Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Mar 25 2024 00:52:14 FOUS30 KWBC 250051 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 851 PM EDT Sun Mar 24 2024 Day 1 Valid 01Z Mon Mar 25 2024 - 12Z Mon Mar 25 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM CENTRAL=20 TEXAS INTO SOUTHEASTERN OKLAHOMA WESTERN ARKANSAS... ....01Z Update... Ongoing convection ahead of a dryline/Pacific cold front centered over western Texas is expected to continue, with additional=20 development along the southwestern flank as strengthening low level flow supports increasing moisture advection into an area of sufficient ascent and instability. Most of the CAMs, including recent runs of the HRRR, appear to be too slow to develop convection, but all generally show the same signal for southwest- to-northeast training convection developing this evening and continuing into the overnight. Recent runs of the RAP show PWs increasing to around 1.25 inches ahead of the slowly advancing=20 surface boundary, while mid-level energy and left-exit region upper jet forcing help support large-scale ascent.=20 With some adjustments made for current radar trends, the Marginal=20 Risk for the 01Z Update reflects the 18Z HREF, which shows high=20 neighborhood probabilities for an inch or more along an axis=20 stretching from the central Texas Hill Country northeastward across North Texas into southeastern Oklahoma and western Arkansas. The=20 HREF indicates the heaviest amounts are mostly likely to fall=20 across southeastern Oklahoma, where it is showing high neighborhood probabilities for amounts exceeding 2 inches. Pereira Day 2 Valid 12Z Mon Mar 25 2024 - 12Z Tue Mar 26 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER MISSISSIPPI VALLEY... ....20Z Update... No significant changes to the placement and orientation of the SLGT risk across the Lower Mississippi Valley as the overall pattern evolution on guidance maintains decent continuity from the previous forecast cycle. There was an expansion of the MRGL up into the central Midwest, mainly covering northern IL into extreme southern WI. This was in conjunction to an increasing signal of convection developing in-of of the warm sector across northeast MO, advancing rapidly northeast with the aid of a strong mid-level steering flow and deeper moist advection thanks to a roaring 55-60kt LLJ extending from the Gulf coast up through the central Midwest. Elevated PWATs on the order of 2 deviations above normal within the environment will be plentiful for locally heavy rainfall falling over a region with fairly low FFG indices thanks to recent precipitation and some urbanization characteristics extending from Rockford over to Chicago. Per request, as well from the Chicago WFO, have expanded the MRGL risk area further north. The Lower to Mid-Mississippi Valley will still be the primary beneficiary of the heaviest rainfall potential over the CONUS. Advancing cold front into a plentiful, buoyant environment will generate periods of heavy rainfall with two main areas of focus; the Ozarks on Monday morning and early afternoon, then across the Mississippi Delta into western AL by nightfall thanks to convection re-firing after the nocturnal LLJ redevelops over the aforementioned area. PWATs between 2-2.5 deviations above normal and sufficient speed shear will help organize cells into a line and slowly trudge eastward through Monday evening. Rainfall rates on the order of 1-2"/hr will be common within the heaviest precip which will offer some localized flooding concerns within poor drainage areas and within any urbanization constructs. Convection will advance eastward into AL by the end of the period with diminishing convective risks thanks to environment stabilizing away from the primary upper trough. This is the reason for the sharp delineation from SLGT to MRGL to nothing for places further east. Kleebauer ....Previous Discussion... ....Lower Mississippi Valley... An line of thunderstorms will be ongoing from Texas and Oklahoma at the start of the period Monday morning. Similar to the Day 1 reasoning for TX/OK, the parent low track turning more northward with time during the day Monday will cause a slowing of the eastward progression of that line of storms. A robust low level jet will advect a plume of deeper Gulf moisture into Louisiana and Arkansas Monday at the same time, which in turn will locally enhance the storms just as their eastward motion slows. Thus, there is good agreement on higher rain fall totals in a rather narrow line from Texarkana north-north east into south-central Missouri. This will act as the pivot point where the line of the storms will be close to stationary. Areas to the north into Illinois will have weaker storms and less moisture due to lack of instability and distance from the Gulf, whereas storms to the south across northern Louisiana will have more moisture and instability, but will be much faster moving. Local rainfall totals to 3 inches in this "Goldilocks Zone" where the line stalls but storms remain strong could cause widely scattered flash flooding. There will likely be an area through central Arkansas with a local minimum of rain as the upper level forcing with the line stays further west, and a second strong impulse of upper level energy tracks eastward further south. For simplicity the Slight includes this area to not have 2 separate Slight risk areas almost right next to one another, and to account for inherent uncertainty as to exactly where this local minimum will ultimately set up. For Mississippi and far western Alabama, the line of storms will reintensify Monday evening as the typical strengthening of the nocturnal low level jet advects environmental moisture characterized by PWATs locally exceeding 1.75 inches across eastern Louisiana and southern Mississippi. Leftover instability from the day will advect into this same area as well. Thus, despite good agreement on a rapid eastward movement of the line of storms, the ingredients supporting the storms will all increase for a time as the line traverses Mississippi and eventually far western Alabama (though uncertainty increases that far east). Rates to 2 inches per hour given the abundant atmospheric moisture will be possible, even as any one area only sees rates that high for up to an hour. Such a deluge of moisture will quickly overwhelm any local streams and creeks with a large influx of water, so the Slight remains in effect. Once the line gets much into Alabama, they will encounter a much more stable atmosphere across the state which will have the benefit of cooling off from the day, increasing the stability before the storms arrive in the wee hours of Tuesday morning. Expect rapid weakening of the storms by this point, however given all the moisture the storms will have to work with, rates will still be high as the storms become increasingly downdraft dominant. The Slight was expanded east to account for a greater eastward translation of the storms in much of the guidance. The faster eastward forward speed should also work to reduce flash flooding potential due to less rainfall into Alabama as well. Wegman Day 3 Valid 12Z Tue Mar 26 2024 - 12Z Wed Mar 27 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE FLORIDA PANHANDLE INTO SOUTHEAST AL AND SOUTHWEST GEORGIA... ....20Z Update... In coordination with the Tallahassee WFO, have upgraded a small portion of the FL Panhandle into the southern portions of AL/GA given the latest trends in convective rainfall anticipated Tuesday evening along the cold front. Recent rainfall across the FL Panhandle has allowed for some decent saturation over areas historically with higher FFG indices. Despite that, there is some concern for locally heavy rainfall within a slow-moving regime to develop Tuesday evening that could drop locally 2-4" totals in-of areas that have been hit recently with heavy rain. Highest theta-E signatures designate a primed area of instability over parts of the Gulf Coast into the very southern portions of AL/GA. Some deterministic are being more aggressive with the setup compared to others, so the door is open for local flood prospects if cells do form and exhibit some training characteristics with the steering flow aligned parallel to the slow-trudging boundary that will enter the area by Tuesday evening. As per the request of the WFO, have added that targeted SLGT risk to cover for the area that has the best opportunity to see some heavier rainfall the back half of the D2 period. Kleebauer ....Previous Discussion... As mentioned in the Day 2/Monday discussion, any lingering convection ongoing at the start of the period Tuesday morning will be encountering an increasingly hostile/stable environment for storms over Alabama. Both rates and storm total rainfall will quickly diminish Tuesday morning. The inherited Slight risk over the entire state of Alabama was downgraded in coordination with all 3 Alabama offices with this update. Guidance suggests storms could refire with afternoon heating over the western Florida Panhandle, southeast Alabama and possibly western Georgia, but those areas are historically quite flood resistant with high FFGs and other than Pensacola are mostly rural. Thus for now, the Marginal appeared to be the best representative risk category, but a smaller and more targeted Slight may be needed for this area with future updates if forecast rainfall notably increases. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-Hi9brS17QbJjCNJsQXRwxmUzGOhVPdZf2_kanjgzGzr= RmSteVIsDqATCL-NDC8LzUUipVx6pAQMMl32uUWuErrOibs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-Hi9brS17QbJjCNJsQXRwxmUzGOhVPdZf2_kanjgzGzr= RmSteVIsDqATCL-NDC8LzUUipVx6pAQMMl32uUWuHefcvLY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-Hi9brS17QbJjCNJsQXRwxmUzGOhVPdZf2_kanjgzGzr= RmSteVIsDqATCL-NDC8LzUUipVx6pAQMMl32uUWu2kOXq3g$=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) .