Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Thu May 22 2025 08:24:00 FOUS30 KWBC 220821 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 421 AM EDT Thu May 22 2025 Day 1 Valid 12Z Thu May 22 2025 - 12Z Fri May 23 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NEW ENGLAND, THE SOUTHERN PLAINS, AND SOUTHEAST FLORIDA... ....Southeastern New England... Energy moving south of an upper low centered over Ontario is expected to assume a negative tilt as it moves through the Mid- Atlantic region to the Northeast Coast later today. This will support a deepening surface low that will track from the northern Mid-Atlantic Coast to the Gulf of Maine. While instability will be modest at best, 30-50kt low level easterly winds on the north side of the circulation, along with the strong forcing aloft will be sufficient for periods of moderate to heavy rain spreading into southeastern New England by this afternoon and continuing into this evening. The 00Z hi-res guidance shows the heaviest amounts centered over eastern Mass, with the HREF showing high neighborhood probabilities for accumulations exceeding 3 inches centered over the area, including the Boston Metro. ....Southeast Florida... In response to the amplifying upper trough over the East, a cold front and corresponding pool of deep moisture (PWs ~1.75 inches) will drop south across Florida today. Much of the hi-res guidance continues to show convection developing later this afternoon across the southern peninsula and then moving east, with increasing rainfall rates as these storms begin to merge with the seabreeze during the evening. The HREF continues to show high neighborhood probabilities for amounts over 3 inches centered over Southeast Florida onto the Gold Coast. ....Southern Oklahoma/North Texas... A boundary settling into the Red River Valley is expected to become the focus for deepening moisture and storm development later today. While the guidance does not indicate overly impressive low level inflow, it is expected to be sufficient for PWs climbing to around 1.5-1.75 inches, which along with ample instability and weak forcing aloft, is forecast to support storm development. While widespread heavy accumulations are not expected, merging cells and repeating development may produce locally heavy amounts and an isolated flash flooding threat. A Marginal Risk was introduced for areas where the 00Z HREF is showing high neighborhood probabilities for accumulations over 2 inches. This also largely coincides with an area of relatively wetter antecedent soils and lower FFGs. Pereira Day 2 Valid 12Z Fri May 23 2025 - 12Z Sat May 24 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN KANSAS and NORTHEASTERN OKLAHOMA INTO THE OZARKS... A multi-day heavy rainfall will start to unfold across the region, with the threat for flash flooding likely increasing by the end of the period. A warm front lifting into the region will become the focus for deepening moisture and storm development as it interacts with weak energy moving off of the top of an upstream ridge into the region. While difference in the details continue, there is increasing agreement that rainfall rates will increase as storms that are forecast to develop further west across the Plains earlier in the day move into the region during the evening and overnight and begin to interact with the deepening pool of moisture (PWs 1.50-1.75 inches). While uncertainty in the details remain, increasing agreement amongst the 00Z guidance, including the GFS, ECMWF, UKMET, along with pre-existing wet soils, provided enough reason to upgrade to a Slight Risk for parts of the region with this issuance. Pereira Day 3 Valid 12Z Sat May 24 2025 - 12Z Sun May 25 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN KANSAS and NORTHEASTERN OKLAHOMA INTO THE OZARKS... With the previously noted front and coinciding pool of deep moisture remaining in place, additional rounds of heavy rainfall are expected, with the threat for heavy accumulations and flash flooding likely to expand across the region. Guidance shows the boundary remaining quasi-stationary through the period, with 30-40 kt southwesterly low level inflow supporting PWs of 1.5-1.75 inches. Models generally agree that the magnitude and expanse of heavy amounts will increase this period as this moisture interacts with a series of weak impulses. Overall, the 00Z models trended further south, with is reflected in the adjusted Marginal and Slight Risk areas. Given the uncertainty, opted not to upgrade beyond a Slight with this package. However, given the potential for consecutive days of heavy rainfall across the same area, an upgrade to a Moderate Risk in this or subsequent periods may be forthcoming. Pereira $$ --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .