Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue May 13 2025 00:48:25 FOUS30 KWBC 130048 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 848 PM EDT Mon May 12 2025 Day 1 Valid 01Z Tue May 13 2025 - 12Z Tue May 13 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AND SOUTHEAST FLORIDA... ....01Z Update... In coordination with MFL/Miami, FL forecast office, the Moderate Risk was downgraded to a Slight for the I-95 corridor from West Palm Beach south. Much of the convection in the area has moved offshore, with only one remaining training band south of Homestead. HiRes models continue to suggest occasional storms forming across south Florida overnight, but agreement is poor on their coverage. Thus, given the expected isolated or widely scattered nature of the overnight convection, flash flooding should be isolated for most areas.=20 Elsewhere, the Slight across the Southeast remains largely unchanged, though coverage of heavy rainfall from storms is quite low, so most areas are in a low-end Slight. That said, a few lines of training storms are locally causing flash flooding, so the potential for additional line formation and training over other areas justifies the continuation of the Slight. The Marginal was trimmed on the southern end across the Southeast behind the most persistent convection, with the upper low and its associated forcing moving north, resulting in a sub-Marginal threat for areas further south. Wegman Day 2 Valid 12Z Tue May 13 2025 - 12Z Wed May 14 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MID-ATLANTIC... No significant adjustments needed to the Day 2 Update, other than expanding the Slight further across portions of central MD based on the latest QPF trends. ---previous discussion--- The slow-moving upper low and occluded surface low will continue to move north, and showers and storms will focus across the Mid- Atlantic on Tuesday. Deep moisture ahead of the system will push PWAT values above 1.5 inches, and there will be enough instability (MUCAPE > 1500 J/kg) across portions of North Carolina and Virginia to support high rain rates (1-2 inch per hour), which may rival 1 and 3 hr FFGs. Upslope flow along the Appalachians from southern Pennsylvania through Virginia will also help to enhance rainfall, with generally 1.5-2.5 inches of rain expected across this area with locally higher amounts possible. After heavy rain on Monday, these areas may be more sensitive to flooding given higher soil moisture, and isolated to scattered flash flooding concerns are expected. This will warrant a continuation of the Slight Risk area from northeastern North Carolina through Virginia and portions of West Virginia, Maryland, and southern Pennsylvania. Isolated flash flooding may also be possible for portions of the Ohio Valley and Appalachians. Moisture wrapping around the northern side of the low pressure system combined with sufficient instability near the surface low will support scattered convection with relatively slow storm motion. Therefore, a Marginal Risk is in place for these regions. Dolan Day 3 Valid 12Z Wed May 14 2025 - 12Z Thu May 15 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHERN PLAINS AND MID-ATLANTIC... ....Northern Plains... A deep upper level trough will push across the Northwest to the Plains on Wednesday, resulting in a strengthening low pressure system in the northern Plains. Stratiform rain with likely some embedded convection on the northern and western side of the low are forecast to produce 0.5-1 inches of rain, with locally higher amounts possible, in eastern Montana and northeastern Wyoming. The main threat will be due to the persistent nature of the precipitation in this area, starting up Tuesday night and continuing through Wednesday into Wednesday night. Rainfall totals will likely come close to the 6 hour FFGs in eastern Montana. For the Dakotas, convective precipitation is forecast to develop ahead of the low along a strengthening frontal boundary. Models are in good agreement on the axis of heaviest precipitation setting up over central North Dakota and eastern South Dakota. PWAT values are expected to be at least 2 standard deviations above normal (1-1.5 inches), and there should be enough instability (MUCAPE > 1500 J/kg) during the day on Wednesday to support at least an isolated flash flood threat. ....Mid-Atlantic... Soils are expected to be thoroughly saturated across the Mid- Atlantic after heavy rain on Monday and Tuesday, which may create a low end flood threat with lingering showers on Wednesday. The slow-moving surface low is forecast to track right over the Mid- Atlantic with scattered precipitation, and there should be enough instability near the low to support some convection capable of producing locally heavy rain. Therefore, a Marginal Risk area was introduced for portions of the Mid-Atlantic. The northern extent of the Marginal was nudged further up into portions of southeast PA and central NJ based on the 12Z guidance. Dolan/Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8-bF188-GdJRPQ-x_DhuXGk4Eic-mP_Br4iOyD1-QsHX= -nNE484Tx79LKE0TSsvuCH0yK4TqL8i81xOqbO0YpHpiFhc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8-bF188-GdJRPQ-x_DhuXGk4Eic-mP_Br4iOyD1-QsHX= -nNE484Tx79LKE0TSsvuCH0yK4TqL8i81xOqbO0Y9yizaeE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8-bF188-GdJRPQ-x_DhuXGk4Eic-mP_Br4iOyD1-QsHX= -nNE484Tx79LKE0TSsvuCH0yK4TqL8i81xOqbO0YAizgwVA$=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) .