Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jun 01 2025 18:40:11 FOUS30 KWBC 011839 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 239 PM EDT Sun Jun 1 2025 Day 1 Valid 16Z Sun Jun 01 2025 - 12Z Mon Jun 02 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST... 16Z Update... Incoming 12Z suite and HREF still support widespread showers and thunderstorms developing within extremely anomalous PWs downstream of a filling mid-level low which will pivot into AZ tonight. The previous discussion is still reflective of today's event, and the inherited MRGL/SLGT risks were modified just slightly to account for the new guidance. Previous Discussion below: Impressive closed low off the northern Baja coast will gradually fill and start to shift northeastward today.Downstream of this low, southerly flow will strengthen, drawing increased moisture and instability northward into the Southwest United States. This will support impressive thermodynamics as precipitable water values surge to 1.25-1.5 inches, above the 99th percentile according to NAEFS (and in some areas above anything measured in the 06Z CFSR climatology). Given the amount of overlap with MUCAPE climbing to 250-750 J/kg, convection is expected to blossom rapidly and then organize through 25-35 kts of bulk shear. This organization within the favorable thermodynamics and upper level dynamics will support at least briefly heavy rain rates that may exceed 1"/hr (10-20% chance). With storm motions slowing as the upper low pivots overhead later this evening/tonight, this could produce pockets of 1-2" of rain, especially in the low deserts of AZ. Farther east, aligned Corfidi vectors with the mean flow will allow for repeating rounds of convection to lift northward, producing other areas of 1-2" of rain despite the faster progression. ....Texas... Low-level flow will gradually back from W to S/SW today, and increase to 15-25 kts across portions of eastern Texas. This will impinge effectively into a wavering warm front, providing ascent through convergence and isentropic ascent later this aftn through tonight. At the same time, a modest shortwave will dig southward towards the Arklatex, amplifying the flow and driving at least subtle height falls. The overlap of this ascent into PWs nearing 2 inches and MUCAPE as high as 3000 J/kg will support expanding convection diving southward along the boundary, potentially growing upscale into clusters or an MCS this evening within favorable bulk shear. There is still quite a bit of longitudinal spread with the placement of this MCS, but where it occurs rainfall rates of 1-2"/hr could train, leading to at least isolated instances of flash flooding. The inherited MRGL risk was modified cosmetically to best match the recent HREF exceedance probabilities. ....South Florida... A cold front dropping slowly southward will serve as a focus for showers and thunderstorms through today. This front is pushed southward by height falls within an expanding eastern CONUS trough, leading to some enhanced ascent, aided additionally by a weak impulse progged to move eastward across the southern peninsula. PWs south of the front will hover around 2 inches today, which is above the 90th percentile of the SPC sounding climatology, which will combine with SBCAPE reaching 2000-3000 J/kg to produce a favorable thermodynamic environment for heavy rainfall. The HREF and REFS probabilities indicate a 30-50% chance, and a 10-20% chance for 1"/hr and 2"/hr rates, respectively, within blossoming convection, and as storms train along the boundary from west to east, or interact with subsequent outflows/sea breeze boundaries, this could produce locally 3-5 inches of rain. This could result in localized instances of flash flooding, especially along the urban SW or SE coasts today. Weiss 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...=20 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=20 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=20 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=20 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...=20 A meandering shortwave trough positioned over the Florida=20 Peninsula will be the impetus for another round of scattered to=20 widespread showers and thunderstorms Tuesday. With thermodynamics=20 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=20 uncertainty into how much convective debris from Monday could limit coverage, the general consensus among the various ensemble camps=20 is for another day of heavy rain, especially across South Florida,=20 where ECENS/GEFS/SREF all suggest at least a low end risk (5-10%)=20 for 3"/24 hrs. This will fall atop ground that will likely be=20 sensitive from heavy rain on D2, and the inherited SLGT and MRGL=20 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!4iPQy-J3mUCuGaLnSlEzbjiNUil1N12CUTE84f-WmrC6= 9huVbcs0T5kNb-6uSVMZ9fDu19wjHfeuTDyYwkAwdm6YmVE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4iPQy-J3mUCuGaLnSlEzbjiNUil1N12CUTE84f-WmrC6= 9huVbcs0T5kNb-6uSVMZ9fDu19wjHfeuTDyYwkAw9N2KJ0U$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4iPQy-J3mUCuGaLnSlEzbjiNUil1N12CUTE84f-WmrC6= 9huVbcs0T5kNb-6uSVMZ9fDu19wjHfeuTDyYwkAwT-5hkXA$=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) .