317 FOUS30 KWBC 011547 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1147 AM 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 HIGH PLAINS AND CENTRAL PLAINS AND OVER THE SOUTHEAST FLORIDA PENINSULA... ...Central High Plains and Central Plains... The mid-level low initially over Baja will continue to weaken and shear out as it becomes embedded within pinched westerlies south of a trough diving out of Montana. This will result in expanding deep layer lift through height falls and modest deformation expanding from the Four Corners through the Northern Plains as well as additional ascent occurring as the northern stream trough drives a cold front southeastward. This synoptic setup will produce an expansive area of lift into increasingly robust thermodynamics, and it is likely that a large area of showers and thunderstorms will develop on Monday...with southerly low- level flow increasing to as much as 40-50 kts at 850mb helping boost precipitable water values increase to 1.00 to 1.25 inches. This overlaps with MUCAPE rising to 250-1000 J/kg, sufficient to produce rainfall rates of 0.5"-1"/hr. While there is still some uncertainty in the placement of the heaviest rainfall, the guidance is in general good agreement in a maximum of 2-3", or locally higher as reflected by <10% probability for 3"/24 hrs from the ECENS/GEFS, from eastern CO and WY through parts of southeast South Dakota or eastern/northeastern Nebraska as well as in parts of west Texas that form along the dryline. Over parts of the Intermountain region...residual moisture from Day 1 is expected to still be in place which enhances the prospect for downpours from any slow moving storms which develop during the late day or evening. This region is sensitive due to the variety of slot canyons and other sensitive terrain, which feature FFG that is as low as 0.5"/1hr. ...South Florida... The 01/00Z model runs tended to show an uptick in the coverage and rainfall amounts over southeast Florida...resulting in the introduction of a Slight Risk area focused along the urbanized corridor. As an upper trough digs south along the FL Panhandle and into the peninsula, it will push a weakening cold front and shortwave southward, enhancing ascent across the area before stalling and potentially getting deflected northward. With PWs approaching 2" overlapped with MUCAPE above 2000 J/kg, this will support scattered to widespread heavy- rain producing convection on Monday with the HRRR and RRFS showing the potential for amounts of 3 inches. Bann Day 3 Valid 12Z Tue Jun 03 2025 - 12Z Wed Jun 04 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE CENTRAL US AND OVER A PORTION OF THE SOUTHERN FLORIDA PENINSULA... ...Texas to the Great Lakes... The front pushing south- and eastward from the Northern Plains will continue to encounter a moist airmass extending from Texas northward towards the western/central Great Lakes region...with maximum precipitable water values approaching 2.2 inches pooling ahead of the front over northern Texas and eastern Oklahoma by late Tuesday evening and values approaching 2 inches nearing the southern Great Lakes by the end of the outlook period. Thunderstorms forming in this airmass should be capable of producing 1 to 2 inch per hour rates. While the storms should be progressive...the alignment of the mean winds with the orientation of the front does open up the potential for repeat convection or training of cells and the associated risk of flash flooding. ...Southeast Florida... Embedded impulses rotating around a building Atlantic ridge will result in showers and thunderstorms lingering across parts of the southern Florida peninsula...although the guidance was struggling with placement. Given their handling of the evolution on Day 2...a small Slight Risk area was embedded within a broader Marginal Risk area and placed based on spaghetti plots of SREF/GEFS 2 and 3 inch amounts. Bann Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt