Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jun 16 2023 00:40:30 FOUS30 KWBC 160040 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 839 PM EDT Thu Jun 15 2023 Day 1 Valid 01Z Fri Jun 16 2023 - 12Z Fri Jun 16 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST...WYOMING/NEBRASKA AND THE NORTHERN PLAINS... ....01Z Excessive Rainfall Outlook North Dakota/South Dakota... Maintained a Slight Risk that was introduced earlier. Some high-resolution guidance still focuses 2 to 5+ inches of rainfall overnight in convection that it focuses along and of a cold front moving across the region. Agree with the previous forecaster that the upper end of this range seems too high given the pattern, but the amount of moisture being drawn in place...a focusing mechanism and dynamics could lead of at least some increase in potential for excessive rainfall. Storm motions of ~10-15 kt quasi-parallel to an approaching cold front with southeasterly to easterly low level flow converging into the region will support convection that persists into the early overnight although the loss of daytime heating should result in rainfall becoming more stratiform in nature. Wyoming... Maintained the Slight Risk area in Wyoming along and ahead of an amplifying upper-level shortwave extending from Wyoming into Utah. Instability remains meager ahead of the wave...and rainfall rates have generally remained under 1+ in/hr but the added dynamical support that could result in locally higher rainfall rates over some areas where rainfall over the past 7 days has been 300 to 600 percent of average. With late afternoon/early evening satellite imagery still showing cooling cloud tops over portions of southeastern Wyoming and the Nebraska panhandle. Moisture is deeper across portions of southeast Wyoming and the western Nebraska into parts of South Dakota...so maintained that portion of the outlook area with little change. ....Lower Mississippi Valley into Southeast... Previous forecast reasoning remains on track with only minor adjustments made to the risk areas based on radar trends and 12Z hires output. There Lower Mississippi Valley through the Southeast... A series of impulses that have ridden the westerly subtropical jet and provided lift a persistent west to east oriented low level frontal zone over the Southeast continued throughout the day with the only a brief respite occurring in response to shortwave ridging. Another shortwave coming across Oklahoma this evening is poised to trigger organized activity that progresses over the lower MS Valley late tonight that guidance takes south and east later. Southern Plains... Mid-level ridging this morning primes OK into northeast TX for heavy and progressive convective activity this evening into the overnight. A marginal risk is warranted to be maintained over these areas with expansion farther south over the Red River where several inches have fallen over the past three days. Ohio... Added a focused Marginal Risk area where slow-moving convection has been producing locally intense rainfall. See WPC Mesoscale Precipitation Discussion 0493 for more details. Bann Day 2 Valid 12Z Fri Jun 16 2023 - 12Z Sat Jun 17 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER THE EASTERN GULF COAST AND FROM EASTERN COLORADO INTO WESTERN KANSAS... Mid-South through eastern Gulf Coast... Longwave troughing will remain over the eastern U.S. from Friday to Saturday morning and with mid-upper level ridging centered over northeastern Mexico, anomalously strong WNW mid-level flow will remain place from the Lower Mississippi Valley into northern Florida. At the surface, a frontal boundary (theta-e gradient) will exist from the Lower Mississippi Valley to the northeastern Gulf Coast and steering flow will continue to be mostly parallel to the frontal boundary orintation. Precipitable water values are forecast to remain high with 1.5 to 2.0 inches across the region with multiple rounds of thunderstorms possible, with one potentially ongoing near/north of the central Gulf Coast at 12Z Friday. While any organized storm clusters are expected to steadily propagate toward the southeast (per Corfidi vectors), short term training and repeating rounds of heavy rain could support rainfall rates of 2-3 in/hr at times, but 1-2 in/hr should be more common beneath diffluent flow aloft. The Slight Risk drawn where the best hires QPF model support for the period overlaps with higher rainfall totals forecast up to the beginning of the period. The Slight was adjusted a little east from continuity based on QPF from the 12Z guidance. Farther north and west into Arkansas, the threat does not appear as widespread when compared to locations along the Gulf Coast but similarly anomalous rainfall over the past week exists from central Mississippi into northern Louisiana and Arkansas. There is some support for locally enhanced precipitation rates in the afternoon/late evening near the frontal boundary boosted through some modest increase in the low level jet ahead of a weak 850 mb circulation forecast moving east from Oklahoma. Colorado Rockies and over the Great Plains... The upper trough axis over the Intermountain West shifts east from Friday into Saturday which takes the QPF focus area farther south and east compared to Thursday - generally over the CO Rockies and adjacent High Plains into Nebraska/South Dakota, east of 100W. A cold front will be the focus for heavy rainfall with anomalous moisture of roughly 1 to 1.5 standardized anomalies of PW forecast by the model consensus. Upslope flow and low level convergence across the High Plains will provide a focus for storms after initial diurnally driven storms move off of the higher terrain. If sufficient storm coverage occurs, clustering may result in eastward propagation from Colorado into Kansas but left exit region upper level jet divergence and strong diffluence aloft should support robust updrafts with potential for repeating rounds, especially southeastern Colorado into western Kansas with added low level convergence near a lee low. Northeastern Pennsylvania into central New York... A pair of mid-level shortwaves advancing across Quebec and the Northeast is expected to result in a a closed low by Saturday morning centered over Upstate New York. Pennsylvania and New York will be north of the better moisture to the southeast but PW values will remain seasonal at 1 to 1.3 inches along with weak instability of 250-750 J/kg. The main concern for flash flooding is recent heavy rain across the region with 2-5 inches over the past 3 days (before which, the region being rather dry) and locally lower FFG as a result. Slow storm motions of possibly 5 kt or less for some locations. Rainfall rates over 1 in/hr will be possible with a localized concern for flash flooding. Southern Sierra Nevada into Desert Regions of California and southern Nevada... While no risk area was drawn, portions of southern California into southern Nevada were considered for a Marginal Risk. PW values are expected to remain highly anomalous for mid-June (2 to 3+ standardized anomalies in CA), though lower through Friday night. The region will lie between an advancing upper level trough axis moving into the Desert Southwest and weak closed upper low offshore of the southwestern coast, resulting in a relative weakness in the mid-level flow. If storms are able to materialize along the higher terrain with daytime heating, forecast cell motions would be fairly slow. However, cloud cover and resulting instability is an unknown with little to no support from hires model QPF for anything concerning. This area will continue to be watched, but for now, no risk area was drawn. Otto Day 3 Valid 12Z Sat Jun 17 2023 - 12Z Sun Jun 18 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF THE GREAT PLAINS TO MID-SOUTH...CENTRAL GULF COAST/FLORIDA AND NEW ENGLAND... Great Plains into Lower Mississippi Valley... An upper trough axis slowly shifts east from the Rockies Saturday into Sunday with a pair of mid-level shortwaves moving through and into the Central Plains. Seasonal to weakly anomalous moisture is expected to be in place from the central to northern Plains as well as the Lower Mississippi Valley. A broad Marginal Risk area was drawn to cover a number of threats, including near a lingering frontal boundary in place from Arkansas into Mississippi and an advancing cold front through the central/northern Plains. While the risk area is likley too broad at this time, there wasn't enough confidence to highlight more precise locations given uncertainties in the guidance for the 48-72 hr forecast time frame. Local upgrades to Slight cannot be ruled out with future forecast cycles, especially considering locations that have wet antecedent conditions from prior rainfall. Central Gulf Coast into Florida... Locally heavy activity looks to linger over the central through eastern Gulf Coast into the Florida Peninsula on Saturday/early Saturday night. Ample moisture with PWs locally eclipsing 2 inches and sufficient instability will be present for this activity that may shift into the Gulf through the day. Given the wet antecedent conditions here, and potential for high end rainfall totals of 3 to 6 inches near the coast, the Marginal Risk was maintained and extended ESE a bit acorss Florida. New England... An anomalous and slow moving closed low is forecast to move from southern New England into the offshore waters from Saturday through Sunday morning. While moisture will remain seasonal for the region (1.0 to 1.4 inch PWs), instability will likely remain weak, perhaps even lower than the previous day. However, moist low level onshore flow will support localized areas of heavy rain with potential for rates above 1 in/hr. Localized 2-4 inch totals will be possible across the New England region, with a focus into the terrain with southeasterly 20 to 30+ kt 850 mb flow forecast and along the coast where convergence may be maximized. Otto Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8NsRYIX75Yde0nukQDaz2NJL9-BBeB630q1TqtFxfRKr= t7YxWB5KaVCjjIN7ML5cTnjAiBHTUmDjNr6_cLi1gau2CWk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8NsRYIX75Yde0nukQDaz2NJL9-BBeB630q1TqtFxfRKr= t7YxWB5KaVCjjIN7ML5cTnjAiBHTUmDjNr6_cLi1uPureCE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8NsRYIX75Yde0nukQDaz2NJL9-BBeB630q1TqtFxfRKr= t7YxWB5KaVCjjIN7ML5cTnjAiBHTUmDjNr6_cLi1KX26FNs$=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) .