Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Sep 20 2023 07:55:43 FOUS30 KWBC 200755 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 354 AM EDT Wed Sep 20 2023 Day 1 Valid 12Z Wed Sep 20 2023 - 12Z Thu Sep 21 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS EASTERN OKLAHOMA INTO WESTERN ARKANSAS, AND CENTRAL TO SOUTHEAST FLORIDA PENINSULA... ....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 SOUTHWEST ARKANSAS... ....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 TO SOUTHEAST VIRGINIA AND THE SOUTHERN DELMARVA PENINSULA... ....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!9pZthgvP2GYO6ut0ZS3kGBT2JYUocGD9wZwuTAPOWNFJ= O8fkm8fMZTwnzTapQqOcUfr9V3YcDjPuj0V-goS5a1ul4Bg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9pZthgvP2GYO6ut0ZS3kGBT2JYUocGD9wZwuTAPOWNFJ= O8fkm8fMZTwnzTapQqOcUfr9V3YcDjPuj0V-goS5NaF6izA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9pZthgvP2GYO6ut0ZS3kGBT2JYUocGD9wZwuTAPOWNFJ= O8fkm8fMZTwnzTapQqOcUfr9V3YcDjPuj0V-goS5-da7Ffs$=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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