Quantitative Precipitation Forecast
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625 FOUS30 KWBC 282030 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 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 SOUTHEASTERN TEXAS INTO 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. An impressive combination of mid/upper forcing, low level convergence, instability and moisture should be present to support a flash flood risk across this region. The 12Z model consensus indicates about 1000-1500 J/kg MLCAPE ahead of what is expected to be a lingering nocturnal MCS from eastern TX into LA. Precipitable water values are forecast to be approaching 2 inches just ahead of the forecast leading edge of the MCS with southerly 850 mb winds of 30 to 40+ kt just ahead, advecting in moisture from the south. The region will also lie within the diffluent and divergent right-entrance region of 100 to 130 kt upper level jet max. Timing and placement differences remain within the 12Z components of the 12Z HREF concerning placement of the expected MCS at 12Z Monday, but the best probabilities for rainfall rates in the 2-3 in/hr range exist from north-central LA into southwestern LA and southeastern TX where backbuilding/training are most probable. The southwestern flank of the MCS is expected to move offshore into the Gulf of Mexico during the day as the leading edge continues to advance eastward across the Lower MS Valley. The forecast evolution of the MCS has it weakening and/or moving offshore by 00Z which should end or at least significant decrease the flash flood threat for the region. Farther north, a broad Marginal encompasses locations in the middle MS and lower OH Valley, eastward into the TN Valley. These locations will be within an anomalous moisture axis (standardized PWAT anomalies of +1 to +2) and at least weak instability, located ahead of a cold front approaching the lower OH Valley during the day. Convective coverage will expand with daytime heating and while most storms should stay progressive enough to limit flash flood concerns, a similar orientation/magnitude of the mean steering and 850 mb flow suggests potential for localized training which may pose some flash flood concerns despite dry antecedent ground conditions. Otto Day 3 Valid 12Z Tue Apr 30 2024 - 12Z Wed May 01 2024 ...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF THE SOUTHERN PLAINS INTO THE UPPER MISSISSIPPI VALLEY...AND OVER THE UPPER TENNESSEE/OHIO VALLEYS INTO THE APPALACHIANS... ...Northeast... A lower end Marginal Risk was maintained for central PA into Upstate NY and portions of VT where localized 1 to 3 inch rainfall totals may occur. Guidance indicates the presence of a NW to SE oriented stationary front across this region on Tuesday, which will likely help enhance lower level convergence, along with the approach of a cold front from the west. Lift ahead of an approaching mid to upper-level shortwave and associated right entrance region of a jet max in southern Ontario/Quebec will aid in convective development. MLCAPE of ~500 to 1000+ J/kg is probable via the 12Z NAM (GFS tends to bias low with instability) over portions of PA and NY, although a bit less certain over VT. PWs are forecast to be quite high for late April/early May, with values from the 12Z NAM/GFS/ECMWF just over 1.25" (approaching climatological 99th percentile). Given the ingredients in place, it seems plausible that at least an isolated flash flood risk could evolve. ...Upper Ohio/Tennessee Valleys into the adjacent Appalachians... Some degree of overnight showers and thunderstorms is expected to continue into Tuesday morning from KY into TN, south and east of a cold front approaching from the west. It`s not clear if storms lingering into Tuesday morning from the overnight will pose a flash flood concern but some redevelopment of convection is likely during the afternoon hours. Moisture anomalies are forecast to decrease during the day as an upper trough passes by overhead, but a small window for localized flash flooding appears possible prior to winds shifting around to the northwest near/after 00Z. ...Midwest to Upper Mississippi Valley... A broad Marginal risk extends from north central TX north into the Upper Midwest. The northern portion of this risk over IA and MN/WI will see the strongest forcing as a low pressure and a negatively tilted shortwave moves across the north-central U.S. The attendant cold front will be progressive, which suggests convection will be moving quick enough to prevent a higher flash flood threat. However do see enough instability and moisture to support briefly heavy rates, which could cause some localized urban flooding and/or isolated flash flood issues where ground conditions are more sensitive from recent rainfall. ...Southern to Central Plains... Further south the front is expected to become stationary over KS with a dryline extending southward from western OK into northwestern TX. Locations near the front could result in a training/backbuilding convective risk, although large scale forcing appears weak due to a lack of height falls across the region. Nonetheless, the presence of the stalling front and increasing low level flow into this boundary does suggest at least some flash flood risk could evolve. Even farther south into OK and TX, while there could be a weak perturbation aloft that helps to initiate convection, daytime heating and erosion of CIN might be the main driver of convective initiation along a dryline. 0-6 km bulk shear is forecast by the 12Z GFS to be somewhat lackluster but seasonably moist airmass and recent heavy rain may be enough to support localized areas of flash flooding, especially if there is overlap with remaining hydrologically sensitive locations. Otto 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