Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 21 2023 08:27:41 FOUS30 KWBC 210827 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 426 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 Valid 12Z Fri Sep 22 2023 - 12Z Sat Sep 23 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF EASTERN NORTH AND SOUTH CAROLINA, SOUTHEAST VIRGINIA, THE SOUTHERN DELMARVA PENINSULA, AND SOUTHEASTERN NEW JERSEY, AS WELL AS ACROSS PORTIONS OF THE NORTHERN ROCKIES AND HIGH PLAINS... ....Mid-Atlantic and Surroundings... Models are continuing to come into better agreement with the evolution of a (still developing) non-tropical/sub-tropical low offshore the Carolinas, in response to the closing off of a mid- to upper-level low center. A well-defined coastal frontal boundary will likely be situated to the north of this developing non-tropical/sub-tropical 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. Strong low-level moisture flux into the coastal front and enhanced upper diffluence on the north side of the mid- to upper-level closed low (and additionally in the right-entrance region of a ~120 kt jet streak) 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 for excessive rainfall has been maintained (and expanded a bit) with the heavy rain potential along the above mentioned coastal regions along and west of the strong coastal front. Areal average QPF of 2-4" is expected in this region with locally higher amounts (considered to be on the upper-end of the Slight Risk probabilistic spectrum). A fairly broad Marginal Risk was also maintained farther inland with this forecast cycle, primarily to acknowledge lingering uncertainty with the NAM/CMC being slightly farther inland outliers (and this Marginal Risk will likely be tightened up with subsequent updates and increasing confidence).=20 ....Northern Rockies into the Northern Plains and Upper Midwest... The strong, slow moving closed-low over the Great Basin will become increasingly progressive on Day 2 as it pushes across the Northern Rockies and into the Northern High Plains. There remains 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. A Slight Risk was maintained (and expanded a bit) across this area, and the probabilities are high enough for localized 2-4" totals across central Montana to be considered a higher-end Slight Risk there. A broad Marginal Risk extends across a larger portion of the surrounding northern Great Basin into the bulk of the Northern Plains and into the Upper Midwest, where more localized excessive rainfall remains possible amid moderate to strong southerly low-level moisture transport. ....Eastern Oklahoma, Western Arkansas, Northeastern Texas... A Marginal Risk was 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 a subsequent targeted upgrade to a Slight Risk, depending on the evolution of heavy rains on Day 1. Churchill/Oravec 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!7-OmaEfPgONtDIU_FxqbPjHiGiISwqnSHRw9Zs83Zpvv= W5S1CQXfkf-_RePn_hRjLJu5Tm1zknqFe-6BVWyxt6blLzY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7-OmaEfPgONtDIU_FxqbPjHiGiISwqnSHRw9Zs83Zpvv= W5S1CQXfkf-_RePn_hRjLJu5Tm1zknqFe-6BVWyxCOCY9Lk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7-OmaEfPgONtDIU_FxqbPjHiGiISwqnSHRw9Zs83Zpvv= W5S1CQXfkf-_RePn_hRjLJu5Tm1zknqFe-6BVWyxJA07aNY$=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) .