Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 15 2025 00:53:53 FOUS30 KWBC 150053 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 853 PM EDT Thu Aug 14 2025 Day 1 Valid 01Z Fri Aug 15 2025 - 12Z Fri Aug 15 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEASTERN VA AND EASTERN NC... 01Z Update... Reduced the footprint of the previous Slight Risk to portions of southeastern VA and eastern NC. In the near-term, convection=20 firing along a slow-moving surface boundary is expected to produce an isolated to scattered threat for heavy rain and flash flooding=20 along the southeastern VA/northeastern NC border before waning=20 later this evening. See WPC MPD #935 for additional information=20 regarding the near-term heavy rainfall and flash flooding threat=20 across this area. Farther to the south, consensus of the hi-res guidance shows a low developing along the southeastern NC coast and becoming the=20 focus for heavy rain as it moves northeast along the Outer Banks=20 overnight. The 18Z HREF shows high neighborhood probabilities (up=20 to 70 percent) for amounts greater than 3 inches over parts of the=20 Outer Banks tonight. Elsewhere, made smaller adjustments to the Marginal Risk areas over the Southwest and the eastern Dakotas/Minnesota. And with the one remaining cluster of storms now moving progressively south over southeastern MA, the Marginal Risk was removed from southeastern New England. Pereira Day 2 Valid 12Z=20 Fri Aug 15 2025 - 12Z Sat Aug 16 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MISSISSIPPI VALLEY, THE SOUTHERN APPALACHIANS, DEEP SOUTH TEXAS, AND FAR SOUTHERN ARIZONA... ....20Z Update... ....Upper Mississippi Valley... Guidance has shifted a bit south and east with the band of heaviest rainfall expected across portions of Minnesota and Wisconsin on Friday. There is reasonable agreement that an MCS will form across Minnesota and Wisconsin, then push south, following the instability, towards Iowa and Illinois. It is really in the formation stage of the MCS during the evening hours when the flooding threat will be maximized, while after that, the MCS should be a fast mover towards the south overnight. There has been some time for the soils to dry in this area, though some rain expected today, D1, could saturate soils in some areas ahead of this new breakout of convection. Overall this area is on the lower end side of the Slight Risk category, but any storms that get hung up on either side of the MCS could make for locally higher impacts, albeit isolated. ....Southern Appalachians... In coordination with GSP/Greer, SC forecast office, a Slight Risk area was added to portions of the Western Carolinas with this update. HREF probabilities for FFG exceedance have increased to 30% across this region, and guidance is in better agreement that clusters of storms will form in this area, and the storms will likely move slowly, chaotically, and be highly influenced by each others cold pools, resulting in cell mergers. On the whole, the storms will generally move south and east with time, so portions of upstate South Carolina may get into the flooding threat later than western North Carolina on Friday. A significant portion of the certainty with the Slight Risk for this area consists of the soils being saturated from recent, and in some cases, ongoing heavy rainfall, which will have local streams, creeks, and rivers already above normal, such that additional rainfall will result in widely scattered instances of flash flooding. PWATs ahead of a southward moving cold front will be between 1.5 and 1.8 inches, allowing for sufficient moisture to be present to support storms capable of locally heavy rainfall, especially when adding any upslope component and forcing tied to the mountains. ....Deep South Texas... In coordination with BRO/Brownsville, TX forecast office, a Slight Risk area was introduced with this update. A healthy looking Invest 98L in the western Gulf is being very poorly handled in the guidance thus far. However, there is good agreement that it will remain on a northwestward trajectory, making landfall somewhere near Brownsville on Friday. Any further intensification of this system will increase the rainfall threat across Deep South Texas. The greatest vulnerability for heavy rainfall will be in the urban areas such as Brownsville and McAllen. There remains much higher than normal uncertainty with the expected rainfall footprint of Invest 98L. Since the guidance has poor handling on this system, most of the guidance is dissipating the rainfall very soon upon landfall, which pattern recognition would suggest does not often happens with a tropical system on the move. Thus, in contrast to much of the guidance, the Slight Risk was hoisted acknowledging the potential for organized convection to impact somewhere along the Texas Gulf Coast, most likely on Friday morning and in Deep South Texas. Outflow and sea breezes may make for slightly elevated chances of heavy rain and resultant flash flooding along all of the Texas Gulf Coast and into Louisiana, where the Marginal was expanded, but remains in place. Due to this high uncertainty, large changes to the ERO risk areas are likely with subsequent updates as the guidance gets a better handle on Invest 98L. Should 98L intensify, additional upgrades may be needed. Please see the National Hurricane Center website at hurricanes.gov for more details about Invest 98L. ....Southern Arizona... Monsoonal moisture in place across Arizona will combine with an upper level vort max to result in greater than normal organization of expected convection across far southern Arizona on Friday afternoon. Given the higher than normal PWATs and potential for organization; in coordination with TWC/Tucson, AZ forecast office, a Slight Risk area was introduced with this update, mainly along the International Border. Convection is likely to break out well into Arizona, mostly along the Mogollon Rim, but guidance has been less than reliable about the coverage of storms, particularly in southern Arizona. Thus, despite some of the guidance being much better, and suggesting a higher end threat, the Slight really focuses where the vort max will be the primary forcing mechanism for convection. ....Pacific Northwest... After extensive coordination with SEW/Seattle, WA forecast office, a Marginal Risk was introduced for all of western Washington with this update. An extremely anomalous moisture plume associated with a robust atmospheric river is forecast to move into western Washington on Friday. PWATs will be as high as 6 sigma above climatological normals for this time of year, which if verified, will set both monthly records for August and threaten all time records, as they near 1.75 inches. This incredible abundance of moisture will translate to a 24-36 hour period of highly efficient rainfall, streaming primarily into the west facing slopes of the Olympics and Cascades Mountains. By far the heaviest rainfall totals will be seen in those areas. For the more populated interior valleys, including the Seattle Metro, the usual downslope shadowing will occur. However, the combination of urbanization and the unusually high amount of moisture is likely to overcome the downsloping, resulting in far lower, but still significant amounts of rainfall into those areas. Contrasting the anomalous atmospheric moisture and rainfall, nearly all of western Washington is in some amount of drought, ranging from Minor to Extreme, as the area is in the midst of its dry season. That dry season will abruptly end tonight through into Saturday morning, as rivers approach record territory with all the rainfall. The drought conditions should certainly dampen any flooding impacts quite a bit, pun intended. Normally with this unusual an amount of atmospheric moisture, the ERO would start at Slight or Moderate and only go up from there, but given a good amount of this rainfall will be beneficial and welcome for filling local reservoirs, the Marginal was the decided happy medium to account for localized impacts from flooding small streams and creeks, as well as urban impacts, but flooding impacts should remain relatively isolated since the atmospheric river has such a large volume to fill before the rainfall becomes impactful. Wegman Day 3 Valid 12Z Sat Aug 16 2025 - 12Z Sun Aug 17 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE UPPER MIDWEST INTO THE GREAT LAKES... ....20Z Update... ....Upper Midwest... Relatively few changes were made to the risk areas on Day 3/Saturday. A higher end Slight is in effect for portions of central Wisconsin. Additional heavy rainfall from MCS development again on Saturday is likely to overlap with areas hit from heavy rains on D2/Friday. This overlap, in addition to prior heavy rains in the area, will keep soils saturated, and the rivers and streams full. Thus, the additional rainfall expected Saturday will fall on top of these saturated soils and full streams. Magnitude-wise, the rainfall on Saturday will also be heavier across Wisconsin than previous days, only increasing the potential impact. For these reasons, the higher end Slight is in effect, and considerations may need to be given, depending on how much rain falls in the interim, as to whether a Moderate Risk may be needed in this area. ....Gulf Coast and Southeast... The inherited Marginal Risk was greatly expanded across portions of central Texas with this update. This is all due to uncertainty with the track and eventual impacts associated with the moisture plume with Invest 98L over the western Gulf. All indications are that the associated moisture will push northward from Deep South Texas on D2/Friday into central Texas on Saturday, but how organized the storms become, their coverage, and heavy rainfall potential remain highly uncertain. Thus, there is higher than normal probability that future upgrades will be needed, there is just no certainty as to where. The guidance is in decent agreement that any upper level disturbance that 98L is able to form prior to its landfall and subsequent dissipation over Deep South Texas will continue up the Rio Grande towards the Big Bend Area. This has the potential to bring heavy rainfall to areas that got the devastating flooding way back on July 4. However, it's very important to note that the guidance is far from agreed on this scenario. ....Southwest... Monsoonal moisture will begin to push east on Saturday, expanding the potential for isolated flash flooding across much of southern New Mexico. However, there remains little to help focus that moisture and those storms into any kind of organization, so while the slopes of the mountains could act as a localized focus, and where the flash flooding threat is highest, this too remains uncertain. Most likely the storms will behave as they've done in past events, remaining widely scattered, reliant on cold pools and cell mergers, and not persist for very long. The inherited Marginal was left unchanged with this update, as any heavy rainfall footprint also remains unchanged. ....Pacific Northwest... The Atmospheric River (A.R.) aimed at Washington State on D2/Friday will begin to shift southward across Oregon on D3/Saturday. Unlike Washington however, the A.R. will be on the move towards the south, greatly limiting the time any one area is under the plume of moisture tracking northeastward into the northern Rockies. Since much of western Oregon is also in a drought, the much lesser amounts of rainfall expected into the Oregon Cascades and coastal ranges compared to areas further north into Washington State should limit any flash flooding to sub- Marginal levels. Considerations for a future Marginal Risk will be needed should more rain impact northern Oregon on D2/Friday than currently forecast. Wegman ....Previous Discussion... ....Central Plains through the Great Lakes... Pinched mid-level flow will become increasingly squeezed between a trough over Ontario and a ridge expanding from the Gulf Coast. Between these two features, nearly zonal flow will force a cold front to generally align west to east and stall, providing a convergent impetus for convection. Along this front, spokes of vorticity will track west to east leading to locally enhanced ascent to support waves of low pressure, while 850mb inflow returns around the high pressure ridge and reaches 25-35 kts to draw elevated thermodynamics characterized by PWs above 1.75 inches and MUCAPE of 4000 J/kg into the boundary. This will support multiple rounds of thunderstorms through the day, peaking during the evening/Saturday night, with rainfall rates of 1-2"/hr or more at times, leading to scattered instances of flash flooding. A SLGT risk has been added for portions of this area where probabilities from the SREF/GEFS/ECENS align for more than 3" of rain. ....Southwest... Continued S/SW mid-level flow around the periphery of a ridge positioned over the Mississippi River Valley will maintain elevated moisture into the Southwest on Saturday. Guidance continues to suggest that weak shortwaves/vorticity maxima will lift northeast embedded within the flow, although the intensity of these features is generally modest, and weaker than on Friday. Additionally, although 850mb flow will remain southerly over NM, it is progged to shift a bit more to the west over AZ, lowering the overall PW anomalies and indicating the greater risk for any isolated flash flooding will shift east from prior days. MUCAPE of 500-1000 J/kg will support rainfall rates that may approach 1"/hr at times, and if these fall across more sensitive features like burn scars or urban areas, isolated instances of flash flooding could result. ....Southeast... A wave of low pressure along a weakening cold front will drift southeast away from the NC coast on Saturday. At the same time, a building ridge over the Mississippi River Valley will lead to anti-cyclonic mid-level flow out of the NW into the Southeast, setting up a situation with moist confluent flow intensifying over the region. PWs will likely remain elevated above 2" along the Gulf Coast, but be lower elsewhere as drier air tries to advect from the NE around the offshore low, and this will result in less widespread coverage of thunderstorms than prior days. While coverage of convection is expected to generally be scattered, some locally enhanced rainfall along any convergence axes is possible due to rainfall rates that will still reach 1-2"/hr at times. Where these fall across vulnerable soils from widespread convection on prior days, impacts could result, and a MRGL risk continues for portions of the area. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8HHCH3abn4tIBvcorwgDx4YB6gP2E3FZE_Uh_9A51u_X= aNaQEEa3VlFbCl6a3EQzKUw0BJJTUNs02cP04ySthVNemD8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8HHCH3abn4tIBvcorwgDx4YB6gP2E3FZE_Uh_9A51u_X= aNaQEEa3VlFbCl6a3EQzKUw0BJJTUNs02cP04ySt8wB5hiE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8HHCH3abn4tIBvcorwgDx4YB6gP2E3FZE_Uh_9A51u_X= aNaQEEa3VlFbCl6a3EQzKUw0BJJTUNs02cP04yStwTvsoG0$=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 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .