Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 21 2023 16:00:47 FOUS30 KWBC 211600 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1200 PM 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 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 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 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 NORTHEAST AND MID ATLANTIC... ....Northeast/Mid Atlantic... Surface low along the eastern sea board will continue to press north with close proximity to the coastal plain and even potentially farther inland as indicated by some guidance (ECMWF/CMC). Increasing ascent on the northern and western flank of closed 5H/7H low(s) will aid in widespread moderate to heavy rainfall across much of the central Mid Atlantic up through the Northeast coastal plain with furthest north impacts being across southeast New England. PWATs on the order of 1-1.5 standard deviations above normal will be found to the east of the Appalachian front within the corridor of maximized ascent with local bufr and regional sounding profiles showing moisture entrenched to up near the tropopause, typically a signature found in very tropical originated airmasses. Global deterministic has begun shifting to agreement on timing with the best chance for heavy rainfall occurring over the span of early-Sat AM through Saturday afternoon with the southern periphery of the outlined area seeing a decrease in heavy rain potential by the end of the D3 period. A general 1-1.5" of rainfall is forecast for area along and west of the I-95 corridor with a sharp decrease once you cross the Allegheny front. 1.5-2.5" is more common to the east of the 95 corridor with local maximum up to as high as 3" as of the latest guidance trends. The threat is higher than other surface reflections due to the expected tropical-like characteristics that would play a role in potential when you couple the ascent with warm rain processes. Given the short time frame of guidance being in agreement, went with a SLGT risk from northeast NC/VA Tidewater up through the central Mid Atlantic and Northeast U.S coastal plain encompassing all the urban corridors from DC to southern Boston. Timing of the surface low and attendant mid-upper forcing will be key to trends in future updates. ....Plains into Mississippi Valley... ULL from the northern Rockies will continue its eastward progression into the northern plains with increasing difluent signature over the area from 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 period is a continuation of the very active pattern focused west of the Continental Divide now making headway into the central plains where moisture advection thanks to a Gulf connection make it possible for widespread heavy rain concerns over the center of the CONUS. No matter the reference in guidance, convective regimes will be applicable based 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. Ensemble bias-corrected precip was stout with widespread 2+" 24 hr totals with much of the precipitation scheme generated from convective potential. This creates periods of high intensity rain rate possibilities that would exacerbate flash flooding concerns. Luckily, a lot of the impact area has climatologically higher FFG's due to soil moisture adaptability, but considering the overall synoptic and expected mesoscale evolution, this will still lead to flooding concerns within the grand scheme as precipitation field will not be isolated but more organized and longstanding over the course of the period. A SLGT risk is in place over much of the central US with extension from northeast OK up through a good portion of the Dakotas.=20 Kleebauer/Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!47RqXGGTvE9vvEl7SvXDUwGqnkZvaveWSaJe3p9_E9cV= mkAJDNx1-HRaAD7TzeFdm0awTx5BwUfNDiEbowMW2lUO9JI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!47RqXGGTvE9vvEl7SvXDUwGqnkZvaveWSaJe3p9_E9cV= mkAJDNx1-HRaAD7TzeFdm0awTx5BwUfNDiEbowMWmvLdgdg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!47RqXGGTvE9vvEl7SvXDUwGqnkZvaveWSaJe3p9_E9cV= mkAJDNx1-HRaAD7TzeFdm0awTx5BwUfNDiEbowMW9XIbF-0$=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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