Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 08 2023 20:10:00 FOUS30 KWBC 082009 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 407 PM EDT Fri Sep 08 2023 Day 1 Valid 16Z Fri Sep 08 2023 - 12Z Sat Sep 09 2023 ....A MARGINAL RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR MUCH OF THE APPALACHIANS, THE I-95 CORRIDOR FROM DC TO NYC, AS WELL AS EAST-CENTRAL FLORIDA... ....16Z Update... Minimal changes to the current ERO placements across the Northeast and Mid-Atlantic, as well as Florida. 12z HREF probabilities continue to pinpoint the Appalachian front from NC up through PA, as well as south-central PA along the Mason Dixon as the primary focus for enhanced rainfall rates for the Northeast MRGL, and the eastern FL coast for the additional MRGL in place. The synoptic scale pattern remains steadfast from the previous forecast package, so no discernible changes necessary. A distinct signal for 2-3"/hr rainfall rates is targeted over FL thanks to expected sea breeze convergence. Models have been doing generally well with the intensity of the precip field over FL, but they are still behind on the coverage as mid-level ascent under the influence of the closed 500mb reflection over the FL/GA line has provided a bit more widespread coverage of shower/storm activity across the northern Mainland of FL. A general enhancement on the northern extent of the precip field along the leading edge of the west coast sea breeze propagation is well defined south of Gainesville with MRMS indicating a points of 2-4"/hr rates within the heaviest cores. Expectation will be similar later this afternoon with hi-res ensembles signaling the period of 18Z-00Z as the timeframe of interest for the heaviest precipitation. The Northeast U.S remains within a SPC slight risk for later this afternoon with the axis aligned southwest to northeast from the Blue Ridge up through portions of New England. Morning sounding out of KIAD shows a deep moist profile with an emphasis focused in the boundary layer. PW indices between 1.5-1.75" are around 1-1.5 standard deviations above normal which is plentiful for prominent rainfall rates during afternoon convection. The faster storm motions will limit the areal extent of any flooding concerns giving credence to maintaining the MRGL risk over the above area. Kleebauer ....Previous Forecast... ....Appalachians Through Southern New England... A digging and increasingly-negatively-tilted trough will move over the OH and TN Valleys today. Meanwhile, deep southerly flow will advect tropical moisture northward up the I-95 corridor. This pattern will increase divergence aloft, allowing storms taking advantage of the abundant moisture to organize and train along the Appalachians. Thus, a marginal flash flooding threat exists for much of the Appalachians, Mid-Atlantic, and into southern New England. The axis of heaviest rainfall is likely along and in the immediate eastern foothills of the Appalachians. The Marginal Risk area was expanded eastward to include the I-95 corridor from DC to NYC. In addition to a small eastward shift in the guidance of the heaviest rainfall, the inherent lower FFGs in the cities will allow even the isolated to widely scattered storms to be capable of 1 to 2 inch per hour rainfall rates, causing isolated flash flooding. For the southern Appalachians, rainfall from storms yesterday (Thursday) caused isolated flash flooding in the Roanoke Valley. With potential for similar coverage of storms again today, the Marginal Risk area was expanded to include more of southwest VA and northwest NC. ....East-Central Florida... Sea breeze convergence will initiate a round of heavy thunderstorms within a narrow corridor near the Melbourne area this afternoon. Hi-res deterministic has been consistent over the past succession of runs for a line of 2-3"/hr rainfall rates to run along the eastern FL coast within the more urbanized corridor extending from Melbourne to up near Daytona beach. There will be a little more enhancement in the upper-levels thanks to a digging shortwave trough forecast to the enter the area, providing a bit more ascent compared to the typical mesoscale boundary convergence. This will lead to additional coverage of storms over the central part of the state, including the Orlando metro. The Marginal Risk area was expanded westward to include potential for isolated flash flooding from very efficient rain-making storms, especially in urban areas. Wegman Day 2 Valid 12Z Sat Sep 09 2023 - 12Z Sun Sep 10 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR THE WESTERN CAROLINAS AND ADJACENT PORTIONS OF VIRGINIA AND GEORGIA... ....2030Z Update... The main change was an addition of a MRGL risk across the Black Hills and adjacent Thunder Basin Grasslands over northeast WY with some extension into southern MT and southwest SD (More information on that below). The remainder of the D2 forecast is relatively unchanged, but did extend the SLGT to cover Columbia proper after collaboration with the local WFO (CAE) given the trends in HREF probabilities for heavy rainfall to potentially reach the most urbanized area within the CWA confines. 