Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Aug 04 2024 15:47:55 FOUS30 KWBC 041547 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1147 AM EDT Sun Aug 4 2024 Day 1 Valid 12Z Sun Aug 04 2024 - 12Z Mon Aug 05 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE FLORIDA BIG BEND THROUGH THE TAMPA BAY METRO... ....16Z Update... ....Southeast/ T.S. Debby... No major changes were made as the short-term forecast for T.S. Debby remains on track and is intensifying as expected over the eastern Gulf of Mexico. With better confidence on the western extent of the sustained rainfall associated with the easternmost bands of T.S. Debby, the Moderate Risk was expanded south and east a few miles to cover all of the rest of the Tampa Bay metro, and extends down the coast about to Venice, FL. Training convection in the easternmost bands have a history of producing rainfall rates as high as 1.5 inches per hour. As the center continues northward these heavier rainfall rates will occasionally impact the Tampa Metro through the afternoon into the evening. Overnight tonight, one of the outer bands is highlighted in the guidance, particularly the 12Z HRRR to persist near or over Tampa through much of the night. This too would greatly increase the potential for flash flooding, especially after nearly steady rain expected through much of the day.=20 The surrounding Slight was nudged eastward as well for expected band development with embedded convection capable of 2 inch per hour rates training over much of the eastern half of the state. Unlike further east, these bands are likely to continue northward with significant breaks in the rainfall rates in between, so the flooding threat today is significantly lower than further west. Further north, the front in the process of capturing T.S. Debby further south is expected to become the focal point for slow moving convection as southerly flow ahead of Debby brings deep tropical moisture north and collide with the stalled out front. This appears most likely to occur over the Low Country of southern SC. For today, this rainfall is not directly associated with T.S. Debby...though Debby's influence bringing deep tropical moisture northward to the east of the circulation center will contribute.=20 ....Mid-Atlantic and Northeast... Other than the expansion of the Marginal into southern VT, NH, and ME, no significant changes were made. The same front influencing Debby over the Southeast will push east with greater speed in the Northeast due to much greater jet support. Nonetheless...the plume of tropical moisture is raising atmospheric moisture and PWATs to up to 2 sigma above normal. This moisture will support upscale growth of the current convection over NY and New England to be much more widespread this afternoon with peak heating. Slow moving storms may cause isolated flash flooding...but it appears the coverage may not be quite high enough to support an upgrade to a Slight. However, the area is close to a Slight upgrade and will be monitored into the afternoon. ....Southwest and Northern Plains... No significant changes made as the forecasted rainfall from showers and storms is largely unchanged. Wegman ....Previous Discussion... ....New England... ....Florida through the Southeast Coastal Plain.... The latest NHC forecast has tropical cyclone Debby strengthening to a hurricane as it approaches the Big Bend of Florida. The latest model guidance and hi-res show areal averages of 4+ inches as Debby nears the coastline. Widespread moderate to very heavy rain will overspread much of Florida, reaching into southern Georgia and South Carolina. Many of the models keep the heaviest rain just offshore (closer to the center). Forecast guidance has had a notable trend slowing the forward speed of Debby, which in turn notably increases the forecasted rainfall along and east of the track. The increased rainfall is for 2 reasons during this period: 1. A slower forward speed means extra time the storm has over the very warm (near 90F) waters of the northeastern Gulf, resulting in additional time the storm has to intensify before landfall, given very favorable meteorological conditions for intensification. 2. The slower forward speed increases the time the storm has to drop prodigious amounts of rain in the area. Despite much of this area being swampy with high flash flood guidance (FFG), the increased strength and longer duration of time for heavy rain prior to landfall should result in more widespread instances of flash flooding...hence the upgrade to a Moderate. Further, wind and storm surge (storm surge just east of the center), should also contribute to more and widespread flooding impacts at the coast, with the wind moving inland resulting in more debris, both natural and artificial, potentially clogging drainage ditches. Thus, even if there weren't also an increase in expected rainfall, the higher wind alone may increase flooding impacts locally, especially where the eyewall comes ashore. Meanwhile, some of the guidance continues to develop a Predecessor Rain Event (PRE) across the Southeast coast thanks to the timing of the system's approach with interaction of the coastline. This something to monitor in future updates as these phenomena are tough to gauge at range, but can be focused significant rain events where they occur. Regardless if a PRE forms or not, the highly anomalous tropical moisture flux and persistent heavy rain banding around the storm circulation will cause locally enhanced flash flood concerns across much of Florida into the Georgia/South Carolina coast. ....Northeast... A Marginal Risk area was maintained for portions of northern Virginia to Massachusetts. A round of convection is expected to fire up along the I-95 corridor from Washington DC to Boston with higher confidence from New York City to Boston. The latest guidance continues to suggest rather fast moving segments, which should keep any flooding to a minimum and in urban and flood- sensitive areas. ....Montana into the Northern Plains and Upper Mississippi Valley... An axis of anomalous TPW will be absorbed in the upper level jet streak and mid- level vort lobe that will cross Montana and the Dakotas. Dynamical forcing along with the thermodynamical profile will favor more widespread showers and thunderstorms, though the shear profiles will certainly favor more progressive/forward- propagating cell motions and thus make for more of an isolated flash flood threat. Areas with FFG equal/less than 1 inch within a 3hour hour period are scattered across the region and some of the storms expected may easily reach or exceed those thresholds. A Marginal Risk was maintained from Montana to southern Wisconsin for this period. ....Southwest... Scattered showers and thunderstorms are expected to develop as monsoonal moisture advects on the underside and backside of the mid-upper level ridge during the afternoon into evening peak heating hours. MLCAPEs of 500-1000 J/Kg will favor pockets of 1"/hr rainfall rates, which will likely lead to localized areas of flash flooding, especially over burn scars and arroyos. Campbell/Hurley Day 2 Valid 12Z Mon Aug 05 2024 - 12Z Tue Aug 06 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST... ....Southeast... Per NHC forecast Debby will have weakened from a hurricane to a tropical storm as it treks from northern Florida into southeast Georgia. Due to the expected slowing of the forward speed of Debby, the forecast rainfall totals have increased. Areal averages for this period range from 2 to 8 inches although a couple of models suggest local maxes of 8 to 12 inches will be possible for this period, particularly near Georgia coastline and far southern South Carolina. A broad Moderate Risk remains in effect from northeast Florida, southern Georgia and southern South Carolina. Expect further refinements in the outlook area over the next several forecast cycles, as there are still some location and timing differences with the models. With Debby's forward speed expected to slow to 6-8 kts over Southeast Georgia, that would put Debby in the 20th percentile of storm forward speed per climatology. The rain fall amounts have the potential to be historic for South Carolina with several instances of significant flash flooding. ....Upper Mississippi Valley to Upper Midwest... A low over the Central Plains, high pressure to the north over Canada, a passing shortwave, and somewhat higher than normal moisture from Minnesota to Michigan will cause training thunderstorms across this region, tracking East-Southeast along a stalled front. Models show localized totals between 3-4", which will likely fall within a 3-6 hour period per the 3-hourly QPF output. Much of this region remains wet after a very wet Spring and first half of summer. A Slight Risk area spans from western Minnesota to central Wisconsin with an encompassing Marginal Risk area from central North Dakota to Wisconsin/northern Ohio. ....New England... The same front draped over the Midwest will focus convection across northern New England, though faster flow should mean the storms move along at a very fast pace, reducing the threat for flash flooding. ....Southwest... The typical monsoon flow will continue in this region. The latest guidance had sparse storm coverage over parts of north-central Colorado and far southeast Wyoming therefore, the northern bounds of the Marginal Risk area was trimmed southward. Campbell/Hurley Day 3 Valid 12Z Tue Aug 06 2024 - 12Z Wed Aug 07 2024 ....THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF GEORGIA AND SOUTH CAROLINA... ....Southeast... Per NHC forecast Debby will continue its very slow trek offshore Georgia/South Carolina Coast. Widespread heavy rain will persist near and north of the central low pressure soaking the Southeast and Mid-Atlantic regions. Multi-day totals will be approaching 20 inches along the coast with the heaviest focused over parts of Georgia, South Carolina and extreme southeast North Carolina. In coordination with the local forecast offices a High Risk was raised spanning from the Savannah metro area to Myrtle Beach vicinity and points west. There will likely be numerous instances of flash flooding/ponding for both rural and urban areas in addition to rising streams in response to multiple days of heavy rain. ....Northeast... Deep moisture streaming north ahead of Tropical Cyclone Debby will be advecting into the Northeast where a front will be slowing/stalled in a west-east orientation. This boost in available moisture should promote increased rainfall efficiency and amounts across the Upper Midwest/Great Lakes/Northeast region. The latest guidance supports this and suggests an area of higher QPF setting up from central Pennsylvania, New Jersey to Connecticut. A Slight Risk area was raised to cover most of eastern Pennsylvania, New Jersey, small portions of Maryland and Delaware, southern New York and Connecticut where there will be an elevated threat for flooding concerns, especially at night. ....Southwest... Monsoonal moisture and diurnal heating will maintain showers and thunderstorms across the Southwest during this period. A Marginal Risk area covers much of the Four Corners region. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4yepCoMs7hj5WskPiz13Ec3Os-OX9xzYQvlQIz1qlfB5= YIvLfS2L1OQ8HsoRH8jT6vzapiaBFN52_rFIsTNWkNxPLHo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4yepCoMs7hj5WskPiz13Ec3Os-OX9xzYQvlQIz1qlfB5= YIvLfS2L1OQ8HsoRH8jT6vzapiaBFN52_rFIsTNW3xXcQMw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4yepCoMs7hj5WskPiz13Ec3Os-OX9xzYQvlQIz1qlfB5= YIvLfS2L1OQ8HsoRH8jT6vzapiaBFN52_rFIsTNWEBgUv_E$=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) .