Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Aug 12 2024 19:46:16 FOUS30 KWBC 121945 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 345 PM EDT Mon Aug 12 2024 Day 1 Valid 16Z Mon Aug 12 2024 - 12Z Tue Aug 13 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF EASTERN COLORADO AND WESTERN KANSAS, EASTERN OKLAHOMA AND FAR WESTERN ARKANSAS, AND THE COASTAL CAROLINAS... ....16Z Update Summary... The SLGT risk across Colorado and KS was expanded back to the west to include the foothills of the Rockies in coordination with WFO Boulder. The SLGT risk across Oklahoma was adjusted to remove areas where flash flooding potential is very low or no longer expected 16z. The threat across OK and AR will likely end after 18z with removal of the risk area anticipated at the 01z update. General continuity across the Carolinas with the current SLGT risk. Elsewhere, only minor adjustments were made to reflect recent radar and hi-res trends in the QPF field(s). There were no significant changes made to the MRGL risk across the west with the heaviest rainfall expected over Northern UT, but away from the more sensitive zones. Kleebauer ....Northeast OK and Western AR... Complex from overnight is currently migrating south with a Flash Flood Warning in effect across a small portions of Southeastern OK. Environment is borderline in terms of relative instability, but upwind propagation vectors do indicate the chance at pockets of training/back-building convection within a small zone over Eastern OK into the OK/AR border. The threat has been handled okay by a few of the CAMs with a general consensus of the threat continuing until about 18z before dwindling as the energy associated with the threat shifts east and weakens through the afternoon. The previous SLGT risk was shrunk about 50% to only cover the current areas impacted at this hour. Expectation is for the threat to dissolve in a few hours with a removal of the risk area likely occurring the next update. Kleebauer ....Eastern CO into KS... 16Z Update: The biggest change in the forecast was a westward expansion of the SLGT risk out towards the Foothills of the Rockies, including the Denver metro in coordination with WFO Boulder. Elevated PWATs located across Colorado with sufficient afternoon destabilization will likely spawn a scattered area of showers and storms capable of producing rainfall rates up to 1-1.5"/hr in the strongest cores. Burn scar locations within the foothills are at more sensitive state with regards to heavier precipitation, so the expansion was to help cover for the localized threat expected this afternoon and early evening before the threat shifts more into the Front Range and points east. A general continuity in the HREF mean QPF and associated probability fields allowed for a maintenance of the SLGT risk area located over Eastern CO into portions of Western KS, mainly along and near I-70. A stationary front was analyzed across the above area with a focus of elevated theta-E's situated within the narrow corridor of the front. This aligns with the CAMs initiation point of convection across Eastern CO, shifting eastward with mean flow and expected cold pool propagation from any cells that develop. LLJ will materialize after 00z with the nose of the jet intersecting the region between 03-05z, a time frame where models do diverge on the intensity of any cells in the region. A solid neighborhood probability for >3" is positioned over East-Central CO (50-70%) with lower, but no insignificant probabilities located across Western KS (25-40%). Rainfall rates between 1-2"/hr will be most common within the stronger cells, but the convection could produce some intra-hour rates higher allowing an enhanced threat, especially across Eastern CO where diurnal destabilization pattern will be at its peak before shifting to a more nocturnal threat downstream. The SLGT risk was maintained across the region from previous forecast given the signals above, but the threat is on the lower end of the SLGT risk threshold due to drier antecedent conditions in place across the outlined area. Kleebauer ...Previous Discussion.. Convection is expected to develop over portions of eastern CO by later this afternoon. Models indicate that a weak shortwave will eject eastward originating from the ongoing convective activity over UT...with an upper jet to the north supporting at least weak divergence aloft in its right entrance region. Meanwhile, easterly flow in the low levels will advect in higher moisture, with PWs forecast to increase towards 1.5", above the climatological 90th percentile for mid August. Given the forcing and moisture in place, the forecast ~2000 j/kg will allow for robust convective development. With deep layer mean flow from the west, and low level flow from the east, cells may initially end up slow moving, with cell mergers a possibility. Given the environment in place, tend to think some of more aggressive high res