Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Mar 16 2024 00:36:52 FOUS30 KWBC 160036 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 836 PM EDT Fri Mar 15 2024 Day 1 Valid 01Z Sat Mar 16 2024 - 12Z Sat Mar 16 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH TEXAS... ....01Z Update... Much of the southern U.S. (from Mississippi eastward) has been removed from Marginal Risk highlights for this outlook update. Convection across Georgia has picked up forward speed, has had marginal rain rates over the past couple hours, and was moving toward areas of higher FFG. Farther west, a Marginal was maintained for portions of southern Louisiana and southeast Texas. A maturing, partially elevated MCS was making slow eastward progress through the region, resulting in a few spots of 1+ inch/hr rain rates. Eventually, this MCS will propagate southeastward toward northwestern Gulf of Mexico waters, but heavy rainfall could last for at least a couple more hours as lingering instability (around 250-1000 J/kg MUCAPE) persists along and downstream of the track of the MCS. Lastly, robust, deep convection continues to mature along the higher terrain of northeastern Mexico just west of the Rio Grande. Models suggest that these storms could cross the Rio Grande and grow upscale while maintaining slow movement amid strong instability (3000-4000 J/kg MLCAPE). A few areas could experience 3+ inch/hr rain rates beneath the slowest and most dominant convection, necessitating a Slight Risk. See the previous discussion below for more information. Cook ....16Z Update... The previous forecast reasoning still holds. The Slight Risk area=20 in the Mid-South has been removed as the flash flood threat is=20 decreasing in this area. The line of thunderstorms moving across=20 the Southeast has been fairly progressive this morning, and=20 instability drops sharply behind the main convective line, limiting rainfall rates in the stratiform ran on the backside of=20 the boundary. Rainfall rates have also begun decreasing in storms=20 on the leading edge and should continue to trend downwards into=20 this afternoon and evening, making it difficult for rainfall rates exceeding flash flood guidance to be realized. However, isolated=20 stronger storms could still cause localized flash flooding concerns in the South and Southeast.=20 There is a second Slight Risk area in effect for portions of=20 Texas where the environment will be favorable for convective=20 development this afternoon. There is quite a bit of uncertainty=20 and spread among the available guidance as to where the heaviest=20 precipitation will develop, but the consensus remains that heavy=20 rainfall is expected. Stronger storms may produce intense rainfall rates, potentially reaching up to 3-4 inches per hour, which=20 could cause flash flooding, especially in urban areas.=20 The broader Marginal Risk remains largely unchanged from Texas to=20 the Southeast Coast with minor adjustments made in the Southeast to adjust for latest radar trends.=20 Dolan ....Previous Discussion... A cold front begins to accelerate to the southeast across the Southeast as a broad upper level trough gains real estate across=20 portions of the Great Lakes and Upper Midwest, with the front=20 moving slower to the south into the Southern Plains as a cold low=20 in the Desert Southwest shows minimal movement and keeps the upper level flow southwest to west- southwesterly which should slow the surface boundary's progression. This combination of systems=20 flattens the ridging across the Southeast and Gulf Coast.=20 Temperatures at 700 hPa suggest that the heaviest rainfall should=20 be north of southern TX, where temperatures are <6C, based on GFS=20 forecasts and ML CAPE should rise to ~3000 J/kg. An axis of=20 anomalous PW values, 1.5-1.75", will be seen near the front.=20 Another day of potentially widespread heavy rainfall is expected, as hourly rain totals to 2.5" and local amounts to 4" remain=20 possible. There is the typical amount of spread with respect to the position and maxima within the heavy rainfall axis, but the=20 overall model consensus suggests potentially heavy rains. Some=20 heavy rainfall is expected across areas of Mississippi, Alabama and Georgia that have had above average precip over the past week=20 or two, resulting in higher stream flows and greater soil=20 saturation which continues to be the reason for the Slight Risk,=20 over other considerations. The pair of Slight Risk areas remain to depict the bimodal distribution of the heavy rainfall pattern in=20 the southern tier of the country, with the eastern