Quantitative Precipitation Forecast
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017 FOUS30 KWBC 280052 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 852 PM EDT Sat Apr 27 2024 Day 1 Valid 01Z Sun Apr 28 2024 - 12Z Sun Apr 28 2024 ...A HIGH RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR A SMALL PORTION OF EAST-CENTRAL OKLAHOMA WHERE NUMEROUS SIGNIFICANT FLASH FLOOD IMPACTS ARE EXPECTED... ...A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF FAR NORTHERN TEXAS INTO CENTRAL AND EASTERN OKLAHOMA, SOUTHEAST KANSAS, SOUTHWEST MISSOURI, AND FAR NORTHWEST ARKANSAS... ...01Z Update... Flash flooding scenario with potentially significant impacts continues to unfold across portions of the Southern Plains states from this evening into the overnight hours. A few minor modifications were made based on radar trends from late afternoon/early evening and the 23Z run of the HRRR...with a slight elongation/expansion to the south and west into western north Texas. HREF guidance from 18Z may not have been quite as bullish on its probabilities but the area with the greatest amount of temporal overlap for probability of 2 inch rainfall in a hour continues to be focused over the High Risk area. Admittedly, on- going convection will make the convective evolution more challenging but the consistent and persistent signals from HRRR and RAP for blossoming of heavy rainfall later tonight certainly highlights the potential. 00Z soundings showed 2500 to 3500 J per kg of CAPE and precipitable water values with a weakly capped environment. This has already led to occurrences of excessive rainfall near the dryline in Texas and western Oklahoma as well as areas farther north and east in proximity to a nearly stationary boundary. The overlap of rainfall intensity and the duration shown by the latest CAMS remains over portions of Oklahoma and supportive of maintaining the High Risk area. Bann ...16Z Update... Confidence continues to be high in a major flash flood event unfolding for portions of east-central Oklahoma with higher end rainfall totals Saturday beginning late this evening into Saturday night. Therefore, a small High Risk was introduced for locations east of OKC along I-40, but higher-end rainfall totals (5 to 10 inch 24 hour totals) and flash flooding is also expected from the Red River Valley into portions of northeastern Oklahoma as well. 12Z components of the HREF show unanimous agreement with 3 to 6+ inches for east-central OK where the High Risk was placed, and while there remains some potential for cold pools to shift the axis of higher rainfall to the south, it is difficult to ignore the consistent (00Z to 12Z cycle) placement of HREF probabilities for higher end rainfall. The probability values listed in the previous discussion below still hold true for the 12Z HREF, with perhaps a 5 to 10 percent increase overall for the EAS (3 inch) and neighborhood probabilities (5 and 8 inch) for the 24 hour period ending 122 Sunday. Elsewhere, changes to the outlook were minor but the Slight Risk across the Midwest was extended east to include northern Illinois into northwestern Indiana, due to the likelihood of cell training from WSW to ENE and 1-2 in/hr rainfall rates late this evening and overnight. Given flash flood guidance for this region of the Midwest is only 1 to 2 inches in 3 hours and after collaboration with LOT and IWX, an upgrade to Slight was introduced for this update. Otto ...previous discussion follows... A significant rainfall event still appears likely Saturday night across portions of north central TX into central/eastern OK and portions of southeast KS and southwest MO. Numerous instances of flash flooding are expected, some of which will likely be significant in nature. Overall not much has changed with the expected setup over the past couple days. The environmental ingredients remain in place for training/backbuilding convection, especially during the overnight hours. Persistent and strong mid/upper level forcing, a significant ramp up in 850mb moisture transport into a stalled boundary, mean flow parallel to the boundary and weak Corfidi vectors (due to the strong low level jet aligned parallel to the mean flow)...all are favorable for training/backbuilding convection. The overlap of impressive upper level divergence and strong 850mb moisture convergence , both of which persist upwards of 6-12 hours, combined with substantial instability and well above average PWs...all point to a numerous flash flood threat, ramping up by later in the day into the overnight hours. Convection will likely get going pretty early in the day and then persist into the overnight hours. The initial convection should be more scattered in nature and moving at a decent clip...so would expect just an isolated flash flood risk initially. However as the low level jet and moisture transport really ramp up this evening into the overnight expect we will see upscale growth of convection into one or more training lines. Of course, the early convection could play a role in where this boundary and most favorable training corridor ends up, but overall models remain fairly well clustered with an axis from north central TX into central/eastern OK. Probabilities from the 00z HREF are pretty impressive. EAS probabilities of exceeding 3" are rather broad and as high as 50-80%. Given EAS probabilities can be considered a smooth point probability...this indicates an event with a widespread 3"+ QPF footprint is probable. Embedded within this 3" area will likely be higher totals...with 5" neighborhood probabilities over 60%, and 8" probabilities around 30%. This generally is in line with expectations from previous shifts...that this event has the potential to drop a swath of 4-8" of rain (isolated 10" max possible)...with the most likely location from just north of the Red River into central/eastern OK..including the Norman to Tulsa corridor. Overall think the 00z HREF QPF and probabilities are reasonable...although tend to think the southwest flank of convection could over perform relative to the HREF...with the 00z GEM Reg and FV3LAM potentially showing plausible outcomes of a QPF max a bit southwest of the 00z HREFbm max. The northeast extent of the MDT risk over Southeast KS and southwest MO may see a bit lower QPF than areas further southwest, however with wetter antecedent conditions here, flash flood impacts are still likely. One thing to continue to consider is that convection should become intense and organized enough to develop a stronger cold pool resulting in some eastward cell progression with time. While we do think there will be some eastward progression, 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 the organized and significant flash flood threat described above. Further north, a Marginal risk extends from northeast CO, across NE and into IA and portions of WI/IL/IN. Gave some consideration to a Slight risk upgrade over portions of NE and western IA given the heavy rainfall over these areas on Friday. However the extent of convective training is a bit more uncertain here, and the higher probabilities of heavy rain end up a bit south of areas hardest hit Friday. Thus tend to think flash flooding will be more isolated in nature today, which fits more into the Marginal risk category. However will continue to monitor. Chenard Day 2 Valid 12Z 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... Convection will likely be ongoing at 12Z Sunday across portions of eastern OK and TX, into western AR/MO. Current expectations are that convection will have enough of a cold pool by this time to result in a decent eastward propagation, while also running into a less unstable airmass with time. While some lingering flash flood risk into Sunday morning is likely over these areas, the risk should be on a decreasing trend. The bigger question becomes what happens by later in the day into the overnight hours. Synoptically, the mid to upper-level trough over the southern/central High Plains on Saturday will lift NNE toward Minnesota through Monday morning. As this occurs, a lack of height falls across the Gulf Coast region will coincide with a retreating dryline in Texas, but locations from Arkansas down the Sabine River Valley will remain beneath diffluent and divergent flow aloft for much of the period (though jet-induced divergence may be focused better across northern locations than those closer to the Gulf Coast). Current thinking is for new convective development along the front/dryline or perhaps remnant outflow (from the early morning convection) in the 18-00Z window over northeastern Texas into southeastern Oklahoma and western Arkansas. The leading edge of the cold pool from the afternoon/evening thunderstorms is expected to be intercepted by 25-45 kt of southerly flow at 850 mb, forcing the development of additional thunderstorms within the unstable environment. Orientation of convection is likely to match mean steering flow from the southwest at times, supporting training and rainfall rates of 1 to 3 in/hr (although localized rates in excess of 3 in/hr will also be possible). Overall this event remains as a higher end Slight risk from east and northeastern Texas into southeastern Oklahoma and western Arkansas. A more focused and concentrated area of numerous and significant flash flooding is still a possibility with higher neighborhood probabilities of exceeding 5 inches in 24 hours (ending 12Z Monday) showing 40-60 percent from near the Arklatex down to a Huntsville to Jasper to Fort Polk line and 20-40 percent for EAS probabilities for 3+ inches along the northern Texas/Louisiana border. Along with the typical convective uncertainties, the 12Z global and hires guidance (FV3, NAM_nest) showed some southward displacement compared to their previous runs, into drier antecedent conditions where flash flood guidance is about 4+ inches in 3 hours. Due to these factors, confidence in placement of a Moderate is not there yet, although Moderate Risk coverage of flash flooding (locally significant possible) is expected somewhere within the Slight Risk. 24 hour QPF from the 12Z model suite ranged from 4 to 7+ inches ending Monday morning. Across the Midwest into Wisconsin and Michigan, a broad Marginal Risk remained in place with minor adjustments from continuity. With northern extent, there remains uncertainty with the availability of instability but a slow moving front with steering flow parallel to the boundary will be in place across northern Illinois/southern Wisconsin into Lower Michigan. While potential for higher coverage of rates over 1 in/hr is not as high for these northern locations, flash flood guidance is low in pockets across the northern half of the outlook area, especially for portions of Nebraska/Iowa due to recent heavy rain. Otto/Chenard Day 3 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 and the main flash flood threat looks to be focused in roughly the first 12 hours of the period. As the closed low from over the weekend moves north, an upper level trough/shortwave is forecast to swing across the Lower Mississippi Valley from the west during the day on Monday. Out ahead of this feature, the 12Z forecast guidance continues to show an impressive combination of mid/upper forcing, low level convergence, instability and moisture to support a flash flood risk. At this time these ingredients look a bit more progressive than previous days, which should result in quicker cell motions and an overall lowered flash flood threat with perhaps the best potential for stalling/training occurring across Louisiana into portions of Mississippi. Rainfall rates should still be quite high given the instability and moisture in place, so 2"+/hr remains probable in spots. This should be enough to drive an isolated to scattered flash flood risk, especially over urban areas. Other than a southward trend in the GFS/UKMET/CMC QPF, no significant changes to mass fields were noted and the models remain in pretty good agreement on the axis of heaviest rainfall. The 12Z ECMWF continues to be a bit of a northern outlier with QPF maxima from southern Illinois, down into eastern AR, just west of the Mississippi River. While a Marginal exists for many of these northern locations, the Slight was kept where the better agreement for higher rainfall existed, from near the Missouri Bootheel to far southeastern Texas and southern Louisiana, overlapping eastern Arkansas and western Mississippi. Otto/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