Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Mon Jun 02 2025 07:25:00 FOUS30 KWBC 020913 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 513 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... ....Central Plains... Mid- to upper-level low over the Southwest will slowly fill while ejecting east-northeast from Arizona today, reaching the Central Plains by late tonight. This evolution will be driven by the amplification of a northern stream trough digging across Montana, which will help shear out the closed low and absorb it into the pinched westerlies. The interaction of these two features will result in widespread convection from the Four Corners all the way to the Upper Midwest as ample moisture from both the tropical Pacific and western Gulf streams northward, and impinges into a cold front being pushed southward by the northern stream trough. As the front sags southeast through the day, it will serve as the primary focus for developing thunderstorm activity this afternoon and evening, generally from Minnesota southwest into Colorado. Hi-res models continue to suggest good agreement that heavy rain producing thunderstorms will be widespread along the boundary, and they've come into better agreement with regard to spread as well (best indicated by relatively high Ensemble Agreement Scale exceedance probabilities for 1" and 2", between 40-80% and 20-40%, respectively). This is due to pronounced ascent through low-level convergence, upper divergence, and isentropic ascent as the LLJ ramps up and pushes PWs to above 1.5 inches, potentially as high as 1.75 inches across NE/SD, well above the 90th percentile. This elevated PW will combine with a ribbon of CAPE above 2000 J/kg to fuel thunderstorms with rain rates that have a high probability (60-70% chance, per HREF 40-km neighborhood exceedance probs) of exceeding 1"/hr. With mean winds expected to be 10-15 kts aligned to the front, but Corfidi vectors becoming increasingly anti- parallel, some backbuilding into the higher instability and resulting localized pockets of training are likely. This will support total rainfall that could exceed 3 inches in some areas (indicated by HREF PMM QPF of 3-5"+), and the SLGT risk largely maintained with little adjustment needed. To the south and west of this SLGT risk, the MRGL risk remains expansive as far back as the slot canyon region of UT and southward to the AZ/NM border (where the MRGL risk was expanded to with this cycle, given persisting convection early this morning in association with the anomalous low-level moisture of the remnants of T.S. Alvin). While convection should largely end by midday over southeast AZ and into southwest NM as the aforementioned trough lifts ENE with drier air in the mid-levels moving in, afternoon thunderstorms are expected to build across the Intermountain West with 0.5+"/hr rates atop sensitive terrain leading to localized instances of flash flooding. 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 eastern OK... Convection blossoming along a surface trough/pseudo-dryline this afternoon and evening may become widespread, although there is still quite a bit of spread in location, coverage, and intensity among the various 00z CAMs. While the coverage may ultimately be somewhat modest, any storms that develop will be capable of producing intense rainfall rates exceeding 2"/hr (per 00z HREF 40-km neighborhood probs of 10-20%) as they track to the E/NE at 15-20 kts. Storms will likely organize through 25-35 kts of bulk shear, forming clusters which could briefly enhance rain rates even further, and may lead to multiple rounds of heavy rain in some areas. FFG from the Rolling Plains southward towards Big Bend are still as low as 2-2.5"/3hrs, despite some drying out over the past several days after an anomalously wet period earlier last week. The inherited MRGL risk remains warranted, and was expanded a good bit (both southwest and northeast into more of TX and eastern OK) to account for uncertainties among the CAMs (encompassing where 2" and 3" exceedance probabilities are greater than 10%). ....South Florida... An active day of thunderstorms is again expected across South Florida today as an impressive shortwave tracks overhead and interacts with a residual front draped across the region. During the period of peak heating (mid to late afternoon) when SBCAPE will likely climb above 2000 J/kg and PWs surge to nearly 2.2 inches (per 00z HREF mean), approaching the daily record for MFL. The simulated reflectivity from the available high-res members have become much more aggressive, likely owing to a more pronounced tail of a jet streak to the east helping to drive ascent. With mean storm motions (using 0-6km mean winds as proxy) expected to be less than 10 kts within modest bulk shear of 20-25 kts, clusters of storms that merge with outflow/sea breeze boundaries could lengthen the duration of rainfall rates that are expected to reach at least 2"/hr, and possibly exceed 3"/hr at times (per 00z HREF neighborhood exceedance probs). This will likely result in heavy rain accumulating to 3-6", and may locally exceed 6" (per both HREF PMM and 5"/24hr neighborhood exceedance probabilities of 40-70% along the Gold Coast and into the FL Keys). The inherited SLGT risk expanded a bit based on the new guidance, but remains capped within the 15-25% probability range (owing to the very high FFGs of the region with low confidence that these higher totals occur directly over more sensitive urban areas, as the best agreement is indicated to be over the less sensitive portions of the southwest mainland and Upper FL Keys). The MRGL was also expanded to include the remainder of the Lower FL Keys. Churchill/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 $$ --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .