Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jun 02 2025 15:59:21 FOUS30 KWBC 021559 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Mon Jun 2 2025 Day 1 Valid 12Z 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 SOUTHEAST FLORIDA PENINSULA... ....Four Corners through the Central Plains...=20 The CAMs have shifted just a bit north/east today with the axis of heaviest rain, owing to subtle evolution changes in the upper=20 pattern. The combination of ascent through height falls,=20 convergence along the southeast advancing front, and increased=20 ascent in the vicinity of a developing low pressure will help focus convection with heavy rainfall into favorable thermodynamics.=20 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=20 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=20 adjustments. Farther Southwest, the ejecting upper low advancing into a region=20 of elevated instability and extremely anomalous PWs (NAEFS PWs the=20 maximums in the 06Z CFSR database) will promote heavy rain=20 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=20 Southern Rockies will likely push additional moderate to heavy=20 rainfall across eastern CO late tonight, and while this round will=20 most likely be less intense due to weaker instability, localized=20 additional flash flooding is possible if the rain occurs atop of=20 primed soils from earlier convection. ....West Texas into Texas Panhandle and western OK...=20 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 rainfall, the generally scattered nature of convection along with uncertainty in placement suggests the MRGL risk remains accurate, 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=20 likely be where multiple rounds occur, but also where storms slow=20 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=20 focused and has been modestly adjusted for new guidance. However,=20 the MRGL risk encompasses much of the southern peninsula where any=20 of these slow moving heavy rain producing thunderstorms could=20 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 AND OVER SOUTH FLORIDA... ....Texas to the Great Lakes... The cold front pushing southeast across the central CONUS Tuesday may become more progressive as the Baja low from Sunday gets even more sheared into the westerlies, producing a more amplified trough axis pivoting east into the Plains. Embedded within this flow, multiple vorticity maxima will rotate E/NE, leading to additionally enhanced lift, aided significantly the latter half of D3 by diffluence within the RRQ of a poleward arcing jet streak downstream of the trough axis. This will result in expansive and impressive deep layer ascent from the Southern Plains through the Great Lakes, with return flow out of the Gulf on 30-50 kts at 850mb drawing impressive thermodynamics northward into the front. This suggests that convection will be widespread across the area Tuesday, which is supported by both the available CAMs (which are largely absent from the latter half of D2 at this juncture, though the RRFS and CMC-reg outputs are impressive) and the global models and ensembles with rainfall rates exceeding 1"/hr likely anywhere along the front. The greatest risk for excessive rainfall will be from northern Texas through southeastern Iowa and western Illinois where deeper warm cloud depths will produce more efficient rainfall rates, which will likely train SW to NE along the front. The SLGT risk was maintained for much of the same area (as models are in very good agreement on the placement of the QPF axis) which continue to match the highest probabilities for 2" and 3"/24 hrs from the ECENS/GEFS. The higher-end probabilities of the SLGT spectrum exist from northern OK into southeastern KS and central MO, where available CAMs suggest the best training axis will occur. ....South Florida... A meandering shortwave trough positioned over the Florida Peninsula will be the impetus for another round of scattered to widespread showers and thunderstorms Tuesday. With thermodynamics remaining robust across the region (PWs 1.75 to 2 inches and MUCAPE above 1000 J/kg), slow moving storms will again have the potential to produce excessive rain rates above 2"/hr. While there is some uncertainty into how much convective debris from Monday could limit coverage, the general consensus among the various ensemble camps is for another day of heavy rain, especially across South Florida, where models remain in good agreement suggesting 3-5" localized totals. This will fall atop ground that will likely be sensitive from heavy rain on D2, and the inherited SLGT and MRGL risks only needed minor adjustments (as the best agreement in a heavy rainfall axis remains in a similar position as D1, over far southern portions of the mainland into the Upper Keys). Churchill/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 INTERMOUNTAIN WEST INTO MUCH OF THE CENTRAL UNITED STATES, AS WELL AS FOR SOME PORTIONS OF THE SOUTHEAST COAST... ....Intermountain West through Central United States... A large MRGL risk was maintained and expanded a bit across a vast portion of central CONUS, while an inherited SLGT risk was removed over TX. While some prior model runs suggested TX could see the QPF maxima of the day, that appears much less likely with the Red River of the South region now conversely having some of the lowest 1" exceedance probabilities (from 00z ECENS/GEFS) of the whole MRGL region. This is due to better separation between the two features of interest, 1) another mid- and upper-level closed low ejecting northeastward from around Southern CA into the Intermountain West and 2) the aforementioned cold front stalling from the Southern Plains into the Midwest, but with the bulk of the favorable forcing from D2 largely having exited the region (though a shortwave pivoting through the larger scale trough is of interest). While moisture and lift looks sufficient across the Intermountain West for 0.5"+/hr rates (with downscaled 00z ECMWF and GFS models depicting localized 1-2" totals amid PWATs in the vicinity of the 90th percentile), the highest probabilities for 1" and 2" exceedance exist from northern AR through southern MI (with notably higher probabilities from the 00z ECENS over the GEFS). With considerable uncertainty at this range, opted to maintain a broad MRGL (though if a SLGT is eventually reintroduced, it's more likely to be located over the Mid MS Valley and Midwest region). ....Southeast Coast... The shortwave and tropical moisture from prior days rainfall looks to lift northward by D3, as the ridge axis over the eastern CONUS that stalled the feature weakens enough in the lower-levels (700-850 mb) to allow for northward progress. There is considerable uncertainty regarding the timing of the lift of this feature and any meaningful organization into a closed low (more likely to remain a surface trough), but models are in overall good agreement in indicating the potential for significant rainfall totals just offshore with this feature. Given the proximity to the coastline, have maintained (and expanded southward) an inherited MRGL risk. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7CmjlJeLPi_EbKi32QCI72pnnfStvkJZIHeYaLNozUie= GC1wjNjEHjYfytkebzFERka65C9YnIQ3XcLidG6Jm7YGeLs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7CmjlJeLPi_EbKi32QCI72pnnfStvkJZIHeYaLNozUie= GC1wjNjEHjYfytkebzFERka65C9YnIQ3XcLidG6JtqVPpow$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7CmjlJeLPi_EbKi32QCI72pnnfStvkJZIHeYaLNozUie= GC1wjNjEHjYfytkebzFERka65C9YnIQ3XcLidG6Jma2ABmY$=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) .