Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 14 2024 19:49:57 FOUS30 KWBC 141949 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 349 PM EDT Thu Mar 14 2024 Day 1 Valid 01Z Fri Mar 15 2024 - 12Z Fri Mar 15 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS AREAS FROM NORTHEAST TEXAS INTO THE MID-SOUTH... ....16z Update... Synoptic setup remains mostly the same for this afternoon/evening's heavy rainfall forecast. Some minor expansions were made to the Marginal and Slight Risk areas. Increasing instability trends over eastern Ohio was enough reason to expand the Marginal into that area. 12z CAMS appear to be suggesting a second round of convection developing late tonight into early Friday morning over southeastern Arkansas where heavy rainfall earlier in the evening would have primed soils for runoff. Similarly over the Midwest (Indiana/western Ohio), antecedent conditions from this=20 afternoon's line of convection propagating through the Ohio Valley will prime soils for another round of convection out ahead of a=20 surface wave. 12z guidance captures the convection well, supported by diurnal processes. Kebede ....Previous Discussion... Starting around 12z Thursday, guidance depicts the potential for=20 training convection within an area of increasingly zonal and unidirectional westerly flow to focus along a warm frontal zone=20 across northern Illinois/Indiana, which should maintain isolated=20 flash flood potential through the morning, although a weakening=20 trend in convection is expected by 18Z. Local amounts towards 3" are possible here per the 00z HREF output. =46rom Thursday afternoon through early Friday, a shortwave moving=20 into the Corn Belt should provide sufficient difluence aloft along with a southward- moving cold front is expected to provide a focus for scattered to numerous thunderstorm development from the Ozarks into western TN. While 500 hPa ridging is increasing, 700 hPa=20 temperatures are in slow decline across AR implying decreasing mid-level capping. Temperatures at 700 hPa (6C+) based on the 03z RAP imply that the heavy rain concerns should be mainly north and=20 west of LA, as does the persistent and strong 850 hPa southwest=20 inflow from the Gulf of Mexico, which should amp up the=20 precipitable water values into the 1.5-1.75" range. ML CAPE should rise as high as 3000+ J/kg in the vicinity of the ArkLaTex. Storms are expected to develop in a difluent area aloft east of a cold=20 low dropping into the Desert Southwest and foster local cell=20 mergers/training to support flash flood potential. The highest=20 potential for flash flooding should reside in and near Arkansas=20 where areas of 2 inch/hr rain totals with local amounts towards 5" are expected to materialize. The Slight risk area continues to=20 depict potential for at least localized flash flood potential=20 through the overnight hours. The guidance has trended in the usual direction -- southward -- so the risk areas have followed suit.=20 The southward guidance trend along with the low chances of 5"+ kept the risk level as a high-end Slight for the time being, though a=20 Moderate Risk near the ArkLaTex can't be ruled out at some point in the future. The Marginal risk area across central Texas continues with the=20 expectation that scattered thunderstorms would focus along a cold=20 front in that area late Thursday and train/repeat, boosting local=20 rain rates. The Marginal risk area across the Ohio Valley coincides with the expectation that isolated to scattered, fast-moving=20 storms could train and promote isolated areas of 2"+/hour rain=20 totals with local amounts to 4" from afternoon through the=20 overnight hours. Roth Day 2 Valid 12Z Fri Mar 15 2024 - 12Z Sat Mar 16 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR SOUTHEAST TEXAS AND PORTIONS OF THE SOUTH... ....20:30z Update... The overall synoptic, moisture and instability setup remains mostly the same. Non- GFS/NAM guidance continues to keep the QPF max over northern/central Alabama, while the 12z CAMs and ECMWF are spread=20 back into Mississippi on Friday. This continues to be the main uncertainty and is likely directly tied to the timing/speed of the surface cold front propagating from the northwest. The 12z HREF=20 mean still has 0.25-0.5/hr rates over southeastern Texas where the other Slight Risk area is currently drawn. Kebede ....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 or=20 two, resulting in higher stream flows and greater soil saturation=20 which continues to be the reason for the Slight Risk, over other=20 considerations. The Slight Risk area was split in two to depict the bimodal distribution of the heavy rainfall pattern in the=20 southern tier of the country, with the eastern portions associated=20 with the progressive frontal zone/850 hPa confluence early on more=20 northerly and the western portions associated with the slow- moving boundary shifting more southerly. A separated Marginal Risk remains in and near portions of WV which is typically more sensitive to heavy rainfall and where flash=20 flood guidance values remain relatively low near a progressive section of the front. Hourly rain totals to 1" and local amounts=20 to 2" are possible there, which could exceed the flash flood=20 guidance values.=20 Roth Day 3 Valid 12Z Sat Mar 16 2024 - 12Z Sun Mar 17 2024 ....A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF=20 SOUTHERN TEXAS... ....20:30z Update... The southward trend in QPF guidance noted in the previous discussion continues into this update. The cold front extending from the Carolina Coast will stall out just south of southern Texas. A potential surface wave over the Texas Gulf Coast may enhance inflow into the state. Synoptic, moisture and instability remains similar to what was forecast overnight. Small details will dictate where the axis of max QPF sets up. Kebede ....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 shifted southwest, which has led to a similar shift=20 in the excessive rainfall areas. Temperatures at 700 hPa imply that the heavy rain threat should be mainly north of Deep South=20 TX, based on GFS forecasts. Inflow at 850 hPa from the Gulf should=20 pulse up to 25-30 kts at times, importing ML CAPE of 2000 J/kg or=20 so. Effective bulk shear should be sufficient for organized=20 convection, capable of producing hourly rain totals to 2" and local amounts to 4". Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7T9Kz0ruqxPjSZ2V3bz6i5nHPG_v1RscJ8srbtkywmRh= 1h8gLzybzzuKtR3GKOH9txIqKii4rad--YRo7GwRgYQSGXA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7T9Kz0ruqxPjSZ2V3bz6i5nHPG_v1RscJ8srbtkywmRh= 1h8gLzybzzuKtR3GKOH9txIqKii4rad--YRo7GwR4ZNVN3M$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7T9Kz0ruqxPjSZ2V3bz6i5nHPG_v1RscJ8srbtkywmRh= 1h8gLzybzzuKtR3GKOH9txIqKii4rad--YRo7GwRvcly3g0$=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. 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