades at this time, and most areas from Arkansas eastward to the Carolinas will experience scattered thunderstorms over relatively high FFGs, suggesting only an isolated flash flood threat. The ongoing Marginal areas adequately cover the threat. See the previous discussion for additional details. Cook ....Previous Discussion... ....Upper Midwest/Mississippi Valley into the Central/Southern Plains... The frontal system that has been discussed in the prior days EROs will gradually push southeastward, with the bulk of the forcing for convection following suit. Anomalously high moisture pooling along the front will present the risk for localized flash flooding across a rather large area, but there is insufficient clustering/agreement to introduce any Slight Risk areas at this time. As CAM guidance starts to enter the period with subsequent updates, this will be further evaluated. Right now, the Marginal Risk area was maintained from the prior Day 4, and generally encompasses areas with the potential for localized 1" exceedance (per the global ensemble guidance). ....Eastern/Coastal Carolinas... Maintained a Marginal Risk area across the Carolinas as well, with the potential for localized 2" exceedance (per the global ensemble guidance). The tail end of a lingering frontal boundary (from the system ejecting into the Atlantic from the Northeast) will likely help to enhance afternoon sea breeze convection, and given prior days rainfall a minimum of a Marginal Risk seems appropriate at this juncture. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8iEjCAYMK2rUehSJtcDOaH7CEiFB0GMrbPmfia22P_NF= CMqcTSiFz58CykZN8dVGkgJqyihhfk649CbYW2ooeVRVZac$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8iEjCAYMK2rUehSJtcDOaH7CEiFB0GMrbPmfia22P_NF= CMqcTSiFz58CykZN8dVGkgJqyihhfk649CbYW2oo6zP79so$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8iEjCAYMK2rUehSJtcDOaH7CEiFB0GMrbPmfia22P_NF= CMqcTSiFz58CykZN8dVGkgJqyihhfk649CbYW2ooHiKRWto$=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) .