Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jun 02 2025 19:43:59 FOUS30 KWBC 021943 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 343 PM EDT Mon Jun 2 2025 Day 1 Valid 16Z Mon Jun 02 2025 - 12Z Tue Jun 03 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL PLAINS AS WELL AS THE SOUTHERN FLORIDA PENINSULA... ....Four Corners through the Central Plains... The CAMs have shifted just a bit north/east today with the axis of heaviest rain, owing to subtle evolution changes in the upper pattern. The combination of ascent through height falls, convergence along the southeast advancing front, and increased ascent in the vicinity of a developing low pressure will help focus convection with heavy rainfall into favorable thermodynamics. The high-res ensemble camps (both 12Z HREF and 06Z REFS) are both still quite aggressive with suggesting more than 3 inches of rain across portions of Nebraska and into IA/MN/SD, as far north as the Coteau des Prairies. The greatest risk for excessive rainfall appears to be across Nebraska, still, however, as convection along the cold front initiates and then builds SW into the greater thermodynamics as the low evolves, leading to enhanced isentropic ascent into the boundary. Fortunately, the area of greatest exceedance probabilities (50-80% chance of 3+ inches from the HREF and REFS) overlaps the Sandhills which are not a favorable flash flood location, or otherwise this event could necessitate a targeted upgrade. Still, with locally as much as 5" of rain possible thanks to slow moving/backbuilding rain rates of 1-2"/hr, at least scattered instances of flash flooding are expected, and the SLGT risk was maintained with only modest cosmetic adjustments. Farther Southwest, the ejecting upper low advancing into a region of elevated instability and extremely anomalous PWs (NAEFS PWs the maximums in the 06Z CFSR database) will promote heavy rain producing convection over the Four Corners, especially during peak heating. Storm motions will be weak in this area beneath the upper low, so as rain rates which may reach 1"/hr (10-20% chance) leading to 15-min rainfall that could reach 0.25-0.5 inches. Should any of these cells, which could be widespread, move across sensitive terrain features like slot canyons, instances of flash flooding could result, and the MRGL risk has been maintained. Additionally, a wave of low pressure developing in the lee of the Southern Rockies will likely push additional moderate to heavy rainfall across eastern CO late tonight, and while this round will most likely be less intense due to weaker instability, localized additional flash flooding is possible if the rain occurs atop of primed soils from earlier convection. ....West Texas into Texas Panhandle and western OK... The surface trough/dry line will focus convective development this aftn/eve as diurnal heating pushes instability to 1000-3000 J/kg.=20 This robust CAPE will work in tandem with PWs exceeding 1.5" as SW=20 return flow reaches 30+ kts, to produce favorable thermodynamics=20 for heavy rain containing thunderstorms. The guidance continues to=20 vary widely with both the timing and spatial coverage of=20 thunderstorms today, but low-level convergence moving into the=20 thermodynamics should support at least scattered convection along=20 this boundary as it pushes east into tonight. While uncertainty is=20 low as far as coverage, it is high that at least a few storms will=20 fire along this boundary, and these will rapidly intensify within=20 this environment. Storm motions of generally 10-15 kts to the WSW=20 will be aligned strongly right of the Corfidi vectors to suggest at least some training is possible as storms re-develop into the more robust instability before tracking ENE. Additionally, increasing=20 bulk shear to 25-30 kts this evening may help organize storms into=20 clusters, and the high-res indicates the greatest chance for an MCS type of event will be into western OK tonight. Although some parts of this region have more vulnerable soils due to recent heavy=20 rainfall, the generally scattered nature of convection along with=20 uncertainty in placement suggests the MRGL risk remains accurate,=20 and this was only adjusted slightly for new guidance. ....South Florida and the Florida Keys... Near record PWs across southern Florida will produce an environment favorable for excessive rainfall today. PWs are progged to reach above 2.2 inches, which would be a daily record for MFL according to the SPC sounding climatology. This moisture will combine with MUCAPE of 2000-3000 J/kg to provide extreme thermodynamics to support intense rainfall rates in any thunderstorms, which are expected to be widespread today. The primary mechanisms for ascent will be a weakening front stalled across the southern Peninsula, as well as a potent shortwave moving across the Gulf and towards the area, with subtle diffluence in the tail of a jet streak over the Atlantic additionally contributing lift. This all suggests widespread convection this aftn/eve, which is supported by the high res CAMs. Within these storms, rainfall rates of 2-3"/hr or more are likely (40-60% chance of exceeding 2"/hr according to both HREF and REFS), which through training or repeated rounds could produce 3-6" of rainfall, with local amounts potentially approaching 8" as reflected by the HREF probabilities. Any amounts above 5" would likely be where multiple rounds occur, but also where storms slow or move chaotically due to boundary interactions and storm mergers. While this part of Florida typically takes a lot of rain to flood, this setup does support a more pronounced flash flood risk today. Despite FFGs that remain around 4"/3hrs, exceedance of this criteria is possible, especially over urban areas that do not drain as efficiently. The focus today is expected to be across the Keys and Gold Coast of FL, and this is where the SLGT risk is focused and has been modestly adjusted for new guidance. However, the MRGL risk encompasses much of the southern peninsula where any of these slow moving heavy rain producing thunderstorms could result in runoff and instances of flash flooding. Weiss Day 2 Valid 12Z Tue Jun 03 2025 - 12Z Wed Jun 04 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE CENTRAL UNITED STATES AS WELL AS SOUTH FLORIDA AND THE FLORIDA KEYS... ....Texas to the Great Lakes... An active day of convection is likely on Tuesday as a cold front sags across the Heartland, providing the focus for widespread showers and thunderstorms. This front will be pushed southeast in response to an amplifying northern stream trough which will dig across the CONUS as far south as Texas. This will drive height falls across the region, with embedded shortwave impulses racing northeast, and a poleward arcing jet streak downstream of the primary trough axis combining to provide additional ascent. This deep layer lift will work into robust thermodynamics characterized by an overlap of PWs above the 99th percentile (NAEFS) and a ribbon of MUCAPE exceeding 2000 J/kg, surging northward on 30-50 kts of 850mb flow emerging on return flow from the Gulf. The CAMs continue to be in excellent agreement in the coverage of convection on Tuesday despite some temporal and spatial differences owing to timing of the front. Despite these modest differences, there is good agreement among the REFS and HREF ensembles to suggest widespread 1"/hr rain rates from the northern TX Hill Country all the way to the eastern Missouri Valley (probabilities exceeding 50% across this entire area). Although mean 0-6km winds of 20-30 kts indicate progressive storm motions, this flow aligned to the front and combined with Corfidi vectors that collapse and veer to the NE indicate a high potential for redevelopment to the SW into the more intense thermodynamics and training of cells to the northeast. Where this occurs, total rainfall exceeding 3" is likely (>60% chance in HREF/REFS) and 5" is possible (>40% chance), with even the global ensembles suggesting a 10-30% chance for 3" in this same region. The inherited SLGT risk was tailored for this new guidance, with the biggest adjustment being an extension SW into the Hill Country of TX where convection that develops along the tail of this front will likely feature slow storm motions and backbuilding to produce heavy rainfall.=20 Although heavy rainfall is also likely in convection that develops northeast into the Upper Midwest and Great Lakes, storms should be more progressive and the Corfidi vectors suggest less opportunity for training, so only a MRGL risk is needed north of Iowa. ....South Florida and the Florida Keys...=20 Shortwave trough over the Gulf may amplify enough to become a weak closed low Tuesday and then spin nearly in place as it gets=20 suppressed beneath a Bermuda- type ridge to its northeast. This=20 will maintain modest synoptic ascent across Florida, but some=20 enhancement is likely as a jet streak over the Atlantic tails back=20 to the SW, placing the favorably diffluent RRQ overtop the=20 peninsula. This forcing will combine with continued, albeit=20 weakening, low-level convergence along a decayed front/surface=20 trough, to result in another day of widespread showers and=20 thunderstorms. PWs in place will again be greater than 2 inches,=20 above the 90th percentile according to NAEFS, combining with 1000=20 J/kg of CAPE to produce robust thermodynamics to support rainfall=20 rates of 2"/hr at times. There continues to be some uncertainty in=20 the coverage of convection, as well as placement, among the various CAMs, somewhat lowering the confidence into D2. However, after=20 heavy rain anticipated D1, any additional rainfall D2 could quickly become problematic and lead to instances of runoff and flash=20 flooding, especially in urban areas. The inherited MRGL and SLGT=20 risks were tailored slightly to encompass the highest probabilities of 1+" and 3+" on D2. Although not included in a risk area at this time, some of the guidance has become a bit more aggressive with a wave of low pressure developing along an inverted trough aligned to the east coast of Florida. If this low were to develop a bit more rapidly or track farther west, some heavy rain could move onshore the northeast Florida coast before 12Z Wednesday. ....Four Corners... A MRGL risk was expanded westward from the Central Plains to the UT/NV/AZ borders with this update. The guidance has become more aggressive with showers and thunderstorms blossoming during the=20 aftn/evening as forcing intensifies from broad divergence=20 downstream of a closed low filling and moving into the Desert=20 Southwest, producing ascent atop a wavering frontal boundary=20 positioned over the region. This lift will impinge into favorable thermodynamics with PWs reaching above 0.75 inches into a bubble of MUCAPE of 750-1000 J/kg. Mean storm motions will be weak, generally around 5 kts, so with rainfall rates which have a 5-15% chance of exceeding 1"/hr, these slow motions could result in locally as much as 1" of rainfall in a short period of time. If this occurs over sensitive soils, urban areas, or terrain features, instances of flash flooding could result. Weiss Day 3 Valid 12Z Wed Jun 04 2025 - 12Z Thu Jun 05 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MIDWEST, THE FOUR CORNERS, AND THE SOUTHEAST COAST... ....Four Corners... Another active day of showers and thunderstorms expected as the upper low over northern Baja fills and ejects northeast atop the Four Corners. The accompanying ascent will move into continued anomalous PWs which will exceed the 99th percentile according to NAEFS. This will fuel widespread convection with rainfall rates of 0.5"/hr, or at times higher, likely., and storms will likely move=20 slowly beneath the upper trough, leading to a longer duration of=20 heavy rain. Although total rainfall will be modest in most areas=20 (24-hr probability from the ECENS/GEFS for 1" peaks around 25%),=20 where the heavy rain impacts sensitive terrain features or urban areas, or should heavier rain impact the wetter soils in the CO/NM high plains, instances of flash flooding could result. ....Mid-Mississippi Valley to Great Lakes... The cold front from D2 will continue its trek eastward on Wednesday, but will start to slow and become aligned more west to east as its tail gets pulled northward by an approaching shortwave over the Four Corners region. Shortwaves embedded within the flow will continue to race northeast, especially from Missouri to Michigan, which is where the greatest concentration of thunderstorms is expected on D3. Storm motions along the front are likely to be progressive, lifting off to the northeast on 0-6km mean winds of 20-30 kts, but weaker Corfidi vectors aligned both to this mean wind and the front suggests a potential for training. As rainfall rates peak above 1"/hr within favorable thermodynamics, this could result in stripes of heavy rainfall for which the SREF/ECENS/GEFS ensembles indicate have a 20-30% chance of exceeding 1", with some low-end probabilities (5% or less) of exceeding 3". The MRGL risk was split, with the Central Plains removed from the risk on D3 due to a more progressive front and some NVA behind impulses, but also extended a bit northeast to reach the Canadian border for isolated flash flood potential. ....Southeast Coast... Guidance has become more aggressive with a wave of low pressure potentially developing along an inverted trough shifting northeast along the FL/GA/SC coasts. While NHC has placed a 10% probability of tropical or sub-tropical development on this feature, regardless of whether a circulation forms or not, there is increasing confidence that heavy rain will spread across the FL Peninsula and then rotate onshore the GA/SC coasts on Wednesday. With PWs in the vicinity progged to exceed 2", above the 90th percentile,=20 supported by warm cloud depths that may exceed 15,000 ft, efficient rainfall rates of 1-2"/hr will be likely in any showers/thunderstorms. It is possible that the heaviest rainfall will remain just=20 offshore, but impressive moisture convergence from Jacksonville, FL to Charleston, SC should support waves of heavy rainfall=20 regardless of development of this system, and although EFI is modest, it does capture the region of greatest potential at this time range. Additionally, plentiful tropical moisture across the=20 rest of the FL Peninsula and a shortwave continuing to pivot across the Gulf could enhance rainfall over much of Florida Wednesday,=20 and the MRGL risk was expanded from Winyah Bay, along the coastal=20 plain of GA/SC, and across much of the FL Peninsula. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8kfhvDsiRtAmK5cPfjeT9H7RUz2N0lVDbUKhH40aGL-V= WIWSGF2x0Hsu88AmbtSC8xBHHOqEiH5OQw-D41BVPNg9uxQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8kfhvDsiRtAmK5cPfjeT9H7RUz2N0lVDbUKhH40aGL-V= WIWSGF2x0Hsu88AmbtSC8xBHHOqEiH5OQw-D41BV4YVDCCw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8kfhvDsiRtAmK5cPfjeT9H7RUz2N0lVDbUKhH40aGL-V= WIWSGF2x0Hsu88AmbtSC8xBHHOqEiH5OQw-D41BV02fiJwg$=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) .