Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Aug 06 2024 00:25:38 FOUS30 KWBC 060025 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 825 PM EDT Mon Aug 5 2024 Day 1 Valid 01Z Tue Aug 06 2024 - 12Z Tue Aug 06 2024 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN SOUTH CAROLINA, EASTERN GEORGIA... ....WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH SIGNIFICANT AND POTENTIALLY CATASTROPHIC FLOODING LIKELY... 01z Update:=20 ....Tropical Cyclone Debby...Western/southwestern quadrants of=20 Debby are starting to experience downward/drying influences from=20 polar stream interaction/upper-level jet dynamics. While isolated=20 showers may increase near diurnal maxima with warm advection along=20 the northern and western sides through out the night into early=20 morning, overall coverage will diminish sufficiently to start=20 carving out some of the higher risk categories across portions of=20 northern Florida and southwestern/south-central Georgia.=20 A Slight Risk was maintained across central Peninsular Florida to=20 account for trailing confluent bands coming off the warm/moist=20 Gulf. Widely scattered, thunderstorms with some training potential=20 will remain through the remainder of the Day 1 period and may=20 aggravate ongoing flooded areas with a quick 1-3" locally.=20 Upper Midwest into Middle Great Lakes... The flash flood risk should increase this afternoon and evening across portions of southeast MN into southwest WI. Thunderstorms are starting to organize into a more surface based MCS and maintains a forecast to track southeastward across MN and WI. The=20 most intense of this activity should be south of the localized=20 training axis this morning, and should also be more progressive in=20 nature. Currently the shortwave driving thunderstorm activity is a a hour or so faster than the core of forecast guidance. LLJ will strengthen but maybe a bit more veered to the convergence axis to limit ideal training profile, but there should be elements through the overnight period for localized rainfall totals of 2-4" and possible flash flooding. As such, still consider the potential to be on the higher side of Slight Risk than any need to introduce a Moderate Risk category for this event.=20 Four Corner States... Marginal Risk appears to be holding well for the activity. There are increased areas of concentration across central Rockies of Colorado and along the AZ/Mexico border regions with enhanced convergence, moisture and slower cell motions; but still remains well within the Marginal Risk category.=20 Gallina/Chenard ....Previous Discussion Below... ....Southeast... Hurricane Debby will usher in a lot of moisture (+4 standard deviation/over 2 inches PW values) as it makes landfall this morning and tracks inland. The slowing of the forward speed of Debby has also increased the likelihood that the storm will be stronger at landfall and allowing more time for the storm to gather Gulf moisture off the sea surface temperatures in the upper 80s to near 90. Further, any wind damage from debris may clog drainage ditches and small creeks and streams, which too could increase the local flooding threat. Areal averages for this period range from 3 to 9 inches although a couple of models suggest local maxes upwards of 15 inches will be possible for this period, particularly near Georgia coastline and far southern South Carolina. A broad High Risk spans from north/northeast Florida, southeast Georgia and southern South Carolina. A Moderate Risk remains in effect from central Florida to central South Carolina. The rain fall amounts have the potential to be historic for South Carolina with several instances of significant flash flooding. ....Northern Plains 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. Training, if fast-moving storms are likely along a stalled front and fast-moving upper level shortwave, which will continue into Michigan and into Ohio. The primary area of focus remains around the Twin Cities to southern Wisconsin, where recent wet weather, the strongest storms happening at peak heating into the evening, and urbanization have all increased the threat for flooding into the higher-end Slight category. ....New England... Convection will focus along the same front that is draped over the Midwest; which will be stronger than typical for late summer. The area has been hard hit in recent days including presently, resulting in low FFGs. With soils near saturation the threat for excessive rainfall and localized flooding concerns will be elevated. However, a limiting factor may be that the storm motion is expected to be fairly progressive. ....Southwest... Monsoonal flow in this region will keep isolated to scattered convection possible for this period. The latest guidance continued to depict sparse storm coverage over parts of north-central Colorado. Campbell/Wegman Day 2 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... ....WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH SIGNIFICANT AND POTENTIALLY CATASTROPHIC FLOODING LIKELY... 20z Update: Only minimal changes to the EROs associated with Debby, as they generally looked in good shape. We were able to trim back some of the southwestern extent over GA and FL, but overall significant impacts are likely to continue across portions of southeast GA into eastern SC and southeast NC. There is a bit of lingering uncertainty with regards to the structure of Debby by this time. Portions of the High risk area could end up within a dry slot at some point Tuesday...but even if this happens the extreme antecedent rainfall and expectation of areas of heavy convective banding near and north of the center more than supports a continuation of the High risk. Also some uncertainty with regards to how far north and inland the convective bands get into NC, so some adjustments to the MDT risk area may be needed...but for now think the portion of southeast NC highlighted in the MDT is where the greater risk currently resides. The MDT risk looked in pretty good shape over portions of the Mid- Atlantic as well. An interesting setup here...with a stationary front draped across the region, and an approaching shortwave and upper jet streak from the west, and some connection to tropical moisture out ahead of Debby. Will likely have an MCS ongoing at 12z Tuesday moving east southeast across portions of MI and the OH Valley. This activity may end up surviving across the Mid- Atlantic, and we may see another convective cluster develop downstream of this MCS along/near the stationary front. The location and magnitude of the flash flood risk depends on a couple factors...does the MCS persist and merge in with frontal convection resulting in a training axis, and where exactly is the front by Tuesday afternoon. Overall this setup does have the look of a potential locally high impact flash flood event. There is enough forcing and instability, and a stationary front that could act as a focus for training/backbuilding cells. Confidence in there being at least some higher end impacts is probably higher than confidence on the location. At the moment the highest threat appears to be from far northeast MD into southeast PA and central NJ, but this is dependent on the eventual location of the front and also the propagation of the upstream MCS...so the MDT risk area could still shift some. The Slight risk over southern AZ was expanded a bit to the north. Still looks like a good setup for convection to develop along the terrain, and then move westward into some of the lower elevations resulting in an isolated to scattered flash flood threat. The well defined shortwave currently over northern MX will approach southern AZ by later Tuesday, likely helping aid in this more organized convective risk. Chenard ....Previous Discussion Below... ....Southeast... A very slow-moving Debby will cause a prolonged period of very heavy rain across this area; in addition to rains from Monday will likely result in widespread and numerous instance of flash flooding with scattered areas of catastrophic flooding likely. The areas most likely to be hit hardest by the rain will be highly dependent on the track of the storm's center, which is uncertain since modest changes in the steering currents result in just enough of a shift in the center's location at any one time to impact where the heaviest rain bands set up. Nonetheless expect a widespread 4 to 8 inches of rain with locally higher amounts just during this time period. There will likely be numerous instances of significant to catastrophic flooding for both rural and urban areas along with rising streams. Feeder bands into Debby will continue...albeit greatly diminished in both intensity and coverage along the west coast of Florida. Due to the likely hard hit nature of this area by Tuesday, even smaller amounts of rain are likely to result in outsized impacts (or at least keep ongoing flooding longer, especially in the likely event of 1-3 nearly stationary feeder bands of rain continuing into the region from the Gulf. The Marginal and Slight Risk areas were expanded further into central and southern Florida to account for this. ....Mid-Atlantic/southern portions of the Northeast... A Moderate Risk area was raised for extreme northeast Maryland, southeast Pennsylvania and central/southern New Jersey where there is a growing signal for a PRE (predecessor rain event) to unfold as Tropical Cyclone Debby impacts the Southeast/Mid- Atlantic region. Training convection will refire with daytime heating Tuesday afternoon, continuing into Tuesday night. From Baltimore northeast up the I-95 corridor to the Boston metro, training storms and urbanization may cause an outsized risk of flash flooding. While there is some spread in the guidance in where the front will stall, the training storms will be capable of dumping 2 to 4 inches, locally 5+ inches which would quickly surpass local FFGs and lead to scattered to possibly widespread instances of flooding. ....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. There is a notable increase with Gulf of California moisture advecting northward into southern Arizona which will further increase potential rainfall efficiency and amounts. A Slight Risk covering portions of southern Arizona was maintained for this period. ....Northern Rockies into Northern High Plains... A renewed round of thunderstorms are expected to be fast-moving, but there will be the potential for training and low 1-hour FFGs. Isolated instances of flash flooding may occur. Campbell/Wegman Day 3 Valid 12Z Wed Aug 07 2024 - 12Z Thu Aug 08 2024 ....THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH AND NORTH CAROLINA... ....WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH SIGNIFICANT AND POTENTIALLY CATASTROPHIC FLOODING LIKELY... 20z Update: Will maintain a High risk over eastern SC into far southeast NC. Debby will continue to move slowly near the Southeast coast, likely resulting in continued significant rainfall over the High risk area. Some uncertainty on the details by this time...but most models continue to support heavy convective bands impacting these areas. The 12z GFS remains a bit of an outlier by this time with a further southwest position of Debby...and given limited support for this solution from other model solutions...continue to lean against this outcome. There is at least some flash flood risk that will extend up into the eastern Mid-Atlantic Wednesday as well...with moisture ahead of Debby likely continuing to interact some with the front to the north and right entrance region of the upper jet. Could very well eventually need an expansion of the Slight risk into portions of the eastern Mid-Atlantic to account for this threat. At the moment think there is just enough model spread to keep the risk level at Marginal, but will continue to monitor and may eventually need to upgrade at least some of this area to a Slight risk. Chenard ....Previous Discussion Below... ....Southeast and Mid-Atlantic... Debby will continue its very slow track northward along the Southeast coastline while dumping additional heavy rain over a very water-logged region. By this point, the multi-day storm accumulation will likely be in the double digits with maxes in the 20 to 30 inch range near the Savannah metro and all along the Carolina Coastal Plain. With rainfall of the caliber catastrophic flooding would be likely/definite and compounded by coastal surge and waves. A High Risk is in effect for much of Carolina Coastal Plain with a Moderate spanning from southeast Georgia to southern North Carolina. ....Southwest... The increased moisture from the Day 2 period will make its way further north/east across the Four Corners region, helping maintain enhanced rainfall during the afternoon and evening. A Marginal Risk area covers much of the Four Corners region and extends into southeast Wyoming and western Nebraska. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7Bqw-eDyl1_8iD3nlOugA_Nu0BhMOmecEzqzJasu9sQk= AneRJgan225UrErho_-yqX8lMvP-2uNslD_mF-B84B3cgEg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7Bqw-eDyl1_8iD3nlOugA_Nu0BhMOmecEzqzJasu9sQk= AneRJgan225UrErho_-yqX8lMvP-2uNslD_mF-B8cygGZEQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7Bqw-eDyl1_8iD3nlOugA_Nu0BhMOmecEzqzJasu9sQk= AneRJgan225UrErho_-yqX8lMvP-2uNslD_mF-B894dqA_o$=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) .