Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Apr 04 2023 20:36:38 FOUS30 KWBC 042036 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 435 PM EDT Tue Apr 04 2023 Day 1 Valid 16Z Tue Apr 04 2023 - 12Z Wed Apr 05 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE GREAT LAKES REGION... ....16Z UPDATE... Several hi-res guidance members this morning have shown a notable uptick in QPF across portions of the Great Lakes Region. Upper-jet support moving in from the southwest this evening supports the development of widespread convection along a slowing northward moving warm front, with the previously noted significant moisture anomalies from overnight still a concern. Areal storm coverage should shift from eastern Wisconsin into the northern Lower Peninsula of Michigan through the overnight hours. Forecast approximately east-northeasterly storm motions along the front will lead to the tendency for numerous storms to train over the same regions for at least a few hours, increasing the risk for heavier rainfall totals of 2-3"+. In addition, many hi-res guidance members also indicate the potential for locally heavier rainfall rates over 1" per hour. FFG values in eastern Wisconsin in particular are relatively low, with hourly values around 1", indicating these rain rates will have a good chance of exceeding these FFG values. In coordination with local offices MKX, GRR, and APX, a Slight Risk upgrade has been added from southeastern Wisconsin into the northern Lower Peninsula of Michigan for the increased concern of some scattered instances of flash flooding. For the Southern Plains to Mid-Mississippi Valley, the forecast looks to remain on track. Expect a line of convection to form along the cold front later this evening into the early morning hours Wednesday. Given the anomalously high moisture streaming northward expect that there will be the chance some thunderstorms will have locally heavier rain rates of 1"+ per hour. However, while the progression of the cold front is expected to slow, storm motions still look progressive enough through the end of the current period that rain totals will likely remain in the 1-2" range, with only isolated instances where shorter-term rates/totals may exceed FFG values. Therefore, the Marginal Risk remains in place with only some slight areal adjustments given latest 12Z hi-res guidance trends. Putnam ....Previous Discussion... ....Western Great Lakes... A powerful cyclone over the Midwest will be responsible for two particular areas where Excessive Rainfall is possible today. Focusing first in the western Great Lakes, the primary surface low is set to track from the Central Plains Tuesday morning and into the Upper Midwest by Tuesday evening. The warm front associated with the low pressure will also push north, tracking through the Middle MS River Valley Tuesday midday and reach the the Great Lakes by Tuesday evening. Between the exceptionally deep upper trough in the West and strong ridging in the Southeast, the integrated vapor transport will steadily increase from South Texas to the Upper Midwest. By 00Z Wednesday, NAEFS shows IVT values will be >750 kg/m/s and above the 99th climatological percentile from southeast TX to northern IL. PWs in the Marginal Risk area will eventually reach 1.10-1.20", which is above the 90th climatological percentile according to NAEFS. The excellent moisture transport and rising PWs does provide enough moisture for developing thunderstorms to tap into, leading to locally heavy rainfall rates. Most guidance is in agreement in MUCAPE >1,000 J/kg, which is high enough to support heavy rainfall rates. The biggest limiting factors in the flash flood threat are storm motions and drier air at upper levels. Area averaged soundings show that the 700-300mb layer is quite dry, and the elevated convection will also contain some modest hail CAPE as well. Mean 850-300mb winds are will be at a progressive 50-60 knots as well so barring the potential for training convection, which is not as likely as convection much farther south, residence time of heavy rainfall rates will be on the shorter side. The wild card in this setup are soil temps, which are still quite chilly. In addition, western MI has also received as much as 300% of rainfall over the past 7 days. FFGs are also as low as 1"/hr in parts of eastern WI, northern IL, western IA, and western MI. With the available instability and anomalous moisture, the Marginal Risk still looks to be in good shape at this time given the fast storm motions will help to keep the flash flood threat much more localized. ....East TX/OK, Arkansas, and Middle MS River Valley... Unlike their neighbors to the north, the atmosphere will be more unstable and feature less intrusion of the dryslot from the powerful cyclone in parts of the South Central U.S. Storm motions will still be rather fast, but this region has the right entrance region of a 150 knot 250mb jet streak positioned overhead, maximizing vertical ascent atop the atmosphere. Over 1,000 J/kg of MLCAPE and PWs approaching 1.5" will coincide within an atmospheric profile Tuesday evening that features 80-90% RH values in the 1000-500mb layer. Unlike the soundings to the north, the atmospheric column will feature deeper warm cloud layers and more saturated profiles, allowing these developing thunderstorms Tuesday night to be more efficient rainfall producers. The slower progression of the cold front and strengthening LLJ after 00Z Wed will prompt thunderstorms to blossom in these areas along a pre-frontal trough initially, followed by the cold front sweeping through these areas early Wednesday morning. It is along the pre-frontal trough with mean winds in the LCL-LFC cloud layer running parallel to it where there is the potential for training convection. However, storm motions will likely be as fast as 40-50 knots, so residency time of hourly rainfall rates up to 2"/hr would be shorter and keep the best flash flood potential to only segments of training storms. East TX may be the most prone location for possible flash flooding given recent 7-day rainfall totals have surpassed 300% of normal. However, the lower 1-hr FFGs are in the Ozarks and into the Middle MS Valley where some values are as low as 1.5"/hr. Given the quick progression of the anticipated convection, have maintained the Marginal Risk but will mention a Slight Risk upgrade may be required should hi-res CAMs begin showing higher QPF totals in future forecast cycles. Mullinax Day 2 Valid 12Z Wed Apr 05 2023 - 12Z Thu Apr 06 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM THE UPPER TEXAS COAST INTO THE OHIO VALLEY... ....2030Z Update... Only minor areal adjustments were made to the current Slight Risk area from the Upper Texas Coast through the Mid-South and into the Ohio Valley. As an upper-level trough and greater forcing shift to the northeast, a cold front will begin to slow as it passes through the region later afternoon Wednesday and into the early overnight hours Thursday. A line of thunderstorms along the cold front as well as some additional thunderstorms ahead of the cold front are expected to be ongoing at the start of the period just to the west of the Slight Risk area. The progression of these storms will slow in tandem with the cold front, leading to more storms training over the same areas and higher forecast rainfall totals. The anomalously high moisture and sufficient instability previously noted overnight will promote areal average rain totals between 1-3", with the potential for locally higher totals, based on the latest hi-res guidance. Several of the hi-res guidance members also suggest the potential for rainfall rates of 1-2" per hour, which again would certainly support these rainfall totals given the tendency for training storms. The surface pattern, available moisture, and forecast guidance QPF does suggest the potential for relatively widespread heavy rainfall across the Slight Risk area, indicating that a higher Moderate Risk category could be warranted. However, FFG values remain high across most of the region, especially to the southwest over Texas and Louisiana, and current forecast amounts aren't significantly above these values outside of more localized/regional areas. In addition, the higher QPF amounts in the guidance are limited in areal coverage to the forecast location of the front, which differs some amongst the guidance, requiring higher confidence in the frontal position to be able to highlight a region of higher probabilities of more significant rainfall totals. Otherwise, forecast QPF amounts did notably come up for portions of the current Marginal Risk in upstate New York. However, there remains questions on the coverage of more appreciable convection with northward extent in the warm sector that keeps confidence on higher totals low at this time. Putnam ....Previous Discussion... The upper trough over the North-Central U.S. and the strong ridging along the Southeast coast will work in tandem to funnel rich Gulf of Mexico moisture north and east from southeast TX into the Great Lakes. The region from east TX to the Mid-South remains favorably positioned beneath the diffluent right-entrance region of a 150 knot jet streak. PWs up to 1.5" are expected as far north as the Ohio Valley with PWs above 1.75" likely in the Lower MS Valley and along the Upper Coast of TX. The latest 00Z HREF shows up to 1,000 J/kg of MUCAPE as far north as the central KY, with upwards of 2,500 J/kg in central LA. Perhaps one of the biggest supporting factors for higher chances for flash flooding lies with a much more saturated profile. The NAEFS showed mean specific humidity values at the 925-850-700-500mb height levels that were all above the 90th climatological percentile through Wednesday afternoon and into Wednesday night, indicating the column through much of the lower-mid levels of the atmosphere is well saturated. The other component supportive of flash flooding is that as the intense cyclone over the Midwest crosses into Canada Wednesday evening, the trailing cold front over the Mid-South and the Upper TX coast slows down to becoming nearly stationary. Mean 850-300mb winds will remain quasi-parallel to the front while southerly 850mb winds continue to pump in Gulf moisture into the frontal boundary. This is a setup that would support training convection, and with warm cloud layers as deep as 10-12k feet in some cases, efficient warm rain processes within these training segments of convection have better odds to cause flash flooding. Antecedent conditions feature wetter soils in some of these areas, most notably in parts of southern AW, northern LA, and central MS. These areas are more prone to potential flooding, but given the slowing forward propagation of the cold front Wednesday night, the primary method for any flash flooding will be most commonly linked to training convection. Hourly rainfall rates >2"/hr are anticipated within the more robust areas of convection, particularly from the Upper TX coast to the Lower MS Valley. Chose to maintain the Slight Risk with just some minor tweaks shifting the at-risk area a bit more west to account for slightly slower trends in the QPF shield from CAMs and deterministic guidance. Farther north, MUCAPE between 500-1,000 J/kg will be present in the Upper Ohio Valley and even into portions of the eastern Great Lakes. With soils growing more saturated it may not take much in the way of 0.75"/hr rainfall rates to possibly cause localized areas with rapid runoff and street ponding. The Marginal Risk remains in place of this forecast update. Mullinax Day 3 Valid 12Z Thu Apr 06 2023 - 12Z Fri Apr 07 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM SOUTHEAST TEXAS TO WESTERN LOUISIANA... ....2030Z Update... Thunderstorms ongoing along and to the north of a stalling cold front along the Texas Gulf Coast overnight Wednesday/early Thursday morning are forecast to continue throughout the day Thursday given continuous moist onshore flow in the low levels from the Gulf. This will promote the redevelopment of thunderstorms and training storms over the Slight Risk area, increasing the risk for higher rainfall totals. Anomalously high precipitate water values 1-2+ standard deviations above the mean will increase in areal coverage over the region given the continuous onshore flow. This increasing moisture will support additional heavy rainfall totals over the same general region of southeast Texas/western Louisiana that saw heavy rainfall during the day 2 period. The global ensemble guidance shows moderate probabilities of areal average rainfall totals of around 2", with the deterministic guidance hinting at the possibility of locally higher totals into the 4-5" range. As noted overnight, an increase in confidence in realizing these more significant totals over more than just isolated locations in the hi-res guidance during the day 2 and/or day 3 period may likely require an upgrade to a Moderate Risk for day 3 given increasingly wet antecedent conditions and the additional excessive rainfall, likely during a relatively continuous timeframe. The forecast for the Pacific Northwest remains on track, with only a small southward expansion of the current Marginal Risk based on updated QPF guidance. Locally heavy rainfall totals of 2"+ over very wet antecedent conditions given the rainfall seen this Winter/Spring in the region may lead to some isolated instances of rapid onset flooding in local streams and rivers.=20=20=20 Putnam ....Previous Discussion... ....South Texas to the Mid-South... Texas on north and east into the Mid-South will contend with a stationary front just off the Texas coast while the region sees a conveyor belt of 500mb shortwaves track northeast into the Mid-Atlantic. Meanwhile, the 250-500mb trough axis is residing over Arizona on south through the Gulf of California. This setup is not only forcing the right-entrance region of the 150+ knot 250mb jet streak to reside over the Mid-South, but a weaker (albeit, still diffluent) left exit region of a 90 knot jet streak over the tropical East Pacific sets up near the Rio Grande. Strong ridging off the East Coast is responsible for an expansive area of southeasterly 850mb winds stemming from the western Caribbean and continuing through the Gulf of Mexico. There is no shortage of atmospheric moisture over southeast Texas with PWs >1.75" expected along the Texas coast and as far north as southwest Louisiana. Compared to recent days, mean wind speeds within the 850-300mb layer are also a little slower at 20-25 knots out of the SW. With a persistent inflow of 850mb moisture flux and a stalled front and upper level pattern, it will be an active day of widespread showers and thunderstorms across the southern tier of Texas and into the Mid-South. There is some uncertainty in how much instability will be present, but as of now, MUCAPE between 500-1,000 J/kg would be available for developing storms. Meanwhile, the 1000-500mb mean RH values are likely to be >90% throughout the Slight and Marginal Risk areas. There was a brief consideration to a Moderate Risk in southeast Texas, but there is still enough uncertainty in rainfall totals, low-end instability, and seeing how saturated soils are after two days worth of rainfall leading up to the event, to keep the threat level topped out as a Slight Risk for now. The Marginal Risk was expanded more into northern AL and southern TN as a result of both lingering PWs >1.5" and thunderstorms likely forming in areas where soils have moistened up after rainfall Wednesday night. ....Pacific Northwest Coast... Chose to expand the inherited Marginal Risk as far north as the Olympics of western Washington as a result of deep 850-700mb moisture flux being oriented along the coast Thursday morning and into the afternoon hours. It is a rather sharp 250-500mb trough axis approaching the coast Thursday morning with excellent diffluence atop the atmospheric column. This is a warmer setup aloft, causing rain to be the primary precipitation type in mountainous terrain that has a deep snowpack. Over 2" worth of rainfall is expected from the coastal range of California and Oregon to the Olympics, where 0-40cm soil moisture percentiles courtesy of NASA SPoRT-LIS are between 70-90% saturated. The combination of heavy rainfall rates up to a 0.5"/hr and some snow melt could result in rapid runoff in complex terrain, as well as rapid rises in creeks and streams. Mullinax Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5cLZZ-KTaV0AHRU54X0rXlasQCmQAYNFtcGrAtuM_L0D= 9I5zbDCY_DBnYOTLF-CRLflMxMX5YjcKkHyUb7XeRxz4srY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5cLZZ-KTaV0AHRU54X0rXlasQCmQAYNFtcGrAtuM_L0D= 9I5zbDCY_DBnYOTLF-CRLflMxMX5YjcKkHyUb7Xek3ZHi_I$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5cLZZ-KTaV0AHRU54X0rXlasQCmQAYNFtcGrAtuM_L0D= 9I5zbDCY_DBnYOTLF-CRLflMxMX5YjcKkHyUb7XeodwUdUc$=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) .