Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 21 2023 00:58:01 FOUS30 KWBC 210057 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 857 PM EDT Wed Sep 20 2023 Day 1 Valid 01Z Thu Sep 21 2023 - 12Z Thu Sep 21 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS EASTERN OKLAHOMA INTO WESTERN ARKANSAS, FAR SOUTHEAST KANSAS, AND SOUTHWEST MISSOURI... ....Eastern Oklahoma/Western Arkansas/Southeast Kansas/Southwest Missouri... No major changes were made for the Ozarks region of eastern Oklahoma and western Arkansas, based on the latest short-term model trends. An MCS is expected to redevelop tonight over many of the same areas that saw heavy rain over the past couple days. While coverage will be reduced and overall storm strength weaker tonight as compared with last night's round of convection, the threat for isolated flash flooding remains anywhere storms can repeat over the same areas. This is most likely from the northeast corner of OK southeast into central AR. Hurley/Wegman =20 Day 2 Valid 12Z Thu Sep 21 2023 - 12Z Fri Sep 22 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHEASTERN OKLAHOMA, FAR NORTHEAST TEXAS AND FAR WESTERN ARKANSAS... ....1830Z Update... No major changes were made with this update. For the Slight Risk area, the biggest change made was to expand the area north to include the KS/MO/OK/AR junction region. This area was hard hit with heavy rain last night, and the potential for additional heavy rainfall, while not as high as areas further south along the OK/TX border, may still cause additional flash flooding overnight Thursday night. The Marginal Risk in IL was expanded southward a bit as a front draws storms northward with the potential for repeating as they track parallel to the frontal interface. While most of the rainfall is likely to be west of Chicago, there remains enough potential in the city itself when adding the lower FFGs there to maintain the Marginal Risk. The Marginal Risk for the intermountain west remains largely the same, with the best potential for heavy rain on the east facing slopes of the mountains of central/western MT due to upsloping. This risk area was separated from the one over the Dakotas, where an isolated flash flooding risk remains. The portions of eastern MT and WY between the 2 areas have largely dried out in recent runs, so the previous Marginal risk was separated into two. Wegman ....Previous Discussion... ....Southeast Oklahoma, far northeast Texas, far southwest Arkansas... A stationary frontal boundary expected to remain oriented west northwest to east southeastward from the Southern Plains toward the Lower Mississippi Valley. Additional shortwave energy moving west to east across the Southern Plains and along this boundary will help to strengthen the low level inflow into this front, supporting potential for additional convection to ride along the front across eastern Oklahoma, northeast Texas, into western Arkansas. There will be potential for training of cells parallel to the boundary and potential for overlap of day 1 heavy rains along this front with day 2 amounts. Over this potential training and overlap region, a slight risk was maintained.=20 ....East coast of Florida into coastal Georgia... A marginal risk was maintained along the east coast of Florida and extended slightly northward from the previous issuance into coastal Georgia. The bulk of the heavy rainfall will likely be offshore day 2 as a surface low forms off the Southeast coast in response to the closing off mid to upper level low along the Southeast coast. Easterly low level flow from the central east coast of Florida, northward into coastal Georgia will support slow moving areas of rain, while another round of convection may form across inland South Florida Thursday afternoon and push eastward to the southeast coastal regions. Concerns for any runoff issues will continue to be primarily over urban areas where short term rainfall amounts of 1-2" are possible day 2. ....Great Basin into the Northern Rockies The strong closed low forming over the northern Great Basin day 1 is expected to move little during the day 2 period. There is good model agreement on an axis of moderate to heavy rainfall in the slow moving region of above average PW values stretching from northeast Nevada, across northern Utah, eastern Idaho, northwest Wyoming into western Montana. No significant changes made to the previous marginal risk across these areas with areal average .50"+ amounts possible and locally heavy totals in the upslope area of western Montana where easterly low level flow will support some localized totals of 2"+ ....Portions of the Northern High Plains... Shortwave energy expected to push north northeastward day 2 on the eastern side of the Great Basin closed low across portions of the Central to Northern High Plains. Strong upper difluence associated with these height fall in an axis of instability stretching across the Central to Northern High Plains will support organized convective potential Thursday from central to northwest Nebraska, northward into central South Dakota and central North Dakota. Only some small changes made to the previous marginal risk area, trimming the eastern portion over South Dakota into Nebraska to better fit the overlap of heavy rain potential and lower ffg values. ....Northeast Illinois into southeast Wisconsin... A small marginal risk area was added for day 2 across northeast Illinois into southeast Wisconsin associated with the northward moving vort across these areas. Strengthening southerly low level flow in the vicinity of this vort may support some locally heavy rains as training of cells in a south to north direction occur.