Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 21 2023 20:08:23 FOUS30 KWBC 212008 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 407 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 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!9kRZ5cnIcgyIuI16ubIwgD7CpON4JRhz_Gh1diebfivW= CwnSfLbq8z24cOMtTfNDc3diioOIbJ6FYDbOvC2AnIdnNcs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9kRZ5cnIcgyIuI16ubIwgD7CpON4JRhz_Gh1diebfivW= CwnSfLbq8z24cOMtTfNDc3diioOIbJ6FYDbOvC2APAB2Pvw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9kRZ5cnIcgyIuI16ubIwgD7CpON4JRhz_Gh1diebfivW= CwnSfLbq8z24cOMtTfNDc3diioOIbJ6FYDbOvC2AQ5BKAZM$=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) .