Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Fri May 30 2025 09:16:00 FOUS30 KWBC 300829 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 429 AM EDT Fri May 30 2025 Day 1 Valid 12Z Fri May 30 2025 - 12Z Sat May 31 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE MID ATLANTIC/OHIO VALLEY AND A PORTION OF ADJACENT NEW YORK STATE... Maintained a Slight Risk area over portions of the Ohio Valley and the Mid-Atlantic region as well as a portion of adjacent New York state as low pressure forms and deepens/strengthens across the area today in response to ample upper forcing. The resulting showers and thunderstorms should be capable of producing heavy rainfall along the immediate track of the surface low pressure center and in the unstable airmass in the warm sector of the low later today and tonight. The risk of flash flooding will be greatest where there is overlap of the heavier rainfall and where soil has been made prone to run off by recent heavy rainfall. With the convection tracking from west to east...the portion of the Slight risk area in Kentucky really focuses early today while the portion in northern New Jersey is primarily focused during the overnight hours from late tonight into very early Saturday morning. Repeating rounds of heavy rain today interacting with the terrain and full rivers should cause onset of flash flooding to occur sooner to the start of the rainfall...making any potential flooding more hazardous in West Virginia while flooding concerns farther east will be based around urbanization. The storms will also occur in the afternoon and evening, during the diurnal warmest time of day, which will add more instability for the storms to feed upon. Bann Day 2 Valid 12Z Sat May 31 2025 - 12Z Sun Jun 01 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NEW ENGLAND... A surface low pressure system across the Mid Atlantic on Friday will continue to strengthen as it makes its way to the northeast on Saturday. Concern for heavy rains lingers across parts of New England with the heaviest rainfall likely over Northern NH and ME, especially within the northeastern flank of the low where the warm conveyor belt advects deeper moisture poleward with weak buoyancy likely remaining over the region into Saturday evening. Additional rainfall amounts of 1 to 2 inches...with locally higher amounts...remain in the forecast. That amount of rain could lead to localized flash flooding given the complex terrain impacted and relatively good dynamics at play. Bann Day 3 Valid 12Z Sun Jun 01 2025 - 12Z Mon Jun 02 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST UNITED STATES... Showers and thunderstorms will become more numerous over parts of the Southwest US as a plume of moisture overspreads the region from the south...with the anomalous moist airmass allowing some of the storms to produce locally heavy rainfall and the risk of flash flooding. Precipitable water values of an inch or greater should already be in place across southwest Arizona and adjacent areas of the California deserts at the start of the Day 3 period...90+ percentile values for this time of year...with moisture continuing to stream into the region. With the flow aloft becoming increasingly diffluent to the east of closed low off the California coast at the same time that moisture continues to stream into the region...storms should be in an environment conducive for heavy rainfall and the potential for repeat convection/training storms leading to the risk of flash flooding. One concern is the increasing moisture may result in sufficient cloud cover to delay or inhibit the convective initiation...but there is enough support in the guidance to support a Marginal risk at this time frame. Bann $$ --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .