Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jun 12 2023 00:49:06 FOUS30 KWBC 120049 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 848 PM EDT Sun Jun 11 2023 Day 1 Valid 01Z Mon Jun 12 2023 - 12Z Mon Jun 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF COLORADO, NORTHEAST NEW MEXICO AND THE OKLAHOMA PANHANDLE...=20=20 =20 ....High Plains and points west... The main focus here will continue to be portions of eastern CO into far northeast NM and the western portions of the OK Panhandle. Slow moving convection will continue to pose an isolated to scattered flash flood risk across this region through the evening hours. Indications that the southern activity near the CO/NM border may have some more longevity into the overnight hours as plentiful upstream instability continues to advect into the area within the low level east southeast upslope flow. The activity further north is more likely to subside as the evening progresses and instability erodes. The southern activity could grow upscale to some extent and not move all that much, with periods of heavy rainfall into the overnight hours over portions of northeast NM into adjacent areas of CO and the OK Panhandle. Recently issued MPD 464 gives some additional details on the near term flash flood threat over these areas. Elsewhere over the West a continued isolated flash flood risk exists over a large geographic extent from CA into western MT. This activity will begin to wane with the loss of daytime heating, but will persist a bit longer into the evening than it otherwise would given the troughing over the area. ....North central TX... Convection has developed over portions of north central TX along a stationary front. This was more of a conditional threat...with some guidance developing convection and others not. Clearly convection has been able to develop...with ongoing activity following along with some of the more aggressive CAM solutions. MLCAPE is as high as 5000 J/KG and PWs are around 1.5"...so these cells will be capable of heavy rainfall rates. Cells should tend to slowly propagate off to the south, but the presence of the stationary front and mean flow parallel to the front suggests we should also see some backbuilding/training near the boundary. Do tend to think QPF will verify on the high end of the model guidance given current observational trends. A localized flash flood risk appears to exist where any slow moving cells are able to persist or any small scale training develops. ....Eastern U.S.... Convection moving across portions of KY/TN/OH will generally stay progressive in nature...and FFG is very high over this region. Thus not expecting anything more than an isolated flash flood threat. Further south from MS into AL/GA and western SC...convective coverage and intensity is expected to increase overnight. The activity currently stretching from central TN into northern MS will become even more organized as it pushes southeastward tonight. The developing convective line should generally stay progressive enough to limit the extent of any flash flood risk. However an isolated flash flood risk still exists where any more discrete downstream activity ends up forming and merging into the approaching convective line. The very high FFG is another factor keeping the risk at only a Marginal level. One area to keep a close eye on is southeast AR into northern MS. There is a bit more of a signal for potential training here with some stronger lower level convergence where the cold front starts to become more stationary in nature. Some increased backbuilding of activity is expected over the next several hours, likely resulting in some swaths of 2-3"+ rainfall. Considered a targeted Slight risk upgrade, but think the drier antecedent conditions and higher FFG keeps this more of a higher end Marginal risk. Would expect some flash flood potential to evolve, but coverage should stay rather low. Chenard Day 2 Valid 12Z Mon Jun 12 2023 - 12Z Tue Jun 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR COLORADO & THE ARKLATEX...=20 ....20Z Update... No big changes were made to the Day 2/Monday EROs, mostly minor tweaks as following the latest guidance. ....CO Area... The inherited Slight Risk was expanded a little bit into the southeastern corner of Wyoming and also southeastward into portions of the OK Panhandle. This follows a fair number of guidance members suggesting storms capable of heavy rainfall rates will impact these areas as well. Otherwise the discussion below remains in effect. The expansion into the OK Panhandle was coordinated with the AMA/Amarillo, TX forecast office. ....OK/AR... The inherited Slight Risk area was expanded a couple rows of counties to the north into OK and AR, but once again few changes needed as little has changed in the guidance. ....Northeast... Small expansions of the Marginal Risk area were made into Philadelphia and north more into the eastern Lake Ontario region for a likely 1-3 inches of training precipitation into this area. Lack of instability and antecedent dry conditions should largely prevent most flash flooding, however rates may get high enough especially over urban areas to cause localized flash flooding in poor drainage areas. Wegman ....Previous Discussion... In and near eastern CO...=20 An upper low moves from the Desert Southwest into the Great Basin with time. Moisture in the High Plains of CO and western KS remains potentially problematic for the region, with PWs of 0.75-1". Daytime heating is expected to lead to 1000-1500 J/kg of CAPE. While there is modest low-level upslope flow, the flow=20 aloft is westerly -- enough effective bulk shear should be in place to promote organized convection capable of heavy rainfall, up to 2" an hour. Activity should tend to move eastward and southeastward per the forecast 1000-500 hPa thickness lines and the forward propagating convective motion vectors which would enhance storm-relative inflow. Local amounts of 2-4" are forecast by the guidance, which is expected so long as convection remains progressive. Heavy rainfall should be aided by storm mergers and short periods of training. The Slight Risk area from continuity has been generally maintained.=20 =20=20 =20 Near the ArkLaTex...=20 Inflow into a front drifting across the region near the apex of a mid-level ridge is expected to maintain a pool of 1.5-2" PWs downwind of 2000-4000 J/kg of CAPE. This is a traditional location for a convective complex, even if it would normally be a bit north of here in mid-June. There is a coherent and fairly agreeable signal for local amounts in the 3-4" range near the eastern border of OK and TX; the 12z ECMWF showed a 6" bull's eye right over the ArkLaTex. Hourly rain totals to 2.5" are possible, and local amounts to 4" are considered possible. Rainfall on Saturday/Saturday night is expected to allow for some saturation across the area. Combined with the guidance QPF and available ingredients, a Slight Risk remains for the area, and has been slightly expanded. Due to the instability pool developing in TX -- which implies some mid-level capping -- think there's more confidence in the western portion of the Slight Risk area remaining stable as we get closer to the event than the eastern portion. =20 =20 Intermountain West...=20 Under the upper level low migrating across the region, enough moisture (particularly at elevation) and instability (up to 1500 J/kg of CAPE) should be available to aid convective development due to daytime heating in the afternoon and evening hours. Hourly rain totals are possible should cells merge and/or train. The=20 ceiling for hourly rain totals should be 1.5", but cell coverage is not expected to be dense enough for more than isolated to widely scattered heavy rain-related issues. The existing Marginal Risk area from continuity still looks good. =20 =20 Northern Mid-Atlantic States...=20 Frontogenesis in the low- to mid-levels combined with some instability will be joining forces from a heavy rainfall perspective. Across the Northeast, the expected rainfall/moisture plume is expected to be progressive enough to be more helpful than harmful (considering recent dryness). However, the 00z HREF had decently high probabilities of 3"+ across portions of PA, while the 00z GFS showed similar in NY. Hourly rain totals up to 2" are possible wherever cells can merge, train, or random mesocyclones can manage to form. =20 Roth=20 Day 3 Valid 12Z Tue Jun 13 2023 - 12Z Wed Jun 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN PORTIONS OF THE MID-SOUTH & SOUTHEAST... ....20Z Update... No significant changes were made to the Day 3/Tuesday ERO areas this afternoon. There remains some spread as to where the axis of heaviest rain sets up...primarily latitudinally. There may be additional north-south adjustments with upcoming updates, but in the meantime there was enough hint in the guidance that the significant convection would start just a hair further east, so the slight was taken out of the corners of TX and OK and added to into western GA. There is at least some potential that an upgrade to a Moderate may be needed with future updates, but that will highly depend both on how the rainfall footprint turns out on Day 2/Monday, and how consolidated along the stalled out frontal boundary the heaviest rains can get. Nonetheless the potential for repeating and backbuilding thunderstorms over a narrow area somewhere in southern AR/northern MS and northern AL is there. QPF has come up a little bit from previous forecasts, and is likely to continue into Day 4 for eastern areas (AL/GA). By this point there should be a better indication as to how the storms will behave and how big an area they will cover. The Marginal Risk area over much of the intermountain West remains unchanged, as afternoon convection is likely to redevelop over many of the same areas as in previous weeks. Wegman ....Previous Discussion... Mid-South/Southeast... A closed low/shortwave is expected to move from the Rockies across the Southern Plains toward the Lower MS Valley. At the surface, a frontal zone is expected to meander across the region, pooling precipitable water values of 1.5-2". CAPE values up to 3500 J/kg are expected to reside within its warm sector. There should be enough low-level inflow and effective bulk shear to organize convection. Cell training from WNW-ESE and occasional mesocyclone formation are the main concerns from a heavy rain/flash flooding standpoint, which in this environment could lead to hourly rain totals up to 2.5". The guidance has a good signal for local 3-5" totals. As this will be the second day of the upcoming three where heavy rain is expected across the region, flash flood guidance should be compromised to some degree from their current values. The Slight Risk was expanded based on the reasonable model agreement in this area, and the Marginal Risk for the region was separated from the Marginal Risk in the Intermountain West.=20 The environment is uncapped, which could lead to some southward adjustment in the eastern side of the risk area, but the high CAPE pool to the west should keep that portion of the heavy rain risk areas relatively consistent from here on out, so long as there is no major model shift in the general flow pattern.=20 Intermountain West... A sharp upper level trough is expected to dive down from western Canada, which should enhance divergence aloft across the region.=20 Pockets of 0.75-1.25" PWs combined with 1000+ J/kg of ML CAPE due to daytime heating should allow for convection to develop in the afternoon and evening hours. Hourly rain totals to 2" are possible within such an air mass where cells merge and/or train.=20 Guidance QPF totals max out close to 2" in spots, so the rainfall in any one place could occur within an hour and pulse out/send out outflow boundaries to initiate new convection. As any heavy rain-related issues are expected to remain isolated to widely scattered, have maintained the Marginal Risk for this region from continuity. Maine... A deep layer cyclone will be moving towards the region from the Great Lakes. Instability and system progression appear to be the limiting factors, with the 00z NAM only indicating MU CAPE rising to 250-500 J/kg in this region. Guidance QPF amounts are exclusively below 2.5". While it's possible that the guidance will trend upward in instability in later runs for this period, right now there's not enough there so the Marginal Risk was removed. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9TFUslSA7qJe3X88QFK4ZUeQ9ZhHxdOGQpJRNGMVIV_z= kb4QqO0PraX5mpPngWUEdv8CC9K9tqotCywW4FatChm52gE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9TFUslSA7qJe3X88QFK4ZUeQ9ZhHxdOGQpJRNGMVIV_z= kb4QqO0PraX5mpPngWUEdv8CC9K9tqotCywW4FatX5TpuLI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9TFUslSA7qJe3X88QFK4ZUeQ9ZhHxdOGQpJRNGMVIV_z= kb4QqO0PraX5mpPngWUEdv8CC9K9tqotCywW4FatvPzCirU$=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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