Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jun 02 2025 00:46:05 FOUS30 KWBC 020044 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 844 PM EDT Sun Jun 1 2025 Day 1 Valid 01Z Mon Jun 02 2025 - 12Z Mon Jun 02 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN ARIZONA... ....01Z Update... No changes were needed across the Southwest with this updated as the rainfall event is well underway across the region, but behaving as forecast. East from there across Texas however, slow-moving and some training storms have developed near and just south of the Metroplex. Isolated flash flooding is possible with any repeating rounds of storms, so the Marginal Risk was shifted west to align with current radar trends. Guidance suggests the storms should begin to turn and move southward with time, dissipating in a few hours with the loss of diurnal heating. The Marginal in Florida was dropped with this update as all significant convective activity in the state has ended. Wegman Day 2 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-level low over Baja Sunday will fill and eject northeast from Arizona, reaching the Central Plains by the end of D2. 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 on Monday from the Four Corners all the way to the Upper Midwest as ample moisture from the tropical Pacific 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 Monday aftn and Monday evening, generally from Minnesota southwest into Colorado. While there is still considerable spread in the longitudinal placement of the heaviest rain due to timing differences of the models, there is good agreement that heavy rain producing thunderstorms will be widespread along the boundary. 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) 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 training is likely. This will support total rainfall that could exceed 3 inches in some areas, and the SLGT risk was adjusted eastward to account for the latest guidance from the high plains of CO eastward towards the Coteau des Prairies. 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 across much of CO and northern NM where afternoon thunderstorms could produce 0.5+"/hr rates atop sensitive terrain leading to instances of flash flooding. Additionally, a wave of low pressure developing in the lee of the Southern Rockies may push additional moderate rainfall across eastern CO late D2, and while this will likely be less intense due to weaker instability, could produce at least localized additional flash flooding if the rain occurs atop of primed soils from earlier convection. ....West Texas... Convection blossoming along the dry line Monday aftn/eve may become widespread, although there is quite a bit of spread both in location, coverage, and intensity, among the various 12Z CAMs. While the coverage may be somewhat modest, any storms that develop will be capable of producing intense rainfall rates exceeding 2"/hr as they track to the E/NE at 15-20 kts. Storms on the dry line will likely organize through 25-35 kts of bulk shear, forming clusters which could briefly enhance rain rates even further, and lead to multiple rounds of heavy rain in some areas. FFG from the Rolling Plains southward towards Big Bend is only 2-2.5"/3hrs due to 7-day rainfall according to AHPS that has been more than 300% of normal. This suggests that the inherited MRGL risk is still warranted for any clusters of storms that move atop these more sensitive soils. ....South Florida... An active day of thunderstorms is again expected across South Florida on Monday as an impressive shortwave tracks overhead and interacts with a residual front draped across the region. During the period of peak heating /late aftn and eve/ when SBCAPE will climb towards 2000 J/kg and PWs surge to nearly 2.2 inches, 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. This will result in heavy rain accumulating to 3-5", with locally higher amounts likely as reflected by both HREF and REFS 5"/24hr probabilities rising above 60% along the Gold Coast. The inherited SLGT risk was modified only modestly for the new guidance, and instances of flash flooding appear likely in the urban areas D2, even moreso if heavy rain falls atop areas impacted by rainfall on D1 as well. Weiss Day 3 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 simulated reflectivity and 24-hr rainfall progs from 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 eastern Iowa 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 tailored to match the highest probabilities for 3"/24 hrs from the ECENS/GEFS, but some eastward expansion of the SLGT was also drawn to account for the AI guidance (AIFS, Graphcast) that is generally east of the deterministic models. ....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 ECENS/GEFS/SREF all suggest at least a low end risk (5-10%) for 3"/24 hrs. This will fall atop ground that will likely be sensitive from heavy rain on D2, and the inherited SLGT and MRGL risks were adjusted only cosmetically from previous. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8QOGplFMwjXA4yCBojILVOtTRNK-AKcEn6kSUmSakCPK= alcS114N2g9BqG4UqcDamXx8MGpTuL24uI0YBiU2XJQ8f-o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8QOGplFMwjXA4yCBojILVOtTRNK-AKcEn6kSUmSakCPK= alcS114N2g9BqG4UqcDamXx8MGpTuL24uI0YBiU2IEFRJLQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8QOGplFMwjXA4yCBojILVOtTRNK-AKcEn6kSUmSakCPK= alcS114N2g9BqG4UqcDamXx8MGpTuL24uI0YBiU2rhWRpsA$=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) .