Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri May 12 2023 20:21:36 FOUS30 KWBC 122021 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 420 PM EDT Fri May 12 2023 Day 1 Valid 16Z Fri May 12 2023 - 12Z Sat May 13 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH-CENTRAL TEXAS... ....16Z Update... ....Texas... The Moderate Risk area was expanded north with this update in coordination with SJT/San Angelo, TX forecast office. A southeastward moving front already draped over western OK and the TX High Plains will sag southeastward as moisture associated with the convection highlighted for the past several days by the Moderate Risk tracks northward into that front. Multiple CAMs models are showing a similar evolution late this afternoon through this evening. These two features are likely to merge over the San Angelo area as the remaining convection tracks northeastward across north central TX and far south central OK. The Slight Risk area was expanded to include these portions of TX and OK. The merger may result in multiple hours of heavy rain as cells from the two boundaries converge, followed by training towards the east. ....Mid-Mississippi and TN Valleys... A Slight Risk area was introduced with this update for portions of southern AR, northern MS, western TN, and north AL. A very slow moving line of storms already causing flash flooding over AR will train east-north-eastward across the TN Valley, where scattered convection has already developed. The line itself has been producing localized rainfall rates approaching 3 inches/hour. While the storms over western TN are occasionally reaching 1.5 inches/hour. Recent rains have saturated the soils in this area, and with the inclusion of Memphis and Nashville, urban flooding will also be a concern. As most guidance shows this convection becoming more progressive and moving southeast through AL and GA by late this afternoon, it's likely this Slight will be able to be cancelled early. ....Northern Plains... The only change of significance was to remove the Sand Hills region of northern NE from the Slight Risk, while some convection is ongoing here across central NE, a "dry slot" of sorts north of the convection should allow the sand to absorb earlier rainfall, so the flooding threat is lower. Expect off-and-on heavy rain across the Dakotas today, as the combination of the wound-up low over NE tracks north and afternoon showers and thunderstorms develop along the northern periphery of the low across southern SD. Meanwhile rain will continue to be drawn north across western ND and SD today, with instability allowing for some embedded heavier elements. ....Marginal Risk Area... Elsewhere, as mentioned below, widely scattered but more disorganized convection is likely across much of the country today, so the Marginal Risk extends from northern Florida to the northern Plains. For the Southeast, southeastward moving storms are expected to develop over southern AL and GA late this afternoon and evening, which despite high FFGs, plentiful moisture and instability will allow that somewhat organized convection to produce heavy rainfall rates which could result in isolated flash flooding in locally flood sensitive areas. For the central Appalachians, low FFGs particularly in far western MD and WV may allow for isolated flash flooding when the Midwest convection reaches that region tonight. Otherwise, plentiful moisture and instability over much of the rest of the eastern half of the country outside of the Northeast will allow any localized convection to briefly produce heavy rainfall. Wegman ....Previous Discussion... ....Texas... A significant flash flood event could evolve Friday night across portions of south central TX. Forcing, moisture and instability all look to be in place to support an excessive rainfall event. Forcing is being driven by a mid level shortwave moving out of Mexico and broadly diffluent flow aloft in between the subtropical jet to the south and a jet streak to the north of the area. This mid/upper forcing persists through the period, and does not show much eastward progression with time. This type of persistent forcing evolution often favors training/backbuilding. Strong southeasterly low level flow off the Gulf of Mexico and a mid/upper level connection to the Pacific will support anomalous PWs and efficient rainfall. Instability should be on the order of 3000-4000 J/kg, with plenty of upstream instability to maintain convection. The main question is the last remaining ingredient for flash flooding...heavy rainfall rate duration. The aforementioned slow moving forcing supports prolonged duration, and deep layer southerly flow, combined with southeasterly low level flow, supports training/backbuilding. As convection organizes it should develop a cold pool and propagate eastward into the low level flow and downstream instability pool. Exactly how quickly this happens is the big question. Initial convection should develop along the Edwards Plateau as forcing and upslope southeasterly flow increases. Think we will see a period of backbuilding/training during this convective phase, before cold pool generation results in an east or southeast progression of activity. At this point leaning towards the slower HREF solutions, as some of the members have a tendency to become cold pool dominant too quickly. Thus would not be surprised if we end up on the higher end of the HREF output for QPF amounts. Thus still thinking this is an event where swaths of 6"+ rainfall are a possibility, with 2-3" an hour rates. Scattered to numerous instances of flash flooding, some of which could be higher end in nature seem possible within/near the Moderate risk area. Convection is also likely along the dryline over west central TX during the afternoon hours. This activity will initially be slow moving and should pose an isolated to scattered flash flood risk. Locally significant rainfall and flash flooding is possible with this activity as well, but coverage should be a bit lower than with the Hill Country activity, and thus a Slight risk should cover the threat. Although guidance has been trending up over this area, and so will need to continue to closely monitor. Overnight this dryline activity should survive eastward into central TX...but the exact magnitude and organization of this activity remains unclear. However a broad Slight risk seems warranted as a flash flood risk could continue into central TX overnight. ....Northern Plains... Isolated to scattered flash flooding is possible over a large portion of the Northern Plains today into tonight. A Slight risk stretches from NE into much of SD and into west central ND. A closed deep layer low moving slowly northward will provide plentiful forcing for convection. A stationary front over the western Dakotas will continue to be one focus for persistent convective activity. Instability is a bit more meager here, however strong low level easterly flow and persistent convergence will still support periodic rainfall totals over 1" in an hour. The 00z HREF supports additional widespread 1" rainfall totals here, with isolated to scattered 3"+ totals likely. The 2nd area of concern is closer to the deep layered low and associated warm front from NE into southeast SD and northwest IA. This activity will have more instability to work with, and thus 2"+ an hour rainfall is a possibility. This activity should be a bit more progressive, but do expect we will at least see some localized swaths of backbuilding/training. HREF probabilities also support isolated to scattered 3"+ rainfall totals here as well. ....MS Valley into the OH/TN valley and Southeast... A broad Marginal risk will cover a lot of this region today/tonight. Would have liked to try to narrow this area down, but found it hard to do, hence the very large Marginal. Above average moisture and instability will cover this entire region, so any convection will be capable of heavy rainfall rates, and where convection persists, could result in localized flash flooding anywhere within this risk area. The actual coverage of FFG exceedance will probably be pretty low over this area, but localized instances could occur just about anywhere within the risk based on 00z HREF and other QPF guidance. Given this HREF output and the favorable moisture/instability parameters...thought going with a large Marginal was the best course of action. Chenard Day 2 Valid 12Z Sat May 13 2023 - 12Z Sun May 14 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF SOUTH TEXAS... ....2030Z Update... ....Texas... Very few changes were made to the Moderate Risk area across south Texas. As mentioned below, it's likely that the event will be ongoing at 12Z Saturday for the start of the Day 2 period, but unfortunately there is not great agreement in the CAMs as to that timing. Most of the CAMs suggest there will be multiple rounds of storms that form to the west towards the Rio Grande, then track with some speed off to the east out to the Gulf as the next round of storms does the same, with this pattern continuing into the night. Of course, depending on which CAM you use, the timing of when each storm will be in a given location varies widely. There's also some disagreement on how far west new storms initiate. As such, the core of the heaviest rainfall in the consensus has shifted east close to the Gulf Coast. Should this verify then Corpus Christi would be on the hook for 3+ inches of rain, with 12Z HREF neighborhood probabilities at over 80% for 3 inches of rain in town, 30% for 5 inches of rain, and a 10% chance of 8 inches of rain in the 24 hour period from 12Z Sat through 12Z Sun. Higher probabilities remain west of the city however, with a wide variety of probabilities from the Gulf Coast to the Rio Grande. Meanwhile, the Slight Risk area was expanded north and west to include all of the TX Panhandle, most of western OK, and some of the OK Panhandle. CAMs guidance suggests a Mesolow will move along the TX Panhandle/OK border, with steady stratiform rain over the Panhandles, with rounds of showers and storms over much of OK. With abundant moisture available with PWATs over 1.5 inches in much of TX and OK Saturday afternoon, and instability values between 1,000 and 2,000 J/kg the storms will have plenty of available moisture and energy for producing heavy rains across north TX and OK. ....Northern Plains and Upper MS Valley... Guidance trends continue to increase for this area, as the convective threat along a stationary/warm front comes into better focus. 3 inch plus neighborhood probabilities from the southwest corner of MN across central IA to near the IA/IL/MO junction are over 45% in the 12Z HREF guidance, and deterministic trends increasingly suggest over 4 inches of rain may fall in a few spots within the Slight Risk area. ....Mid-Atlantic... A back door cold front moving in from the north will collide with the plentiful Atlantic moisture in place across southern VA and northern NC to produce locally heavy rains in the Tidewater region and the southern Delmarva. While FFGs are high, the combination of over 2,000 J/kg of CAPE and over 1.5 inches PWAT will be more than enough to result in thunderstorms that produce heavy rains that may cause isolated flash flooding, especially in urbanized areas such as Richmond and Hampton Roads. Wegman ....Previous Discussion... ....Texas... The event discussed in the day 1 discussion should be ongoing at 12z Sat across portions of south TX. The expectation is that the convective complex will be making slow eastward progress by this time. The forecast by this time is already becoming lower confidence, as convection on day 2 will be heavily dependent on the location of cold pool driven outflows and ongoing convective activity. Guidance has trended a bit quicker with convective progression, so we did trim back some on the western extent of the Moderate risk with this update. The overall synoptic environment will be conducive to excessive rainfall over central/southern TX, so really comes down to outflow and instability recovery, which will be tough to pin down at even this lead time. Do think the greatest flash flood risk will be over south TX associated with the likely continuing convective complex, but some renewed development behind this can not be ruled out either. Just like on day 1, extreme rainfall rates of up to 3" in an hour seem probable with this convection over south TX. Still thinking the potential is there for 6"+ rainfall amounts in spots across south TX downstream of the day 1 convection. Think this is a solid MDT risk level event, with locally significant flash flooding possible. Can not rule out a High risk level event, but the trend for a slightly quicker convective progression seems to more solidly put this more in the MDT level for now. A second area of flash flooding is possible across portions of north TX into OK associated with retrograding mid level shortwave and persistent upper level diffluence. Just like over south TX, we have some questions regarding the degree of instability and boundary locations here as well. Seems like a solid Slight risk level event with some backbuilding/training potential, and an embedded more organized threat could exist. Just too early to pin that down at the moment, but will continue to monitor trends. ....Northern Plains into Upper MS Valley... The closed mid level low will finally begin to open up into an open trough on Saturday, however enough forcing will persist to drive a convective threat near the stationary/warm front. It seems likely that we will get upwards of 2000 J/kg of instability into this boundary, which will be enough to support robust convective development near the front where convergence is maximized. Overall the ingredients appear to be in place to drive a heavy rainfall threat, and now model QPFs continue to suggest a narrow heavy rainfall event as well. Even through it only goes through the first half of day 2, the 00z HREF is already showing 3"+ neighborhood probabilities over 40%, and 00z ECMWF and GEM Reg QPFs are locally over 3" as well. This QPF support, combined with the favorable ingredients, increases confidence in a narrow heavy rainfall event. On top of that, it currently appears like there will be decent overlap with the portion of southern MN and northern IA that just recently saw heavy rainfall and some flooding impacts. Given all this, a Slight risk upgrade was warranted...with isolated to scattered flash flooding seeming more likely Sat into Sat night. ....OH/TN valley into the Mid Atlantic... Above average moisture and instability will cover portions of this region on Saturday. Guidance is also indicating a weak shortwave which could help focus convective development. The details remain unclear, and convection may end up fairly quick moving, however felt like the overlapping moisture/instability and convective signal in the 00z CAMs supports the potential for localized flash flooding. Chenard Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8if599_6Mxho5GsKbRV-M9gooZJsV5ULDojGrJbvRRZ-= BPznSi6cmWnJvd8b-wN0Xj0hbmr_-STmFxxcZYFrAESQ1ig$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8if599_6Mxho5GsKbRV-M9gooZJsV5ULDojGrJbvRRZ-= BPznSi6cmWnJvd8b-wN0Xj0hbmr_-STmFxxcZYFrR7ncwp8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8if599_6Mxho5GsKbRV-M9gooZJsV5ULDojGrJbvRRZ-= BPznSi6cmWnJvd8b-wN0Xj0hbmr_-STmFxxcZYFrbloZYCU$=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) .