Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 15 2025 16:00:26 FOUS30 KWBC 151600 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Mon Sep 15 2025 Day 1 Valid 16Z Mon Sep 15 2025 - 12Z Tue Sep 16 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF EASTERN NORTH CAROLINA AND SOUTHEASTERN VIRGINIA... ....Eastern North Carolina and Southeast Virginia... WV imagery off the NC coast indicates convection associated with a deepening low pressure. This feature is deepening in response to=20 potent closed low (500mb heights below the 10th percentile=20 according to NAEFS) digging across the Carolinas and interacting=20 with a stationary front offshore. Subtle upper diffluence in the=20 distant RRQ of a jet streak near New England is additionally=20 providing ascent, leading to pressure falls within the surface low. This should continue to deepen the wave offshore, leading to=20 intensifying onshore moist flow (850mb winds reaching -3 sigma in=20 to the west) driving intense moisture confluence into NC/VA through tonight as the low lifts very slowly north/northwest, likely=20 moving onshore the Outer Banks before 12Z Tuesday. While there continues to be some uncertainty into the exact timing and track of this low pressure system, guidance has trended stronger with more pronounced moisture advection to the W/NW. This will result in heavy rainfall spreading onshore from southeast North Carolina through the Tidewater region of VA and potentially reaching as far north as southern Delaware before Tuesday morning. The heaviest rainfall is likely to fall along an axis of strong 900-700mb fgen aligned just NW of the surface low, with additional heavy rain likely anywhere north of the low center due to persistent onshore flow and convergence. In these areas, this onshore flow will additionally surge instability to 250-500 J/kg, especially this evening/tonight, which, while modest, will be sufficient in the very moist environment to improve rainfall efficiency and support rainfall rates of 1-2"/hr. Where these train/repeat, total rainfall of 3-5" with locally higher amounts are likely. Although this area has been dry recently (7-day rainfall from AHPS generally less than 5% with few exceptions) leading to elevated FFG, this type of rainfall can still overwhelm the soils in scattered locations. This is reflected by 12Z HREF 3-hr FFG exceedance probabilities rising to around 20%, so despite the antecedent dryness, and after coordination with WFOs MHX/RAH/AKQ, a targeted SLGT risk was introduced with this D1 update. ....Ozarks... A weak mid-level impulse pushing into a short-wavelength but high amplitude ridge this evening will likely result in slow moving showers and thunderstorms. While coverage is expected to be widely scattered, rainfall rates within convection will likely reach 1-2"/hr in response to impressive thermodynamics characterized by MUCAPE of 2000-3000 J/kg overlapping PWs of 1.5 to 1.75 inches. These storms will additionally move quite slowly as 0-6km mean winds will be just around 5 kts, with additionally chaotic and slow propagation vectors. While locally this could result in 2-3" of rain (HREF and REFS probabilities for exceeding 3" peak around 20-30%), this will fall atop dry soils leading to only an isolated chance of FFG exceedance. Opted to hold off on a MRGL risk for this area as the probability appears to be less than 5% for excessive rainfall today. Weiss Day 2 Valid 12Z Tue Sep 16 2025 - 12Z Wed Sep 17 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST, THE CENTRAL HIGH PLAINS, AND VIRGINIA/NORTH CAROLINA... ....Southwest... Scattered convective pattern will initiate on Tuesday afternoon and early evening before fading with the loss of diurnal heating. Current progs are for convection to be tied closer to the terrain across southeast AZ into southern NM with isolated signatures of >1"/hr plausible in the stronger cell cores. These areas generally prone to runoff due to terrain complexities, as well as remnant burn scars present in some locations. Maintained continuity from inherited MRGL risk with some minor adjustments to remove El Paso proper. ....Central High Plains... Strong shortwave trough ejection out of the interior Northwest will lead to convective development and regional enhancement across the Central High Plains. Greatest ascent is aligned with the diffluent region of the mid-level shortwave with left-exit region dynamics likely to maintain a prevalent heavy rain signature across the western third of NE down into neighboring northeast CO and northwest KS. Heaviest rain could remain just outside of the risk area according to some guidance leading to low probability of flash flooding due to the occurrence within the western Sand Hills. There's still a good agreement on localized maxima within the hi- res ensemble means, pin-pointing the area west of North Platte, NE as one of the favorable spots for convective impacts. FFG's in this corridor are lower than the neighboring Sand Hill domain, so the MRGL risk remains relatively small. ....Virginia and North Carolina... Our surface low will mature and occlude by Tuesday afternoon as the closed height fields indicate the 850/700/500 mb lows becoming vertically stacked leading to a slow decay of the surface low by the second half of the period. Persistent heavy rain potential will linger across eastern NC up into the VA Tidewater leading to multi-day totals reaching 2-4" with locally upwards of 6-7" over the span of 48 hrs. Despite the drier soils leading into the event, the cumulative effect will lead to areal flooding concerns for portions of the above locations. Some westerly push of the heavier QPF is progged in the front half of D2 leading to some areas in north-central NC up through south-central VA possibly sneaking into the flash flood threat. The only good news is the weakening surface low will lead to degrading rates eventually, and slow wane on the threat. The MRGL risk inherited was maintained with some minor westerly expansions to account for recent trends in the ensemble means. Kleebauer Day 3 Valid 12Z Wed Sep 17 2025 - 12Z Thu Sep 18 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE NORTHERN AND CENTRAL PLAINS, SOUTHERN HIGH PLAINS, AND SOUTHEAST FLORIDA... ....Northern and Central Plains... A strong mid-level shortwave trough will eject into the WY Front Range and eventually close off across NE generating a maturing lee cyclone across the Central High Plains. The setup will likely cause quite a robust deformation signature on the northwest flank of the cyclone with ensemble mean QPF placement situated over western NE up through southwestern SD, arcing back into the nose of the warm- conveyor belt on the northeast side of the system. This is a classic mid-latitude lee cyclogenesis setup with a strong jet coupling leading to broad large scale forcing and widespread precip across the Plains. Eastern side of the cyclone will exhibit the best convective potential while the deformation axis provides a solid precip field with moderate to locally heavy rates for several hrs. in that northwest side of the low. Totals >2" are forecast within the means across western NE into SD with scattered elevated QPF across the Missouri River basin located in eastern SD and NE. This setup is conducive for at least some isolated to scattered flash flood prospects considering the synoptic evolution. This was enough to warrant a MRGL risk in those areas deemed the best chance for heavy rain prospects. ....Southern High Plains... Maturing lee side low to the north will help drag a trailing cold front through the Central and Southern High Plains on Wednesday. The front will slip south and southwest once down near the latitude of the OK Panhandle, leading to a more textbook backdoor frontal progression as it enters into the northern TX Panhandle and northeast NM. Flow begins to run more parallel to the boundary in these zones when assessing forecast soundings from relevant global deterministic. The key in the setup is a relatively solid isentropic ascent pattern in a small zone bordering CO/OK/NM/TX with cross-section views over the area displaying a classic sloped low-level surface FGEN with strong omega correlating to rapid ascent as the front progresses through the area. Locally enhanced corridor of precip will transpire along and in wake of the front with local maxima of 1.5" to as much as 3" plausible during the event. This could be enough for an isolated flash flood threat, especially in those smaller urban areas situated across the 4 state intersection. A MRGL risk has been added to account for the threat. ....Southeast Florida... Stalled front with an enhanced moisture return across South FL will lead to a heightened threat for flash flooding over the urban centers of southeast FL. Deterministic output is scattered in the exact location of the heaviest precip, ranging from the Keys up to West Palm Beach and everywhere in-between. The setup is historically favorable for flash flood concerns over the urban zones of southeast FL with PWATs hovering between 2-2.3", a solid 1/1.5 deviations above normal. In coordination with the Miami WFO, a MRGL was maintained for that urbanized coastal corridor between West Palm down to Miami proper since this is the most likely location for flash flooding in this type of setup. Will monitor convective trends as we get closer to see if this necessitates any expansion, or even a targeted upgrade. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5XirESeycZHKTGmXQrnNZeYcMGs_T-snrqpmIK8plRZz= -g6Pu6sf39dQGpjIg2_Ui59ceZ1iCryILj2Yp2l0xrj5KPA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5XirESeycZHKTGmXQrnNZeYcMGs_T-snrqpmIK8plRZz= -g6Pu6sf39dQGpjIg2_Ui59ceZ1iCryILj2Yp2l0bCFsr9M$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5XirESeycZHKTGmXQrnNZeYcMGs_T-snrqpmIK8plRZz= -g6Pu6sf39dQGpjIg2_Ui59ceZ1iCryILj2Yp2l0dOHsYi0$=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 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .