Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Aug 01 2023 20:46:03 FOUS30 KWBC 012045 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 445 PM EDT Tue Aug 01 2023 Day 1 Valid 2043Z Tue Aug 01 2023 - 12Z Wed Aug 02 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF EASTERN MISSOURI... ....Eastern Missouri... In coordination with LSX/St. Louis, MO forecast office, a Moderate Risk area was introduced for portions of eastern MO with the late morning update. Much of the guidance continues to indicate a second round of training convection, which is expected to be a bit more prolific with rainfall totals than the area dying out over central MO currently. Using this morning as precedent, where there were scattered instances of flash flooding, this second round is likely to produce similar impacts. The good news is that the area expected to be impacted with flooding rains tonight will be displaced to the east just enough such that there is only limited overlap with the rainfall that occurred overnight last night. There remains considerable uncertainty as to the level of impacts in the St. Louis metro itself, as any notable eastward deviation of the line of storms could result in significant impacts to the city itself. For now, will stick with the guidance consensus that the heaviest rains will remain just west of town, though that will certainly bear watching. It's worth noting that the latest HRRR guidance does bring the axis of heavier rains into town, but is a fair bit weaker in total rainfall than both its predecessors and other current guidance. Expect there will be a bit better upper level forcing tonight, and the west-southwesterly LLJ will be pumping more moisture into the region tonight as compared with last night, so the expectation of greater impacts is well-founded. As regards timing, we remain in this pattern with a dichotomy between a large majority of the heaviest rainfall associated with tonight's rainfall happening before the 12Z cutoff for the Day 2/Wednesday period, but with enough rainfall (especially in southern areas) ongoing past 12Z, along with most of the major impacts, as river rises lag behind the heaviest rainfall, that the impacts of tonight's Moderate lasting into the Day 2/Wednesday period. That will be reevaluated with the Day 2 update this afternoon. For now, there is good confidence that enough rain will occur before 12Z, especially using this morning as a guide, that flooding impacts will be ongoing during the Day 1 period, which required the risk upgrade. ....Northeast CO into southwest NE and far northwest KS... The Slight Risk area was maintained today with minimal changes. The guidance hinted that heavy repeating storms may persist further eastward along the NE/KS border, so the Slight was expanded a bit. Of course this is expected to occur in areas that have been hit with heavy-rain-producing thunderstorms the past couple afternoons, so soils have been primed to flood with today's rainfall. This same disturbance causing the afternoon convection today will track downstream and force the third consecutive night of heavy rains across IL/KY Wednesday night. ....UT/NV/AZ/CA... With the morning convection, and the likelihood for additional storms this afternoon, the Marginal Risk was expanded to include more of the Las Vegas, NV area, extending east into southeastern Inyo County, CA. Afternoon thunderstorms have broadened the risk across UT and AZ. Locally heavy rains could produce localized flash flooding especially in any canyons, dry washes, and burn scars. ....Treasure Coast of FL... In coordination with MLB/Melbourne, FL and MFL/Miami, FL forecast offices, a Marginal Risk was introduced for the I-95 corridor east of Lake Okeechobee up through southern Brevard County. Another afternoon of locally heavy rains from slow moving thunderstorms forming along the sea-breeze front is likely, with the storms impacting areas hard hit in previous days. The greatest threat for localized flash flooding will be where any nearly stationary storms form over urban and flood-sensitive areas. Roth/Wegman Day 2 Valid 12Z Wed Aug 02 2023 - 12Z Thu Aug 03 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID MISSISSIPPI VALLEY... ....2030Z Update... ....Eastern Missouri through Southern Illinois... A significant rainfall event for all or part of the St. Louis metro area continues to be suggested by much of the latest guidance this afternoon. The ingredients coming together include: 1) A strong low level jet (LLJ) colliding with a 2) nearly stationary frontal boundary with 3) some upper level support in the form of a shortwave trough and the LLJ transporting 4) highly anomalous moisture in the 95th to 99th percentile of climatology along with 5) MUCAPE values starting out the night Wednesday night at over 2,000 J/kg are all looking to come together around or near St. Louis Wednesday night into Thursday morning. This will be on the back of a separate training event that will be occurring overnight tonight into Wednesday morning. This event is likely to impact areas of eastern Missouri just west of both the Mississippi River and the St. Louis metro. Areas north and west of St. Louis that are likely to be impacted by both nights' events are most likely to see the most extreme impacts, but it's worth noting that the urbanization around St. Louis could exacerbate the impacts locally with just one night's event of heavy rain. The front forcing the storms will be nearly stationary, and the LLJ being oriented orthogonal to the front will allow constant moisture replenishment of the storms as they track southeastward along the front, making for a nearly ideal scenario for training of heavy-rain-producing thunderstorms. There is potential for locally extreme amounts of rain, with values approaching 12 inches for just Wednesday night's event, with 2 day totals likely higher in areas where the 2 training events overlap. In coordination with LSX/St. Louis, MO and PAH/Paducah, KY forecast offices, the Moderate Risk was shifted a bit north and east in following with the latest guidance consensus, along with CAMs simulating the full day's rainfall fully. The most extreme solution is the NAMnest, which usually is a high outlier, but has a bullseye of 7 to 9 inches of rain over a swath covering the area of land between the Mississippi and Missouri rivers at the mouth of the Missouri River north of St. Louis. HREFbm and HREFpmm guidance are slightly lower in the amount of rainfall, but are still suggesting 5-7 inches of rain in this same area, which includes all of metro St. Louis. Referring to previous events, the last extreme event in St. Louis on July 26, 2022 produced upwards of a foot of rainfall, and the HREFbm and HREFpmm were noted as being particularly useful and consistent in this area. All-too-recent experience says these extreme amounts of rain are very possible in this kind of pattern, with the main uncertainty being where that bullseye of rain occurs, which often isn't clear until the event is underway. Nonetheless a consensus of the guidance suggests anywhere along the Mississippi from Iowa to Kentucky could potentially be hit with these extreme rainfall amounts, but it's highly likely that the area seeing that much rainfall will be very localized. Thus, there's likely to be a large to extreme gradient of rainfall with just 25 miles being the difference between occasional light rain and the deluge feared likely somewhere in the Moderate Risk area. It's also worth noting that much of the guidance suggests heavy rain will also impact extreme western Kentucky, which was hit with historic rainfall not 2 weeks ago. While there's a bit more certainty that the most extreme rainfall amounts will remain north of that area, even the several inches of rain forecast for this area could hamper recovery efforts and potentially cause additional damage in this already hard-hit area. ....Great Basin to North/Central Rockies and High Plains... The Slight Risk area inherited was expanded a little bit to merge the previous 2 Slights in north central CO with the larger Slight over northeast UT, southeast ID, and western WY, to include the rest of central WY. The rainfall signal in this area has increased with the latest guidance, and PWAT values as high as 1.25 inches is well above the 95th percentile of climatology in this area. A slow-moving but potent shortwave trough will traverse the area, providing forcing for the anomalous moisture to produce scattered thunderstorms capable of heavy rains that could flood dry washes, canyons, burn scars and other sensitive areas of this region, that are largely spared heavy rain events. Wegman ....Previous Discussion... ....Mid-Mississippi and Lower Ohio Valleys... Convection will be ongoing at the start of the period across portions of Missouri, associated with the nighttime low level jet interacting with a frontal boundary draped in the region and shortwave energy moving through the northwest flow aloft. Favorable moisture axis and sufficient elevated instability will continue the threat of locally significant heavy rainfall and flash flooding early on in the period Wednesday morning where the latest deterministic and ensemble guidance supports potential for several inches. This should fall slightly displaced from the heavy rainfall on Tuesday morning, but also could be more significant given the more favorable environmental ingredients. In coordination with WFO St. Louis, a Moderate Risk was introduced to account for the potential for locally but significant flash flooding early Wednesday. Another round of heavy rainfall is also then likely to develop again Wednesday night into Thursday morning, though with a warm front beginning to advance more eastward, this axis of heavy rainfall looks to be displaced eastward yet again but could fall near or around the St. Louis metro area and into southern Illinois and with similar environmental ingredients (high PWs, warm cloud depths, strengthening low level jet), swaths of heavy rainfall will be likely at the end of the period into Thursday morning. ....Great Basin to North/Central Rockies and High Plains... Widespread showers and thunderstorms are expected Wednesday into Wednesday night aided by shortwave energy and right entrance dynamics with a strengthening jet streak over the northern Rockies. Moisture will be very anomalous, upwards of 1-1.25" of precipitable water and nearly 3 standard deviations above normal, based on the latest model guidance and with the daytime heating and terrain influences, localized intense rain rates exceeding 0.5"/hr will be possible. The 00Z guidance continues to highlight portions of eastern Idaho, western Wyoming, and northern Utah with the greater probabilities of seeing the higher rainfall totals as well as portions of Colorado and southeast Wyoming where Slight Risks were maintained from the previous forecast cycle. Taylor Day 3 Valid 12Z Thu Aug 03 2023 - 12Z Fri Aug 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE TENNESSEE VALLEY AND NORTHERN AND CENTRAL PLAINS... ....2030Z Update... ....Tennessee Valley... The Slight Risk across the TN Valley was shifted north and east with this update, consistent with the latest trends in the guidance for both the Day 3/Thursday period and prior periods as well. Expect a widespread 2 to 4 inches of rain across much of eastern TN primarily, but extending into southern KY and the Great Smokies of extreme western NC. The rain will be associated with the same disturbance expected to produce significant rains over the mid-Mississippi Valley Wednesday night, with the bulk of this event occurring Thursday night into Friday. While upslope may play a role in far eastern TN, the bulk of the event will continue to be a moisture-laden air mass colliding with a front over the area producing a more widespread swath of 2 to 4 inches of rain. The ingredients mentioned that are likely to cause more significant flooding in the mid-Mississippi Valley are not expected to be quite so coordinated by the time the heaviest rain reach the TN Valley, but widely scattered flash flooding is possible, especially where terrain will help focus the rainfall into narrow streams and creeks. There will likely be a local minimum of rainfall across central KY and TN, where daytime heating typically disrupts the advection associated with the LLJ so any storms that form will not be nearly so persistent as the nighttime convection. ....Northern and Central High Plains... A Slight Risk area was introduced in many of the same areas as have been seeing rain the past couple days again for the Thursday afternoon period from far northeastern CO into southern NE and northwestern KS. Rainfall amounts may not be quite so prolific as the maintained Slight across western SD and northeast WY, but antecedent conditions are likely to make up for that differential. The break between the 2 Slight Risk areas was due to the Sand Hills making flash flooding much less likely locally, though similar amounts of rain are expected in that area between the 2 Slights. Rainfall is also likely to redevelop again Thursday afternoon and evening across southeastern ID and western WY, which may also need a Slight Risk upgrade should prior day's rainfall exceed current forecasts. The movement of this time frame into the short-range, when CAMs guidance may shed more light on the rainfall potential should help clarify if additional upgrades will be needed. Wegman ....Previous Discussion... ....Tennessee Valley... A pair of robust shortwave troughs are expected to move through portions of the Mid Mississippi to Tennessee Valleys Wednesday night (early Thursday) and then again Thursday night (early Friday), bringing a couple rounds of heavy rainfall to the area. Convection is expected to ramp up late in the evening and overnight hours associated with the strengthening low level jet where the latest model guidance shows upwards of 30-40 kts at 850 mb impinging on a boundary draped across the region. With the mean flow oriented nearly parallel to the expected storm motions, some repeating rounds and backbuilding will be possible and given the anomalous moisture expected (PWs 1.75-2"), the highly efficient rain producing thunderstorms combined with potential longer duration could bring localized swaths of higher end rainfall totals. There still remains large model spread, particularly spatially with some north/south uncertainties but the consensus and ensemble data point toward portions of southern Kentucky through middle and eastern Tennessee having the grater threat for both rounds to overlap for the period, so a Slight Risk was introduced this forecast cycle. ....Great Basin to Northern High Plains... Shortwave energy lifting through the region combined with favorable right entrance jet dynamics will support a broad area of forcing for ascent from portions of the Great Basin to the Northern High Plains. Moisture anomalies will remain above normal, nearly 2-2.5 standard deviations above normal, particularly across portions of northeast WY, southeast MT and western SD. Convection is expected to develop initially over portions of the Great Basin before moving eastward through WY/MT and pushing into the Plains later in the period where it'll interact with the greater instability/moisture. Localized rain rates 0.5-1"/hr will be possible across ID while further east rain rates up to 1.5"/hr will be possible in the Slight Risk area, where the 00Z guidance suggests localized rainfall totals over 2" will be possible. The inherited Slight Risk continues to look reasonable with only minor adjustments made. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6hu1I__PMaIuo4X6ki3WnXpKt2DUQ2ZZQeeU4gLeSpHu= X3EnP1UDu8pSE4hoFH0Huf5c33Jp0IiXZ7avrcGL0QV7wZw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6hu1I__PMaIuo4X6ki3WnXpKt2DUQ2ZZQeeU4gLeSpHu= X3EnP1UDu8pSE4hoFH0Huf5c33Jp0IiXZ7avrcGLyjZSPZA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6hu1I__PMaIuo4X6ki3WnXpKt2DUQ2ZZQeeU4gLeSpHu= X3EnP1UDu8pSE4hoFH0Huf5c33Jp0IiXZ7avrcGLlyAWhG4$=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) .