models may end up closer to reality for this event. This would support localized rainfall upwards of 3-5" over portions of eastern CO and far western KS. HREF QPF, environmental ingredients in place, and the CSU machine learning ERO are all supportive of an isolated to scattered flash flood threat. Thus we will go ahead and upgrade this region to a Slight risk. This activity is likely to grow upscale overnight and then move eastward across KS. Convection will likely be quicker moving by this time, although can not rule out a west to east training axis setting up. If this were to occur then Slight risk level impacts may continue overnight across KS. However given some lingering uncertainty on these convective details, and the fact that soil saturation and streamflow conditions across KS are running below average...will keep the risk level at Marginal for now. ....Carolinas... 16Z Update: Little to no change was made to the SLGT risk across the Eastern Carolinas as HREF blended mean QPF distribution of heaviest rainfall and relative probability fields remain fairly unchanged from previous forecast issuance. Expect the main threat to initiate between 18-21z with ongoing convection likely through 03z before dissipating overnight with the loss of diurnal heating. Kleebauer ...Previous Discussion.. Upgraded portions of the eastern Carolinas to a Slight risk for today. We should see another round of convection near the stalled front this afternoon, with HREF 3" neighborhood probabilities in the 40-70% range. Totals of this magnitude will likely stay isolated to scattered in nature...however given the increased sensitivity over the area after Debby and post Debby rainfall...expect another day of isolated to scattered flash flooding. Today is a bit trickier of a forecast with a weak low along the front. While the higher probabilities of excessive rainfall are along the coast this afternoon, we could see at least a localized flash flood risk a bit further inland this morning into the early afternoon as well. Chenard ....Southwest... A broad Marginal risk remains across much of the Southwest, with isolated flash flooding possible over this large geographic region. The coverage/organization of intense rainfall may be a bit less than what we saw on Sunday, but nonetheless an isolated risk likely continues. Do see some more elevated probabilities over southern AZ, so may end up with a bit better concentration of convection there, but still seems to fall shy of Slight risk levels. Portions of central/northern UT also stand out as potentially seeing greater convective coverage...but this signal is generally north of the more sensitive basins in UT...and thus not thinking a Slight risk is needed at this time. Chenard Day 2 Valid 12Z Tue Aug 13 2024 - 12Z Wed Aug 14 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF EASTERN KANSAS INTO MISSOURI AND FAR NORTHERN ARKANSAS... ....20Z Update Summary... Previous SLGT risk across the Central Plains into the Mid- Mississippi Valley was generally maintained, however there were some adjustments on the northwest and eastern flank with the latter being trimmed away from the Mississippi River as trends in guidance indicate less of a threat further east with the setup. The southern portions of the SLGT was also trimmed with the primary axis of heavy rainfall in the period likely to occur further north, especially with the secondary complex. There was little no adjustment necessary across the Southeast, however the area near Charleston continues to be the focal point in the setup with an upgrade entertained. The Southwest and Inter Mountain West was relatively unchanged, but did broaden the MRGL risk over the northern and western periphery to account for the recent trends in areal QPF/convective coverage forecast. Kleebauer ....Portions of KS/MO/AR... 20Z Update: The overall synoptic pattern from previous forecast is relatively unchanged and limited any adjustments to the previous SLGT risk to mainly cover the trends within the overall QPF and probabilistic fields within the 12z hi-res ensemble (HREF). The prospects for flash flooding along the eastern flank of the inherited forecast took a drop based on the lowered QPF footprint in Eastern MO and was reflected in the lower probabilities for the 1" and 2" total precip fields. The highest corridor of enhanced 2+" probabilities is split between two areas of interest. The first is located over Southeastern KS with the first MCS potential in the period that will be ongoing in the D1 period, bleeding into the front 3-6 hours of D2. 12z CAMs mostly highlighted a narrow corridor to the north-northeast of Wichita with upwind Corfidi=20 vectors still aligned as such to create a small window for back- building potential leading to training that contributed to the=20 makeup for flash flooding concerns. The 3 and 6 hour FFG exceedance probabilities were up between 40-50% respectively with an extension of lower, yet insignificant probabilities extending southeast towards the KS/MO border. This setup has been well documented in successive runs of the CAMs that reach out to that period in question, and there is consensus within the latest First Guess fields indicating a SLGT risk over the area highlighted. This was enough to maintain continuity with an adjustment further northwest to encompass the higher FFG exceedance probabilities in KS.=20 The second setup has a lot of merit synoptically with an=20 increasing mid-level ascent pattern likely to develop across the=20 Central Plains, pivoting southeast around the northeast flank of=20 the ridge located over the Southern Plains. Guidance continues to=20 harp at the idea of an axis of heavy rainfall across portions of=20 Southeastern NE into MO by the back-end of D2, but there is still=20 some discrepancy on exactly where the pivot southeast will occur.=20 The general premise for some kind of elevated flash flood threat is there within the ensembles, and the HREF neighborhood=20 probabilities have a broad coverage of >25% for at least 3" located across much of Northwest and Central MO. The previous rainfall=20 from last night into this morning lends credence to some primed=20 soils across the Northwest portion of MO, so any repeating MCS=20 pattern in the area should spur a better risk for flash flooding.=20 This area is likely the main focus for the threat and well within=20 the SLGT risk bounds. As mentioned in the previous discussion=20 below; the corridor for heavy rainfall will be more narrow in=20 coverage, but could easily account for a line of flash flood=20 concerns, especially towards the end of the period when LLJ is at=20 peak and the prospects for training convection increases.=20 Kleebauer ...Previous Discussion.. Likely to have some ongoing convection at 12z Tuesday across portions of southeast KS and southwest MO. This will be what remains of the Monday night organized convection that is expected to move across KS. Most indications are that this activity should be on a weakening trend after 12z, so while an isolated flash flood threat may persist, not currently anticipating a widespread threat. The greater threat is the likelihood of training convection Tuesday night across this corridor. This appears to be evolving into a potentially significant training convective setup. Robust instability will be advecting in from the southwest, likely resulting in a tight instability gradient somewhere in the vicinity of eastern KS into western to central MO. Meanwhile the low level jet will increase, resulting in strong and persistent moisture transport into the region. Corfidi vectors end up becoming quite weak, and what movement they do indicate is generally parallel to the expected moisture convergence axis and instability gradient. Thus we very well may end up with repetitive backbuilding upscale convective growth over this area Tuesday night. There remains some uncertainty on the exact details of convective evolution and placement of highest totals...but confidence is increasing in an excessive rainfall threat, with potentially significant localized impacts. The coverage of excessive rainfall will likely be quite a bit narrower than the Slight risk area...however it remains broader to account for uncertainty in the maximum rainfall axis. ....Western U.S... 20Z Update: Only minor changes were made to account for the convective outlook across the Western US, especially in the Interior from Utah up through Western WY and the Tetons of ID. Convection will remain more progressive overall limiting the threat to a general MRGL risk, but the pattern still remains favorable for isolated flash flood concerns for the areas highlighted. Kleebauer ...Previous Discussion.. Scattered convection is again likely from portions of AZ/NM northward into ID/WY. There will be a bit more in the way of westerly flow compared to earlier days, which may allow cells to move along at a faster pace. This should keep the flash flood risk isolated in nature...however heavy rainfall rates will again be probable in the more intense cells...which could result in a localized flash flood within the broad Marginal risk area. Chenard ....Southeast... 20Z Update: The prospects for flash flooding across parts of the Southeast remains focused within the coastal plain of SC and GA within proximity of the stationary front bisecting the area. A narrow corridor of elevated neighborhood probabilities for both 3" (50-70%) and 5" (30-40%) are present across the central SC coast over Charleston with lower probabilities extending further west away from the coast. The overall setup is likely to cause a training signal somewhere within the vicinity of the front and right now that area seems to be Charleston, SC in the latest 12z CAMs and ensemble bias corrected QPF footprint. A SLGT risk was entertained and could still occur, but the small footprint of the heavy rain area with a majority of flash flood concerns likely to be the urban setting in Charleston contributed to not upgrading the area for the time-being. A very targeted SLGT risk could be entertained in future updates, so this will be a setup to monitor. Kleebauer ...Previous Discussion.. Convection is again likely Tuesday across portions of GA/SC/NC. Overall it is an interesting setup, with model guidance suggesting a more well defined low over SC by this time, with stronger forcing and flow aloft moving in as well. This should seemingly allow for some strong convective development...with the stronger flow aloft supporting quicker cell motions off to the southeast. These cell motions would typically suggest a lowered flash flood risk...however we will need to keep an eye on how the convergence axis east of the low plays out. This boundary could act as a focus for some training convection, which would offset the quicker cell motions and still produce a flash flood risk. Overall think enough uncertainty on these details remains to keep the risk level at Marginal for now. However will continue to monitor trends and would not rule out an eventual Slight risk upgrade over portions of the area. Chenard Day 3 Valid 12Z Wed Aug 14 2024 - 12Z Thu Aug 15 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MIDDLE MISSISSIPPI VALLEY AND MIDWEST... 20Z Update: Confidence in increasing on the potential for heavy rainfall across portions of the Midwest and Mid-Mississippi Valley for D3 with a solid ensemble QPF footprint located over portions of MO/IA/MN. There still some discrepancy on the northward extent of the heaviest rainfall with the latest ECMWF very aggressive across Southern MN and Northern IA with some of the other deterministic better aligned further south. Recent UFVS First Guess field was consistent from its previous iteration with a SLGT risk forecast across Northern MO up through Western IA and Southwest MN. Recent AI models (AIFS and Graphcast) from the ECMWF and GFS are both consistent with an axis of heavy precipitation (>1.25") located=20 over IA with a bit of an extension into Northern MO and Southern=20 MN, respectively. The southern footprint of the current SLGT risk=20 across Eastern MO is in artifact of the initial MCS propagation at=20 the beginning of the period, bleeding over from D2 as the complex=20 moves southeastward. The primary target for the period lies within=20 the convective pattern later that afternoon and evening through the overnight hours into Thursday morning. The synoptic pattern=20 depicted would yield a fairly broad area of enhanced precip thanks=20 to increased large scale forcing ahead of a stronger shortwave=20 trough to the west. An area of mid-level diffluence downstream of=20 the disturbance will play a pivotal role in the setup, so where=20 that aligns will likely determine where the heaviest precip will=20 occur. For now, a broader SLGT risk was proposed given some=20 uncertainty, and to cover for the two-part setup this period.=20 Kleebauer ...Previous Discussion.. The threat of excessive rainfall shifts into Plains and MS Valley on Wednesday. Likely to have some ongoing risk to start the day across portions of central MO...with the threat likely shifting to the north by late in the day into the overnight hours. It again looks like a favorable environment for training/backbuilding convection with an increasing low level jet into a warm front. Strong and persistent moisture transport into this boundary is typically supportive of backbuilding convection and a flash flood risk. There is some model QPF spread, with convection and locally heavy rainfall likely all the way from the Dakotas and MN into MO. However it looks like the best ingredients for excessive rainfall are over MO to start the day, and then across portions of IA and western IL later in the day/overnight. This is where the better instability and most persistent moisture transport is currently forecast. And while the location of greatest QPF differs from model to model and run to run...this axis is favored by the majority of solutions. Thus will place the Slight risk here for now, with the understanding that some adjustments are likely as the event nears. So while confidence on the exact location is only average...do think we are getting towards above average confidence in the impacts from this event...with at least isolated to scattered flash flooding expected, some of which could be locally significant. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6O_h7WqifB461DopJUm8480nPA3D3y2JKKdNYnxojtNh= vnnvs_lnDpGOND3yx7M2PAopZuJZO9cmRVDa8zwRWWyN7hQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6O_h7WqifB461DopJUm8480nPA3D3y2JKKdNYnxojtNh= vnnvs_lnDpGOND3yx7M2PAopZuJZO9cmRVDa8zwRHxHZfhw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6O_h7WqifB461DopJUm8480nPA3D3y2JKKdNYnxojtNh= vnnvs_lnDpGOND3yx7M2PAopZuJZO9cmRVDa8zwRBiccNNs$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .