Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Oct 24 2023 20:00:27 FOUS30 KWBC 242000 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 359 PM EDT Tue Oct 24 2023 Day 1 Valid 16Z Tue Oct 24 2023 - 12Z Wed Oct 25 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS AND OVER PORTIONS OF THE UPPER MISSISSIPPI VALLEY INTO THE UPPER GREAT LAKES... ....16Z Update... Overall synoptic scale pattern will remain steadfast through the D1 period with an active convective scheme across both our targeted areas. Progressive MCS over WI this morning has moved into northern MI with a convective rainfall footprint situated along an axis extending from southeastern MN across central WI with totals between 1-2" and max around 2.75" where the convective pattern lingered the longest. Crest max unit streamflow depictions show a well-defined area of return flows along I-90 in western WI extending into Green Bay which is right around where the stationary front settled overnight into this morning. Mesoanalysis indicates that convergence point very well where the LLJ interacted with the boundary and the organized convective complex developed and moved along the theta-E gradient over the northern Midwest. This area and places just to the north of the previous complex will be under fire again this afternoon and evening as a persistent LLJ signature and increased large scale ascent downstream of an amplified longwave trough pattern will keep advecting a series of shortwaves up from the central plains into the Midwest. Forecast instability is at peak now through the afternoon but will decrease closer to 00z and beyond as area cold pools from earlier convection will stabilize the regional environment and previous shortwave energy exits, leading to the convective scheme shifting to more of a stratiform event with integrated convective clusters as forecast soundings over central WI show some lingering instability within the highest theta-E positioning. 12z HREF probabilities have not backed off from previous forecast with the 2"/24-hr EAS probabilities running between 30-40% across central WI which is quite the signal within the mean as the EAS is notoriously conservative in its depictions. A swath of 30-40% neighborhood 10-year ARI exceedance probabilities are also within that corridor of central WI, even up into the Michigan UP where area convection will pivot into later in the period. 100-year ARI exceedance probability is also non-zero across central WI, aligning where the previous morning QPF footprint aligns with the next round of expected convection. Further to the south across the southern plains, remnant moisture and mid-level energy from Norma will continue to advect northeast downstream of the ULL centered over northern MX creating the secondary flood potential over TX/OK into eastern KS later today. Regional rainfall footprint is well documented across west TX into the Red River Valley and southern OK with max totals reaching 3.5-4" across portions of the Permian Basin up into Wichita Falls and southwestern OK. Those areas will once again see more impact from the next round of anticipated convection as the main energy kicks north and a secondary axis of convection will fire across NM and move eastward into a more favorable environment. Nocturnal LLJ pattern will evolve again across the southern plains with 850mb wind forecast across guidance reaching between 35-45kts across the Lower Pecos river valley up through the Concho into north TX creating a better opportunity for upstream convective development to remain organized and even redevelop within the confines of area cold pools generated from previous convection. The entire corridor from far southeastern NM through the Permian Basin into west-central TX (SJT/western EWX forecast areas) will be the focus for heaviest axis of rainfall within a fairly unstable environment (MUCAPE between 1250-1750 J/kg). HREF probabilities are fairly robust for exceeding 1" across southeast NM into the Permian Basin with the higher probabilities (>70%) for exceeding 1" located over OK into eastern KS. The highest probability for >3" within the 24 hr time frame is located over eastern KS where instability maximum this afternoon will couple with vorticity advection from the southwest creating a favorable convective environment where 1-2"/hr rainfall rates will be possible over a 3-6 hr period before decaying. HREF probabilities for 1-2"/hr rainfall has a secondary maximum over far southeast NM where large scale forcing within the difluent axis on the lead side of the ULL will initiate a round of thunderstorms off the terrain to the west and propagate overhead. This will lead to the enhanced flooding potential over west TX where heavy rains from yesterday have already caused flood issues and FFG's have lowered significantly, especially within the Permian Basin and Lower Pecos river valley. Another area of interest will reside in the lower Concho valley into part of the Edwards Plateau where some guidance is indicating a series of training heavy thunderstorms tomorrow morning where LLJ and regional cold pools work to provide an enhanced convergence zone across the area. This is forecast to materialize at the very end of the D1 period with the highest impact likely within the early D2, but wanted to make mention considering the setup could initiate within the D1 window.=20 Kleebauer ....Previous Discussion... An expansive area of anomalous PW values...2 to 3+ standard deviations above the mean... expected to stretch across the Southern to Central Plains into the Upper Mississippi Valley and Upper Great Lakes region day 1 in the broad southwesterly mid to upper level flow across these areas. There is good model agreement for two primary precipitation areas in this anomalous PW axis. One across the Southern to Central Plains into the Lower Missouri Valley and another in the vicinity of a west southwest to east northeast oriented frontal boundary stretching from the Upper Mississippi Valley into the Upper Great Lakes... ...Southern to Central Plains into the Lower Missouri Valley... The current organized area of rains across the Southern Plains will be lifting farther northeast day 1 into areas from north Texas, across large portions of Oklahoma, save for the Panhandle and southeast OK, and into eastern Kansas and northwest Missouri, associated with lead areas of height falls ahead of the primary strong upstream closed low moving across northern Mexico. The risk area is depicted as marginal with this lead area of precip as this region has been well below average in precip over the past few weeks, with resultant low soil moisture and below average stream flows. We did trim the slight risk area from the previous outlook out of far southeast Kansas into north central Oklahoma. This will be followed by a second surge of heavy precip developing across the Southern High Plains Tuesday afternoon ahead of the aforementioned upstream strong closed low moving across northern Mexico. The slight risk area was drawn from southwest Oklahoma, southwestward into Northwest Texas and toward the Big Bend of Texas to encompass areas that have or will receive potentially heavy rains through 12Z Tuesday and overlap areas that will then see another round of potentially heavy rains moving into the Southern Plains Tuesday night/early Wednesday. The HREF neighborhood probabilities for 1"+ hourly amounts are the most defined with this next surge of heavy precip into far southeast NM and west Texas from approximately 0300 to 1200 UTC Wednesday with probabilities ranging from 20-50%+. In comparison, the HREF neighborhood probabilities for 1"+ hourly amounts for the lead area of precip into the Central Plains and Lower MO Valley top out at 20%+. ....Upper Mississippi Valley into the Upper Great Lakes... Current surface analysis is showing a well defined front stretching along the MN/IA border eastward into southern WI and the southern portion of the L.P. of MI. This front is forecast to remain well defined as it lifts slowly northward day 1. This front will likely remain an active focusing mechanism for overrunning convection from early Tuesday morning as the current area developing along the MN/IA border pushes east northeastward into south central WI and the northern portions of the L.P. into early Tuesday morning. This will be followed by potential for additional organized areas of convection to fire along and north of the front as 30-40 kts west southwest h85 flow continues into the boundary. Simulated hi res radars show potential for additional rounds of convection to move parallel and along and just north of the front, with a period of training with each round. The slight risk area was drawn where the HREF neighborhood probabilities were high for 1, 2 and 3"+ totals from the southeast MN/southwest WI border area, east northeast across north central WI and into the northern L.P. of MI. Hi res models are in good agreement on this axis, reflected by high EAS probabilities of 1"+ amounts, 50-70% and 10-25% for 2"+ amounts through the slight risk area. In areas of training hourly totals of 1-2"+ are possible with potential for more than one round of training during the day 1 period. Oravec=20 Day 2 Valid 12Z Wed Oct 25 2023 - 12Z Thu Oct 26 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS... ....20Z Update... Main adjustment within the forecast period was a shift eastward of the risk areas and an expansion northward to the SLGT into northeastern OK given the latest hi-res ensemble guidance QPF footprint and ensemble bias-corrected QPF which has translated further eastward to include central TX and the western periphery of the DFW metroplex. The shift is correlated with a bit more progression of the mean trough to the west with convective development slightly further downstream within the best difluent axis. The convergence area along a slow-moving cold front in the southern plains will be the inflection point(s) within the heavy rain forecast as area convection will develop along and just ahead of the frontal progression through the D2 forecast period. 12z HREF probabilistic guidance has really trended up with the potential across the northern extent of the Edwards Plateau up into the northern Hill country located from Weatherford down to just north of the Rio Grande near Del Rio. The higher end signals in the probabilistic realm are the neighborhood QPF exceedance probabilities over 24 hrs with 3"/24-hrs maxing at 80-90% within the aforementioned area, 5"/24-hrs peaking at 50-60% across central TX, and even an area of 25-40% for exceeding 8" across the Edwards Plateau stemming from the convergence signature shown at the end of D1 translating into the beginning of this period. Rates will be generally within 1-2"/hr with some probabilistic signatures for locally up to 3"/hr in the heaviest storms. This is all a product of very anomalous moisture located within a large scale ascent pattern and substantial surface convergence axis within an above normal theta-E axis in lead of a deep upper trough. Decided to maintain the SLGT given the higher FFG's within the 3 and 6-hr FFG exceedance intervals, as well as the dry antecedent conditions where the heaviest precipitation is forecast. This is a higher end SLGT within the grand scheme and short term upgrades could be possible if the evolution warrants. Further to the north, the trend was for convective potential to be focused a bit further south over the central Midwest into the Upper Mississippi Valley. 10-year ARI exceedance probability was not very robust by any means across the outlined area, but a tongue of higher theta-E's being advected northward towards the back end of the period will likely bring a better chance for locally heavy rainfall within a broadening area of convection. With rainfall rates generally between 1-2", at best within the forecast cycle, maintained the MRGL risk, but just shifted the impact area closer to the overall QPF forecast of the HREF blended mean and NBM/Ensemble bias-corrected data blend. This setup will translate to the northeast into the northern Midwest for the D3 period. Kleebauer ....Previous Discussion... The strong height falls moving across northern Mexico day 1 will be ejecting northeastward into the Southern Plains day 2, continuing the multi day heavy rain event for portions of the Southern Plains. The day 1 axis of above average PW values across the Southern to Central Plains into the Lower MO Valley will continue during the day 2 period with anomalies remaining 2 to 3+ standard deviations above the mean across these regions. This and the overall strong upper dynamics associated with the northeastward ejecting closed low will support potential for additional widespread heavy rains from the Big Bend, northeastward into Northwest TX, much of OK, save for the Panhandle and southeast OK. There are still some differences in this heavy axis, but a continued strong signal for the potential of overlap of heavy rains from the day 1 period and day 2. Only some minor changes to the previous outlook areas, trimming of the west side of the marginal risk and narrowing of the slight risk area. With the potential for heavy rain overlap days 1 and 2, an upgrade to moderate is possible with later issuances, depending upon where the heavy axis occurs day 1. The day 2 slight area continues to=20 match fairly well with CSU first guess fields, although having a slightly different orientation to the axis to match the latest day 2 qpf. ....Upper Mississippi Valley into the U.P. of Michigan... The expected active day 1 convection along the frontal boundary stretching across the Upper MS into the Upper Lakes may begin to refire along this boundary late day 2, early hours of Thursday,=20 as southwesterly low level flow of 30-40 kts restrengthens into this front. This will favor the potential for another round of convection to move parallel to the boundary early Thursday and possibly across areas that received heavy amounts during the day 1 period. This heavy rain will likely continue into the day 3 time period, with a higher threat risk depicted for day 3. For day 2, we continue to show only a marginal risk, but this may need to be increased to slight in subsequent issuances depending on where the training axes set up day 1. Oravec Day 3 Valid 12Z Thu Oct 26 2023 - 12Z Fri Oct 27 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER MISSISSIPPI VALLEY INTO THE UPPER GREAT LAKES... ....20Z Update... No major changes to the previous forecast as the overall synoptic scale pattern and anticipated thermodynamic environment across the northern Midwest remains consistent from previous forecast issuance. Convective setup across the Upper Mississippi Valley into the Great Lakes remains fairly stout within a corridor of very high PWATs and tight theta-E gradient along and south of a frontal boundary positioned over WI and northern MI. The SLGT risk was maintained in generally the same area, although the northern bounds were expanded to include areas that will have lower FFG's due to today's convective impact. The western extent was also shifted a bit to encompass the axis of best instability and forecast QPF of at least 1.25" or more. Deterministic guidance is hinting at precip maxes around 2-3" possible within the corridor of expected lower FFG's, so will have to monitor if any further shifts are necessary once the HREF and associated CAMs come into play in the next 24-48 hrs. Kleebauer ....Previous Forecast... The well defined front stretching from the Upper Mississippi Valley into the Upper Great Lakes will continue to be the focus for additional heavy rains day 3. The next round of convection moving along and to the north of this front that is expected to begin late day 2, will continue into the day 3 time period as surface low pressure moving along this front helps to maintain strong west southwesterly flow into this front. This will likely support widespread heavy rains from northwest IA, across far southeast MN, much of WI into the northern L.P. of MI and the central to eastern U.P. of MI. The slight risk was narrowed from the old day 4 outlook to match current qpf consensus. There will be potential for training of cells along and north of the front and potential for overlap of day 1 heavy amounts with expected day 3 amounts. This may be especially so from northeast WI into portions of the south central U.P. into the northern L.P. where multi day totals of 3-5"+ are possible. This may be an area where an upgrade to moderate may be necessary with later issuances depending upon how the day 1 qpf axis verifies. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7_0zgEL0XjyewLc2Cv0st4VbiXooZHKBFdGghj7wlTeG= 1Fy09ISK2P4WNpsc66KZ7phqccfK_9yfwGq7aAIZCIw77Hk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7_0zgEL0XjyewLc2Cv0st4VbiXooZHKBFdGghj7wlTeG= 1Fy09ISK2P4WNpsc66KZ7phqccfK_9yfwGq7aAIZP4biEF8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7_0zgEL0XjyewLc2Cv0st4VbiXooZHKBFdGghj7wlTeG= 1Fy09ISK2P4WNpsc66KZ7phqccfK_9yfwGq7aAIZ8Mm2NWQ$=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) .