Quantitative Precipitation Forecast
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Issued by NWS
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002 FOUS30 KWBC 271618 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1218 PM EDT Sat Apr 27 2024 Day 1 Valid 16Z Sat Apr 27 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... ...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 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. Ingredients do appear to be in place for upscale convective growth and some training/backbuilding potential. There is a slowing front over the area, with strong and persistent upper level divergence moving over top of this boundary. Moisture transport is not as strong as what we will see Saturday night over the Plains, but still note a pretty strong low level jet and 850mb moisture convergence signature. Overall this event remains as a higher end Slight risk from northeast TX into much of AR, and would expect to see scattered flash flooding across this corridor. A more focused and concentrated area of more numerous and significant flash flooding is still a possibility. Based on the ingredients in place, the most likely location for this appears to be in the vicinity of the Arklatex. This is also where the 00z GEM reg, UKMET, ECMWF and FV3LAM have a relative max in QPF. However there remains some spread with this scenario, and model QPFs are generally lower than what we are seeing on Saturday night over the Plains. Thus this event will probably have a bit less flash flood coverage. Still think a MDT risk upgrade may eventually be needed, with the Arklatex region the most likely location. However we will have much more high res guidance to evaluate over the the next 24 hours than we do now, so would prefer to hold off on any upgrade and continue to evaluate trends. Either way some flash flooding is expected, some of which could be locally significant in nature. A broad Marginal risk extends from MO northward into MN, WI and MI, 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, some of these areas will have seen heavy rain on previous days...so can not rule out an eventual need for an embedded Slight risk pending soil and stream response from this earlier rain. 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 MS Valley on Monday. Still seeing 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. Nonetheless, 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. Models are in pretty good agreement on the axis of heaviest rainfall, generally trending a bit further east compared to previous runs. The greater uncertainty lies with where to place the northern end of the Slight risk. The ECMWF remains further north than the GFS, GEM reg and UKMET. For now will keep the northern edge of this Slight risk over western TN, as this better aligns with the model consensus, with even the machine learning EC AIFS further south than the deterministic ECMWF. 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