portions=20 associated with the progressive frontal zone/850 hPa confluence=20 early on more northerly and the western portions associated with=20 the slow-moving boundary shifting more southerly. Despite the=20 slowness of the progression in TX, the guidance appears weaker in=20 its support for this heavy rainfall. Kept the Slight Risk there as a precaution since the ingredients still appear to support a=20 greater than isolated flash flood risk. The Marginal Risk for areas north of approximately Chattanooga TN=20 was removed as convective progression through 06z and MU CAPE trends the last several hours suggested that any threat north of there would be more or less done by 12z. Roth Day 2 Valid 12Z Sat Mar 16 2024 - 12Z Sun Mar 17 2024 ....A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF=20 SOUTH AND SOUTHEAST TEXAS... ....20Z Update... The synoptic setup remains favorable for heavy rainfall in=20 portions of Texas Saturday into Saturday night as a slow-moving=20 frontal boundary moves across the southern half of the state. Two=20 upper level shortwaves will move overhead during the period, which will support a couple rounds of convection that will be responsible for heavy rain and could potentially lead to flash flooding.=20 Latest guidance has been trending slightly wetter and northwards=20 with the QPF, and the Slight Risk area over portions of Texas was=20 expanded slightly north and east accordingly. The Marginal Risk=20 area was also pulled north to include part of North Texas,=20 including the Dallas/Fort Worth metro area. Although conditions=20 will be more favorable for strong convection in south-central and=20 southeast Texas, there may be enough instability to support a=20 decent round of convection in the afternoon/evening in north Texas. Dolan ....Previous Discussion... Difluence aloft ahead of an upper level low moving slowly across=20 Arizona teams up with a sagging front in southern TX to continue=20 the potential for heavy rainfall in the front's vicinity. The=20 guidance has become broadly wetter when compared to this time yesterday, which led to some expansion in the Slight Risk area.=20 Temperatures at 700 hPa imply that the heavy rain threat should be mainly north of southernmost TX, based on GFS forecasts. Inflow at 850 hPa from the Gulf should pulse up to 25-30 kts at times,=20 importing ML CAPE of 2000 J/kg or so. Effective bulk shear should=20 be sufficient for organized convection, capable of producing hourly rain totals to 2" and local amounts to 4". Roth Day 3 Valid 12Z Sun Mar 17 2024 - 12Z Mon Mar 18 2024 ....A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE NORTHERN GULF COAST... The forecast remains on track. A potent upper level shortwave will move along the stalled frontal boundary over the Gulf Coast,=20 producing heavy showers and thunderstorms. This feature looks to be fairly quick-moving, which should help to limit the risk of flash=20 flooding, so there is currently only a Marginal Risk area in place from the Texas Gulf Coast across the northern Gulf Coast to the=20 Georgia Atlantic Coast. The risk of flash flooding may increase if stronger convection impacts an urban area or if training storms=20 develop, which are details that will be better defined in the 1-2=20 day period when hi-res guidance is available.=20 Dolan=20 ....Previous Discussion... Organized convection overriding a front is expected to lead to=20 heavy rainfall for portions of the northern Gulf coast. Sufficient 850 hPa inflow off of the Gulf of Mexico, up to 40 kts, will lure=20 precipitable water values of 1.5-1.75" into the region and MU CAPE up to 2000 J/kg. A slight amount of warming at 700 hPa could lead=20 to some northward shift in the convection, when compared to=20 Saturday. The main issue from an excessive rainfall perspective is the progression noted, as an upper trough sharpens across the Ohio and Mississippi Valleys and helps act to shift the moisture=20 plume/best 850 hPa inflow east with time. Hourly rain totals to=20 2.5" with local amounts to 4" are possible where limited cell=20 training occurs and/or mesocyclones happen to develop. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4xf4Is3Tgxd9nbmPvV4bdn7W_B8ZUF5w_rtiB94QD9U5= iZB0leJk2S798PE-j_AzZawgVFTpj2R0FTO9W2O3mcTQKM8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4xf4Is3Tgxd9nbmPvV4bdn7W_B8ZUF5w_rtiB94QD9U5= iZB0leJk2S798PE-j_AzZawgVFTpj2R0FTO9W2O3jgUNdwY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4xf4Is3Tgxd9nbmPvV4bdn7W_B8ZUF5w_rtiB94QD9U5= iZB0leJk2S798PE-j_AzZawgVFTpj2R0FTO9W2O3un8ywpU$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .