Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jun 11 2023 16:04:01 FOUS30 KWBC 111603 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1203 PM EDT Sun Jun 11 2023 =20 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF COLORADO AND THE OKLAHOMA PANHANDLE...=20=20 =20 ....16Z Update... ....Colorado and the Oklahoma Panhandle... Few changes needed to the outlook today for this area other than a small expansion of the Slight southeastward into the Oklahoma Panhandle in coordination with the AMA/Amarillo, TX forecast office. The pattern as described in the previous discussion provided below remains in effect. The guidance is suggesting some of the stronger storms may drift into the western OK Panhandle this evening, and the antecedent wet conditions there make that area a bit more vulnerable to flash flooding. ....Tennessee Valley... In coordination with GSP/Greer, SC and MRX/Morristown, TN forecast offices, the Slight risk area for portions of the Southeast was downgraded to a Marginal Risk with this update. Ongoing convection across north GA and into western SC have been lackluster, generally holding below the 1 inch per hour threshold with the strongest cells. More importantly, the entire MCS is moving at a decent clip, 15-20 kts, which is reducing the time any one area is seeing the heaviest rainfall rates. This theme is expected to continue today, despite plentiful atmospheric moisture with PWATS to 1.75 inches and instability eclipsing 2,000 J/kg. The fast movement of the storms, which according to most of the CAMs guidance is likely to be in the form of a southeastward-moving squall line, will prevent anything more than isolated flash flooding to occur, especially given how dry the area has been the past few weeks. This morning's rain may actually reduce the flash flooding potential this afternoon, as there will be several hours for the rain to soak into the parched soil, making it more receptive to soak up the additional rainfall expected. ....Midwest... No significant changes were made to the Marginal Risk area for the Ohio Valley and points north. A widespread 1-3 inch rainfall event is expected as a slow moving front allows a large area of stratiform rain to develop along the Ohio River today into tonight. Antecedent dry conditions and little instability remain likely to prevent flash flooding despite potential banding leading to slightly heavier rainfall rates. The greatest threat for isolated flash flooding and ponding will be in the urban areas such as Louisville, KY and the I-71 corridor through Cincinnati, Columbus and Cleveland, OH. Further west, much of MO was removed from the Marginal Risk area as the convection ongoing there wanes and only an isolated thunderstorm or 2 may possibly develop this afternoon with diurnal heating over the southern half of the state. ....CA/NV border... A high-end Marginal Risk remains in effect for this region, the combination of ongoing snowmelt out of the high Sierras, and showers and potentially a few thunderstorms capable of rainfall rates up to a half inch per hour moving over the mountains have the potential to cause isolated to widely scattered flash flooding. Wegman ....Previous Discussion... =20=20 Great Lakes/Midwest/Ohio Valley/Tennessee Valley...=20=20 A broad area of locally heavy rainfall is expected in the eastern portion of the country -- much of it welcome because of recent dryness -- as a deepening closed cyclone drops into the Great Lakes and phases with a shortwave moving eastward from the southern Plains. The best signal for heavy rainfall is in the TN Valley, where strengthening low-level inflow, CAPE of 2000+ J/kg, and PWs of 1.5-2" are forecast. There appears to be enough effective bulk shear for convective organization. These ingredients allow for hourly rain totals to 2.5". The guidance has a good signal for local 3-4" amounts. Outside of northernmost GA, the region has seen much below rainfall over the past week, if not two months. However, with the rain rates expected, particularly in urban areas, widely scattered to scattered instances of flash flooding are expected despite the recent dryness. The Slight Risk has been shifted southeast to best fit the available guidance.=20=20 =20 To the north, a front moves in while 1.5"+ PWs pivot in and around portions of the OH Valley, quite a different look than seen in the guidance 24 hours ago. Increasing 850 hPa frontogenesis, and the low-level inflow exceeding the mean 850-400 hPa wind are expected to lead to better than average rainfall efficiency. Outside of portions of southern IL, southern IN, and perhaps southwest OH, residence time of the highest moisture in OH is a bit more limited than seen on last night's guidance. Within this region that's been dry, some areas for quite a while, there's a concern that in some spots soils will be hard/brick-like initially, with rainfall all running off until soils can soften and allow infiltration.=20 Hourly rain totals to 2" should be possible, which would be problematic in urban areas. In order to avoid constant flip-flopping on the risk area in this region, and due to the moderately large mass field change over the past 24 hours, have left the level at Marginal in the Ohio Valley.=20 =20=20 =20=20 In and near Colorado...=20=20 Upslope flow behind a frontal zone imports moisture from the southern and central Plains into eastern Colorado, with PWs exceeding 1" in easternmost CO in a col point in the mid-level flow. CAPE is expected to rise towards 1000 J/kg. With the mid-level flow opposing the low-level flow, enough effective bulk shear should exist for organized convection, which could allow hourly rain totals to rise to 2" where cells train and/or merge.=20 Once cold pools becoming dominant, organized convection should plow east to southeast into the low-level flow, increasing storm scale inflow. The guidance has a good signal for local amounts of 2". Flash flood guidance is rather low, so a scattered risk of flash flooding is anticipated as both hourly and three hourly flash flood guidance values are exceeded. The Slight Risk area remains similar to continuity and matches the overnight model guidance.=20=20 =20=20 =20=20 Remainder of the West...=20=20 An anomalously deep closed low pivoting through southern CA into southern NV will help cause pockets of 1000+ J/kg of CAPE and PWs of 0.75-1.25". This is expected to cause isolated heavy rain concerns particularly at elevation across portions of the West and northern Rockies. Where cells train and/or merge, hourly totals of 1.5" are possible. Considering recent wetness, the existing Marginal Risk area looks reasonable at this time as cell coverage should not be dense enough to cause scattered flash flood issues.=20 Some expansion was made to the Marginal Risk area to account for the 00z HREF guidance.=20 =20=20 Roth 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 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... 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!6SnKkxBXwGTKuSGuxhHafHPrPt51weYouw5vKayKEO83= 4yy7C7AKBgjsEa4JuE_gltCFK8Buqe_rb9f_VogVYmfDCiM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6SnKkxBXwGTKuSGuxhHafHPrPt51weYouw5vKayKEO83= 4yy7C7AKBgjsEa4JuE_gltCFK8Buqe_rb9f_VogVVAKAp1M$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6SnKkxBXwGTKuSGuxhHafHPrPt51weYouw5vKayKEO83= 4yy7C7AKBgjsEa4JuE_gltCFK8Buqe_rb9f_VogVX0CNETQ$=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) .