Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Apr 14 2023 15:53:40 FOUS30 KWBC 141553 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1152 AM EDT Fri Apr 14 2023 Day 1 Valid 16Z Fri Apr 14 2023 - 12Z Sat Apr 15 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH FLORIDA & THE MID-ATLANTIC/SOUTHERN APPALACHIANS... ....16Z Update... Very few changes needed for both ERO risk areas for this morning's update. ....South Florida... Typical afternoon sea breeze related convection is possible along the coast from Ft. Lauderdale to Miami today. Given the extreme sensitivity of this area to any additional rainfall, what little rain is possible today would still be enough to pose an isolated flash flooding risk. The Marginal Risk introduced overnight is unchanged for this midday update. ....The Carolinas to the southern Appalachians... Widely scattered convection is ongoing across central NC into southwest VA this morning. With additional daytime heating, expect MUCAPE values across the Marginal Risk area to approach 1,000 J/kg in the next few hours as the vertically stacked low over the Southeast slowly progresses northeast this afternoon. The low will cause temperatures to cool aloft, which will further destabilize the atmosphere as well as provide a forcing source for storms to develop along. Thus, expect scattered convection to develop over the Marginal Risk area over the next few hours, which while staying largely disorganized, has the potential to train since the low will continue moving very slowly. Local FFGs over central NC are a bit lower than the surrounding areas, with values to 1.5 inches in 3 hours. Given this afternoon=C2s storms have some potential to become strong, these values may be exceeded. Nominally higher chances for heavy rain are expected on the south and east facing slopes of the southern Appalachians as any convection tracks northwestward, but FFGs are also higher in these areas. Thus, the broad Marginal Risk is largely unchanged, other than including far western SC and far northeastern GA in the Marginal Risk area for this update. Wegman ....Previous Discussion... ....South Florida... Heavy rains yesterday caused some of the major roads in the Fort Lauderdale to Miami corridor to become flooded. Additional showers and thunderstorms are expected to fire up near the coast during the afternoon and evening hours, which will aggravate the already sensitive area. A Marginal Risk area was raised for the coastal/urban areas of southeast Florida. ....Mid-Atlantic and Central Appalachians... A deep mid/upper level low is expected to lift northward and weaken/open into Friday night. Moist easterly flow ahead of the advancing warm front will bring a surge of higher moisture through the region, characterized by PWs of up to 1.5 inches. With available instability (~1000 J/kg CAPE), locally intense rainfall will be possible across the Piedmont region. A secondary round of convection is possible during the afternoon/early evening associated with the main upper level shortwave moving overhead. The combination of steeper lapse rates and peak heating could bring another round of heavier showers and thunderstorms. Some of the hi-res guidance is depicting hourly rainfall of 1 to 1.5+ inches/hr across the Carolinas, as well as, eastern Kentucky, western Virginia and southern West Virginia. The 3-hr FFG are as low as 1.5 inches in the areas of higher elevation and around 2-2.5 inches elsewhere. The inherited Marginal Risk area was reshaped to trim out parts of South Carolina where the latest trends have lowered and then expanded east-west into western Kentucky, Tennessee and southern West Virginia where intense rainfall may quickly surpass local FFG and lead to isolated flooding concerns. Campbell Day 2 Valid 12Z Sat Apr 15 2023 - 12Z Sun Apr 16 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE LOWER AND MIDDLE MISSISSIPPI RIVER VALLEY... ....Western U.P. of Michigan, northern Wisconsin and the Arrowhead of Minnesota... Recent snowmelt across the region has lowered local 6-hr FFG to 0.25 inch. While the rain-snow line may play a role in northeastern Minnesota, rain will be the dominant precipitation type further east. 24-hour forecast rainfall totals are expected to generally range between 1/2 and 3/4 inch through early Sunday morning, but the potential with moisture advection of exceeding the 1/4 inch in 6 hour threshold for meeting FFGs is significant. There is unanimous model agreement that temperatures will remain above freezing in this area between now and the rainfall on Saturday, so there's no reason to expect snowmelt will not continue around the shores of Lake Superior. While the rain is never expected to be heavy, the low FFGs and the forecast rainfall alone are worthy of consideration of a Marginal Risk introduction for this area with future forecast updates. Wegman ....Lower to Mid-Mississippi River Valley... A subtropical jet with an embedded shortwave trough is expected to track through the central U.S. which will provide the large scale forcing for ascent early in period. PW values will be near 1.5 inches with possible surges of 1.75 inches along the Gulf Coast of Louisiana, Mississippi and Alabama. With deep moisture drawing northward the environment will be conducive for locally heavy rainfall to spread across the Lower and Middle Mississippi Valley. Later on, a stronger shortwave trough will approach as it departs the Rockies. Showers and thunderstorms will fire up along and ahead of this approaching cold front mainly after 00Z and have the potential to dump 1 to 3 inches over the Mississippi Valley. Some of the this region has had recent rains, and are 150 to 600 percent of normal. These sensitive soils could quickly become over saturated and lead to isolated/scattered areas of flooding. The Marginal Risk area that is already in effect covers much of this sensitive region. Given the latest QPF trends and WPC forecast, the western and northern bounds of the Marginal were expanded. Campbell Day 3 Valid 12Z Sun Apr 16 2023 - 12Z Mon Apr 17 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-XdPfOMUUjY75iXCJJn5RCu3pVf7fZ1gfIeNmpyP9ByW= O50eDiCQqou3q-wq-YhKwkXTmXL5vDuptr_uQW7zkdUWd70$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-XdPfOMUUjY75iXCJJn5RCu3pVf7fZ1gfIeNmpyP9ByW= O50eDiCQqou3q-wq-YhKwkXTmXL5vDuptr_uQW7zYfyEyEM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-XdPfOMUUjY75iXCJJn5RCu3pVf7fZ1gfIeNmpyP9ByW= O50eDiCQqou3q-wq-YhKwkXTmXL5vDuptr_uQW7ztKqqx2g$=20 $$ --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .