Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 02 2023 00:32:38 FOUS30 KWBC 020032 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 831 PM EDT Tue Aug 01 2023 Day 1 Valid 01Z Wed Aug 02 2023 - 12Z Wed Aug 02 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF EASTERN MISSOURI... ....Eastern Missouri... A Moderate Risk area remains for portions of eastern MO. Much of the guidance continues to indicate a second round of training convection as a potentially organized convective cluster/possible MCV moves east out of CO towards the region and interacts with the nocturnal low level jet. On Tuesday morning there were scattered instances of flash flooding, this overnight/early morning round is likely to produce similar impacts, with the 18z HREF indicating non-zero chances for 8"+ by Wednesday morning. Because the signal for this rainfall is farther north, extended the Slight Risk area up into southern IA. The area expected to be impacted with flooding rains tonight should be displaced to the east just enough to limit overlap with the rainfall that occurred overnight Monday night, though there is no guarantee of such given the usual north and east bias of the mesoscale guidance. 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 idea of day shift which is well supported by the 18z HREF. A large majority of the heaviest rainfall associated with tonight's rainfall should happen by Wednesday morning, 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 Risk lasting into the Day 2/Wednesday period. ....Northeast CO into southwest NE and far northwest KS... The Slight Risk area was maintained today with minimal changes (perhaps a slight adjustment northward based on radar reflectivity trends). The guidance hinted that heavy repeating storms may persist further eastward along the NE/KS border, if not beyond, so the Slight Risk remains in that area. 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. ....UT/NV/AZ/CA... Convective trends led to some changes to the Marginal Risk areas across AZ (somewhat reduced) and southern ID (new). 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 until CIN sets in for most areas. The exception would be southern NV where an upper level low appears to be tracking towards which could tap elevated instability and led to new overnight development. ....Northern Plains... A frontal boundary extends from eastern MT all the way to a surface low located in far northwest MN. There is also a warm front oriented N-S that has helped spark storms over northwest MN. Visible satellite also depicted an outflow boundary propagating north towards the front triggering new thunderstorms. The atmosphere sports plenty of instability (MLCAPE of 1,000-2,500 J/kg) and moisture content (PWs of 1.25-1.5", highest values over the Red River of the North). The special 20Z BIS sounding showed a strong 200-300mb jet aloft with increasing sfc-6km shear (easterlies in the sfc-3km layer, westerlies 3-6km layer). This kind of sheared environment will support storms capable of containing healthy mesocyclones, which in the presence of upper 60s to low 70s dew points and PWs that are near the 90th climatological percentile can still be efficient rainfall producers. The 850mb jet over SD will strengthen out of the south and intersect the stationary front over ND. Meanwhile, 850-300mb winds will be oriented quasi-parallel to the front, suggesting storms forming along the front and other potential outflows could result in training storms. It is worth noting the 850-300mb winds are progressive (20-30 knots), so cases where flash flooding have the best odds of occurring would be where training storms takes shape. The setup could favor possible training in select areas along or just ahead of the cold front, especially as storms in western ND approach ongoing storms in eastern ND and western MN. Over time, the 18z HREF appears to show coalescing convection across ND as instability slowly erodes from the sides and outflow boundary collisions appear to take over, a process which has appeared to have begun. AHPS shows some parts of southern ND and western MN have picked up 300-400% of normal rainfall in the past 7 days. Flash flooding is possible this evening and overnight with areas with poor drainage, more urbanized communities, and locations with more saturated soils most at-risk. Roth/Mullinax/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!5tPH5iAZWJwtPOQt1Op3uudAqcwDurdbKzS-hpMnL2Lb= 5jIBLZnLsR59BWv91lVOLrgmpMu_r9UO31IL0R1oaRmr6c4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5tPH5iAZWJwtPOQt1Op3uudAqcwDurdbKzS-hpMnL2Lb= 5jIBLZnLsR59BWv91lVOLrgmpMu_r9UO31IL0R1oCasGXiA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5tPH5iAZWJwtPOQt1Op3uudAqcwDurdbKzS-hpMnL2Lb= 5jIBLZnLsR59BWv91lVOLrgmpMu_r9UO31IL0R1oIa18XvU$=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) .