Subj : HVYRAIN: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri May 12 2023 00:49:47 FOUS30 KWBC 120049 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 849 PM EDT Thu May 11 2023 Day 1 Valid 01Z Fri May 12 2023 - 12Z Fri May 12 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL HIGH PLAINS... ....Central High Plains... Anomalous and symmetric deep layer low continues to slowly moving across Eastern Colorado providing steepening lapse rates aloft.=20 While the dry line surges northeastward, low level moisture and strengthening LLJ will continue to advect across west-central to northwest KS likely to maintain sufficient ascent for active arched bands of convection centered across northwest KS into southwest NEB through the overnight period. LLJ increases to 40-50kts and with boundaries from ongoing convection, there may be stronger isentropic ascent to maintain additional bands though the overnight period. These areas have already seen sufficient heavy rainfall over the last 24-30hrs with upper soil moisture ratios pushing upwards of 60-70%, so very little addition rainfall will continue the risk for flash flooding conditions with these further bands, including a shift northward toward the Platte River Valley. As such, small adjustments of the Moderate Risk were made, though the largest change was the removal of the western portion across the East to east-central CO, where mostly stratiform rains should fall; though with wet grounds, maintained the Slight Risk though the wrap around portion of the TROWAL over the Front Range and connection toward the northern High Plains. Strong isentropic/FGEN ascent across E WY and the NEB Panhandle warranted the Slight Risk, though given the rates are not likely intense enough, the Sand Hills will remain in the Marginal Risk of Excessive Rainfall. ....Northern High Plains... North of the main upper-low a deforming trof across central MT along with the apex of the synoptic ridge across MN/NDak will continue to support very favorable upper-level divergence ascent pattern across the northern High Plains of E MT and the western Dakotas. This is directing the deepest moisture axis/warm conveyor belt across the Midwest and into the Dakotas with 1.25-1.4" advected on strengthening LLJ to 25-30kts by 06z. This will intersect fairly orthogonally across the 850mb north-south boundary and produce broad area of ascent and high moisture.=20 Depending on the strength of the deepening shortwave (given the favorable pattern aloft), some Hi-Res CAMs continue to suggest 2-4" totals with higher localized maxima, while it is difficult to pin-point such maxima, a broad Slight Risk over west-central NDak remains warranted. Overall instability values/strength is likely to be the limiting factor in terms of the highest magnitudes of rainfall.=20 ....Mid to Lower Mississippi River Valley into the Southeast... The broad shortwaves that have been over the Southern Plains and Western Gulf over the past few days, have lifted and stretched along the Mississippi River with northern most wave now crossing NE MO. The warm conveyor continues to have ample deep layer moisture along and ahead of the shearing mid-level trof with 1.75-1.9" total PWat values with highly confluent low level flow through the axis, while at the tail-end of the shearing trof, the larger MCC/squall line that crossed LA has weakened significantly. Yet, a narrow line of thunderstorms through the shear axis into the northern portion of the squall line across W TN into N AL continues to activate stronger, highly efficient thunderstorms.=20 Forward propagation has been slow, but given the narrowness of the lines, the duration is likely going to be limited to 2-3" locally potentially inducing widely scattered flash flooding instances as the instability wanes of the early overnight period as it crosses the Mississippi River into S IL, W KY, W and Middle TN, N AL.=20 Further south, the anticyclonic rotor of the larger MCC/squall line has remained active (as noted by MPD 266) with and WSW to ENE line of stronger thunderstorms with 2-2.5"/hr rain-rates across far E LA/S MS into S AL. Thunderstorms reside in an area of enhanced instability with some strengthening convergence/warm air advection coming off the central Gulf of Mexico. There was a consideration for a small Slight Risk of Excessive Rainfall for this area, but instability is starting to be exhausted and there is some suggestion for recent downward trend in intensity; plus the heavy rainfall is falling across an area of higher FFG values. Scattered instances of flash flooding remain possible within the next few hours, but will maintain the Marginal Risk here for the uncertainty of the ongoing longevity into the overnight period. ....Oklahoma/Red River Valley... Strong thunderstorms have developed in the vicinity of an older E-W outflow boundary or effective warm front that was lifting north through the Red River Valley ahead of the dry line. Strong moisture flux convergence along the strengthening low level jet has led to solid convective initiation across central OK toward the Red River. HRRR solutions have been hinting at an axis/es of heavier rainfall across central OK into eastern OK for the remainder of the night aided by the LLJ. However, there is run to run inconsistency to placement of the rainfall axis across the state. With that stated, efficient rainfall production with rain rates of 2-2.5"/hr there is sufficient coverage/confidence to pull a broader Marginal Risk of Excessive Rainfall back to account for this growing convective area. Please refer to MPD 268 for more details. Gallina 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!6Xv1zLVNTJdtsXIOUYV6wjkfbE40jZtMZ5OSiHcH-Y2B= DuZjlEQoCSrvpmeeebkT1KYFL-4n2s3SrgxZeF0JzlUryQM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6Xv1zLVNTJdtsXIOUYV6wjkfbE40jZtMZ5OSiHcH-Y2B= DuZjlEQoCSrvpmeeebkT1KYFL-4n2s3SrgxZeF0JFSSVeOQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6Xv1zLVNTJdtsXIOUYV6wjkfbE40jZtMZ5OSiHcH-Y2B= DuZjlEQoCSrvpmeeebkT1KYFL-4n2s3SrgxZeF0JDJ6Ybas$=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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