Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri May 05 2023 20:10:35 FOUS30 KWBC 052010 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 408 PM EDT Fri May 05 2023 Day 1 Valid 16Z Fri May 05 2023 - 12Z Sat May 06 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL GULF COAST, THE CENTRAL/NORTHERN INTERMOUNTAIN WEST, AND CENTRAL TEXAS... ....16Z Update... ....Central Texas... This Marginal Risk area was introduced with this midday update. Much of the Hi-Res guidance shows some convection initiating late this afternoon across the Marginal Risk area, then moving southeastward towards the Gulf Coast. There is tremendous disagreement between the various guidance as to how long the storms persist, where they form, and how strong they get, so there's unusually high uncertainty as to where the threat is greatest. The Marginal Risk area covers where most of the guidance suggests there will be convective initiation...generally in the central to western part of the Marginal Risk area, followed by tracking off to the southeast. Assuming convection makes it southeast of the Marginal Risk area, they were by then expected to be faster moving and weakening due to loss of diurnal heating. The primary reason for focusing on convective initiation and shortly thereafter is due to the incredible amounts of low level atmospheric moisture, with the entire Marginal Risk area showing dewpoints currently in the mid-70s on average, and model forecasts of instability growing to as much as 5,000 J/kg by this evening ahead of the line. Certainly there's potential for hail, as SPC's analysis shows, but that level of moisture should still result in nearby rainfall rates easily exceeding the 1 hr FFG values, particularly in the middle of the Marginal Risk area, where FFGs are lower from recent rainfall. Due to the uncertainty as to how widespread any convection gets, the Marginal Risk was determined sufficient for now, but there most certainly is potential if rain rates approach 3 inches per hour that a localized short-fused Slight may be needed...with confidence in such an upgrade likely only reaching there once convective initiation occurs and the strength and movement of the storms can be much more reliably determined. ....Eastern LA and Southern MS... The Marginal Risk in this area was trimmed out of southern AR, northern LA, and central MS with this update. As with further west into TX, there is also tremendous guidance disagreement on the nature of convection in this area that will develop later this afternoon as well. However, with this morning's squall line now clear of the area, and a bit of clearing trying to develop behind it, think convective initiation is more likely than further west, along a stationary boundary draped across the Marginal Risk area. The guidance has somewhat better agreement that when showers and thunderstorms develop later today, they will train southeastward along the boundary along the corridor from Baton Rouge to New Orleans, the former of which picked up over 2.5 inches of rain in less than an hour just this morning. Thus, think FFGs are high since heavy rainfall in this area isn't yet been translated into new FFG guidance, and the potential for not only training storms, but the abundance of atmospheric moisture here too with dew points well into the 70s will provide more than enough juice for the storms to potentially result in similarly high rainfall rates in many of the same areas. This area too will need to be monitored if convection is more widespread than expected for potential upgrades. ....Intermountain West... No changes were made to the inherited Marginal Risk area. A completely different pattern is set up across this area as compared to the Marginal Risk areas near the Gulf. Here, snowmelt and a slow-moving upper level low are both major contributors to the flooding potential, with snowmelt priming the area. Convection is expected to increase in coverage later today with daytime heating, moving over areas dealing with rapid snowmelt. While the storms won't be anywhere near as strong as closer to the Gulf, the antecedent conditions and terrain implications are much more favorable to rapid-onset flash flooding here. ....Northwestern NE and Southwestern SD... The inherited Marginal Risk area for this region was eliminated with this update. While there is some potential for heavy rain, the extremely high FFGs associated with the Sand Hills should limit any flash flooding to very isolated nuisance flooding in any more developed towns. The focus of the heaviest rain continues to drift south across NE, centered right over the Sand Hills, so any flash flooding threat in this area was determined to be lower than the Marginal Risk threshold. Wegman ....Previous Discussion... ....Mid-south and Tennessee Valley into the Southern Appalachians and Parts of the Southeast... There is still uncertainty in the exact track of where the highest QPF will set up although the trends noted in the yesterdays Day 2 ERD still seem to be holding out with lighter amounts being forecast farther south than in previous runs. With the downward trend continuing in QPF...the Marginal risk area over the Tennessee Valley was removed and refocused more along an instability axis extending from portions of Arkansas southward to the Gulf coast from southeast Louisiana eastward to roughly the Alabama/Florida panhandle line. The models were struggling with what appear to be convective feedback that showed little run to run consistency in placement. Even so...there was enough of a signal for the 05/00Z HREF to depict a sub 15 percent risk of 2 inch per hour rainfall accumulation by early evening in coastal Mississippi and adjacent Louisiana. Extended the Marginal a bit farther north than QPF suggested based on expectations of where on-going rainfall at the start of the Day 1 period may provide some priming for later rainfall. By later in the evening...models tend to show decreasing coverage of rainfall as mid-level ridging sets in.=20 ....Central and Northern Intermountain West to Adjacent Western High Plains,.. Upper level pattern changes very little across these areas through late tonight with broadly difluent upper level flow east of a deep-layered trough axis (though the favorable upper level forcing does shift a bit northeastward into western-central MT). The favorable forcing and weak deep-layer instability (relatively steep mid-level lapse rates) along a ribbon of above-average PW will favor isolated hourly rainfall rates of 0.50-1.00". These rates, along with the recent snowmelt and possibility of occurring over burn scar areas, could foster localized short-term runoff issues. As a result, the Marginal Risk that was inherited from yesterday's Day 3 ERO continues in today's Day 2 ERO. Given an uptick in the deterministic QPF and increasing risk of 2 inches of accumulation in 3 hours from the Black Hills into portions of northern Nebraska...introduced a Marginal Risk but avoided expanding too far into the driest parts of the states. Bann Day 2 Valid 12Z Sat May 06 2023 - 12Z Sun May 07 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER MISSISSIPPI VALLEY AND NORTH-CENTRAL TEXAS... ....20Z Update... ....Lower Mississippi Valley... A Marginal Risk area was introduced in many of the same portions of LA and MS as are under threat this evening. The stationary boundary currently in place will remain parked in the same areas again on Saturday. Afternoon and evening storms are likely to develop along the boundary once again, and move southeast over flood sensitive portions of LA and MS, including Baton Rouge, New Orleans, and Biloxi. The expected convection later today will keep the soils in the area primed for additional flash flooding when the next round of storms move through Saturday. ....North Central Texas... A subtle shortwave disturbance ejecting out of the desert Southwest will race along in the southwesterly flow into north-central Texas Saturday evening. Thunderstorms are expected to break out along the same boundary as today in many of the same areas as today as a result. There's somewhat better agreement in the guidance for Saturday's storms than this evening's, that the focus for convection will be just a hair further north in Texas as compared to today. Unfortunately, this will put the Dallas-Fort Worth Metroplex in more of a target area. Given previous days' rains, expected rain in this region later today, and then yet another round again Saturday, a Marginal Risk was introduced. Just like in the lower Mississippi Valley, the meteorological conditions will be essentially unchanged again tomorrow, which will support many of the same areas seeing renewed showers and thunderstorms again on Saturday. Wegman Day 3 Valid 12Z Sun May 07 2023 - 12Z Mon May 08 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MIDWEST... ....20Z Update... ....Midwest... Confidence remains low on the area of convection associated with a warm front moving over the Midwest on Sunday. Total forecasted rainfall has not changed much from the previous forecast, but the area under most concern for the potential for isolated flash flooding has shifted a little bit more to the northeast with the latest 12Z cycle of model runs. Thus, the inherited Marginal Risk area was adjusted northeastward, and now extends from central WI southeast to far western MD, and all areas in between. Low FFGs in this area are playing their part in keeping the Marginal Risk area intact, considering the weak signal in the guidance as to how strong any convection that develops in this area will be. Dew points will be around 60, give or take a few degrees across the Marginal Risk area, with a steady southwesterly flow adding atmospheric moisture to the area behind the warm front. Expect healthy instability as well, as MUCAPE values approach 2,000 J/kg Sunday afternoon and evening, so any storms that fire will have a favorable environment to work with. However, inside the broad area encompassed by the Marginal Risk, there remains little confidence as to which areas have a greater threat for flash flooding. Low FFGs in the central Appalachians suggest that area may have the highest threat, but a larger distance from the real forcing further west may negate that. Expect continued adjustments with future updates as the details become more clear. Wegman ....Previous Discussion... Opted to introduce a Marginal Risk area from portions of the Lower Great Lakes into Kentucky where various models have been developing QPF bullseyes anywhere from southern Wisconsin into Kentucky with below average run to run coherency/consistency in placement. In the broad-scale picture...there is initially mid- and upper-level ridging that will be working against much in the way of heavy to potentially excessive rainfall. The various global models latch on to an ill-defined shortwave trough embedded within the broad southwest flow to the west of the ridging axis.=20 Some of the guidance lowers heights in response to the shortwave trough to the point where convection forms underneath difluent flow late Sunday night/early Monday morning. The spread of model spread really limits confidence in any one solution...at various times there have been localized convective bull eyes over Kentucky or Illinois...while the UKMET has favored areas farther north.=20 But the NAM/GFS/ECMWF all develop a convective QPF max of 2 to 4 inch amounts somewhere in the area...felt a Marginal was warranted (even discounting the presumed wet-outlier UKMET). Further adjustments and modifications are expected in subsequent EROs/ERDs. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-v5QpNlSi6bdOhOVQoIT78Y7a4AFKpBWV-zcqYGB4mK2= bs6u3tV7AHOZ4OKYZjCGMWa_urlyN3GybA_zBvZbDNBK6fs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-v5QpNlSi6bdOhOVQoIT78Y7a4AFKpBWV-zcqYGB4mK2= bs6u3tV7AHOZ4OKYZjCGMWa_urlyN3GybA_zBvZbvYCVaoU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-v5QpNlSi6bdOhOVQoIT78Y7a4AFKpBWV-zcqYGB4mK2= bs6u3tV7AHOZ4OKYZjCGMWa_urlyN3GybA_zBvZbYlLYTjw$=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) .