Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Tue Jun 03 2025 09:01:00 FOUS30 KWBC 030830 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 AM EDT Tue Jun 3 2025 Day 1 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 expected today 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 1.8-2.0" PWs (above the 99th percentile per NAEFS) and a ribbon of MUCAPE exceeding 2000 J/kg (through northern MO), surging northward on 30-50 kts of 850mb flow emerging on return flow from the Gulf. The CAMs have continued to be in excellent agreement in the coverage of convection, while also recently coming into better agreement on temporal and spatial differences (owing to timing of the front). CAMs indicate widespread 1"/hr rain rates from the northern TX Hill Country all the way to the eastern Missouri Valley (40-km neighborhood 1"/1-hr probabilities exceeding 50% across this entire area), and these probabilities increase from north to south overtime today (as overnight convection likely remains active in the morning, then grows upscale during the day). 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, localized rainfall totals exceeding 3" are likely (50-80% 3"/24-hr neighborhood exceedance probs across the entire Slight risk area, per 00z HREF). The best spatial agreement for 2"+ totals (per 00z HREF EAS 2"/24-hr exceedance probs) are located within the core of the central and northern portions of the Slight risk, from central OK through southeast KS and central MO (where 5" neighborhood exceedance probabilities are also greatest, as high as 30-50%). The inherited SLGT risk was tailored to this new guidance, but very little change was necessary. The Slight risk over TX is considered the lowest confidence, as spatial agreement (per HREF EAS probs) is lowest here (though convection that does manage to develop along the tail of this front well into the night will likely feature slow storm motions and backbuilding, with 5" neighborhood exceedance probabilities still as high as 20-30%). 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 maintained north of IA. ....South Florida and the Florida Keys... A shortwave trough over the Gulf has amplified enough overnight to become a weak closed low early this morning, and is expected to spin nearly in place as it gets suppressed beneath a Bermuda- type ridge to its northeast. This will maintain modest synoptic ascent across Florida, but some enhancement is likely as a jet streak over the Atlantic tails back to the SW, placing the favorably diffluent RRQ overtop the peninsula. This forcing will combine with continued low-level convergence along a decayed front/surface trough, resulting in another day of widespread showers and thunderstorms. PWs in place will again be greater than 2 inches, above the 90th percentile according to NAEFS, combining with 1000 J/kg of CAPE to produce robust thermodynamics to support rainfall rates of 2"/hr at times. There continues to be some uncertainty in the coverage of convection among the various CAMs, but better agreement in the placement of 3"+ amount (per HREF EAS probs) along the southwest coast of FL has necessitated an expansion of the SLGT to just south of Tampa Bay. While FFG exceedance probs are highest across this area, have also maintained the SLGT for the remainder of South FL (including all of the FL Keys and the Miami metro) as localized 5" totals remain possible (with HREF 40-km neighborhood 3"/24-hr exceedance probs of 30-50% in very close proximity). Although not included in a risk area at this time, some of the guidance has continued to be 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 further expanded westward with this cycle, now encompassing southeastern portions of CA through NV/UT/AZ and the Four Corners region into the the Central Plains. The guidance has continues to be more aggressive with showers and thunderstorms blossoming during the afternoon/evening as forcing intensifies from broad divergence downstream of a closed low filling and moving into the Desert Southwest, producing ascent atop a wavering frontal boundary 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 10-40% chance of exceeding 1"/hr (per 00z HREF 40-km neighborhood 1"/1-hr exceedance probs), 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. Churchill/Weiss Day 2 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, MIDDLE MISSISSIPPI VALLEY, SOUTHERN PLAINS, THE FOUR CORNERS, AND THE SOUTHEAST COAST... ....Four Corners into Central/Southern High Plains... 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 (per NAEFS). This will fuel widespread convection capable of localized rainfall rates of 0.5"/hr (or at times higher). Storms will likely move slowly beneath the upper trough, leading to a longer duration of heavy rain. Although total rainfall will be modest in most areas (as reflected by relatively low 24-hr probability from ECENS/GEFS for 1" exceedance), where the heavy rain impacts sensitive terrain features or urban areas instances of flash flooding could result. Probabilities have also increased a bit (particularly from the new 00z ECENS suite) further east into the Central/Southern High Plains and the MRGL risk has been expanded accordingly. Should models come into better agreement, additional expansion may be needed further south into West TX (and with additional CAM guidance and agreement, an upgrade to SLGT may also be needed). ....Southern Plains and Mid-Mississippi Valley to Great Lakes... The cold front from D1 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 D2 (though some models are also indicated potential for isolated convective development farther southwest into portions of the Southern Plains). 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 with ensembles indicating 10-40% chance of exceeding 1" and up to 10% probabilities for exceeding 2" (with the new ECENS being the most aggressive). The inherited MRGL risk was maintained (and expanded a bit southwest) with this update. ....Southeast Coast... Guidance continues to be somewhat aggressive with a wave of low pressure potentially developing along an inverted trough shifting northeast along the FL/GA/SC coasts. While NHC continues to indicate a 10% probability of tropical or sub-tropical development with this feature, regardless of whether a circulation forms or not, there continues to be increasing confidence that heavy rain will spread across the FL Peninsula and then rotate onshore the GA/SC coasts on Wednesday. There also has been a clear model trend of potential increased convective development farther inland to the FL Panhandle and into southeast AL and southwest GA (likely due to a stronger closed low and the associated dynamics), so the MRGL was expanded to reflect this. With PWs progged to exceed 2", above the 90th percentile, 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 certainly remains possible that the heaviest rainfall in association with the potential surface low will remain just offshore, but impressive moisture convergence from Jacksonville, FL to Charleston, SC should support waves of heavy rainfall regardless of development of this system. Additionally, plentiful tropical moisture across the rest of the FL Peninsula and a shortwave continuing to pivot across the Gulf could enhance rainfall over much of Florida Wednesday, and the MRGL was maintained accordingly (with the exception of the FL Keys, which were removed due to model consensus that convection will be pulling away northward by 12z Weds). Churchill/Weiss Day 3 Valid 12Z Thu Jun 05 2025 - 12Z Fri Jun 06 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN KANSAS AND NORTHERN OKLAHOMA... ....Central/Southern Plains into the Midwest... Modest return flow from the Gulf overnight on D2 looks to result in elevated PWs once again into the Southern Plains by D3, as DPVA in association with the aforementioned upper-low (now shortwave) may help to spur more significant convective organization and growth. While models are still in rather substantial disagreement on the scale of organization and ultimate placement of higher QPF amounts, there's enough agreement to reintroduce a SLGT to portions of KS/OK (where ECENS best overlaps the stronger GEFS signal). Downscaled GFS and ECMWF solutions suggest the potential for localized 2-3" totals (as do the respective ensemble exceedance probs). ....Southeast/Southern Mid-Atlantic Coast... Guidance continues to indicate the potential for further development of the aforementioned wave of low pressure. Regardless of any tropical or sub-tropical development of this feature, heavy rainfall is likely to remain in close proximity to the coast into D3. The MRGL was expanded a bit to cover the range of solutions from the models at this juncture. Churchill $$ --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .