Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Sun Apr 28 2024 15:20:00 FOUS30 KWBC 281629 CCA QPFERD Excessive Rainfall Discussion...CORRECTED NWS Weather Prediction Center College Park MD 1229 PM EDT Sun Apr 28 2024 Day 1 Valid 16Z Sun Apr 28 2024 - 12Z Mon Apr 29 2024 ....A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS EASTERN PORTIONS OF THE SOUTHERN PLAINS INTO THE LOWER AND MIDDLE MISSISSIPPI VALLEY... ....16Z update... 15Z radar imagery and surface observations showed a broad area of stratiform rain with weakening embedded convection from central AR into east-central TX. There is good agreement in the 12Z hires model suite for redevelopment/reintensification of thunderstorms between 18-00Z this evening. It is still a bit unclear if remnant outflow will help to initiate storms and/or the dryline farther west in central TX. Either way, a broad region of thunderstorms aligned from NNE to SSW from eastern OK/western AR into eastern TX appears likely. Within this area of thunderstorms will likely be portions that align with the steering flow supporting training. Current thinking is there could be a bi-modal distribution of heavy rain, first across portions of eastern TX into western LA and far southern AR where inflow/instability should be maximized. This is in line with the 12Z HRRR, ARW, ARW2 and FV3. A second area farther north is possible from far eastern OK into southern MO/northern AR where synoptic scale lift will be greatest, with a relative minimum in precipitation over central AR. Confidence for a Moderate is still not there but perhaps a late update to Moderate is possible as the convective situation becomes clearer later this afternoon. Otto ....previous discussion follows... Convection will be ongoing at 12z this morning across portions of eastern OK and TX, into western AR/MO. Still looking like activity will be progressing off to the east at a greater clip by this time, and the overall convective intensity should also be on a gradual downward trend. Thus, while some lingering flash flood risk into this morning is likely over these areas, the risk should be on a decreasing trend. By this afternoon expect to see additional convective development along the front/dryline across TX into eastern OK, with this activity pushing eastward into AR and southern MO by this evening. This will be supported by strong and persistent upper level divergence, and increasing 850mb moisture transport/convergence into the front/dryline. The 850mb moisture transport axis is a bit more progressive compared to Saturday, which combined with cold pool generation from the organized convection, should result in an eastward propagation of convection. Maybe just enough of an eastward motion to prevent a more widespread flash flood event. However, with the orientation of convection likely to match mean steering flow from the southwest at times...there will still likely be some brief training of cells. This will support rainfall rates as high as 1-3"/hr at times, and do expect we will at least see isolated to scattered flash flooding over a fairly large swath from eastern TX into southern MO. HREF EAS probabilities of exceeding 2" get as high as 40-70% over the area, but 3" EAS probabilities drop to 15-30%. Meanwhile neighborhood probabilities of exceeding 3" are over 50% over a large area, but 5" probabilities drop more into the 15-30% range. With EAS probabilities more indicative of QPF coverage, and neighborhood probabilities higher end maximum potential...this all suggests that areal averaged rainfall will tend to stay in the 1-3" range...with localized swaths of 3-5" likely as well. However 5"+ totals are a bit more uncertain...and while I'm sure we will see a swath or two of over 5" of rain, exactly where remains unclear. Overall would like to have seen a more concentrated and higher 5" probabilities to upgrade to a MDT risk. From a hydrology perspective, soil saturation and streamflows are elevated over some of this area, and not as much of other locations. Thus not quite enough sensitivity to justify using this as a reason to do a categorical upgrade at this time. Thus, overall still think this event remains as a higher end Slight risk across eastern TX, western LA, southeast OK, central and western AR and southern MO. Both the 00z and 06z HRRR runs have been aggressive with convective training and QPF amounts on the southwest flank of the area of convection over southeast TX into far western LA. This is a typical favorable location for training/backbuilding, so the HRRR evolution is possible. However the HRRR is further south of the higher HREF probabilities...while the experimental NSSL MPAS runs are actually further south of the HRRR, and even indicate the possibility ongoing convection in the morning persists and propagates fairly quickly through the region. Thus think there is still a bit too much uncertainty to go with a MDT risk upgrade...although this appears to be the most likely location for one...and so will continue to monitor observational and model trends today. Across the Midwest into Wisconsin and Michigan, a broad Marginal Risk remained in place with minor adjustments from continuity. Not a much instability here, and convection should generally be moving along at a decent clip. Although like further south, some brief along track training is possible given steering flow parallel to the boundary. Chenard Day 2 Valid 12Z Mon Apr 29 2024 - 12Z Tue Apr 30 2024 ....A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF THE LOWER MISSISSIPPI VALLEY.. The convective and heavy rain threat shifts eastward into the lower Mississippi Valley on Monday. Thunderstorms with heavy rain are likely to be ongoing at the start of the period Monday morning across portions of far southeast TX into portions of LA and MS...and the main flash flood threat looks to be focused in roughly the first 12 hours of the outlook period. Models continue to show an impressive combination of mid/upper forcing, low level convergence, instability and moisture to support a flash flood risk across this region. Most indications are that convection will be progressing off to the southeast Monday morning given a gradual eastward push of the convergence axis and cold pool prorogation. This is expected to limit the extent and magnitude of any flash flood risk. However, depending on the exact orientation of the convection by this time, we could see some backbuilding/training on the southwest flank of the MCS. This is most likely over LA, and this could drive a more organized flash flood risk. Even without any more pronounced training, the expected organized MCS will still be capable of hourly rainfall as high as 2-3"/hr given the expected instability and moisture in place. This should be enough to drive an isolated to scattered flash flood risk, especially over urban areas. It should be noted that there is some uncertainty on the timing of the MCS, with the 00z and 06z HRRR suggesting the organized convection is almost offshore by 12z. While this is a possibility (and would require shrinking the Slight risk), this is more of an outlier scenario at the moment....with most models supporting the MCS more over central LA at 12z. Additional convection is likely from MS into portions of the TN and OH Valley. This activity will have the support of a compact shortwave pushing east across the region. The progressive nature of this feature, and more limited instability to work with, both suggest any flash flood risk should stay localized in nature across these areas. The Slight risk was cut back over these areas. Chenard = = = --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .