Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Mar 16 2024 19:57:36 FOUS30 KWBC 161957 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 357 PM EDT Sat Mar 16 2024 Day 1 Valid 01Z Sun Mar 17 2024 - 12Z Sun Mar 17 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH-CENTRAL AND SOUTHEAST TEXAS... ....16Z Update... No changes were made to the inherited Slight and Marginal risk areas across Texas with this update. Ongoing slow moving convection may pose an isolated flash flooding risk into the early afternoon, with a waning of storms through the rest of the day.=20 The main area of convective activity will be during the overnight=20 hours. Multiple rounds of storms through the late evening and early morning hours will evolve into a more organized MCS through early=20 Sunday morning. Unlike ongoing convection, by tonight the storms=20 will be much faster moving, but more widespread. Corfidi Vectors=20 are parallel to the flow, which could support isolated training=20 convection, especially given the moisture-rich atmosphere with=20 PWATs above 1.75 inches. This is as much as 2.5 sigma above normal=20 for this time of year.=20 Guidance remains highly variable as to how the convection will evolve tonight. The Fv3, GEM, and CONEST all suggest there may be more training of fast-moving storms in various streaks across the Slight Risk area, particularly just inland from the coast. Meanwhile the historically more reliable HRRR and ARW members are much lighter in the overall rainfall footprint, alternatively suggesting that one more organized MCS will race across south Texas and into the Gulf, perhaps clipping far southwestern=20 Louisiana before 12Z. Should this scenario occur, then the flash=20 flooding risk is much lower. Given this inherent uncertainty, have=20 opted to leave things as status quo. The average look in the=20 guidance suggests this to be a lower end Slight, largely boosted by earlier heavy rains from last night across portions of the Slight=20 Risk area. FFGs remain rather high despite the recent rains, so it=20 appears it will require training convection to get the widely=20 scattered flash flooding consistent with the Slight. A fast-moving=20 MCS/squall line scenario would keep any flash flooding to a minimum and would require a Marginal risk downgrade. Wegman ....Previous Discussion... Difluence aloft ahead of an upper level low moving slowly across=20 Arizona teams up with a quasi-stationary front to continue the=20 potential for heavy rainfall in the front's vicinity. The guidance has shifted northward when compared to this time yesterday, which=20 led to a similar shift in the Slight Risk area. Slight warming in=20 the temperatures at 700 hPa imply that the heavy rain threat should edge northward, which appears to be beginning per recent=20 radar reflectivity trends in southeast TX, but how far north=20 remains the question. Inflow at 850 hPa from the Gulf should pulse up to 25-30 kts at times, importing ML/MU CAPE of 2000 J/kg or so. Effective bulk shear should be sufficient for organized convection, capable of producing hourly rain totals to 2" and=20 local amounts to 4". The guidance indicates a couple rounds of=20 convection this period, early and late. Dispersion continues=20 concerning where within southern and eastern TX the heavy rainfall=20 is most likely to occur, but the amounts still appear high enough=20 to maintain the Slight Risk threat in the outlook. There was some=20 accounting for the heavy rainfall that fell on Friday near the=20 southern border of LA/TX in helping to define the eastern edge of=20 the Slight Risk area. Roth Day 2 Valid 12Z Sun Mar 17 2024 - 12Z Mon Mar 18 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF SOUTHERN LOUISIANA AND COASTAL MISSISSIPPI... ....2030Z Update... A low level jet with abundant Gulf moisture will run into an advancing cold front moving down the Mississippi Valley Sunday. The clash of these 2 air masses will provide a focus and lifting mechanism for the moisture, supporting strong storms capable of heavy rainfall rates to 2 inches per hour. The front will allow for the storms both to form and to track east along the frontal interface, which will extend along all of the northern Gulf Coast. In coordination with LIX/Slidell, LA forecast office, the Slight Risk has been expanded well to the east to account for the convection that will be ongoing Sunday morning. The 2 inch per hour rainfall=20 rates will be capable of rapid-onset flash flooding, particularly=20 in urban areas such as Baton Rouge and New Orleans. Recent heavy=20 rainfall over and upstream of these areas have made local streams=20 and rivers high, which will lower the threshold needed for flash=20 flooding to occur. Once the cold front moves through, conditions=20 should rapidly improve, to include rapid drying of the atmosphere.=20 This will end the flash flooding threat caused by rainfall. Wegman ....Previous Discussion... Organized convection in the vicinity of a front, which should be=20 somewhere the southern border of TX/LA Sunday morning, is expected to lead to 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 precipitable water values of 1.5-1.75" into the region=20 and MU CAPE up to 2000 J/kg. Difluence aloft should be fostered by increasing troughing across the Great Lakes, Midwest, and=20 Mississippi Valley. There is some signal in the guidance for=20 cooling temperatures at 700 hPa, which is of concern as the=20 1000-500 hPa thickness diffluence is now very close to the LA=20 coast. This means that there is some chance that organized=20 convection could shift slightly offshore LA at some point in its=20 evolution, leading to decreasing confidence in heavy rainfall the farther east you go in LA. Both the 00z NAM and 12z-00z ECMWF are=20 on the north side of the guidance envelope (which usually isn't=20 good news for the ECMWF solution) while the remainder of the=20 guidance lies closer to the Gulf coast, including the normally=20 northward- biased 00z UKMET. The guidance shows a bit of=20 progression to the convective pattern, as an upper trough sharpens across the Ohio and Mississippi Valleys and helps act to shift the moisture plume/best 850 hPa inflow east with time. Hourly rain=20 totals to 2.5" with local amounts to 4" are possible where limited cell training occurs and/or mesocyclones happen to develop. Because of the heavy rainfall that occurred on Friday near the southern=20 TX/LA border, and per coordination with the LCH/Lake Charles LA=20 forecast office, added a targeted Slight Risk area to areas where=20 soils recently became more sensitive. Roth Day 3 Valid 12Z Mon Mar 18 2024 - 12Z Tue Mar 19 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8lPBouFUtcB5EWkEP0pXulFAhaIKwEszOAAbtCpfu86w= 5XMvwZLKLVEGcf75NjVrHRkJSPVNDkOQP8PDrjjROUq3mEs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8lPBouFUtcB5EWkEP0pXulFAhaIKwEszOAAbtCpfu86w= 5XMvwZLKLVEGcf75NjVrHRkJSPVNDkOQP8PDrjjRgHni7JI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8lPBouFUtcB5EWkEP0pXulFAhaIKwEszOAAbtCpfu86w= 5XMvwZLKLVEGcf75NjVrHRkJSPVNDkOQP8PDrjjR-AxiCxQ$=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) .