Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jul 22 2023 08:16:47 FOUS30 KWBC 220816 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 415 AM EDT Sat Jul 22 2023 Day 1 Valid 12Z Sat Jul 22 2023 - 12Z Sun Jul 23 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST, CENTRAL PLAINS, SOUTHEAST, AND SOUTH FLORIDA... ....Southeast... A continued amplification of the eastern CONUS longwave trough will drive subtle height falls southward, pushing a cold front towards the Gulf Coast. This front will slow or stall in a generally west to east fashion today as it becomes entrenched within more zonal flow at the base of this broad cyclonic flow. Convergence along this boundary aligned with right entrance region upper jet dynamics will be enough to support and sustain convection Saturday into Saturday night over the Southeast. HREF mean shows instability along and south of the front will be substantial, generally 2000-3000 J/KG, and PWs should generally be near or over 2". Given this overlap of favorable ingredients not surprising that the HREF shows substantial probabilities of hourly rainfall over 2" across this area. Convection is expected to be ongoing at 12z Saturday across portions of southern AR into MS and AL. This activity should survive southeastward in some fashion across portions of AL/GA through the morning hours. The expected forward progression of this activity suggests only an isolated at best flash flood risk. By this afternoon we should see additional convective development closer to the central Gulf Coast along the low level convergence axis. The flattening of the trough atop the front will result in nearly unidirectional low and mid-level flow, becoming more parallel to the front, which could result in some training of this convection. However low level flow is rather weak, and not much in the way of deeper layer shear either. So we are most likely looking at a pulse convective mode, with propagation along outflows. This scenarios can still produce heavy rainfall, but the duration should be somewhat limited given the pulse convective nature and gradual erosion of instability. The most concentrated area of convection is expected to be from southeast LA into southern MS/AL and the western FL Panhandle. HREF neighborhood probabilities of exceeding 3" are 50-70%, but drop off to 10-25% when looking at the 5" threshold. HREF EAS probabilities of 1" and 2" do support a clustering of 1-2"+ totals along the aforementioned corridor. For most areas of the country this type of rainfall would solidly support a Slight risk. However FFG is very high over this region, and despite the aforementioned rainfall totals seen in the HREF, the HREF FFG exceedance probabilities never really get above 10%. Thus while we do think there will be some instances of flash flooding today/tonight, tend to think the coverage will be too low to support a Slight risk. Thus a Marginal risk was maintained with this update. ....Eastern North Carolina... A Marginal risk was added across portions of eastern NC with this update. Not all that confident in any flash flooding occurring, but there was a notable uptick in HREF probabilities across this region. A shortwave will traverse the area today, and guidance indicates a north south boundary over eastern NC, both enhancing convergence and maintaining higher instability air to its east. Guidance is not unanimous in how things evolve today, but HREF 3"+ neighborhood probabilities are 40-70%, with 5" probabilities over 15% as well. FFG is high over this area, but if some of the wetter scenarios unfold some exceedance is possible...thus think a Marginal risk is warranted for. ....Southwest into the Plains... A Marginal risk covers portions of AZ/NM into the TX/OK Panhandle and southwest KS. The impressive mid level ridge will persist with subtle lobes of vorticity rotating around the periphery continuing to produce waves of enhanced ascent. This lift will occur within still favorable thermodynamics as PWs are slightly above normal and MUCAPE reaching 1000+ J/kg in spots. This will result in scattered, primarily aftn/eve, convection with rainfall rates possibly exceeding 1"/hr. Over NM, organization and intensity may be somewhat muted compared to Friday, but storm motions will remain quite weak and variable. Over AZ, PWs are a bit higher compared to Friday, so would expect a bit higher convective coverage and rainfall rate potential. Over both NM and AZ any heavy rain producing cell could yield more than 1" of rainfall, which could result in isolated instances of flash flooding, especially if these cells move across more vulnerable areas. Subtle shortwave energy riding over top the impressive western CONUS ridge, weak upper divergence on the periphery of the ridging, and an area of lower level convergence will work together to trigger convective development Saturday afternoon/evening over the central Plains. Organized convection will tend to drop southward, and PWs are only around climatological average values. So certainly not a high end or widespread flood threat. However we'll have plenty of instability and shear to work with, so some intense supercells and/or convective clusters are possible. The low flash flood risk is probably mainly confined to areas that saw heavy rainfall the other day over southwest KS, so trimmed back the eastern extent of the Marginal risk to only include this area. This convection may also eventually get into the TX Panhandle, with some potential of cells propagating into this region from NM as well. ....South Florida... Anther day of afternoon thunderstorms is expected Saturday as an overlap of impressive instability of 2000-3000 J/kg and PWs around 2" support diurnal convection. Activity on Friday afternoon generally under performed compared to HREF guidance...and that could certainly happen again today. However what we have today that we did not yesterday is slightly stronger westerly low level flow, which may favor localized stationary cells along the urban east coast of the state. So do think the threat today is a bit higher than Friday, so will maintain the Marginal risk, with HREF neighborhood probabilities showing a 40-70% chance of some 3"+ rainfall totals. Chenard Day 2 Valid 12Z Sun Jul 23 2023 - 12Z Mon Jul 24 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST AND SOUTHEAST... ....Southeast into the Mid Atlantic... Generally a continuation of the pattern from day 1 with a broad longwave trough, upper jet, and a low level front in place with high PWs and instability along and south of it. Northern FL will need to be watched closely, as the ingredients are in place to potentially support an organized area of training/backbuilding convection. An uptick in low level flow along the front and a more favorable position of the upper jet to the north would support organized convective development. Meanwhile unidirectional flow parallel to the front could support a training and backbuilding threat. Although given the high FFG over this area everything does need to come together just right to really drive any more organized flood threat. While that may happen, there are some indications that the front will have just enough of a southerly push to keep convection propagating south and east with time, and other indications that the heaviest convection could end up focused offshore. For these reasons think the risk remains at a Marginal level, but will continue to monitor. Over the Carolinas into VA the convective threat appears to be increasing compared to prior forecasts. Shortwave energy within the approaching trough axis is trending a bit stronger in most guidance, which is driving a bit better mid/upper forcing and lower level convergence. This has resulted in a notable uptick in recent GFS and ECMWF QPF over this region. The 00z HREF shows instability upwards of 1000-2000 J/kg and PWs increasing towards 1.5", with generally scattered/disorganized diurnal convective development. There are some slow/chaotic cell motions evident in HREF simulated reflectivities, which would support some cell merging and a localized flash flood threat given the thermodynamics in place. The aforementioned trends evident in the GFS/ECMWF actually support a bit higher PWs over the region along with a more focused corridor of lower level convergence. Thus will be interesting to see trends in the HREF to see if it also begins to trend upwards with the potential organization of convective activity. Can not rule out eventually needing a Slight risk somewhere over the Carolinas or VA, but for now think this is a solid Marginal risk. ....Southwest... Not much change here compared to Saturday, although the better moisture does migrate westward by this time, which should lower the risk over NM. But marginally above average PWs over AZ will persist, with another day of isolated to scattered diurnally driven convection. Any storms could drop a quick inch of rain, which could drive a localized flash flood risk within any more susceptible basins. Chenard Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!71Htc0dj5Pagc_IcOrPpHW9zdFZtsSEZkIIDykS6Cxnt= nYD5vpa_yElg6j8Y4ay_Bx51rgRyLbgi0vjRPgrg6XO82NU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!71Htc0dj5Pagc_IcOrPpHW9zdFZtsSEZkIIDykS6Cxnt= nYD5vpa_yElg6j8Y4ay_Bx51rgRyLbgi0vjRPgrgs2uMaA0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!71Htc0dj5Pagc_IcOrPpHW9zdFZtsSEZkIIDykS6Cxnt= nYD5vpa_yElg6j8Y4ay_Bx51rgRyLbgi0vjRPgrgR_pxxCU$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .