Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
116 FOUS30 KWBC 242010 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 410 PM EDT Tue Sep 24 2024 Day 1 Valid 16Z Tue Sep 24 2024 - 12Z Wed Sep 25 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE VIRGINIA/NORTH CAROLINA PIEDMONT NEAR THE STATE LINE... ...16Z Update... ...VA/NC border... In coordination with RNK/Blacksburg, VA; RAH/Raleigh, NC; and AKQ/Wakefield, VA forecast offices, a Slight Risk area was introduced with this update along the VA/NC border in the Piedmont area. Tonight, much of the guidance, including the 12Z HRRR, HREF, and both ARWs are suggesting an area of training convection will develop. The heavy rain will be forced by a stationary surface trough over the area combined with a local maximum of atmospheric moisture with PWATs over 1.75 inches. A strong and deep upper level trough over the Mississippi Valley will greatly increase the upper level divergence to its east. This too will greatly add lifting support to the storms as they form with the convective maximum after sunset this evening. HREF probabilities are up to 30% of exceeding FFGs in the western part of the Slight risk area, as well as over 80% of exceeding 3 inches of rain in the neighborhood probabilities and almost 50% of exceeding 5 inches through tonight. The factors working against heavy rain and flash flooding are some antecedent dry conditions, with at least average soil moisture across this area, and marginal instability generally around 500 to 1,000 J/kg. The instability will limit to some extent the widespread coverage of the heavy rain, but the excellent forcing may be able to make up for that. Portions of the Slight Risk area saw heavy rainfall last week, which at least has kept those area from having totally dry soils...but average soil moisture can sometimes work against flash flooding as some clays can be hydrophobic when they`re really dry, resulting in extra runoff. Such is not expected to be the case here. ...Midwest... Elsewhere across the Upper Ohio and Tennessee Valleys expect continued off and on showers and storms, but general struggles with organization. Very dry soils north of the Ohio River should also generally limit the flooding threat...whereas along the west facing slopes of the Appalachians, localized upslope may enhance it a bit. Regardless any flash flooding in these areas should be confined to urban and flood-prone locations. Wegman ...Previous Discussion... One area of showers and thunderstorms that produced spotty moderate to heavy rainfall overnight along a quasi-stationary front overnight should be weakening as it reaches portions of the Mid- Atlantic region by morning. However...shortwave energy in the northern stream will be developing a closed low that deepens with time as it tracks from the Upper Midwest into the Mid-Mississippi Valley later tonight. This backs the flow aloft and supports some convection with locally heavy rainfall rates in a diffluent flow regime over the eastern Ohio Valley as well as thunderstorms ahead of the low track and ahead of a trailing cold front. Model guidance continues to show the potential for locally heavy amounts while not presenting a signal for widespread heavy rainfall totals...including portions of the central and southern Appalachians where relative soil moisture is slightly higher and where flash flood guidance was lower. Bann Day 2 Valid 12Z Wed Sep 25 2024 - 12Z Thu Sep 26 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN GEORGIA THROUGH THE SOUTHERN APPALACHIANS... ...2030Z Update... ...Southern Appalachians... In coordination with FFC/Peachtree City; GSP/Greer, SC; and MRX/Morristown, TN forecast offices, a Moderate Risk upgrade was introduced with this update. There was unanimous agreement in the 12Z guidance of an eastward shift in the axis of heaviest rainfall associated with training storms ahead of an advancing cold front across Tennessee, Alabama, and Georgia. This will be the making of a predecessor rain event, or PRE. The PRE will take advantage of increasing deep tropical moisture streaming north ahead of what is forecast to be Hurricane Helene, which will be moving northward into the eastern Gulf. The tropical moisture running into the front from the east will oppose the westward movement of the front, and frankly the upper level trough associated therewith as well. The expected stalling of the front will allow the storms that form along it to train northward. This pattern alone is favorable for flash flooding, but once the storms reach northern Georgia, western Carolinas, and far eastern Tennessee, then they will have the terrain of the southern Appalachians to contend with. The mountains will help uplift the moisture and storms, resulting in localized enhancement of the rain. Lower FFGs in the terrain will mean flash flooding will be more common there. Further south, the axis of the PRE`s storms will likely be in or very near Atlanta. Urban effects will also increase the flash flooding potential, as well as the impacts deriving therefrom. Thus, the Moderate Risk includes all of the Atlanta Metro as well. While the PRE, as the name implies, is not expected to be nearly as impactful for this region as the rainfall from Helene expected Thursday, the potential for multiple inches of rain so soon before the arrival of rain associated with a major hurricane will greatly exacerbate the impacts from flooding from Helene`s rains. The aforementioned eastward shift in the guidance has significantly worsened the potential threat from Helene`s rains, as the areas now expected to see the worst of the rains from the PRE will also be within the core rain plume from Helene. ...Florida... The Slight Risk in the Florida Big Bend Region was left alone as this area will be in the crosshairs for some of Helene`s worst rains. The rains will be fairly light when they become steady through the evening, but pick up in intensity through the early morning hours. Enough rain may fall there before 12Z for localized flooding impacts, though by far the worst impacts will hold into the Day 3/Thursday time period. A Slight Risk area was introduced for the urban centers (Tampa through Ft. Myers) of Florida`s West Coast with this update. Training convection associated with multiple back-to-back bands of rain on the outer fringes of Helene`s circulation may cause local flash flooding, especially in the urban areas. Given antecedent wet conditions and the potential for frictional effects to result in band development along the coast, the Slight Risk was added with the somewhat higher confidence of flash flooding there. Wegman ...Previous Discussion... Only minor adjustments were needed to the previously issued Slight risk area from portions of Alabama and Georgia into southeast Tennessee...while a targeted Slight Risk area was introduced along the coastline of the Florida panhandle. It appears that a predecessor rainfall event will take shape somewhere in the Southeast US as moisture streams northward from the Gulf of Mexico ahead of Potential Tropical Cyclone Nine and interacts with a cold front dropping southward from the Tennessee Valley. There is still some disagreement among the various models with respect to where the axis may set up but the area covered by the Slight risk has the best overlap of different solutions so only minor adjustments were needed to what is effectively a high-end Slight Risk area. Along the coast...model QPF has increased along portions of the Florida coastline to the point where a Slight seems warranted. Bann Day 3 Valid 12Z Thu Sep 26 2024 - 12Z Fri Sep 27 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHEAST... ...2030Z Update... In coordination with all of the impacted offices, the Moderate Risk was expanded north to include much of the east facing slopes of the Southern Appalachians with this update. What is forecast to be Major Hurricane Helene is forecast to make landfall in the Big Bend Region of the Florida Panhandle Thursday evening. Around and well ahead of the arrival of the center of circulation, bands of locally very heavy rain will impact all of the Florida Panhandle and portions of the west coast of the Florida Peninsula. PWATs with Helene are expected to be nearly off the charts, as the much larger than normal hurricane draws nearly unlimited moisture from the much warmer than normal eastern Gulf and efficiently converts it to heavy rainfall. Areas of the Florida Peninsula east of the track will contend with storm surge along the coast, which will impact drainage from the heavy rain...resulting in excessive rainfall flooding due to poor drainage. Thus, the Slight continues along the Florida Peninsula, with the Moderate closer to the track. Fortunately, Helene is likely to be moving at its fastest forward speed when it makes landfall in the Big Bend region. This should somewhat reduce the impact potential of the heavy rain. Thus, for now, a High Risk along the Gulf Coast is not yet anticipated, but will certainly continue to be considered with new and changing guidance. Further north, the intensifying southeast flow ahead of Helene`s center will advect increasingly deep tropical moisture into Georgia and the Carolinas. The previous day`s PRE will give way to the primary rainfall shield with Helene, only resulting in increasingly heavy rainfall into north Georgia and the Carolinas, especially overnight Thursday night. The Moderate Risk was expanded north to account for the PRE, Helene`s rainfall, and the much more dangerous nature of the impacts from nighttime flash flooding. Mudslides and landslides will become increasingly common in the southern Appalachians as rainfall amounts approaching 12 inches are expected. Despite recent dry conditions in this area, PWATs exceeding 3 inches in some areas will support storms easily capable of overwhelming the soils resulting in a very healthy percentage of the rainfall converting to runoff. The area from metro Atlanta, much of north Georgia, the western tip of South Carolina, and much of the mountains of western North Carolina are considered in a higher-end Moderate Risk, with increasing potential of eventually needing a High Risk upgrade for this area. This is for a few reasons: 1) The PRE shifting east (as mentioned in the D2 discussion) will prime many of the same southeast facing slopes much more efficiently than in previous forecasts. 2) Increasing rainfall totals with PWATs potentially getting as high as 3 inches in some areas means that much more efficient warm rain processes. 3) Southeast flow perpendicular to the southwest to northeast orientation of the southern Appalachians will maximize the upslope component of the flow, resulting in more rain. 4) Terrain issues, especially after multiple inches of rain Wednesday could mean multiple mudslides and landslides which could cut off whole communities from the road network. For Atlanta, any ERO risk upgrades will be dependent on significant rain from the PRE on Day 2/Wednesday in the city, as otherwise on Day 3 the rainfall in Atlanta will likely be similar to surrounding areas. The surrounding Slight Risks have generally been expanded in all directions: In the Florida Peninsula, convergence along the East Coast with the expansive wind field may result in heavy rain in urban areas from Orlando to Jacksonville. Given the eastward shift in the guidance and that much of Helene`s moisture will shift northeast well after landfall, the Slight has been expanded to include all of South Carolina and central North Carolina. Finally, the Slight has also been expanded west to cover nearly all of Tennessee. Much of the westward shift in the guidance will be with some of Helene`s moisture as it dissipates being absorbed into a cutoff low over the Mississippi Valley, resulting in a conveyor belt of moisture and heavy rain over much of Tennessee. That heavy rain will continue westward with a jet eventually moving into Missouri and Arkansas by Day 4/Friday. Flash and urban flooding across the major Tennessee cities from Chattanooga, Nashville, Knoxville and maybe as far west as Memphis will be possible. Wegman ...Previous Discussion... There should be an increasing threat of widespread and potentially significant rainfall as Potential Tropical Cyclone Nine approaches the coast and eventually makes landfall sometime around 27/00Z based on the latest guidance from the National Hurricane Center. Moderate to heavy rainfall may develop across portions of Florida panhandle into the Southeast US well before PTC Nine make landfall as strong transport of Gulf moisture interacts with deepening mid- and upper-layer closed low over the central/southern Mississippi Valley. Primary changes were to extend the Moderate risk area northward along the NHC path of PTC Nine where the guidance had shown an increase in rainfall amounts...with the Moderate risk now into the southern Appalachians where terrain effects look to result in locally enhanced rainfall amounts. With an unusually deep low best seen in the mid- and upper-levels located to the west...at least some of the moisture being drawn inland by Nine will begin to get drawn westward over portions of the Tennessee Valley and into the Mississippi Valley. As a result...part of the Slight risk area from the Southeast US gets pulled westward into the Tennessee Valley around the mid- and upper-level low. Given the model spread and the poor run to run consistency...refrained from taking the Slight risk area too far westward at this point. Bann Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt