Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jun 11 2023 00:43:46 FOUS30 KWBC 110043 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 842 PM EDT Sat Jun 10 2023 Day 1 Valid 01Z Sun Jun 11 2023 - 12Z Sun Jun 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE PLAINS AND MS Valley... Two main areas of focus for the most organized heavy rainfall...one over southeast TX and the other along an axis from eastern NE into western MO. Over TX, some west to east training along a surface trough is expected to continue as convection only slowly shifts southward this evening. FFG is quite high across this corridor, which will be a limiting factor for flash flooding. However with upwards 4000 J/KG of MLCAPE overlapping ~1.7"+ PWs, cells will be capable of very heavy rainfall rates. This combined with the aforementioned slow movement/training, should allow for at least some instances of localized flash flooding...with a good signal for swaths of 2-4" of rainfall. Slow moving convection from eastern NE into western MO will also pose some flash flood risk into tonight. Continue to see a good signal for localized 2-4" rainfall totals here as well. Should end up with at least some isolated FFG exceedance over portions of eastern NE into adjacent portions of southwest IA and northeast KS...but might be a bit tougher over MO where FFG is quite a bit higher. Overall this appears to be on the lower end of the Slight risk definition (which is 15-40%), with isolated to widely scattered flash flooding the expectation within the slow moving convective regime. Elsewhere a localized flash flood risk continues over portions of CA/NV and over WY/MT, but coverage of any impacts should stay pretty localized in nature. Also can not rule out a localized concern over portions of MS within any cell mergers with the ongoing convective activity. Chenard Day 2 Valid 12Z Sun Jun 11 2023 - 12Z Mon Jun 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF COLORADO & THE TENNESSEE VALLEY...=20 ....1830Z Update... The only significant change with this afternoon's ERO update was to eliminate the Slight Risk area across the Ohio Valley, while the dynamics of a strong upper level low moving into the area are impressive, especially for this time of year, the lack of instability is a huge detriment to getting rainfall rates needed to cause both significant runoff on the very dry soils in this area, and to get the rates needed for flash flooding. Think instead this area will see a prolonged period of light to moderate rain, which will fall at a slow enough pace that it will have time to soak into the dry soils. Initially the Slight Risk area was trimmed out of northeast Ohio/western Pennsylvania, but with even areas to the southwest which will have some instability (though only to 500 J/kg) not expecting flooding, instead opted to drop the Slight entirely. This was coordinated with the ILN/Wilmington, OH and LMK/Louisville, KY offices. Further south, QPF trends across the TN Valley have come down a bit with the latest guidance trends, but it will be worth noting whether this remains the trend with upcoming guidance. In the meantime, the Slight Risk area was expanded a little towards the southeast down the GA/SC border, given a shift in the local maximum of average rainfall towards the south and east. There should be plenty of instability in this area, but the problem is that any rounds of convection are expected to move through the area quite quickly, so any one area may not see heavy rain for more than a few minutes. This too will limit the flash flooding potential. Given several urban areas including Memphis, Nashville, and Atlanta are in the Slight risk, which all inherently have lower thresholds for flash flooding, have opted to leave the Slight up as a low-end Slight for the time being. The portion of the Slight Risk area over central AL was also trimmed towards the north in coordination with the National Water Center, due to soils being less favorable for flash flooding in that area (minus any urban areas) than points further north into far northern AL. Finally, the Slight risk area over Colorado was nudged a hair towards the west to focus more of the area towards the Front Range. This aligns with the most recent updated average QPF. Any strong or severe thunderstorms over this area will be slow moving with the upslope flow and will struggle to move too far from the mountains. No significant changes were made to the surrounding Marginal risk area over much of the intermountain West, as this area generally has been quite wet, so the lighter forecasted amounts of rain here may still pose an isolated flash flooding threat, particularly on the slopes of any mountains impacted by the shower and thunderstorm activity. Wegman ....Previous Discussion... =20 Great Lakes/Midwest/Ohio Valley/Tennessee Valley...=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=20 effective bulk shear for convective organization. These ingredients allow for hourly rain totals to 2.5". The 00z UKMET and 00z NAM guidance indicate local 3-4" amounts, while the 00z Canadian Regional advertises local 4-6" 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, so raised the TN Valley remains in a Slight Risk.=20 To the north, a front moves in while 1.5"+ PWs stay over portions of OH for 12+ hours. Some instability is expected to sneak in to southwest OH from KY with time, but the combination of lowish instability, 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. Within this region that's been dry, some areas for quite a while, there's a concern that soils will be hard/brick-like initially, with rainfall all running off until soils can soften and allow infiltration. With hourly rain totals maxing out in the 1.5" range at the southwest portion of the new Slight Risk, and closer to 0.75-1" in the northeast portion of the new Slight Risk, the three hourly flash flood guidance should be able to be exceeded on a scattered basis, hence the new risk area. Coordination with JKL/Jackson KY, LMK/Louisville KY, and ILN/Wilmington OH forecast offices led to the current Slight Risk configuration.=20 =20 =20 In and near Colorado...=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 which matches the overnight model guidance.=20 =20 =20 Remainder of the West...=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 Roth Day 3 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... ....1830Z Update... No major changes were made to either Slight Risk area on Day 3. The one in Colorado was expanded a bit per both latest guidance trends and the expectation of rains causing widely scattered flash flooding on Day 2 (Sunday). The Slight over the Arklatex region was shifted north a bit, again following the latest ensemble guidance, but overall changes were small. The strong front over the Northeast and Mid-Atlantic will have little instability, just like over the Midwest on Day 2, so think much of the 1-2 inch totals over PA and NY will be stratiform rain which will be largely welcomed after a very dry and smokey period of weather. Wegman ....Previous Discussion... In and near eastern CO... 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 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 Near and west of the ArkLaTex... 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. Outside of the 00z NAM, there is a coherent and fairly agreeable signal for local amounts in the 3" range near the eastern border of OK and TX. 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 is raised for the area. Intermountain West... 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 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. Northeast/Great Lakes... With instability expected to be minimal across the region, frontogenesis in the low- to mid-levels would be doing the heavy lifting 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) despite the wet 00z Canadian Regional and 00z ECMWF QPF solutions showing 2-4" totals over portions of NY, so the Marginal Risk area was dropped, a conversation started by the CTP/State College PA forecast office. The situation is trickier across MI, as frontogenesis will move minimally across the Upper Peninsula of MI. The center of cold lows sometimes surprise if enough daytime heating is around. Still, hourly rain totals are expected to max out in the 0.5-0.75" range and aren't expected to pile up above 2". Since it's been so dry, per coordination with APX/Gaylord MI and MQT/Marquette MI forecast offices, went ahead and dropped the inherited Marginal Risk area for MI. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_jaoWRkVd97ntX5mjX9pP7bTJrqt5nfxx9JyTBy55GGM= cA9fBFeB53eRXI2zg8IffyYDntSRaI-dYngwhWRvcriNQ8Y$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_jaoWRkVd97ntX5mjX9pP7bTJrqt5nfxx9JyTBy55GGM= cA9fBFeB53eRXI2zg8IffyYDntSRaI-dYngwhWRvG4jEB9k$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_jaoWRkVd97ntX5mjX9pP7bTJrqt5nfxx9JyTBy55GGM= cA9fBFeB53eRXI2zg8IffyYDntSRaI-dYngwhWRvLbTD03o$=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) .