Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu May 11 2023 20:21:46 FOUS30 KWBC 112021 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 421 PM EDT Thu May 11 2023 Day 1 Valid 2013Z Thu May 11 2023 - 12Z Fri May 12 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL PLAINS AND THE LOWER MS VALLEY... ....2030Z Special Update... ....Northern Louisiana... The Moderate Risk area across northern Louisiana was removed with this update. The heaviest rains have become a progressive squall line and have largely cleared the area. The widely scattered cells over the previous Slight Risk area may locally cause flash flooding, but the threat is no longer expected to be widespread. ....Northern Iowa/Southeastern Minnesota... The stationary line of storms over a small portion of north central Iowa and the southeastern corner of Minnesota have entirely dissipated, so the Slight Risk area was downgraded to a Marginal, for the potential that shower activity late tonight may move over these newly flood-sensitive areas and cause very isolated flash flooding. ....Central Plains... (No changes from the previous 16Z forecast issuance, the forecast remains on track here) In coordination with BOU/Denver, CO forecast office, the Moderate Risk area in eastern CO was expanded to near the I-25 corridor in north central CO to highlight the ongoing showers and thunderstorms that are tracking northwestward across the state. Flash Flood Warnings have already been issued near Colorado Springs, and the expectation that a developing low will keep rain ongoing across north central CO right through tonight with little to no break should result in more widespread flash flooding, especially in those areas getting hit harder now with rainfall rates approaching an inch per hour at times. 6 hourly FFGs along and east of the I-25 corridor are 1.5-2 inches per hour broadly, which will likely be exceeded with the slow moving bands of embedded heavy rain. Wegman ....Previous Discussion... ....Lower MS Valley into the OH/TN Valley and Southeast... A broad mid level trough with embedded vorticity centers is located over the Southern Plains and Lower MS Valley this morning. Convection is likely to be ongoing at 12z across portions of AR/LA and far northeast TX. There is some concern that a locally significant flash flood risk could exist into this morning across portions of far northeast TX into northern LA and possibly portions of AR. The south/southwest flanks of these shortwaves can often be a favored region for training/backbuilding convection. Persistent convergence on this flank, along with plentiful upstream instability can allow for continued repeat/training convection. The environment is certainly conducive for heavy rainfall with a deep saturated layer and high freezing levels supporting efficient warm rain processes. Thus 2-3" in an hour rainfall is possible...which combined with any backbuilding/training can quickly result in excessive rainfall totals. Many of the 00z HREF members do show an evolution like this...which has resulted in storm total 3"+ neighborhood probabilities over 70% and 5"+ over 40%. The environmental setup and 00z HREF signal does raise a concern for a potential organized, and locally significant, flash flood event into the morning hours. There remains some uncertainty with regards to coverage and organization, so we will need to closely monitor observational trends. However do think the threat is enough to go ahead with a Moderate risk in the new ERO. This is mainly for the morning hours, with the expectation that convection should begin to forward propagate eastward a bit more by later this morning into the afternoon...reducing the flash flood risk. Elsewhere a broad Marginal risk extends from OK east into the OH/TN valley and southeast into FL. An isolated flash flood risk likely exists anywhere within this region, although at this time not seeing a strong signal for a more organized threat. .....Central Plains... A broad area of rain will likely be ongoing at 12z across eastern CO. A strong deep layer low will provide ample forcing for heavy rainfall, and PWs will remain well above average...so the main driving factor for where flash flooding may become an issue will be instability. The 00z HREF suggests activity will become increasingly convective in nature through the morning, with 1"+ in an hour probs quickly increasing. The better chance of these higher rates is expected to be from northeast CO into northwest KS and southwest NE...with rates probably gradually dropping off as you head west in CO. Thus think a Slight risk should suffice for most of north central CO, where prolonged rainfall with embedded higher rates should produce an isolated to scattered flash flood risk...but the lack of more significant rates preventing anything more widespread. However we are a bit more concerned with portions of southwest NE, northwest KS and immediate adjacent areas of CO. Much of this area has already received locally heavy rainfall over the past 24 hours, likely making the region a bit more susceptible to additional flooding. The 00z HREF has neighborhood probs of 40-60% chance of exceeding 3" in 24hr here, and as mentioned above, 1"+ an hour probabilities ramp up between 16z-00z. There is some chance the heavier convection today ends up moving through pretty quickly...but given the persistent slow moving forcing, still worried there could be some repeat/backbuilding activity. Given this additional rainfall potential on top of what has already occurred...do think more numerous flash flooding could occur, some of which could be locally significant. Thus we will go ahead and upgrade to a small MDT risk over this area. ....Northern Plains... Plentiful forcing and moisture will result in widespread moderate to heavy rainfall across portions of eastern MT into the western Dakotas. Persistent low level convergence will help focus areas of convection...with instability likely the main differentiator between areas that see a prolonged steady rainfall vs higher rates and a potential flash flood risk. Some higher rates are possible over eastern MT early in the period, but then the focus is expected to shift eastward into ND/SD. There has been a notable eastward shift in the 00z guidance...putting more of a focus into central/western ND and western SD instead of eastern MT. This isn't to say the threat is gone in MT, as scattered to widespread 1" rainfall is excepted with localized amounts of 2-3". So still enough to result in an isolated to scattered flash flood threat. However the greater risk for more prolonged inch plus an hour rates, and thus higher total rainfall amounts, now appears to be a bit further east into ND and SD, where forecast instability is higher. The 00z HREF is highlighting a maximum near or just west of Bismarck, along what appears to be a corridor of lower level convergence maximum on the nose of the better instability. Chenard Day 2 Valid 12Z Fri May 12 2023 - 12Z Sat May 13 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH-CENTRAL TEXAS... ....2030Z Update... ....Texas... No significant changes were made with this update. The guidance has been in good agreement that portions of south central Texas may receive several inches of rain Friday night as a strong LLJ advects significant Gulf moisture and instability up the terrain into the Texas Hill Country. Since the forecast remains on track, please refer to the previous discussion below for more details. ....Dakotas... The most significant change with this update was to expand the Slight Risk area north across western ND. Pockets of heavy rainfall are likely as an upper level low tracks northward into NE/SD. A nearly stationary front/trough will be centered over the western Dakotas, which will uplift a 30 kt southeasterly jet of moisture with PWATs to 1.25 inches into the front. This steady forcing should allow for widespread 1-2 inch rainfall totals, which in addition to the rain that falls this afternoon and tonight may result in widely scattered flash flooding. ....OH/TN Valley... Model guidance remains very diffuse in how the convection over the area will organize on Friday. One of the changes made was to expand the Marginal a bit to the east to include low FFG portions of the Appalachians of far western MD and WV. Otherwise think there will be less forcing further west in IL/northern IN, so the Marginal Risk area was trimmed a bit on the northwest side. ....Southeast... Very few changes were made to the inherited Marginal Risk area. The previous discussion remains in effect. Wegman ....Previous Discussion... ....Texas... A significant flash flood event could evolve Friday night across portions of south central TX. Forcing, moisture and instability all look to be in place to support an excessive rainfall event. Forcing is being driven by a mid level shortwave and favorable upper jet. This mid/upper forcing persists through the period, and does not show much eastward progression with time. This type of forcing evolution often favors training/backbuilding. Strong southeasterly low level flow off the Gulf of Mexico and a mid/upper level connection to the Pacific will support anomalous PWs and efficient rainfall. Instability should be on the order of 3000-4000 J/kg, with plenty of upstream instability to maintain convection. The main question is the last remaining ingredient for flash flooding...rainfall duration. The aforementioned slow moving forcing supports prolonged duration...and the increasing 850mb southeasterly low level flow results in very weak Corfidi vectors, indicative of a backbuilding/training risk. With that said, the 00z 3km NAM depicts a forward propagating convective line, and thus while it would still support some flash flood risk, it's evolution would likely not result in a significant widespread event. However, while this evolution can not be ruled out...we do think the overall environment is conducive to more backbuilding/training than this CAM would suggest. Model QPFs from the GFS/ECMWF/GEM Reg/UKMET also support something more extreme. Thus still thinking this is an event where swaths of 6"+ rainfall are a possibility, with 2-3" an hour rates. Scattered to numerous instances of flash flooding, some of which could be higher end in nature seem possible. There remains some spread with the exact location of highest rainfall this period. The inherited Moderate risk area was generally in between some of the further south and north solutions, and still seems like a good compromise and region of most likely occurrence. Thus little to no adjustment was made to that area. The Moderate risk is surrounded by a broader Slight and Marginal risk, where flash flooding is possible, but coverage and magnitude look lower at this time. ....Northern Plains... Pockets of heavy rainfall will continue across portions of the Northern Plains through the day Friday as a slow moving closed low drifts northward. The best combination of forcing and instability currently looks to be from central NE into central SD and south central ND. Some of these areas will have been hit by heavy rainfall in the preceding day or two as well. So the threat of excessive rainfall still seems Slight risk worthy across this corridor. The exact area is still subject to some adjustment...but for now aligned it with the best model consensus. The heaviest rain this period may actually end up over the sand hills of NE...but given the absorption efficiency of this landscape...a Marginal risk should suffice. ....OH/TN valley... The shortwave over the lower MS Valley as of 12z Thursday will make its way northeastward into the TN/OH Valley by Friday. The exact track of these shortwaves are always tricky (as they can be greatly influenced by convective features), and forecasts such as this are typically subject to pretty large location errors. Thus not really all that confident on how things will play out at this time. However will say that the better instability will be to the southwest...and sometimes the global models can track these waves too far off to the north away from the instability. Thus my guess is that if there is a flash flood threat with this feature it will tend to be further south over the OH/TN valley...and if the wave does get further north it will tend to be a more stratiform or only weakly convective rain. But for now will keep a rather broad Marginal risk to account for the uncertainty. ....Southeast... Opted to introduce a small Marginal risk across portions of southern AL/GA into the FL Panhandle. It looks some a small piece of vorticity associated with the broader trough to the north will try to drop into the region Friday. This could act as a focus for convective development and a localized flash flood risk. The 00z HREF shows pretty good 3"+ neighborhood probabilities over this area through 00z Sat, which combined with some low level convergence and above average PWs suggests some flash flood risk could exist. Chenard Day 3 Valid 12Z Sat May 13 2023 - 12Z Sun May 14 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF SOUTH TEXAS... ....2030Z Update... ....Texas... The latest 12Z guidance trends are shifting the heaviest rainfall south. Thus, the confidence that the heaviest rain will fall across much of southern Texas is increasing, with a bit lower certainty for significant heavy rains further north. In collaboration with BRO/Brownsville, TX and CRP/Corpus Christi, TX forecast offices, the Moderate Risk area was expanded to include all but small portions of coastal south TX. It's important to note that in the small area included in both Day 2 and Day 3 Moderate Risks, San Antonio is in both days Moderate Risks. Thus, the impacts of flash flooding may still be substantial in the San Antonio area. Further, with the heaviest rains shifting south, that happens to be downstream of the Day 2 Moderate, and as such it's likely that any main stem river flooding initiated on Day 2/Friday may be further worsened by the heaviest rains following the Rio Grande southward. Thus, while the area of overlapping rain has diminished somewhat from previous forecasts in terms of the area of overlap of both days' Moderate Risk areas, the combination of both the overlap that remains and that the new area is downstream of the previous one still supports the possibility of needing a High Risk upgrade in a portion of the Moderate Risk area with future updates. Since there has been a fair bit of "movement" as to where the heaviest rains are expected, and the high-resolution models may be needed to confirm the area of the highest risk, no upgrades were considered with this update. ....Northern Plains into Upper MS Valley... Agreement remains good for a Marginal Risk potential in this area, so the previous discussion remains in effect with no changes made. ....Previous Discussion... ....Texas... The event discussed in the day 2 discussion should be ongoing at 12z Sat across portions of south central TX. The expectation is that the convective complex will be making slow eastward progress by this time. There is still a question as to how quickly it will propagate eastward, and will be interesting to watch when we get into the high res model time frame. It will be a battle between the upstream forcing and deep layer flow trying to keep convection anchored, and cold pool development trying to propagate convection southeast into the instability pool. Tough call, but unfortunately it seems more likely that the slowing factors generally win out, and we only see slow eastward propagation with significant training/backbuilding. Either way, extreme rainfall rates of up to 3" in an hour seem probable with this convection. So even a best case progressive area of convection would likely result in at least a locally significant urban flash flood risk as the convection moves across the Hill Country and towards San Antonio and Austin. Best guess at the moment favors the potential for 6"+ rainfall amounts in spots across this area. We will need to closely monitor trends, especially as we get more CAM guidance to evaluate. There remains some potential that this setup could produce a High risk event over the Hill country vicinity. ....Northern Plains into Upper MS Valley... The closed mid level low will finally begin to open up into an open trough on Saturday, however enough forcing will persist to drive a convective threat. There is some signal for a bit better instability getting into the area along the low level stationary/warm front, which could support some more robust convective development. The ingredients might actually be there for a Slight risk type event, however these decaying closed lows are tough to forecast, so would like to see a bit more consistency before upgrading. In addition, antecedent rainfall may play a role in the eventual flash flood risk coverage. Will this day 3 axis overlap areas that see rainfall in the preceding days or not? Given these questions will keep a Marginal risk for now and continue to evaluate. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-iZkzkME_8tGjpVVT1Q5YqP10nO9tGRluqah9Opa_mN2= E9-lnkTWPpn3lo6nv_RxRIalc74DBrFa_vR91P-TTQUIW3E$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-iZkzkME_8tGjpVVT1Q5YqP10nO9tGRluqah9Opa_mN2= E9-lnkTWPpn3lo6nv_RxRIalc74DBrFa_vR91P-TJuIFXCc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-iZkzkME_8tGjpVVT1Q5YqP10nO9tGRluqah9Opa_mN2= E9-lnkTWPpn3lo6nv_RxRIalc74DBrFa_vR91P-TqyB8Z6I$=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. 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