Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 21 2023 20:37:26 FOUS30 KWBC 212037 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 436 PM EDT Thu Sep 21 2023 Day 1 Valid 16Z Thu Sep 21 2023 - 12Z Fri Sep 22 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEAST OKLAHOMA INTO NORTH TEXAS, AND SOUTH-CENTRAL SOUTH DAKOTA AND NORTH-CENTRAL NEBRASKA... ....Portions of the Northern and Central Plains... Shortwave energy continues to eject east from a closed-low located over the Great Basin, crossing the north-central Plains today. This will prolong an ongoing heavy rainfall event over Nebraska through the South Dakota border today which should expand south into Kansas tonight. Strong upper-level diffluence associated with height falls in an axis of instability (1000-3000 J/kg MUCAPE) that extends north from the southern Plains through western Nebraska. 12Z HREF CAMs are mightily struggling with this activity, but recent HRRRs do have some semblance of reality and feature repeating activity along the central Neb/SD border into this afternoon which warrants maintenance of the Slight Risk in that area. Late afternoon through tonight, further impulses which should track a bit south of the current one is progged by most 12Z HREF CAMs to allow development in the increasing southerly LLJ pumping moisture and instability up the Plains. Hedged between guidance and expanded the Marginal Risk over eastern KS through the KC metro which connects to the southern Plains Marginal Risk. There is a fair amount of uncertainty with this activity with a risk for some high rain rates, but much of this area is in high drought categories, so a Marginal Risk should suffice for now. ....Southern Plains... A quasi-stationary frontal boundary is expected to remain oriented west-northwest to east-southeastward from the Southern Plains toward the Lower Mississippi Valley today. Additional shortwave energy moving from west to east across the Southern Plains will help to strengthen the low-level inflow into this front again today, likely supporting additional convection along the front across eastern Oklahoma, northeast Texas, into western Arkansas and far southeast Kansas and southwest Missouri (with localized rainfall totals of 2-4" having fallen yesterday). There will be potential for training of cells parallel to the boundary, with the best chance of overlap with prior days rainfall occurring across southeast Oklahoma and into portions of North Texas. Over this potential training and overlap region, a Slight Risk was maintained.=20 ....East Coast of Florida through Coastal Georgia and South Carolina... The Marginal Risk was ticked north to include the rest of the SC coast per regional radar this morning. The bulk of the heavy rainfall will likely be offshore today 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 and South Carolina will support slow moving areas of rain, while another round of convection may form across inland South Florida this 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, localized rainfall amounts of 2-4" are possible (per 00z HREF 40-km neighborhood exceedance probabilities as high as 30-60% for the 3" threshold). ....Northern Great Basin through the Northern Rockies... The strong closed-low over the northern Great Basin is still expected to move very little today. 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 were made to the previous inherited outlook with areal average 0.50"+ amounts expected with locally heavy rainfall in association terrain supporting localized totals of 1-3" (with the upslope areas of western Montana where easterly low-level flow is maximized supporting some localized totals of 2"+). ....Illinois and St. Louis... The Marginal Risk area was focused more over central IL with maintenance over the St. Louis metro. This area is in association with the northward moving vorticity maxima, and some weak convective activity is ongoing across the region early this morning. 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. The bulk of the area remains quite dry from a soil moisture perspective, leading to mostly beneficial rainfall, with just a localized threat for excessive rain. Jackson/Churchill Day 2 Valid 12Z Fri Sep 22 2023 - 12Z Sat Sep 23 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID-ATLANTIC, AND PORTIONS OF THE NORTHERN ROCKIES AND NORTHERN PLAINS... ....Mid-Atlantic... Improved agreement among 12Z guidance for the potential tropical cyclone track north-northwest from off the Southeast coast to eastern North Carolina Friday night. Strong low-level moisture flux in advance of the low spreads up the Mid-Atlantic coast and enhanced upper diffluence on the north side of the mid- to upper-level closed low will support an axis of heavy rains across eastern North Carolina, northward into southeast Virginia and the southern Delmarva Peninsula. A Slight Risk for excessive rainfall has been maintained and trimmed east a bit given confidence increase from 12Z guidance. Areal average QPF of 2-4", local 6", is expected by 12Z Saturday in this region which is on the upper-half of the Slight Risk probabilistic spectrum. The broad Marginal Risk was trimmed a bit east/south given the non-CMC preferred solution for timing and intensity of QPF.=20 ....Northern Rockies across the Northern Plains... The closed-low over the Great Basin ejects east across the northern Rockies Friday night. Good agreement with the evolution of this system remains with model consensus for a broad axis of heavy rain from central MT through central SD which is where the PW anomaly is around 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 plains from northern Nebraska, through much of South Dakota, southwestern North Dakota, northeast Wyoming into eastern Montana with strong low level moisture convergence present with sufficient instability. The Slight Risk was expanded southeast to the central SD/Neb border where heavier QPF would fall over areas that have received heavy rain today. A broad Marginal Risk extends across a larger portion of the surrounding northern Rockies and Plains with a trim west over MN/IA based on latest frontal progs. ....Southern Plains to Ozarks... The Marginal Risk was expanded north a bit into KS which is 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 Friday. 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 a targeted upgrade to a Slight Risk, depending on the evolution of heavy rains through tonight. Jackson Day 3 Valid 12Z Sat Sep 23 2023 - 12Z Sun Sep 24 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS THE NORTHERN AND CENTRAL PLAINS INTO THE MISSISSIPPI VALLEY, AND OVER THE MID-ATLANTIC TO SOUTHERN NEW ENGLAND... ....Mid-Atlantic into New England... PTC 16 is forecast to track north just inland from the Mid-Atlantic coast/over the Chesapeake Bay Saturday/Saturday night. The leading trough will bring the tropical plume of moisture across the northern Mid-Atlantic/southern New England coast with increasing ascent on the northern and western flank of the low will aid in widespread moderate to heavy rainfall. PWATs of 1.75" to 2" (1.5 to 2 standard deviations above normal) will spread east of the Appalachian front within the corridor of maximized ascent. 12Z guidance has slowed the tropical low a bit more allowing a more expansive Slight Risk over the central Mid-Atlantic, but limited to the southern New England coast. There is a risk of continued heavy rain near the tropical low center which is currently eastern NC and eastern VA which is where a future Moderate Risk may be needed, but for now is considered a higher end Slight Risk. ....Plains into Mississippi Valley... The upper low will slow as it crosses the northern Plains Saturday with increasing difluence over the Dakotas down through the central Mississippi Valley. Deep, anomalous moisture transport up through much of the central US will be key in the heavy rain potential on Sat/Sun with NAEFS standardized anomalies a solid 1-2 deviations above normal for the Mississippi Valley up through the central Midwest and 2-3 deviations above normal for areas of the Dakotas, mainly across ND into MT thanks to dynamic influences coupled with the moisture advection regime. This area is receiving heavy rain today and the continued western Gulf connection will make it possible for repeating widespread heavy rain. Organized convective activity is likely within the corridor of deep moisture transport and upper forcing as shortwaves pivoting around the mean trough and ULL will create a large footprint of well-above normal rain chances and totals. Periods of high intensity rain rates that would exacerbate flash flooding concerns, particularly for repeating areas such as near the OK/KS/MO border and over the Dakotas. A broad Slight Risk remains across this area with a bit of an expansion south per the 12Z consensus. In addition, continued terrain enhanced rain near the comma head on the north slopes of the Bighorn Mtns along the MT/WY border into Saturday warranted a Marginal Risk addition. Jackson Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8pDZ2xjr-FHm4qB_Ku0DlscU_O-Z665fnHjJLiKAoX4V= AGGSuNanq9nzJ3QD0KBOjTsFqCg0Y2OaiXTr0VjJI8dIcuQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8pDZ2xjr-FHm4qB_Ku0DlscU_O-Z665fnHjJLiKAoX4V= AGGSuNanq9nzJ3QD0KBOjTsFqCg0Y2OaiXTr0VjJc3sn9ic$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8pDZ2xjr-FHm4qB_Ku0DlscU_O-Z665fnHjJLiKAoX4V= AGGSuNanq9nzJ3QD0KBOjTsFqCg0Y2OaiXTr0VjJxJD4Ky0$=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) .