Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Sep 20 2023 18:07:24 FOUS30 KWBC 201807 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 206 PM EDT Wed Sep 20 2023 Day 1 Valid 16Z Wed Sep 20 2023 - 12Z Thu Sep 21 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS EASTERN OKLAHOMA INTO WESTERN ARKANSAS, MUCH OF THE FLORIDA PENINSULA, AND PORTIONS OF SOUTHEAST WASHINGTON STATE INTO NORTHEAST OREGON... ....16Z Update... ....OK/AR... No major changes were made for the Ozarks region of eastern Oklahoma and western Arkansas. 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. ....FL... A stalled out frontal boundary with a few passing upper level disturbances will combine with ample atmospheric moisture in an environment supported by PWATs of over 2 inches. Convection currently out over the Gulf will organize into a line of strong and moisture-laden storms this afternoon as the aforementioned upper level disturbances approach. Meanwhile, convection associated with the typical sea-breeze and afternoon instability will also form over the Peninsula. The two areas of convection will merge over the northern part of the peninsula before shifting northeastward into northeast FL and the GA coast. Any urbanized and flood-sensitive areas, including the Orlando and Jacksonville metros are at a higher risk for localized flash flooding as the storms are likely to locally generate 3 inches or more of rain, based on the latest 12Z HREF neighborhood probabilities. ....WA/OR... A new Marginal Risk area was introduced with this update across southeast WA and northeast OR. A heavily dynamically driven upper level low and associated jet streams will track southeastward across WA and into OR this afternoon into tonight. Wraparound rainfall will develop starting in Idaho and then shift westward across WA and OR. While there will be little in the way of instability, low FFGs and a steady moderate rain could cause localized flash flooding issues, especially tonight after 06Z. Wegman ....Previous Discussion... ....Eastern Oklahoma into western Arkansas... An upper vort expected to move slowly east northeastward from central Missouri into central Illinois during Wednesday. While convection has been active in the vicinity of this vort during Tuesday, the latest hi res runs are not showing much in the way of convection along the track of this vort across south central Missouri into southwest Illinois during the day 1 period. At the moment, this lessening trend looks reasonable given very little in the way of instability along and ahead of the track of the vort across Missouri into Illinois. With precip below average across these regions over the past few weeks and soil moisture low, the previous marginal risk area was removed from this region. Better instability is expected to persist on the southwest side of this vort across the Southern Plains . Latest hi res guidance is showing potential for additional convection to fire in the vicinity of eastern Oklahoma/western Arkansas along the position of a stationary frontal boundary. 850-700 mb moisture flux continues to be anomalous, 1 to 2 standard deviations above the mean, into this front day 1. This will support potential for additional convection to push east southeast from eastern Oklahoma into western Arkansas/far northeast Texas. HREF neighborhood probabilities are high for 1"+ amounts across this area, 40-70%, decreasing to 20-50% along the Oklahoma/Arkansas border area.=20 ....Central to Southeast Florida... No changes made to the previous broad marginal risk area across central to southeast Florida. PW values will remain high, 2"+ in the vicinity of the stationary front currently across the central portion of the peninsula. An upper trof will begin to amplify day 1, supporting another round of organized convection across the central to southern Florida Peninsula Wednesday afternoon. Low confidence in any qpf details, but localized runoff issues possible, especially in urbanized regions. HREF neighborhood probabilities are high for 1, 2 and 3"+ amounts in the upcoming day 1 period in the marginal risk area. Oravec 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 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!7ai43XYr5wjrprHM_l1S3sSf1YmL-yR4zXVzADPV0Y_2= dRpgchgqEKbZBwU4OQWSB93DQvruIh3p6RPPsJRbiGrNwj8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7ai43XYr5wjrprHM_l1S3sSf1YmL-yR4zXVzADPV0Y_2= dRpgchgqEKbZBwU4OQWSB93DQvruIh3p6RPPsJRbtSXu4C8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7ai43XYr5wjrprHM_l1S3sSf1YmL-yR4zXVzADPV0Y_2= dRpgchgqEKbZBwU4OQWSB93DQvruIh3p6RPPsJRbHwFLkEA$=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) .