Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 21 2023 08:25:39 FOUS30 KWBC 210825 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 423 AM EDT Thu Sep 21 2023 Day 1 Valid 12Z Thu Sep 21 2023 - 12Z Fri Sep 22 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHEASTERN OKLAHOMA AND NORTH TEXAS, AS WELL AS SOUTH-CENTRAL SOUTH DAKOTA AND NORTH-CENTRAL NEBRASKA... ....Portions of the Northern and Central Plains... Shortwave energy is expected to eject north-northeastward today into portions of the Northern and Central Plains, on the eastern periphery of a closed-low located over the Great Basin. Strong upper-level diffluence associated with height falls in an axis of instability (~1000-3000 J/kg of MU CAPE, per the HREF mean) stretching across this region will support organized convective potential from east-central Nebraska, northward into central South Dakota and central North Dakota. The 00z HREF has odds for localized 5" exceedance (via 40-km neighborhood probabilities) maximized at 20-40% across a small section of south-central South Dakota and north-central Nebraska, and a Slight Risk was introduced in this area accordingly (as HREF EAS neighborhood probabilities for 2" exceedance are 20-40%, indicating relatively good spatial agreement among the ensemble as well). A Marginal Risk was maintained surrounding the larger region where convection is possible. ....Southeast Oklahoma, North Texas, western Arkansas, far southeastern Kansas and southwestern Missouri... 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 (per 00z HREF PMM suggesting additional localized totals of 2-3" in this region). Over this potential training and overlap region, a Slight Risk was maintained.=20 ....East Coast of Florida into Coastal Georgia and South Carolina... A Marginal Risk was maintained along the east coast of Florida and extended slightly northward from the previous issuance into coastal South Carolina. 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 into 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"+). ....Northeast Illinois into southeast Wisconsin... A small Marginal Risk area was shifted southwest for the new Day 1 outlook, now centered across much of Illinois and into east-central Missouri and far eastern Iowa. 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. While the bulk of the area remains quite dry from a soil moisture perspective (leading to mostly beneficial rainfall), there have been some recent heavy rains over northeast Illinois (with soils become locally saturated in those areas).=20 Churchill/Oravec Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7J7hJwQseXdg5M28wBxibRM9U3pkQHZSqcIiASS-4MO1= 7uBRaDIGWhmYZC0UkPHuU0AW3WaSwPGEzPbuNsBvkZoMek0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7J7hJwQseXdg5M28wBxibRM9U3pkQHZSqcIiASS-4MO1= 7uBRaDIGWhmYZC0UkPHuU0AW3WaSwPGEzPbuNsBvjsqwE4I$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7J7hJwQseXdg5M28wBxibRM9U3pkQHZSqcIiASS-4MO1= 7uBRaDIGWhmYZC0UkPHuU0AW3WaSwPGEzPbuNsBvcdcHlLQ$=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) .