WeatherWheeler35 Posted May 13 Share Posted May 13 Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Sat May 13 2023 Day 1 Valid 16Z Sat May 13 2023 - 12Z Sun May 14 2023 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF COASTAL TEXAS, THE TEXAS PANHANDLE, AND PORTIONS OF THE UPPER MS RIVER VALLEY TO THE NORTHERN PLAINS... ...16Z Update... ...Coastal Texas... The evolution of the storms ongoing along coastal Texas has continued to move offshore into the Gulf with the strongest convection in Deep South Texas, and while there is some training and slow moving storms along the middle Texas coast, this area of convection is rather isolated. The storms north and east of there approaching Houston are nearly stationary, but the rainfall rates are either not there or very isolated. To summarize the above, the flash flooding risk is far from widespread across south Texas. Thus, the Moderate Risk has been downgraded to a Slight with this update. Further, as for guidance for the rest of today into tonight, few CAMs show showers and thunderstorms redeveloping at all, and for those that do, that convection moves quickly, and therefore even if the storms are able to produce heavy rain, the rates will be short-lived over any one area. Thus, the Slight Risk was also shifted eastward closer to where the ongoing convection is, with little convection noted along the Rio Grande except well to the north close to Rocksprings. This rather isolated convection is training, but rates for now are 1.5 inches/ hour or less. Isolated flash flooding may be possible, consistent with the Marginal Risk over the area. ...Texas Panhandle to Oklahoma... The Slight Risk over much of central and eastern Oklahoma was downgraded to a Marginal with this update. The convection that was over southeastern OK is dissipating. This afternoon, isolated to widely scattered convection is likely to develop to the east of a mesolow tracking over west TX and the TX/OK border. The storms may be strong enough to produce heavy rainfall, but there's good agreement that such storms will be moving quickly, and will therefore be unlikely to produce anything more than isolated flash flooding. Further west towards the TX Panhandle, stratiform rain is likely to continue for much of the day into tonight, and while rates are unlikely to ever get too high, the long period of steady rain across this region west of the aforementioned mesolow may cause isolated to widely scattered flooding. ...Iowa/Southern Minnesota... Somewhat better agreement exists among the guidance that showers and thunderstorms will redevelop in the Slight Risk area over IA and southwestern MN this afternoon, which is likely separate from the convection ongoing over eastern IA. The front will pivot northwestward, which will allow the storms to train north/northwestward with time. This area has also seen heavy rainfall lately, particularly on the Minnesota side of the border. The Slight Risk was expanded a bit to the north and east to cover areas with lower FFGs that are likely to pick up additional heavy rainfall late this afternoon through tonight. Wegman ...Previous Discussion... ...Texas and Oklahoma... Tricky excessive rainfall forecast for today into tonight. The inherited Moderate risk looks to be in some trouble, as convection overnight into early this morning has been forward propagating at a greater clip than anticipated. Thus the greatest flash flood risk may be past by the time the new day 1 starts at 12z. It will be close, but still think coastal areas of south TX should still be getting heavy rainfall at 12z, and the eastern portion of the line should still be onshore and producing some very heavy rainfall rates over 2" in an hour. Although even this portion of the line is forecast to begin a weakening trend by mid morning. So while heavy rainfall rates will probably be ongoing at 12z, the forward progression of convection and gradual weakening trend should limit the magnitude/extent of the flash flood risk. With that said, the heavy rates and short duration backbuilding could still pose a locally significant flash flood risk into the morning hours, especially over any urban areas that are impacted. Once this activity clears, the question becomes do we get another round of convection tonight. Initially instability will be eroded, so we'll have to see if the continued southeasterly inflow can advect instability back into the area. Additional weak shortwave energy may move into the area out of Mexico, and so could envision convection forming in Mexico and moving back into the area tonight. Several of the 00z HREF members depict a solution like this, which results in 3"+ neighborhood probabilities of 40+ percent between 00z-12z Sunday. However it is not unanimous, and the 06z HRRR notably does not develop additional convection. This is a plausible outcome as well, and so will just need to monitor observational and model trends through the day. Given some potential lingering convection at 12z this morning, and at least some potential for redevelopment overnight, did not want to completely remove the MDT risk at this time (although certainly some chance this is eventually dropped). Instead opted to just reduce the size and trim back the western portion of the risk area. A second area of convective focus should be across portions of northern TX into OK. This is related to the retrograding shortwave/MCV that is expected to lift north northwest across the region today into tonight. Even here there remains uncertainty with regards to the degree of instability and location of boundaries helping focus convection. The overall setup does appear to favor some swaths of training convection, so just a matter of whether we get enough instability to drive rainfall rates high enough, and exactly where this better convective concentration will end up. For now left a pretty broad Slight risk over the area. A bit broader on the southern extent to cover where some overlap of additional convection and heavy antecedent rainfall may occur, and a bit sharper with the TX Panhandle cutoff as this areas has been drier and instability should generally be lower the further north and west you go. Oklahoma is interesting and could very well be the focus for heaviest rainfall today over the Southern Plains within training banded features east of the MCV/low. However ongoing convection this morning likely complicates boundary locations and instability later today, so even here confidence is low...but still within Slight risk levels. ...Northern Plains into Upper MS Valley... Training convection this morning into the afternoon should pose an isolated to scattered flash flood risk over portions of eastern SD, southern MN and central/eastern IA. Convection is focusing along the 925/850mb boundary east of slow moving deep layered low, and convergence along this boundary should remain pretty persistent through the morning hours. Low level southeasterly flow should maintain instability into the region as well...with 1000-2000 J/KG of CAPE. Expect that we will continue to see periodic backbuilding/training convective segments along this corridor, which should be enough to drive some flash flood risk. HREF guidance supports a broad area of 1-2" and localized amounts over 3". In fact trends support some 3-5" potential within swaths across this corridor. Portions of southern MN are saturated from recent heavy rainfall which would allow for a quicker development of potential flash flooding. The heaviest rain today may end up just south of this saturated area, but it will be close. Other areas of the Slight risk have been drier and so it will likely take longer training for issues to develop...but even here there is enough of a signal to support at least an isolated to scattered flash flood risk. ...OH/TN valley into the Mid Atlantic... Above average moisture and instability will cover portions of this region on Saturday. A weak shortwave currently over OH will move into the Mid Atlantic and help drive a convective threat. Individual cells will be quick moving, but broad forcing is supportive of multiple convective rounds. On top of that these cells should be capable of dropping a quick 1-2" in localized swaths. So do think at least an isolated, mainly urban, flash flood risk could exist. Chenard Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt Quote Link to comment Share on other sites More sharing options...
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