395 FOUS30 KWBC 260823 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 423 AM EDT Fri Apr 26 2024 Day 1 Valid 12Z Fri Apr 26 2024 - 12Z Sat Apr 27 2024 ...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF THE SOUTHERN PLAINS INTO THE LOWER TO MIDDLE MISSISSIPPI VALLEY... A Slight risk was maintained with this update from portions of northeast TX into MO, although there remains some uncertainty regarding the intensity and organization of this activity. Convection will likely be ongoing at 12z this morning across portions of TX and OK, with this activity gradually pushing eastward across these two states and into portions of MO and AR through the afternoon hours. With time today the better synoptic forcing lifts to the north, resulting in a downward trend in forcing across the southern half of the Slight risk, with weakening lower level convergence and also less pronounced mid/upper level ascent. These larger scale factors would generally favor weaker and less organized convection with time today into tonight over these southern areas. Countering this is a rather favorable thermodynamic environment, with plentiful instability and moisture. Thus it seems most likely that if convection this morning over OK and TX can grow upscale enough to generate an organized outflow/cold pool, then this mesoscale feature could be enough to sustain an axis of more robust convection through today. Any convection that is able to sustain and grow upscale may exhibit some training and backbuilding characteristics into the strong low level jet in place. More recent observational and HRRR trends suggest portions of MO have the best chance of seeing multiple convective rounds today with stronger upper forcing and WAA...with one round this morning and another through the afternoon hours. Thus we did expand the Slight risk into more of MO to account for this risk. Further north a Marginal risk is in place from northern MO into SD and MN. Elevated showers and embedded thunderstorms will move across this region this morning, but will be outrunning the better instability...which should keep rainfall rates low enough to prevent any more organized flood threat. As the surface low and strong mid/upper forcing ejects into the Plains this afternoon additional convective development is likely from eastern NE/SD into portions of IA and MN. Some of this activity should be intense, but generally should be moving at a quick enough pace to limit the flash flood risk. Chenard Day 2 Valid 12Z Sat Apr 27 2024 - 12Z Sun Apr 28 2024 ...A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF FAR NORTHERN TEXAS INTO OKLAHOMA AND FAR SOUTHEAST KANSAS... A significant rainfall event still appears likely Saturday night across portions of north central TX into central/eastern OK. Overall not much has changed over the last couple model cycles, and thus much of last nights discussion still holds true. Large scale forcing really ramps up Saturday night with the approach of the mid level trough and 250mb upper jet streak. With strong ridging to the east, these features lift more northward than eastward during the overnight hours, resulting in a prolonged period of impressive upper level divergence centered over Oklahoma. The persistence of this large scale forcing is typically indicative of a favorable training convective setup. In the lower levels we have a very strong 850mb jet in excess of 50 kts resulting in impressive moisture transport and convergence near a slow moving cold front. This low level flow is nearly equal and parallel to the deep layer mean flow, which typically supports a backbuilding convective risk. The expectation is that with deep layer mean flow nearly parallel to the low level boundary, convective cells will tend to train and also backbuild into the strong low level jet. Convection should become intense and organized enough to develop a stronger cold pool resulting in some eastward cell progression with time. This eastward progression is the main wild card in determining the magnitude of the flash flood risk with this event. We do think there will be some eastward progression, but the persistent forcing and strong low level jet both will be opposing a significant eastward push...and thus tend to think we will see enough persistence of convection to result in an organized flash flood threat. Convection will be capable of 2-3"/hr rainfall, and thus it will not take much persistence to start getting some significant rainfall totals. While widespread 5"+ totals may not occur, this event does appear to have the potential to produce a swath(s) of as much as 4-8" of rainfall. This is expected to result in an increasing, and locally significant, flash flood threat Saturday night Model guidance remains in decent agreement, all generally focusing the highest swath of QPF from north central TX into central/eastern OK. Although, as would be expected with any convective forecast, there is still some uncertainty on where exactly the highest QPF ends up, as the swath of the most excessive amounts will likely be rather narrow. The GEM REG has been pretty consistent with a swath heavy rainfall, and while the magnitudes and areal extent are probably overdone in that model, do think it represents a reasonable evolution of how things may play out Saturday into Saturday night. The 00z ECMWF remains a bit further southeast than the consensus. The 00z GFS has a secondary max over eastern KS into western MO, and while heavy rain is expected here, tend to think the GFS is overdoing this northern swath, and under doing the swath over OK (this is a typical GFS bias). The experimental machine learning GFS Graphcast actually appears to be correcting this bias, and has an axis more similar to the GEM reg (albeit lower magnitudes). The current placement of the MDT risk is where the best overlap in ingredients and the model QPF consensus resides, and was extended a bit more southwest into north central TX with this update. Across eastern KS into northwest MO and southern IA locally excessive rainfall is also expected along an axis of stronger low level convergence. But in general the thermodynamic ingredients here are less favorable for a prolonged period of heavy rain compared to the swath over OK, and thus a Slight risk should suffice. A Marginal risk extends westward across NE into northeast CO along/near the low track where convection near the warm front may result in a localized flood risk. Chenard Day 3 Valid 12Z Sun Apr 28 2024 - 12Z Mon Apr 29 2024 ...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF THE SOUTHERN PLAINS INTO THE LOWER AND MIDDLE MISSISSIPPI VALLEY... Convection will likely be ongoing at 12z Sunday across portions of OK and TX, a continuation of the Moderate risk day described above for Saturday night. Current expectations are that convection will have enough of a cold pool by this time to result in a decent eastward propagation. Thus the flash flood risk should be decreasing into Sunday morning, although some continued flash flood risk is probable. The bigger question becomes what happens on Sunday night. There is some model disagreement on this, although the majority of solutions would suggest an additional round of upscale convective development is likely. This is also generally supported when looking at things from an ingredients based perspective. With the main low lifting off to the northeast over the Upper MS Valley, the portion of the front over TX and AR should become nearly stationary. Meanwhile we see a strong and persistent upper level divergence signature centered near the TX/AR border, and a trailing mid level shortwave ejecting into the southern Plains should only help to enhance lift near the stalled low level convergence axis. Overall, ingredients are similar to what we will see Saturday night over OK, thus training and backbuilding convection appears probable. Still think this event may very well eventually need a Moderate risk given the setup and ingredients in place. However there remains a bit of uncertainty with regards to where the best convective training will be...with anywhere from northeast TX into AR within the range of possibilities. Given this uncertainty, and the fact that the overall synoptic setup, while similar to Saturday night, is probably not quite as favorable (also noting slightly lower model QPFs compared to day 2)...think sticking with a higher end Slight risk is the best course of action for now. Plus, rainfall Friday will likely play a role in if/where soil and stream conditions will be more sensitive for flash flooding come Sunday...so knowing how convection plays out Friday may help with any eventual MDT risk placement Sunday. Thus the Slight risk remains for now, and will continue to evaluate on future shifts. A broad Marginal risk extends from MO northward into MN and WI, generally along and east of the strong low and mid/upper forcing. Convection here, while locally intense, should be quick moving, limiting the extent of any flash flood risk. However this will be the second strong low pressure and convective threat within a 3 day period...so can not rule out an eventual need for an embedded Slight risk pending soil and stream response from round 1. Chenard Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt