Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Fri Jun 09 2023 18:52:00 FOUS30 KWBC 092012 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 411 PM EDT Fri Jun 09 2023 Day 1 Valid 16Z Fri Jun 09 2023 - 12Z Sat Jun 10 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF MONTANA... ....16Z Update... ....MT/SD... A few notable changes were made with this morning's update. First, the Moderate Risk area across north central Montana was expanded a bit towards the southeast, but still following the areas of MT with very wet antecedent conditions based on AHPS guidance. The Moderate Risk area includes those portions of eastern MT that have been 300% or more above climatology as far as rainfall amounts go over the past 1-2 weeks. In addition, the models are trending towards there being heavier rainfall amounts into southeast MT and especially into northwest SD. A strong and fast moving cold front will dive out of Manitoba and Saskatchewan into ND this evening, reaching the SD border by early Saturday morning. Meanwhile, northeastward moving convection on the leading edge of a stalled out front will form convection across MT. When these 2 boundaries collide, expect cell mergers to be common, and while the one front will be moving quickly, the storms it forms will not be, so nearly stationary convection may also pose a flash flooding threat. There is general model consensus that the heaviest rainfall amounts will be across portions of northwestern SD, where there will be greater forcing ahead of the cold front racing southward across ND. Once the two fronts combine, somewhat lighter, but still steady rain will continue along a northwest to southeast oriented line that will remain stationary across southeast MT and northwest SD. While these areas have been much drier than areas further west, the threat for both heavy and much steadier rainfall is significant, and it's likely widely scattered instances of flash flooding will develop, in accordance with the Slight risk area already out. The heavier rain should be offset by the drier antecedent conditions a bit, allowing for a more widely scattered flash flooding risk. ....OR/ID... The other large change with this morning's update is to upgrade much of eastern OR and far western ID to a Slight Risk. Nearly stationary convection is already ongoing across the area, underneath a stationary upper level low. With diurnal heating, instability will increase, and when added to highly anomalous PWAT values over 1 inch, will result in more widespread showers and thunderstorms that will remain nearly stationary. Rates up to 2 inches per hour will be possible. Given this area also has low FFGs and is therefore prone to flash flooding, this scenario should result in more widely scattered instances of flash flooding, coincident with a Slight Risk. This upgrade was coordinated with the PDT/Pendleton, OR and BOI/Boise, ID forecast offices. ....Elsewhere... The 2 Marginal Risks in the central Plains and north Florida remain unchanged with this update, as the guidance remains in good agreement for widely scattered storms that may cause isolated flash flooding. Wegman ....Previous Discussion... ....Sierra through the Pacific Northwest and Northern Rockies... A weakening mid-level trough will continue to lift north as it navigates around the east side of a somewhat phased trough just offshore the West Coast. The incoming system has an extremely moist environment to work with as PWs remaining as high as +4 standard deviations near the Canadian border, while a plume of 1000+ MUCAPE advects westward from the High Plains. This setup should result in scattered to widespread heavy rainfall, which will occur on top of soils that will be highly vulnerable to runoff due to a very wet week/month/two months. The inherited Moderate Risk area was left alone as it is an area with decent 3"+ probabilities and non-zero 5"+ probabilities over an area of saturated soils apparently driven from the 00z Canadian Regional maximum near 6", using the 300-600%+ weekly rainfall from AHPS as a guide. The Slight Risk in western SD encompasses the highest risk for 3"+ totals with non-zero probs of 5"+ totals -- guidance shows local 4-6" totals. However, this region has been quite dry so left their risk level as Slight for the time being. Farther to the southwest, another day of scattered slow moving showers with isolated thunderstorms is forecast for portions of OR and ID. The organization and duration of convection is expected to be less than points northeast, but given the available ingredients, hourly rain totals to 1.5" are possible, particularly at elevation. This could result in at least isolated instances of runoff or flash flooding on Friday. ....Southern to Central High Plains... A shortwave in the southern stream of the Westerlies moves out of the West and through the Southern Plains Friday into early Saturday. This should strengthen the southerly flow across the region, noted by increasing low level moisture transport through the Southern Plains. There is some consensus that a stripe of showers and thunderstorms will develop and push eastward beneath this shortwave into the better moisture, particularly from CO east and southeast through portions of KS, the TX Panhandle, OK, and western AR. Model spread with location and amounts remain an issue, but there remains a signal for local 2-4" of rain in this area. The available ingredients suggest 2.5" an hour totals would be the high bar, which would be problematic in urban areas. The inherited Marginal Risk was split in order to differentiate this area from activity expected farther north across the Northern Plains. Heavy rain-related issues here are expected to be isolated to widely scattered. ....Northeast Florida... The 00z NAM and 00z Canadian Regional models have local 3-6" values advertised across this region near a segment of the polar front. PWs rise to ~1.75" and instability of ~1500 J/kg is expected to lie across the region while under an upper level trough though somewhat east of its axis. Since the flow is generally out of the west, this should concentrate activity towards the eastern portion of the Marginal Risk area. Still, with the front in the vicinity, it could focus activity along its length should CIN build in and any instability remains aloft. Flash flood guidance values are high, but hourly rain totals rising towards 2.5" potentially would be a problem in urban areas within this region. The Marginal Risk area was refined somewhat, but retained in this region. Roth 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. 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