Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Sat May 31 2025 10:12:00 FOUS30 KWBC 311131 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 731 AM EDT Sat May 31 2025 Day 1 Valid 1125Z Sat May 31 2025 - 12Z Sun Jun 01 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NEW ENGLAND... 12Z Update... Pivoting bands of moderate rain continue to bring 2"/3hr rainfall over southern NY where the Marginal is expanded through. Expanded Slight Risk over the rest of Vermont where 06Z guidance is highlighting 1.5 to 2.5" rainfall through mid-afternoon. Jackson ....New England... A deep low pressure system will continue to track northeast today and tonight over New England in response to favorable ascent from the right inflow region of an upper jet streak, height falls, and residual baroclinicity along a cold front while the low- level flow downstream advects greater moisture and instability northward. Thermodynamics/static instability look to remain modest as reflected by forecast PWs of 1-1.25 inches, around the 75th percentile according to the SPC sounding climatology, and MUCAPE of around 250 J/kg. This will support generally modest rainfall rates of 0.5"/hr, but locally higher is possible, especially within any organized convection on 30-35 kts of bulk shear...with swaths of 1 to 2 inch amounts of rainfall. Maintained the previously issued Slight Risk area in the complex terrain aiding orographic processes and the greatest potential for persistent moderate to heavy rain overlaps the sensitive terrain, and where the highest HREF probabilities remain for more than 3" of rain exist. ....Southern Plains... Expansive trough centered over the Great Lakes will lead to lowering heights over the Plains, with an embedded but potent shortwave progged to track southward from Nebraska to Texas this evening and later tonight. This shortwave will impinge into a destabilizing environment reflected by a corridor of 1000 J/kg or more MUCAPE, collocated with precipitable water values surging to around 1.25 inches as the southwesterly low level jet increases to to 20-25 kts by evening. Thunderstorms are expected to blossom along a decaying cold front being pushed south by this shortwave, with upscale growth and organization possible thanks to 40-50 kts of bulk shear. Rainfall rates of 1-2"/hr are progged within this maturing convection, leading to locally as much as 3" of rain (10-20% chance). This could produce isolated instances of flash flooding despite the general progressive nature of this convection. Bann Day 2 Valid 12Z Sun Jun 01 2025 - 12Z Mon Jun 02 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST... Maintained the Marginal Risk area over parts of the Southwest US on Sunday as a persistent closed upper level low off of the Baja Peninsula interacts with a plume of deep moisture that has its origins in the subtropics. As the upper low begins to fill and slowly eject northeast on Sunday...subtle height falls in the mid- levels will reinforce the northward transport into the area as reflected by NAEFS PWs above 1.25 inches, or above the 99th percentile surging into AZ/CA/NV late Sunday night/early Monday morning. This will overlap of at least modest instability, potentially as high as 250-500 J/kg, to support increasing convective coverage late in the period. There remains some concern as to whether or not the increased amount of mid-level moisture leads to enhanced cloud cover and what impact that might have on the amount of coverage/intensity of convection. There was some uptick in the 31/00Z model guidance with an a corresponding increase in ECENS/GEFS probabilities for even 1" of rainfall. Even so...those probabilities seem low considering the precipitable water anomalies in place and the amount of diffluent flow aloft to the east of the closed low east of the Baja peninsula. The expectation is that once showers to develop they could contain rain rates of above 0.5"/hr, which will support at least an isolated potential for flash flooding where storms move most slowly beneath the upper low. Bann Day 3 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 ON MONDAY/MONDAY NIGHT... ....Rockies to the Plains... Moisture that had been streaming northward into the Southwest US on Sunday will continue to be drawn northward on Monday as a northern stream trough along the US/Canadian border amplifies and a cold front makes its way southward. A 35 to 45 kt southerly low level jet develops and leads to enhanced moisture flux convergence along the front from Colorado into Nebraska and a part of South Dakota Monday night/early Tuesday morning. With added upper support from the entrance region of an 80 to 100 kt upper jet in the form of increasing divergence aloft and precipitable water values in the range of 1.5 to 1.75 inches pooling along the front...at or above the 90th percentile for this time of year and over 2 standardized anomalies...rainfall rates in excess an inch per hour and the potential for repeat convection may result in instances of flash flooding. ....South Florida... Introduced a Marginal Risk of excessive rainfall mainly across the urban areas of the southern Florida peninsula as an east to west oriented front settles into the area from the north and stalls. Mid and upper level heights should be falling in response to the approach of a mid/upper level wave from the eastern Gulf during the afternoon...helping set the stage for scattered late day showers and thunderstorms capable of producing locally heavy rainfall. Precipitable water values of 1.75 inches or greater should be in place across the peninsula...some 1.5 to 2 standardized anomalies along the coast or just off shore over the Florida Strait. Localized rainfall rates of 1 to 2 inches per hour are possible. Bann $$ --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .