Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Oct 25 2023 00:48:33 FOUS30 KWBC 250048 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 847 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... ....01Z Update... The primary changes made with this update were to trim the Slight across Texas to focus mostly on west Texas a bit east of the NM border. In this area the combination of increasing moisture and instability out of central TX, an approaching shortwave out of Mexico, and ongoing convection across NM will allow for new storms to develop in the Slight Risk area, which will initially be slow moving and capable of 2 inch/hour rainfall rates, before gradually congealing into a faster moving line, which will diminish the overall flash flooding threat, so the focus of the Slight is for the initial storm formation over the next few hours. Some of this area, including the Midland-Odessa area has been recently hard hit with heavy rains, which will locally increase the flash flooding potential if storms move over those towns. No significant changes were made further north across southeast KS, where most of the CAMs continue to indicate new training storm development from Kansas City south and west through the overnight. The storms will track northeastward, which could result in localized flash flooding if the storms move over any cities/towns such as Kansas City. For the upper Midwest, a line of training storms is nearly stationary from far southeastern MN across central WI. Overall storm coverage should continue gradually decreasing overnight, but additional impulses of upper level energy will likely keep light to moderate rain going. Thus, the Slight is largely unchanged except on the southeastern side which appears to be out of the main area of rain. This area is likely on the lower end of the Slight category now, but the several inches of rain that have already fallen combined with expected ongoing rain through the overnight should keep the flash flooding threat above a Marginal, so the Slight remains. Wegman 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!6JPXMop8cQc3Oxi3ZDU1GMaUAY2EB0TxGOgyBLXbSmYr= LvK8m2YbNeFnvTVo8zHbDP1tyfHq4zdelU5wB77BFRnwBJg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6JPXMop8cQc3Oxi3ZDU1GMaUAY2EB0TxGOgyBLXbSmYr= LvK8m2YbNeFnvTVo8zHbDP1tyfHq4zdelU5wB77BB9odSh0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6JPXMop8cQc3Oxi3ZDU1GMaUAY2EB0TxGOgyBLXbSmYr= LvK8m2YbNeFnvTVo8zHbDP1tyfHq4zdelU5wB77BAgk8XTo$=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) .