=20 Recent heavy rains over northeast Illinois into southeast Wisconsin have increased soil moisture and lowered ffg values, with a marginal risk denoted across these lower ffg values areas where additional locally heavy rains are possible. Oravec Day 3 Valid 12Z Fri Sep 22 2023 - 12Z Sat Sep 23 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN ROCKIES INTO THE NORTHERN HIGH PLAINS AND EASTERN NORTH CAROLINA, SOUTHEAST VIRGINIA, THE SOUTHERN DELMARVA PENINSULA, AND SOUTHEASTERN NEW JERSEY... ....1830Z Update... ....Mid Atlantic... The latest guidance trends for the coastal storm/Nor'easter have trended both westward and faster, with many solutions now bringing the center of the low over the Outer Banks of NC. Accordingly...the Slight and Marginal Risk areas have been expanded well north and west from the previous forecast. From Atlantic Beach, NC north up through Hampton Roads, VA through about Rehoboth Beach, DE, the heaviest rains are expected. Expect widespread 2 to 4 inch totals, along with strong onshore flow resulting in potential tidal flooding in addition to the rainfall. Thus, this area is considered in a higher end Slight risk, with further increases in rainfall with future guidance potentially necessitating a Moderate for portions of this area. The Slight Risk area was also expanded westward to include Richmond, VA and Raleigh, NC, as well as the southern Jersey Shore. Meanwhile the surrounding Marginal Risk was expanded to include the I-95 corridor from DC through NYC. ....Eastern Oklahoma, western Arkansas, and northeast Texas... The Marginal risk for this area remains largely unchanged. The biggest concern is that the rainfall from Day 2/Thu will be ongoing at the start of the period, although most of the rain is likely on Day 2. Thus, most of the impacts from the rain should be ongoing at the start of the period. Depending on timing, portions of this area may need to be upgraded to a Slight. ....Northern Rockies and Northern Plains... The main changed made with this update was to shrink back the Slight out of central SD, as the better potential for flooding rains will be in the Black Hills and areas where there's more significant topography. While it should be a nice soaking rain for central SD, it's not expected given the antecedent dry soil conditions that there will be any more than isolated flash flooding. Westward into MT, the forecast remains largely the same, and will be a continuation of rain from Day 2/Thu. However, unlike the previous day, much of the rain with the system in this area is likely to fall in the Day 3/Fri time frame. Wegman ....Previous Discussion... ....Northern Rockies into the Northern High Plains... The strong slow moving closed low over the Great Basin day 2 will become increasingly progressive day 3 as it pushes across the Northern Rockies and into the Northern High Plains. There is overall good agreement with the evolution of this system, with model consensus for a broad area of heavy rain from portions of the Northern Rockies into the Northern High Plains in an axis of much above average PW values, 1.5 to 3+ standard deviations above the mean. The heavy rains will be comprised of a well defined comma head/deformation precip area across western to central Montana and another round of convective rains across portions of the northern High plains from northern Nebraska, through much of South Dakota, southwestern North Dakota, northeast Wyoming into eastern Montana. No significant changes to the previous slight risk for this region, with the slight risk continuing to fit well with the overlap of day 3 heavy rains and lower ffg values. The southern portion of the previous marginal risk area was reduced across much of the Lower Missouri Valley region given relatively high ffg values and only modest qpf.=20 ....Eastern Oklahoma, western Arkansas, northeast Texas... A marginal risk maintained in the vicinity of the stationary frontal boundary expected to remain oriented west northwest to east southeast across portions of the Southern Plains toward the Lower Mississippi Valley. Additional anomalous low level moisture flux into this boundary will support potential for additional moderate to heavy rainfall amounts from eastern Oklahoma into western Arkansas, northeast Texas and northern Louisiana. There is potential for this area to be upgraded to a slight risk depending where the days 1 and 2 potential heavy qpf axes occur. ....Eastern North Carolina, southeast Virginia into the southern Delmarva Peninsula... There is overall agreement in the latest models for a developing potential sub-tropical/tropical low along the Carolina coasts in response to the closing off mid to upper level center. A well defined coastal frontal boundary will likely be situated to the north of this low along the North Carolina/Mid Atlantic coast and will be pushing westward into eastern North Carolina, southeast Virginia and the southern Delmarva from Friday afternoon into early Saturday. While there are still detail differences with the overall evolution of this system, there is agreement that strong low level moisture flux into the strong coastal front and enhanced upper difluence on the north side of the mid to upper level closed low and in the right entrance region of a upper jet across the northern Mid-Atlantic will support an axis of heavy rains across eastern North Carolina, northward into southeast Virginia and the southern Delmarva Peninsula. A slight risk was introduced for the heavy rain potential along the above mentioned coastal regions along and west of the strong coastal front.=20 Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7Ng5AeDgdONAMYSKM3wm0IOiQoV7ACQD5BJ0DfaHD4st= 33x4FemLH3HJN2Sv_I8Bdpca0fY2c0UruknWNKS5LyRSGr4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7Ng5AeDgdONAMYSKM3wm0IOiQoV7ACQD5BJ0DfaHD4st= 33x4FemLH3HJN2Sv_I8Bdpca0fY2c0UruknWNKS5kPCS-gE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7Ng5AeDgdONAMYSKM3wm0IOiQoV7ACQD5BJ0DfaHD4st= 33x4FemLH3HJN2Sv_I8Bdpca0fY2c0UruknWNKS5kYCo8eU$=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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