Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 842 PM EDT Fri Jun 09 2023 Day 1 Valid 01Z Sat Jun 10 2023 - 12Z Sat Jun 10 2023 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH DAKOTA... The only Slight risk area remaining is across portions of central and western SD. Slow moving convection is ongoing along an axis of enhanced lower level convergence near a surface trough and ahead of southward dropping cold front. Shear profiles are pretty weak, but strong upper level diffluence is noted across the region...likely allowing cells to persist/organize a bit more than the weak shear would otherwise allow. This combined with the persistent convergence will continue to support some slow moving convection across the area for the next several hours. As the evening progresses instability will gradually erode, and the front will push south of the area...thus the heavy rainfall risk should decrease by the overnight hours. In the meantime 18z HREF neighborhood probabilities of 3"+ are over 40%, and EAS probabilities of 1" are 20-30%. This supports the idea of scattered 1" QPF coverage, and localized amounts exceeding 3" across central/western SD. Amounts of this magnitude are also supported by recent HRRR runs and MRMS QPE estimates. While this region has not been as wet over the past couple weeks as MT, isolated to scattered FFG exceedance is expected, which may result in some flash flood impacts. Elsewhere a broad Marginal risk covers much of MT into the Northwest, and much of the central Plains. Over MT/ID/WA/OR the risk appears to be on a downward trend as instability gets eroded, thus think a Marginal risk should cover the remaining threat...with mainly isolated flash flooding still a concern over these areas. Convection over KS/NE should organize into one or more convective lines that will progress southeastward through the overnight hours. In general the expectation is that this activity will become more progressive with time...thus while non-zero, the flash flood risk should stay generally localized in nature over this region. Chenard Day 2 Valid 12Z Sat Jun 10 2023 - 12Z Sun Jun 11 2023 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE PLAINS, ARKLATEX, AND THE MIDWEST... ...2030Z Update... ...NE/IA... In coordination with LSX/St. Louis, MO; EAX/Kansas City, MO; DMX/Des Moines, IA; and OAX/Omaha, NE forecast offices, the inherited Slight Risk was shrunk quite a bit to favor the MO River Valley between NE and IA. This area of the country has been running very dry, with 14-day AHPS Climatology guidance suggesting much of western IA and MO at under 50 percent of normal precipitation and it's even drier off to the east towards St. Louis. Further, there is some question as to how widespread the storms will be, and as such the signal for heavy rain is lower. MUCAPE values look to only peak between 1,000 and 2,000 J/kg, though PWATs will be sufficiently high at around 1.5 inches, which isn't unusual for this part of the country, but certainly is plenty of moisture to support thunderstorms. Given the very dry antecedent conditions, the uncertainty as to how widespread the storms will be, and that much of the area (especially in IA) is agricultural land that is hoping for rain, the flash flooding threat is determined to be low, so much of the area has been downgraded to a Marginal. For along the MO River through Omaha, rainfall today has moistened things up a bit, and some of the storms have been robust, dropping 1" of rain in 20 minutes, so the Slight was maintained in and around Omaha. ...Arklatex Region... The Slight Risk area is largely the same, but there was a small expansion southward included as the guidance hints at more widespread 2-4 inch rainfall totals from strong/severe thunderstorms on Saturday through this region. Energy moving along the southern branch of the jet stream will be enhanced by the approaching positively tilted trough which will be causing the convection further north. With additional moisture available due to proximity to the Gulf, the storms are likely to build to the south with time, requiring the southward expansion. FFGs are high in the region due to recent dry weather, so the flash flooding threat even with those higher amounts of rain should be limited. ...Intermountain West... No significant changes were made to the inherited Slight and surrounding Marginal Risk areas. Very wet antecedent conditions will help to make up for lesser amounts of rain expected Saturday as compared with previous days, as such, the inherited risk areas look good. Wegman ...Previous Discussion... Northern High Plains... Anomalous moisture continues early on before dropping off and southward as an upper level shortwave potentially splits, with some energy moving north into Canada and some undercutting the ridge and dropping southward. Instability lingers early on, so the existing Slight Risk still makes sense, with storms forming initially across MT before dropping southeast across portions of WY and western SD/NE. Midwest... A shortwave drops south around a forming closed low across southeast Ontario, with the overall system on a strengthening trend. A low- to mid-level low drops southeast and lures PW values up to 1.5-1.75" into portions of KS, MO, NE, and IA. Inflow at 850 hPa and effective bulk shear should be increasing, particularly late in the period when the low-level jet tends to ramp up Saturday night into Sunday morning which portends organized convection. CAPE values upstream are forecast as high at 3000 J/kg on the 21z SREF guidance during daytime heating, falling towards 1000 J/kg overnight. The guidance (outside the 00z NAM) has a cohesive signal for heavy rainfall in and around the offset KS/MO/NE/IA border junction where mid-level heights are diffluent, with the guidance having a decent signal for local 3-5" amounts. Flash flood guidance values, particularly outside MO, are modest, but the Kansas City urban area would naturally be susceptible to heavy rainfall. The ingredients available could support hourly rain totals to 2.5", which would be a problem in such an urban area. The Slight Risk area for these region remains similar to continuity per the above parameters. A broad Marginal Risk area continues to surround the Slight due to some lingering spread seen in the 00z NAM and 00z UKMET solutions. Portions of CA/NV... An anomalously strong closed low drops into CA Saturday into Sunday. Precipitable water values remain elevated, with moisture increasing in the Central Valley as the cold low develops and moves in. Instability shouldn't be a problem as temperatures aloft decrease. Believe a Marginal Risk of excessive rainfall/flash flood should continue Saturday into Sunday. Hourly rain totals up to 1.5" should be possible where cells backbuild, train, and/or merge. ArkLaTex to the Central Gulf Coast... As a southern stream shortwave phases with a deepening positively tilted upper trough, PWs rise towards 2" along a section of the polar front making some progress eastward. Low-level inflow/effective bulk shear appear to be sufficient for convection with some level of organization, which would support hourly rain totals to 2.5" somewhere between eastern TX, LA, and MS. The guidance is honing in on a cohesive heavy rainfall signal with local 4-6" amounts in the ArkLaTex, so introduced of a new Slight Risk area in that region. Roth Day 3 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... ...2030Z Update... No big changes were made to either of the risk areas on Sunday. The guidance remains in good agreement so much of the changes were cosmetic, including expanding the TN Valley Slight risk area just a bit to the north into KY. Otherwise the surrounding Marginal risk area was shrunk a bit to account for recent dry weather allowing much of the expected rainfall to be absorbed into the ground or by the flora of the area. Afternoon convection is likely to redevelop across much of the Intermountain West and along the CO Front Range, where the Slight was left unchanged with this update. Continued antecedent wet conditions will favor lesser amounts of rain still being capable of producing isolated flash flooding from the Sierras of CA up into MT. Wegman ...Previous Discussion... Great Lakes/Midwest/Ohio Valley/Tennessee Valley... A broad area of locally heavy rainfall is expected in the eastern portion of the country 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". 00z UKMET guidance indicates local 4"+ amounts, while the 00z Canadian Regional advertises local 7"+ amounts. Outside of northernmost GA, the region has seen much below rainfall over the past week. 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 risk level in the TN Valley to a Slight Risk. Coordination with JKL/Jackson KY and the LMK/Louisville KY forecast offices kept the Slight Risk area out of southernmost KY. In and near Colorado... 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. 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 was shifted southward from continuity to better match the current model guidance. Remainder of the West... 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. Roth Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt