Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jul 29 2023 20:02:03 FOUS30 KWBC 292002 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 401 PM EDT Sat Jul 29 2023 Day 1 Valid 16Z Sat Jul 29 2023 - 12Z Sun Jul 30 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHEAST AND THE CENTRAL APPALACHIANS/OHIO VALLEY... ....16Z Update... Only minor areal adjustments were made to the ongoing risk areas based on updated 12Z hi-res model guidance. The MRGL Risk for South Carolina has been extended southwestward into southeastern Georgia given trends in the 12Z guidance that support more expansive thunderstorm chances. Given notably high pw values running 2+ standard deviations above the mean, some locally very heavy downpours are possible that could lead to some minor flooding issues in urban areas. The Slight Risk for the Northeast/New England was also adjusted further to the west in New York given the trend for locally heavy rainfall with ongoing storms in western New York expected to continue eastward through the day with increasing instability ahead of the cold front. Otherwise, the current forecast for the ongoing Slight Risk looks strong given the 12Z guidance continues to indicate significant probabilities of rainfall amounts of several inches, especially for terrain enhanced areas along and to the southeast of the White Mountains. There is less confidence this morning in storm development later today over the Central Appalachians, particularly with northeastward extent into West Virginia. However, these areas are notably more sensitive to any additional rainfall, so the Slight Risk has been maintained assuming any storm development would carry a locally higher risk for flash flooding. Putnam ....Previous Discussion... ....Northeast and Mid-Atlantic... A well-defined shortwave associated with ongoing convection over the Great Lakes region is forecast to move east of the lower Lakes, with storms reintensifying as the wave interacts with deepening moisture and increasing instability supported by daytime heating and amplifying southwesterly flow. Recent runs of the RAP show PWs increasing to at or above 2 inches within an area of strong ascent afforded in part by favorable upper forcing. Ahead of the system, a downstream wave now moving across Pennsylvania may bring some scattered showers and storms to parts of southern New England later this morning. This will likely be followed by more organized strong storms, with intense rainfall rates likely, and training possible from eastern New York and Pennsylvania to southern and central New England during the afternoon into the evening hours. The Slight Risk area reflects where the HREF guidance centers its heaviest amounts from the Catskills eastward into southern and central New England, with significant probabilities for amounts exceeding 3 inches.=20 Some locally heavy amounts are possible farther south as well as storms are expected to develop along the trailing cold front as it presses south through Mid-Atlantic this afternoon and evening. ....Central Appalachians to the central Plains... A Slight Risk area was maintained across portions of the central Appalachians and extended west into portions of the Ohio Valley.=20 Storms developing along the previously noted cold front may create at least isolated flash flood concerns, especially across areas in West Virginia and eastern to central Kentucky that have recently been impacted by heavier rains and where FFGs are relatively low.=20 Some of the CAMs show a fairly good signal for slow-moving/training storms, especially from central Kentucky into the Appalachians, increasing the threat for heavy amounts and flash flooding concerns.=20 Meanwhile, convection developing over the central Plains may intensify and spread southeast across the mid Mississippi and into the lower Ohio Valley later today. While these storms are expected to be generally progressive, some locally heavy amounts producing isolated flash flooding cannot be ruled out.=20=20 ....South Carolina... An area of low pressure will continue drifting slowly northward, producing additional showers and thunderstorms along the South Carolina Coast. Ample moisture and instability will continue to support a localized flash flood threat across this region before the wave begins to turn to the east early Sunday ahead of the amplifying trough to the north. ....Eastern and south-central Colorado and the Sangre de Cristo Mountains of New Mexico... Easterly low level inflow is expected to contribute to an increase in moisture and the development of showers and thunderstorms over the mountains. Weak flow aloft is expected to increase the potential for slow-moving storms, which may result in locally heavy amounts with isolated flash flooding concerns. ....Southeastern Arizona/Southwestern New Mexico... Deepening moisture ahead of an inverted upper trough tracking west across northern Mexico is forecast to help support increasing coverage of showers and thunderstorms across the region. Models shows PWs returning to 1-1.5 standard deviations above normal across portions of the region, raising the threat for showers and thunderstorms with locally heavy amounts and isolated runoff concerns. Pereira Day 2 Valid 12Z Sun Jul 30 2023 - 12Z Mon Jul 31 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST, THE NORTHERN PLAINS, THE SOUTH CAROLINA LOWCOUNTRY, AND THE FLORIDA PENINSULA... ....2030Z Update... A couple additional Marginal Risk areas were included for this update based on incoming hi-res model guidance. In Florida, weak upper-level flow will allow for transient sea breeze-related storms. Relatively higher surface moisture over the Peninsula, with pw values 1-1.5 standard deviations above the mean, and MU CAPE values ranging between 2000-3000 J/KG will support the risk for some very heavy downpours. This may cause a few localized flash flooding issues for more urban areas. Further north, additional storms are expected to develop ahead of a southward moving cold front/surface wave over the South Carolina Lowcountry. Embedded shortwave energy rounding the base of an upper-level trough centered over the East Coast will also likely help encourage storm development. Similar to today, pw values remaining 1.5-2 standard deviations above the mean ahead of the front will help promote some locally heavy downpours, which may also lead to the risk of some minor flooding, particularly for urban areas.=20=20 The current Marginal Risk area over the Northern Plains was extended further northwest into eastern Montana with hi-res guidance indicating the potential for additional storm development in moist, southeasterly post-frontal upslope flow. The chance for a locally higher risk over portions of northeastern Wyoming and southwestern South Dakota is apparent with notable incoming 12Z HREF probabilities for 2"+ rain totals supported by rain rates upwards of 1-1.5" per hour. However, this outcome looks to be tied to upscale convective growth that remains rather progressive in upper-level northwesterly flow which may ultimately limit the duration and coverage of heavier rain totals, keeping the risk more isolated. Any further southeasterly adjustment of storm coverage would also keep heavier rain totals over the previously noted less flood prone Sandhills in Nebraska. Putnam=20 ....Previous Discussion... ....Southwest... An inverted upper trough will continue to lift northeast from northern Mexico into the southwestern U.S., pushing the subtropical ridge farther to the east and allowing deeper moisture to advect across Arizona through the Four Corners region and into the central Rockies. The general consensus of the guidance continues to show PWs increasing to over 1.5 standard deviations above normal across southern California and along the southern Colorado Basin into the Four Corners region on Sunday. This moisture along with decreasing heights aloft is likely to support more widespread afternoon and evening thunderstorms and an increased threat for locally heavy amounts and isolated flash flooding. ....Northern Plains... Mid-level shortwaves moving off of the top of the ridge may pose a localized heavy rainfall threat as they interact with a plume of deeper moisture fed by southeasterly low level flow. These storms could create isolated flash flooding concerns across the northern High Plains into central South Dakota before moving into the less flood prone Sandhills of central Nebraska. Guidance continues to differ significantly on the magnitude and location of any heavy amounts that may develop across this region. Pereira Day 3 Valid 12Z Mon Jul 31 2023 - 12Z Tue Aug 01 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE SOUTHWEST TO THE NORTHERN HIGH PLAINS, THE MID MISSOURI TO THE LOWER OHIO VALLEY, AND OVER PORTIONS OF NORTHEAST FLORIDA... ....2030Z Update... The prior forecast remains on track, no modifications were needed for this update. Putnam ....Previous Discussion... ....Southwest to the northern High Plains... A favorable monsoon pattern will continue to support storms across portions of the Southwest, especially Arizona and interior southern California, while expanding farther north and west across the Great Basin and the central Rockies. Deepening moisture and energy moving along the western periphery of an eastward-shifting upper high is likely to support the increasing coverage of showers and thunderstorms across southern Nevada, Utah, and western Colorado. Energy moving over the top of the ridge may interact with low level easterly flow over the central Plains, supporting storms and potentially locally heavy amounts spreading east of the mountains into eastern Wyoming and Black Hills region. Storms that develop across the region will have the potential to produce intense rainfall rates and raise flash flooding concerns, especially in urbanized and areas of complex terrain. ....Mid Missouri to the lower Ohio Valley... Mid-level energy embedded within northwesterly flow interacting with a deep moisture pool associated with a low level front may produce rounds of convection with locally heavy amounts. While the signal for potentially heavy amounts remain, models continue to disagree on their magnitude and location. ....Northeast Florida... Deep moisture (PWs around 2 inches) associated with a stalled frontal boundary is likely to enhance shower and thunderstorm coverage and the potential for heavy rainfall amounts across the area. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9XSxJCpSsjzbuGgiQIud4MQo5KUPoiN77MHUJlq6HWgD= _KPb7JwuRTQE3OM2MdM7AcZFX1oh4887xZlH5LoGQRkx5XI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9XSxJCpSsjzbuGgiQIud4MQo5KUPoiN77MHUJlq6HWgD= _KPb7JwuRTQE3OM2MdM7AcZFX1oh4887xZlH5LoGcoouFOc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9XSxJCpSsjzbuGgiQIud4MQo5KUPoiN77MHUJlq6HWgD= _KPb7JwuRTQE3OM2MdM7AcZFX1oh4887xZlH5LoG0kn8h-g$=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) .