Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 30 2024 20:18:37 FOUS30 KWBC 302018 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 418 PM EDT Fri Aug 30 2024 Day 1 Valid 16Z Fri Aug 30 2024 - 12Z Sat Aug 31 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG THE IMMEDIATE GULF COASTAL AREAS OF SOUTHEAST TEXAS AND SOUTHERN LOUISIANA... ....16Z Update... The Slight risk along the Gulf Coast was expanded east to include metro New Orleans in coordination with LIX/Slidell, LA forecast office. Ongoing waves of heavy rainfall moving over New Orleans has resulted in several flash flood warnings over the past couple days, and the heavy rain will continue today and into tonight as well. Given the plentiful atmospheric moisture in place the atmosphere remains supportive of additional storms capable of heavy rainfall. The storms over the Gulf have been associated with a nearly stationary trough, which continues to funnel Gulf moisture into the Mississippi Delta region. The Marginal in Virginia and the Carolinas was expanded westward to include the southern Appalachians with this update. Additional heavy rainfall is expected in this region after yesterday's storms. They produced localized flash flooding yesterday, and today's activity should be more widespread. The terrain effects will support more impactful flash flooding where storms produce sufficient rainfall due to the fast moving nature of the flood waters. The front from New Mexico northeast to the lower Great Lakes remains unchanged. A few higher signals in west Texas as well as Missouri suggests local effects could reach Slight risk levels, but the signal and forcing should be limited enough to prevent a more widespread flooding threat. No significant changes were made. Wegman ....Previous Discussion... ....Gulf Coast... Moderate to heavy rainfall expected along the Gulf coast as deep moisture advects northwards as a tropical low drifts in the northwest portions of the Gulf of Mexico. Repeated rounds of slow- moving heavy rain cells from the Gulf into the Texas/Louisiana border area east of Houston. Guidance depict 1 to 4 inches for areal average along the coast with isolated higher amounts possible. Some of the hi-res guidance have very isolated maximums of 6 to 11 inches. Areal averages of The focus for any potential flooding in this small area will be mainly in the urban areas around Beaumont/Port Arthur and points east. A Slight Risk is in effect from the Houston area east to Vermilion Bay. The=20 surrounding Marginal risk was expanded to include areas near/west=20 of Mobile. ....Great Lakes to Southern Rockies/Southern High Plains... Convection will continue to be driven by deep gulf moisture that is streaming into the north/northeast towards the Great Lakes ahead of a well defined cold front pushing southward and eastward. Precipitable water values of 2 to 2.25 inches are widespread from eastern Texas into Louisiana and Arkansas with an axis then extending north and east ahead of the front being some 2 to 2.5 standard anomalies above climatology being an indication of the moisture-laden atmosphere still in place. There continues to be some variance on where the deepest moisture gets funneled... with the above- mentioned upper level feature playing a key role. Although the overall upper forcing still does not appear to be terribly robust, the diurnal instability supports at least the potential for locally excessive rainfall. A broad Marginal Risk was maintained for this period from New Mexico/Texas to Wisconsin/New York. ....Mid-Atlantic... Portions of the region will have PW values greater than 2 inches along with slow moving storms near the front. This environment looks to be quite favorable for localized heavy rain with some of the guidance showing probabilities of 3 inches/3 hour 30-50% across Northeast North Carolina. A Marginal Risk area was raised for portions of southeast Virginia and northeast North Carolina. Campbell Day 2 Valid 12Z Sat Aug 31 2024 - 12Z Sun Sep 01 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE GULF COAST FROM GALVESTON TEXAS THROUGH THE MISSISSIPPI COAST AS WELL AS FOR THE CENTRAL APPALACHIANS... ....2030Z Update... ....Mid-Atlantic... 12Z HREF guidance have increased in the potential for flash flooding across much of the Appalachians, especially Pennsylvania and New York. A slow-moving cold front will interact with the terrain as the front funnels well above normal atmospheric moisture northward along the chain. Given heavy rainfall in some portions of Pennsylvania, western Maryland and the eastern Panhandle of West Virginia yesterday, soils in some areas are well-saturated and therefore will be more prone to flooding. While chances for flooding decrease into east-central West Virginia, the potential for heavy rain and terrain influences may still result in widely scattered instances of localized flash flooding, despite the recent dry weather. In coordination with CTP/State College, PA; LWX/Sterling, VA; BGM/Binghamton, NY; and RLX/Charleston, WV forecast offices, a Slight risk area was introduced for the=20 central Appalachians for Saturday. ....Gulf Coast... Ongoing heavy rain along much of the Louisiana Gulf Coast has saturated soils in the area today. A slow moving trough of low pressure will continue to funnel abundant Gulf moisture northward into the coast in the form of heavy and slow-moving showers and thunderstorms. Multiple flash flood warnings have been issued around New Orleans the past few days, and the same pattern with the same heavy rain threat for that area will continue into Saturday. Thus, in coordination with LIX/Slidell, LA forecast office, the Slight Risk area for the Gulf Coast was extended eastward to cover New Orleans and the Mississippi Gulf Coast. ....Florida... In coordination with MFL/Miami, FL forecast office, a Marginal Risk area was introduced with this update for the urban I-95 corridor in southeast Florida. Similarly heavy rain will continue into the Keys, so the Marginal also includes the Keys. A separate slow moving area of showers and storms associated with a tropical wave will move into South Florida Saturday. Heavy rains both in recent days and ongoing in Florida have saturated soils, with some areas seeing up to 7 inches of rain in the last 24 hours. Thus, with similar or more widespread rainfall expected Saturday, isolated flash flooding is probable. ....West Texas... No significant changes were made, but the threat seems to have shifted south out of the Panhandle, so the Marginal was trimmed on the north side. Wegman ....Previous Discussion... ....New Mexico and Texas... Showers and thunderstorms are expected to fire across portions of New Mexico and Texas as the trailing cold front slows its progression over the region. Some of these storms will be capable of producing hourly rain rates greater than 0.50/1 inch per hour. A Marginal Risk area was maintained for this part of the country. ....Gulf Coast... Showers and thunderstorms will persist along the Gulf coast along the nearly stationary tropical low and frontal boundary. The Slight Risk area that was in effect has bee maintained as well as expanded eastward to just east of Vermilion Bay. A broad Marginal risk area extends northward and eastward as gulf moisture streams northward ahead of...and eventually intersects with...a cold front. Guidance shows the highest amounts occurring from Saturday into early Sunday over portions of the Upper Texas coast and nearby Louisiana where a moisture plume with precipitable water values in excess of 2.25 inches should be positioned at the start of the period (a nearly 2.5 standardized anomaly greater than climatology for this time of year and above the 90th climatological percentile along the gulf coastline shown by both the ECENS and GEFS). ....Lower Mississippi Valley to New England... Two inch precipitable water values are forecast to extend from eastern Texas northeastward in a corridor to far southern Indiana. During the ensuing 24 hours...the northern portion of the front reaches portions of New England and the Mid-Atlantic region but the southern portion of the front makes little progress into Texas and Arkansas. Moderate to heavy rain will be possible for areas along and ahead of the advecting boundary. Given the amount of moisture and weaker steering flow, though, the southern portion of the Marginal area has a better chance for excessive rainfall while storms to the north should be more progressive leading to a nominally low/more isolated excessive rainfall risk. The question remains whether or ot the stronger dynamics farther north will enhance amounts/rates. Some of the guidance do signal a concentration of QPF and increasing threat for excessive rainfall over portions of central pennsylvania. Antecedent conditions will have lowered FFGs across this part of the region thus becoming more favorable for growing flooding concerns. After coordination with the local forecast office decided to hold off for another cycle to reassess the need for a possible upgrade to a Slight Risk for central portions of Pennsylvania. Campbell/Bann Day 3 Valid 12Z Sun Sep 01 2024 - 12Z Mon Sep 02 2024 ....2030Z Update... No major changes were made as the highest threat for flash flooding Sunday will be isolated areas in west Texas, along the Gulf Coast, and into the central Appalachians and mid-Atlantic. The Marginal in the Mid-Atlantic was expanded north over much of West Virginia due to increasing signal for heavy rain and expectation of some heavy rain there on Saturday.=20 The rain will shift southward enough along the Gulf coast that much of the guidance suggests the heaviest rains will fall off the coast. For sure additional showers and storms will impact the Upper Texas and Louisiana Gulf Coasts, but should not be quite as heavy or widespread as in previous days. Conversely, signals have been increasing across much of Far West Texas on Sunday. A higher-end Marginal is considered in place in and around Midland, and there's potential a Slight may need to be considered with future updates. Given how dry the area has been a bit higher signal and better consistency in the guidance was needed before committing to such an upgrade.=20 Wegman ....Previous Discussion... ....New Mexico and Texas... Showers and thunderstorm activity is expected to linger with the capability of producing excessive rainfall. The threat for localized flood concerns will remain elevated thus a Marginal Risk was maintained for this period. ....Gulf Coast... The highest rainfall in this multi-day heavy rain event will have likely already occurred however showers and thunderstorms are expected to linger over the region near the coastline. A Marginal Risk remains in affect from the southeast Texas coastline to eastern Louisiana coastline. ....Eastern Tennessee Valley and Mid-Atlantic region... Moderate to locally heavy rainfall will persist across portions of the Mid-Atlantic as a cold front approaches the region. Some of these areas will have lowered FFGs by this period and may be more susceptible for additional rainfall to increase the threat for local flooding concerns. A Marginal Risk area was maintained from eastern Tennessee Valley to the Mid-Atlantic region. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7_WFoe5XWFEW7eCx8JVkAcWVTzZoD2byagIANnFEh0od= UXQYpSc5dUKolAykautQwHrKDQU_iInClOtIptJ6HY9XMN8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7_WFoe5XWFEW7eCx8JVkAcWVTzZoD2byagIANnFEh0od= UXQYpSc5dUKolAykautQwHrKDQU_iInClOtIptJ6eIdup44$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7_WFoe5XWFEW7eCx8JVkAcWVTzZoD2byagIANnFEh0od= UXQYpSc5dUKolAykautQwHrKDQU_iInClOtIptJ6-oIa45Q$=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) .