Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Aug 01 2023 20:27:34 FOUS30 KWBC 012027 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 426 PM EDT Tue Aug 01 2023 Day 1 Valid 16Z Tue Aug 01 2023 - 12Z Wed Aug 02 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF EASTERN MISSOURI... ....16Z Update... ....Eastern Missouri... In coordination with LSX/St. Louis, MO forecast office, a Moderate Risk area was introduced for portions of eastern MO with this 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/CA... With ongoing 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, where 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. Wegman ....Previous Discussion... ....Southwest to the central High Plains... Another round of afternoon to evening convection expected today with the monsoonal moisture axis primarily confined to portions of eastern Nevada through much of Utah and into Wyoming/Colorado. Probabilities of seeing 0.5-1" hourly totals ramp up considerably by afternoon, peaking upwards of 30-50 percent across portions of NV/UT and above 60-70 percent for the CO foothills eastward through the central High Plains by late tonight. Some of these areas have received heavy rainfall Monday and early this morning, so soils will become increasingly saturated, especially across portions of CO. So the combination of intense rain rates (1-2"/hr), isolated rainfall totals of 3-4", and wetter antecedent conditions may lead to another round of scattered flash flooding. ....Northern Plains... Localized heavy rainfall will be possible during the period as an axis of higher moisture surges ahead of an approaching front combined with favorable forcing and enough instability to produce localized intense thunderstorms that may lead to a few instances of training thunderstorms that could produce flash flooding. ....Mid Missouri to the lower Ohio Valley... Very active, unsettled, and potentially significant heavy rainfall event beginning to set up across portions of Missouri the next couple of days. Upper ridge axis remains anchored over Texas early this morning with the ring of fire convection developing along its periphery. The northwest flow aloft positioned over the Mid Missouri to Mid Mississippi Valleys will interact with the strengthening low level jet early this morning to produce a narrow/localized swath of heavy rainfall that is just beginning to develop per recent radar and satellite imagery. This activity is likely to train/repeat as it slowly drifts southeast across central Missouri. The 00Z HREF suggests localized higher totals exceeding 3-5" will be possible through late morning before the low level jet wanes. This generally accounts for the western half of the Slight Risk area, which is on the higher end of the Slight Risk probabilities.=20 After a break/lull expected mid day to early evening, another round of convection will develop across portions of central/eastern Missouri after midnight tonight through Wednesday mid morning. The setup continues to look very conducive for heavy rainfall with anomalous moisture in place, sufficient elevated instability, and a strengthening low level jet positioned favorably into the region. The 00Z HREF and CAMs show potential for another localized/significant heavy rainfall swath, which is likely to be displaced just enough eastward of this morning's heavy rainfall, limiting the overlap and this threat accounts for the eastern area of the Slight Risk outlook. That threat will continue into the Day 2 period with additional heavy rainfall going beyond 12Z Wednesday. Taylor 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 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_A2vu0O5sIltMVl3-PucRgsQ4vhyJS_V_gZ5Wt6DMpqo= WCr3Y1Wgggk6jlDHk2mZdGGnjLP2sHnhjcQDoGDSGdB4BtY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_A2vu0O5sIltMVl3-PucRgsQ4vhyJS_V_gZ5Wt6DMpqo= WCr3Y1Wgggk6jlDHk2mZdGGnjLP2sHnhjcQDoGDSunmyJAI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_A2vu0O5sIltMVl3-PucRgsQ4vhyJS_V_gZ5Wt6DMpqo= WCr3Y1Wgggk6jlDHk2mZdGGnjLP2sHnhjcQDoGDScdM8RGc$=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) .