Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Apr 06 2023 20:31:33 FOUS30 KWBC 062031 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 PM EDT Thu Apr 06 2023 Day 1 Valid 16Z Thu Apr 06 2023 - 12Z Fri Apr 07 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM SOUTHEAST TEXAS TO WESTERN LOUISIANA... ....16Z Update... The overall forecast remains on track for Texas and the Lower Mississippi Valley. An ongoing line of storms from central Texas into northwestern Louisiana, southeastern Arkansas, northwestern Mississippi, and western Tennessee is forecast to continue for the next few hours given jet coupling support for vertical assent aloft and very moist onshore flow over a stalled frontal boundary to the southeast along the Gulf Coast northeast into the Lower Mississippi Valley. The expectation is for the northern extent of the front to slowly push further southeastward while the southern portion over the Gulf remains stalled, leading to a bit more progressive storm motions over the Marginal Risk area in Louisiana/Mississippi into central Tennessee while a wavey band of storms continues over the Slight Risk area in Southeast Texas. The hi-res model guidance indicates the potential for areal average rainfall totals between 2-3", with locally higher totals of 4-5" certainly possible. In fact, MRMS Q3 Multi-Sensor rainfall estimates support these higher rainfall totals given some localized amounts in this range over the past 12 hours. One hour MRMS estimates also support rainfall rates over 1" per hour which will add up as storms tend to train over the same areas given only modest expected southeast movement of storm coverage during the day 1 period. Only minor updates were made to the ongoing Slight Risk over Southeast Texas into western Louisiana given updated hi-res guidance trends in notable QPF coverage. This included a small southwestward expansion into South Texas where additional storms are expected to form later this evening into the overnight hours to the west of a surface wave over the Gulf of Mexico and additional lift from a weak short-wave passing over the region aloft. One area of note outside of the current Slight Risk would be portions of southeast Louisiana into southwest Mississippi to the southeast of the frontal boundary. Moderate buoyancy in an uncapped environment will support the development of numerous heavy rainfall producing storms during the day. However, the regional coverage of any more significant higher rainfall totals compared to the Slight Risk area to the west looks to remain isolated enough for now. Additional locally heavy rainfall is expected with thunderstorms developing later this afternoon along a cold front and potential outflows over the Mid-Atlantic. Dry antecedent conditions and the progressive movement of the front should limit any locally heavy totals to very minor instances of urban road flooding/ponding. Putnam ....Previous Discussion... ....Southeast TX to Lower MS Valley... A frontal boundary off the Texas coast and a conveyor belt of 500mb shortwaves tracking northeast into the Mid-Atlantic will lead to an active day of showers and thunderstorms from Texas on north and east into the Mid-South. In addition, the 250-500mb trough axis is residing over Arizona on south through the Gulf of California. This setup is not only positioning the right-entrance region of the 150+ knot 250mb jet streak to reside over the Mid-South, but a weaker (albeit, still diffluent) left-exit region of a 100 knot jet streak over the tropical East Pacific sets up near the Rio Grande. This jet coupling will foster plenty of vertical ascent in the upper levels of the atmosphere. Strong ridging off the East Coast is responsible for an expansive area of southeasterly 850mb winds stemming from the western Caribbean and extending through the Gulf of Mexico. This provides no shortage of atmospheric moisture over southeast Texas with PWs >1.75" expected along the Texas coast and as far north as southwest Louisiana. Mean wind speeds within the 850-300mb layer are also a little slower at 20-25 knots out of the SW, while also oriented parallel to the front off the coast. This lays the groundwork for potential training of storms within the Slight Risk area. With persistent inflow of 850mb moisture flux and a stalled front and upper level pattern, it will be an active day of widespread showers and thunderstorms across the southern tier of Texas and into the Mid-South. Guidance has continued to key in on southeast TX to western LA with the best odds for heavy rainfall totals >3" on the day according to the 00Z HREF. In addition, this setup is dependent upon where heavy rainfall from Wednesday occurs, and how much instability is available in these regions. The 500-1,000 MUCAPE values and PWs that according to NAEFS would top the 99th climatological percentile from Corpus Christi and San Antonio to areas southeast of Houston, giving this event a good floor as a Slight Risk. Soils have gradually grown more sensitive after rounds of storms today, most notably just west of the TX coast. That said, much of southeast TX came into today's event with as low as 10% of normal rainfall, and despite the most saturated atmospheric profiles tomorrow (GFS shows 1000-500mb RH values >90% throughout southeast TX) instability will also be harder to come by farther inland. There remain enough question marks for identifying significant rainfall amounts to not introduce a Moderate Risk, but given the dearth of moisture present and weak but sufficient MUCAPE, there does remain a good opportunity for heavy rainfall and potential flash flooding in Southeast TX and western LA. Maintained the Marginal Risk areas into the Mid-South could see PWs approach 1.5", MUCAPE near 500 J/kg, and over soils that are more sensitive and saturated compared to locations in western LA and southeast TX. It is worth mentioning that while there is no Marginal Risk in place, portions of the Mid-Atlantic could witness strong thunderstorms during the mid-late afternoon hours. PWs are likely to be at or above 1.5" with up to 1,500 MUCAPE available, which would support hourly rainfall rates close to 2"/hr in the most intense cells. The primary reasons for no Marginal Risk this forecast cycle are because cell propagation will be to the east with mean 850-300mb winds not running parallel to a more progressive frontal boundary. Northern VA to eastern MD is also sporting as low as 25% of normal rainfall over the last 7 days according to AHPS, indicating the soils in the area could use the rain. Still, a more urbanized corridor from central VA to the I-95 corridor of northern MD could be at risk for 1-3" worth of rain close to the afternoon rush hour. Should confidence in those higher end totals increase, a Marginal Risk may be necessary for the 16Z update. ...Pacific Northwest Coast... The Marginal Risk along the coast of the Pacific Northwest remains in place as guidance is in good agreement on rich 850-700mb moisture flux being oriented along the coast Thursday morning and into Thursday afternoon. A sharp 250-500mb trough axis approaching the coast Thursday morning is set to provide excellent diffluence atop the atmospheric column. This is a warmer setup aloft, causing rain to be the primary precipitation type in mountainous terrain that has a deep snowpack. Rainfall amounts as high as 2-3" (locally higher amounts in the Olympics possible) are expected from the coastal range of California and Oregon to the Olympics, where 0-40cm soil moisture percentiles courtesy of NASA SPoRT-LIS are up to 70% saturated in some locations. The combination of heavy rainfall rates topping out close to 0.5"/hr and some snow melt in higher elevations could result in rapid runoff along complex terrain, as well as rapid rises in creeks and streams. Mullinax Day 2 Valid 12Z Fri Apr 07 2023 - 12Z Sat Apr 08 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM CENTRAL TO SOUTHERN LOUISIANA AND SOUTHERN MISSISSIPPI... ....2030Z Update... The focus Friday will remain on an axis of heavy rainfall producing storms over the Southeast, to northwest of a frontal boundary draped over the Southeast into the western Gulf of Mexico, with additional storms likely to develop to the south of the front from southeast Louisiana into southern Mississippi, Alabama, and Georgia. The hi-res guidance paints a bit of an uncertain picture on where exactly this axis of slowly progressing storms, and thus risk for heavier rainfall totals, will set up. This is likely tied to the forecast position of the front which will continue to meander a bit with southwestward extent given the lack of appreciable upper-level support to help progress it along. However, the Slight Risk continues to cover the region most likely to see heavier rainfall totals both southeast of the boundary over southeastern Louisiana as well as to the northwest over central Louisiana and southern Mississippi. Hi-res guidance suggests a shortwave aloft will help to kick off an initial round of convection south of the front over southeastern Louisiana while helping to maintain storm coverage north of the front into central Louisiana and southern Mississippi. Storms will likely linger a bit to the west over eastern Texas/western Louisiana but expect that the heavy rainfall threat will move offshore as the front pushes a bit eastward over the Gulf, with the northern extent of these storms also pushing eastward into the Slight Risk area over southern Louisiana. Additional post-frontal storms are expected to the northeast into the Southern Appalachians and Carolinas. However, as was noted overnight, the cold front is forecast to push southward much quicker over this region with little expected lingering instability aloft to support heavier rain rates. Uncertainty over storm coverage grows significantly further east into Alabama and Georgia as well, with the noted progressive nature of the front limiting areal coverage of the warm sector with northward extent rather quickly Friday afternoon/evening. Putnam ....Previous Discussion... The Mid-South and as far south and west as east Texas remains under the influence of the diffluent right-entrance region of a roaring 160 knot jet streak over the Northeast. In addition, the mean 250-500mb trough over northern Mexico is also producing its own diffluent flow aloft over southeast Texas. The frontal boundary that has acted as the trigger for rounds of heavy rainfall will continue to stick around off the coast while southerly 850mb flow out of the Gulf of Mexico continues to deliver anomalous moisture to the region. The atmosphere looks to remain quite saturated (1000-500mb RH values >90% for most of the Slight Risk and Marginal Risk areas) and soils in these areas will have continued to have grown more sensitive from multiple days worth of rainfall. One limiting factor is that MUCAPE remains below 1,000 J/kg and mean 850-300mb mean winds remain around 20 knots, allowing for storms to remain on the progressive side. That said, the added soil sensitivity, anomalous moisture content, and just enough instability aloft to allow some storms to produce rainfall rates up to 2"/hr is enough to keep the Slight Risk in place over the Lower MS River Valley. The Lower MS Valley will reside directly in front of an approaching 500mb vort max, while the region also features better odds for seeing more modest instability levels. The Marginal Risk extends as far south and west as the Houston metro area to as far east as the southern Appalachians. The latter could see MUCAPE range between 500-750 J/kg Friday afternoon and PWs as high as 1.5". Revisited the Marginal Risk potential all the way to the Carolinas, but the cold front sinking further south into GA made it seem less likely sufficient instability and high enough PWs would be present to promote a more sufficient flash flood setup. Should there be a trend for more instability and moisture to become available in future forecast cycles, an extension of the Marginal Risk into portions of the central Carolinas may be necessary. Mullinax Day 3 Valid 12Z Sat Apr 08 2023 - 12Z Sun Apr 09 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM THE FLORIDA PANHANDLE TO THE CENTRAL CAROLINAS... ....2030Z Update... Only very minimal adjustments were made to the current Marginal Risk area based on the updated global guidance. As is the case for Friday, there is a bit of disagreement on the axis storm coverage and thus heavy rainfall threat through the Southeast as the meandering frontal boundary finally begins to push offshore over the Atlantic. However, southeasterly low-level flow setting up over the Atlantic and over the boundary will help to continue to support the threat for areal average rainfall totals of 1-2", with locally higher amounts prompting an isolated threat for flash flooding. Putnam ....Previous Discussion... Saturday features another favorable setup for a very wet day for much of the Southeast with a pesky frontal boundary still positioned from the Gulf Coast to the Carolina coast. A strong dome of high pressure over the Northeast will work with lower pressure near the Bahamas to produce strong onshore winds, providing the region with some Atlantic moisture. Aloft, the Southeast remains beneath the diffluent right-entrance region of a 250mb jet streak located over the Northeast. There is also a 250-500mb trough traversing the Mid-South that will enhance vertical motion atop the atmosphere too. This trough will tap into a stream of 850mb moisture flux and direct it at the front, providing additional moisture into the region and aid in the development of numerous showers and thunderstorms. PWs will generally hover between 1.25-1.50" while MUCAPE generally struggles to eclipse 500 J/kg. These values don't necessarily scream flash flood potential, but 1000-500mb RH values will be >90% for much of the area and soils will have grown more saturated from rainfall that transpired on Friday. Given these factors, the inherited Marginal Risk still appears to be on track, but did take out portions of the Carolina beaches where, barring a bigger increase in instability and moisture content, have the soil type to be able to better manage the steady periods of rain throughout the day. Mullinax Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6qtHglSidx07g4BRtK0i056yskG3jQDD4W8mQj-EbOls= AYIsTnFOLVB75RDbRIn-19W3Op81EjV3eIptF4tDJnJDkSI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6qtHglSidx07g4BRtK0i056yskG3jQDD4W8mQj-EbOls= AYIsTnFOLVB75RDbRIn-19W3Op81EjV3eIptF4tDMKlL5ww$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6qtHglSidx07g4BRtK0i056yskG3jQDD4W8mQj-EbOls= AYIsTnFOLVB75RDbRIn-19W3Op81EjV3eIptF4tD_vl62RQ$=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) .