Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 28 2024 20:19:47 FOUS30 KWBC 282019 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 419 PM EDT Sat Sep 28 2024 Day 1 Valid 16Z Sat Sep 28 2024 - 12Z Sun Sep 29 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE OHIO AND TENNESSEE VALLEYS AS WELL AS FOR CENTRAL AND SOUTHWEST FLORIDA... ...16Z Update Summary.. Only minor changes were necessary for each respective MRGL this period. Kleebauer ....Ohio/Tennessee Valleys... 16Z Update: The remnants from Helene continue to spin over the Ohio Valley with the surface reflection becoming increasingly diffuse as noted by the 12z observations. The upper support is still present, but the limited instability is noted within the latest 12z soundings out of KILN and KBNA with modest moisture but meager lapse rates and weak capping across the boundary layer. Persistent cloud cover will limit destabilization at the surface leading to less of a convective concern over the region and more towards lingering stratiform with embedded rates between 0.25-0.5"/hr at peak intensity. Antecedent conditions are the main driver for isolated flash flood concerns after yesterdays rainfall with FFG indices running closer to 0.5-1"/hr for potential opening the door to a low-end threat likely ~5% threshold necessary. The threat will subside in full after sunset as any destabilization potential quickly fades and rates fall to well-below any threat for flash flood concerns. This will likely be the last forecast with a MRGL over the Ohio Valley for D1 with a removal at the 01z update. Across the Central and Southern Appalachians, persistent flooding allows for an extremely low threshold for any flooding risks as additional rainfall is "Insult to injury" for the region. A maintenance of the MRGL was warranted to cover for any threat lingering over the region. Kleebauer ....Florida... 16Z Update: Current radar/sat composite indicates the best convective concerns remain confined over West-Central and Southwest FL within the prevailing southwest flow off the Gulf into the western shore. Modest SBCAPE within the 12z KTBW sounding maintain a positive posture for convective initiation and potential that will last through the afternoon. PWATs between 1.9-2" run near +1 deviations according to NAEFS, a signal that usually points towards a low-end threat, at best over the FL coasts with less of a chance further inland. This is primarily an urban flash flood risk from Tampa to Fort Meyers with a time frame between now and 23z as the period of interest before the threat dwindles. HREF probs for >3" run between 35-50% over the above area, just enough for the area, especially after dealing with rainfall from Helene. Kleebauer Day 2 Valid 12Z Sun Sep 29 2024 - 12Z Mon Sep 30 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF=20 THE INTERIOR MID-ATLANTIC... ...20Z Update Summary.. An upgrade to SLGT risk was added across portions of the interior Mid Atlantic, including the Shenandoah Valley and Blue Ridge of VA. Only minor adjustments were necessary for the southern extent=20 of the MRGL across FL.=20 Kleebauer ....Mid-Atlantic/Central Appalachians... 20Z Update: Remnant moisture from Helene will remain caught up in=20 the upper trough pattern across the Ohio Valley while the mean=20 trough begins to slowly advance eastward through the period.=20 Lingering mid-level energy will rotate around the base of the=20 trough entering into the interior Mid Atlantic during peak diurnal=20 destabilization. The combination of increased upper forcing, modest SBCAPE, and PWAT anomalies between +1-2 standard deviations will=20 generate an environment capable of scattered to numerous convective pulses that will unfortunately exhibit slow mean storm motions=20 given the recent bufr soundings across the Central Appalachians.=20 Recent ensemble trends, especially within the bias corrected QPF=20 and HREF outputs depict a relatively small max area within the=20 Shenandoah and adjacent Blue Ridge with the areal mean QPF coming=20 in between 1-1.5". Typically this wouldn't be a huge concern, but=20 the issue is the antecedent conditions of the area play a pivotal=20 factor in the potential flood concerns across much of Western VA,=20 Eastern WV, down into the Apps of NC. Helene has degraded much of=20 the FFG indices to abysmal levels (<1" necessary for flooding=20 concerns) leading to a heightened state for flash flooding, an area still recovering from the catastrophic impacts of Helene. 6hr FFG=20 exceedance probabilities are up between 60-80% across much of the=20 above area between the 18-06z time frame in the forecast window=20 according to the latest HREF, a signal more pronounced than the=20 previous forecast iteration. In coordination with the local WFO's,=20 a SLGT risk was added with heightened wording despite a less=20 primitive signal than normal. A targeted upgrade across parts of=20 the SLGT risk are also not out of the question in future updates.=20 Kleebauer ....Florida... 20Z Update: Persistent southwest regime off the Gulf and slightly above normal PWATs will elevate the convective threat for another period across Central FL Peninsula into the urban corridors of Southwest FL. Relatively unstable airmass coupled with lingering=20 deep layer moisture will promote isolated to widely scattered=20 thunderstorm activity capable of localized flooding, especially=20 within urbanized zones. A quick 2-4" are plausible in any cells=20 that pulse up, especially near the coast where sea breeze focal=20 points could provide the necessary element to obtain a maximum in=20 the current setup. The risk from previous period was maintained=20 with a small shift south for the southern end of the MRGL.=20 Kleebauer Day 3 Valid 12Z Mon Sep 30 2024 - 12Z Tue Oct 01 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF=20 THE INTERIOR MID ATLANTIC... 20Z Update: The biggest change from the last forecast was the uptick in the ensemble QPF output across the same areas as the D2 time frame; the Shenandoah Valley into the adjacent Blue Ridge of VA and far Eastern WV. There's a growing consensus for a repeat type setup across the interior Mid Atlantic, continuing the trend of locally heavy rainfall in the same areas as the period prior, only exacerbating the flood recovery efforts from Helene. High pressure to the north, and a developing surface reflection off the Mid Atlantic coast thanks to the upstream trough will create a persistent northeasterly flow setup with greater upslope emphasis within the terrain across Western VA into the Eastern confines of WV. The convergence signal is prevalent when assessing the 850-700mb Omega panels within global deterministic, a well-defined axis of strong VV's situated across the interior located right into the best zone of elevated moisture in the column. It's a similar premise to the last period that despite a non-textbook indication for prolific rates, the combo of continued rainfall near 1-1.5"/hr at times in the stronger cores, plus the antecedent soil moisture and continued flooding will enhance the flash flood prospects. This was enough to add a targeted SLGT risk over parts of VA/WV with a chance for broader coverage if the signal becomes more favorable with the addition of CAMs input.=20 Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4Oe5N3Ip9PXIIA09zBYaK3V9hz034NEs0pRHU4o8Gbtn= kP2LpZVYpCIqIkgNALIO28Gh4PReUtZ8UEe9Ks24BIAgQgQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4Oe5N3Ip9PXIIA09zBYaK3V9hz034NEs0pRHU4o8Gbtn= kP2LpZVYpCIqIkgNALIO28Gh4PReUtZ8UEe9Ks241lEUcOA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4Oe5N3Ip9PXIIA09zBYaK3V9hz034NEs0pRHU4o8Gbtn= kP2LpZVYpCIqIkgNALIO28Gh4PReUtZ8UEe9Ks24ek3d5e8$=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. 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