Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 02 2024 20:13:16 FOUS30 KWBC 022012 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 412 PM EDT Mon Sep 2 2024 Day 1 Valid 16Z Mon Sep 02 2024 - 12Z Tue Sep 03 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE HILL COUNTRY/CENTRAL TEXAS AND ALONG THE TEXAS GULF COAST... ....16Z Update... ....North Central Texas... No significant changes were made to the inherited ERO risk areas. Heavy rain ongoing across the area will continue through the day. The CAMs guidance has continued to increase across the Hill County and portions of central Texas, generally west of Ft. Worth, southeast of Lubbock, and around and northeast of San Angelo. For that reason, that area of north central Texas is considered in a higher-end Slight risk. Widely scattered to scattered instances of flash flooding are probable with locally significant and considerable flash flooding possible. The rocky soils and some terrain will convert much of the rainfall into runoff...increasing the potential for flash flooding. ....Texas Gulf Coast... The inherited Slight Risk along the Texas Gulf Coast remains largely the same, with minor tweaks this morning. Heavy rainfall associated with westward moving convection with a stationary tropical low may cause localized flash flooding, particularly in any low-lying and urban areas today. Thus far the most organized convection has been stationary over Galveston, extending west to the south of Houston. However, these kinds of localized heavy rains that may train will continue to be a flooding concern for the rest of the day into tonight. ....Carolinas... The biggest change to the ERO this midday is to trim nearly all of SC and GA out of the Marginal Risk. The heaviest rains today are now expected to remain offshore of SC and GA based on the latest CAMs guidance. Heavy showers are impacting the North Carolina beaches and the southern Outer Banks, which remain in a low-end Marginal, but once again the vast majority of the heaviest rains are expected to remain offshore over the Gulf Stream. Wegman ....Previous Discussion... ....Texas... An upper level trough with a mid-level vort will be advancing into western Texas during this period. Meanwhile a surface low hugging the South Texas Coast will maintain easterly flow from the Gulf allowing PW values of 2+ inches to pool over much of the state. The latest guidance continues to depict areal averages of 1 to 5 inches of QPF across the Hill Country into east-central Texas and along much of the Texas coastline. Recent rainfall has increased soil saturation across the state thus lowering FFGs. The hi-res guidance is showing training of slow moving convection capable of 1 to 2.5 inches/hour rain rates which could easily lead to scattered areas of excessive rainfall and localized flash flooding. A Slight Risk areas remains in effect for west-central and central Texas. Additionally, a Slight Risk was added for the coastline today where storms will be producing rates of 1 to 3 inches/hour. ....Carolinas... A cold front sinking through the Mid-Atlantic region will continue to move showers and thunderstorms southward over the Carolinas today. Guidance keeps the bulk of the QPF offshore during this period where there are better dynamics in place however there will still be an isolated threat for localized excessive rainfall and flooding. A Marginal Risk area was maintained for eastern portions of the Carolinas. Campbell Day 2 Valid 12Z Tue Sep 03 2024 - 12Z Wed Sep 04 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL TEXAS AND THE MIDDLE TEXAS COAST.. ....2030Z Update... ....Texas... A new Slight Risk area was introduced for the Middle Texas Coast with this afternoon's update. A slow moving tropical low will continue to bring periods of heavy rain in the form of slow-moving showers and thunderstorms from the Gulf into the adjacent Texas Coast. Any rainfall from today will saturate the soils for the next round of rain on Tuesday. Similar amounts of rain are expected on both days, so any potential flooding from today will be worse on Tuesday. Most areas will not see steady rain the entire time, but very localized areas of training storms resulting in flash flooding. For north central Texas, very few changes were needed as continued rain from Monday is still expected to hit the portion of north central Texas between Ft. Worth, Abilene, and San Angelo. Lesser amounts of rain are expected Tuesday as compared with today, but with scattered flash flooding ongoing currently, additional rain in these same areas will continue or worsen existing flooding. The Slight risk area was expanded a row of counties eastward in keeping with the latest guidance trends. ....Mississippi and Alabama Gulf Coast... A low end Marginal was introduced with this update for the potential of slow moving and training storms to the east of the low along a stalling front into coastal Mississippi and Alabama. The area has been somewhat drier than points further east, but the abundance of atmospheric moisture and instability as well as the slow-moving nature of the synoptic systems did increase the confidence that an isolated flash flood or two could develop in these portions of MS & AL where heavy rain from slow moving storms persists the longest. Wegman ....Previous Discussion... ....Texas... The mid-level vort is expected to exit the region although precipitable water values will remain high (+1 to +2 sigma initially, then decreasing late Tuesday). A large portion of the flooding potential will be contingent on a verified forecast from Day 1/Monday, as Monday will have the heavier rain/stronger storms of the two days. However, even the lesser amounts of rain expected Tuesday should still cause flooding issues in the area with a widespread 1 to 1.5 inches forecast with locally higher amounts. A Slight Risk remains in effect for central portions of the state. The heaviest rains in the state are expected to be along the Gulf Coast from Galveston south and west through Corpus Christi. Locally heavy rains to 2 inches per hour will be possible. For now the greatest flooding threat remains confined to any urban areas, but should Monday's rains overperform here then a Slight may be needed for the hardest hit areas with future updates. ....Idaho/Montana... A potent shortwave trough will interact with up to 2 sigma above normal atmospheric moisture on Tuesday. PW values rising to near 1 inch will support storms capable of heavy rain that may produce isolated flash flooding should the storms impact particularly flood sensitive or more urban areas. Campbell/Wegman Day 3 Valid 12Z Wed Sep 04 2024 - 12Z Thu Sep 05 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE MISSISSIPPI DELTA REGION AND ADJACENT GULF COAST... ....2030Z Update... The combination of a stalled out front, tropical low, and abundant moisture and instability will all make for a rainy day along nearly the entire Gulf Coast from south Texas all the way to the Florida Peninsula on Wednesday. The focus for the heaviest rains will be in the central Gulf Coast from Louisiana east through the Florida Panhandle. Locally heavy rains are likely, especially from New Orleans east through Pensacola, where a higher-end Slight is in place. With the greatest forcing pushing south with time, rainfall amounts further north into the mid-Mississippi Valley have come way down. So the biggest change to the ERO was to downgrade the Slight and Marginal risk areas into MO/KY/TN, much of AR, and northern MS. Given the multiple days of scattered shower and storm activity into Louisiana and the adjacent Gulf Coast late last week, the Slight was expanded both west and east along the Gulf Coast to highlight the potential for heavy rain and adjacent flooding, especially in urban areas.=20 With increasing amounts of rain forecast in and around New Orleans, continued increases in rainfall there may require an eventual targeted Moderate Risk should trends continue. Further, potential for heavy rain may continue into the Middle Texas Coast Wednesday, so another Slight for coastal sections may also need to be considered with future updates. Wegman ....Previous Discussion... ....Gulf Coast and Lower Mississippi Valley... The influx of anomalous Gulf Moisture will persist all across the southern tier states while the upper trough and vort exits the plains and into the Mississippi Valley. The surface front will continue to be draped west to east across the South continuing to provide focus for showers and thunderstorms across the region. During the late afternoon and evening hours there will be an intensification of instability and forcing to boost rainfall rates across the Lower Mississippi Valley, particularly over Louisiana, western Mississippi and southeast Arkansas. Most of the guidance are suggesting that 1 to 3 inches will concentrate over this part of the region thus increasing the risk for excessive rainfall and local flooding potential. There is the potential for a few locations for local maximums to exceed 6 inches near northern Louisiana/Arkansas. Onshore flow across southeast Louisiana and southern Mississippi could yield 1 to 4 inches as well. A Slight Risk area was raised for parts of Louisiana, Mississippi and Arkansas. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8SlyYVFagzmS0_85wU6WEhruy7i7rIsSExm7UBu745kj= j0lw3Zc4zIeWQ3pix9KYvbSBD4WC6DQe-MZcDm4FvT7npGc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8SlyYVFagzmS0_85wU6WEhruy7i7rIsSExm7UBu745kj= j0lw3Zc4zIeWQ3pix9KYvbSBD4WC6DQe-MZcDm4FR5zZbBQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8SlyYVFagzmS0_85wU6WEhruy7i7rIsSExm7UBu745kj= j0lw3Zc4zIeWQ3pix9KYvbSBD4WC6DQe-MZcDm4FCfNxFgw$=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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