Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jun 15 2023 20:28:29 FOUS30 KWBC 152028 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 427 PM EDT Thu Jun 15 2023 Day 1 Valid 16Z Thu Jun 15 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... ....1600Z update and 1620Z amendment... North Dakota/South Dakota... Added a Slight Risk for this update given 12Z HREF model output for 3 to 7+ inches over the 24 hour period from southern North Dakota into northern South Dakota. While the upper end of this range seems too high given the pattern, a region of the Northern Plains will see increased 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 later this afternoon and evening, continuing into the early overnight. Beyond 03Z or so, instability for North Dakota is expected to weaken but more of a stratiform rain could add to the 24 hour totals with 3 to 5 inches locally possible. Wyoming... Given consensus forecasts of instability only up to ~500 J/kg for portions of central and west-central Wyoming, rainfall rates may not be high enough to support widespread coverage of 1+ in/hr rates within the Slight Risk area for the middle portions of Wyoming but based on collaboration with RIW, the preference is to keep the Slight in place from the 09Z ERO issuance with 12Z HREF probabilities for 2+ inches showing 60 to 80 percent for the 24 hour period ending 12Z Friday. Rainfall over the past 7 days across a good portion of Wyoming has been 300 to 600 percent of average which will increase susceptibility to flash flooding. The southeastern portion of the Slight Risk for southeastern Wyoming into far western Nebraska looks reasonable with storms likely to form in the 18-21Z window ahead of an approaching front within an axis of anomalous moisture (1 to 2+ standardized anomalies of PW). ....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. The ongoing axis of convection from Mississippi to Georgia looks to continue much of the day while sinking southward with time. There could be a secondary flare up overnight, especially as upper level shortwave(s) appraoch from the west into the region. Otto ....previous discussion follows... Lower Mississippi Valley through the Southeast... The 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 continues this morning. Brief ridging builds in later today from the west which should provide a short break in activity, though a shortwave trough currently crossing into New Mexico from Arizona is poised to trigger organized activity that progresses over the lower MS Valley late tonight. Regarding morning activity, the western edge to the impulse series looks to be entering southern AR. Training convection extends east from southern AR across north-central MS and over southern AL. Much of this activity should continue a southeast progressive trend through the rest of the morning with the main threat areas after 12Z expected to be in a zone from central MS through the western FL Panhandle. Pooling of 1.75 to 2" PWs persists through this corridor extending north from the eastern Gulf Coast with abundant instability and deep layer shear maintaining organized activity to repeat over this already saturated section of the Southeast (as well as get into newer areas farther south such as around Mobile Bay. The Slight Risk extends from central MS down to the MS/AL Gulf Coast and east across southern AL/GA and far northern FL. 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. Intermountain West onto the Great Plains... An upper trough digs south over the northern Rockies this morning in the wake of the departing upper low into the Canadian Prairies, connecting a trough axis through southern CA. Lee-side cyclogenesis off the WY Rockies will draw Gulf-sourced moisture up over the High Plains and promote heavy rain (good potential for 1"+ hourly rainfall) over central through southeastern WY where a Slight Risk is maintained. A QPF axis over UT into the CO Rockies, similar to yesterday is expected with maintenance of the marginal risk there. Farther east on the northern Plains, Gulf moisture drawn up around the lee-side low and slowing cold front allows for increased PW around 1.25" (1.5 sigma above normal) and diurnally enhanced development of locally heavy thunderstorms. Slower southwesterly mean flow should allow some cell mergers, continuing to warrant a marginal risk over much of the Dakotas. Southern Sierra Nevada... Weak instability combined with PW of 0.75 to 1" (2 to 3 sigma above normal) will support locally heavy rainfall rates in diurnal convection. Some instances of flash flooding are possible, particularly in areas where soils are overly saturated from multiple recent days of rain and in rugged terrain warranting maintenance of the Marginal Risk. Jackson 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 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!-uU_rdLHBJnAuMiUw48YKIBcMqCtTNg-mg1sEcVu7ELA= IG6YO6zWFzrAfqhW2tPvH7hpNDLgVifpyP4HSrj-vmz4lVw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-uU_rdLHBJnAuMiUw48YKIBcMqCtTNg-mg1sEcVu7ELA= IG6YO6zWFzrAfqhW2tPvH7hpNDLgVifpyP4HSrj-lY4cv3Y$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-uU_rdLHBJnAuMiUw48YKIBcMqCtTNg-mg1sEcVu7ELA= IG6YO6zWFzrAfqhW2tPvH7hpNDLgVifpyP4HSrj-X73l6jg$=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) .