12z HREF neighborhood probability was fairly gung-ho for 2-3"/hr rates within the outlined area, as well as isolated 3-5" totals thanks to a stout upslope flow pattern across the NC/SC Piedmont, as well as low-level convergence signature neared the stalled front likely draped overhead. Synoptic evolution shows very little deviation from previous runs of global deterministic which adds confidence in the potential for flash flooding concerns within the terrain of the Carolinas into northeast GA. Further to the north, more convection is anticipated across the Mid-Atlantic and Northeast U.S where signals for locally heavy rainfall of 1-2"/hr will be plausible within a continued, anomalous moist environment and enhanced mid and upper ascent thanks to a meridional flow pattern setup east of the Mississippi. There is a small chance a targeted slight risk could be added in future updates, but a lot is contingent on today's rainfall of the "where" the heaviest rains occur, and could those areas get impacted 2 or 3 times in a row. For now, will monitor the trends in obs over the next 12 hrs and proceed from there. Kleebauer ....Previous Discussion... ....Western Carolinas... In coordination with RNK/Blacksburg, VA; GSP/Greer, SC; and FFC/Peachtree City, GA forecast offices, a Slight Risk area was introduced for much of the western Carolinas and small portions of adjacent VA and GA with this update. A newly cutoff upper level low will be stuck over eastern TN Saturday and Saturday night. Continued southerly flow up the Gulf Stream will continue advecting a 1.5 to 1.75 inch PWAT northward up the east side of the Appalachians. The combination of upper level divergence over the western Carolinas, plentiful moisture that will be replenished, an upslope component, and antecedent storms in recent days over portions of the area will all favor the development of widely scattered instances of flash flooding, with the heaviest rain likely in the foothills of the Appalachians. Rainfall totals of 1-3 inches are likely with the heaviest rains, but much of that will be from storms producing 1-2 inch per hour rainfall rates. Training and slow-movement of the storms are expected, since the upper level forcing will be nearly stationary. This too will favor flash flooding, particularly in any flood-sensitive and urban areas. ....Mid-Atlantic into the Northeast... Another afternoon of showers and thunderstorms is expected Saturday along the I-95 corridor and the central Appalachians. Like previous days, the storms will be only loosely organized, but given the stagnant weather pattern with a negatively-tilted longwave trough off to the west, expect storms to once again move over many of the same areas again Saturday afternoon as on Friday. The Marginal Risk remains largely unchanged, though a bit expanded to the east. Despite the repeating days of storms, antecedent conditions remain largely dry over the area, which should preclude most flash flooding. With that said, should Friday's storms become more numerous than expected, it's possible an upgrade may be needed for portions of this area, particularly around eastern PA, which should be in the axis of heaviest rainfall both Friday and again on Saturday. Wegman ....Northeast Wyoming/Black Hills of South Dakota... Mid-level vorticity will round the northern extent of the ridge positioned across the western CONUS providing enough ascent to initiate storms off the Big Horn of southern MT, propagating southeast along a narrow theta-e gradient aligned across northeast WY down into northwest NE. ML-CAPE on the order of 500-1000 J/kg within the zone above and sufficient shear will generate a period of severe thunderstorms capable of locally heavy rainfall within their path of impact. Precipitable water values approaching 1-2 standard deviations above the climatological norm signifies an moisture laden column, backed up by forecast bufr soundings located within the corridor outlined. 12z HREF probabilities also indicate a non-zero potential for 2"/hr rates located within the corridor of higher theta-E's near the SD/WY line as convection pivots southeast around the eastern edge of the mid-level flow. When coupled with low FFG's, a MRGL risk was added with collaboration from all offices encompassed. Kleebauer Day 3 Valid 12Z Sun Sep 10 2023 - 12Z Mon Sep 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL PLAINS... ....2030Z Update... Little change to the Slight Risk imposed over the central plains. Ensemble guidance and global deterministic indicate a targeted area over western KS with emphasis where convergence along the expected cold front will generate the best environment for training convection over the course of Sunday afternoon and evening. Ensemble probabilities are fairly robust on D3 for neighborhood precip totals exceeding 2" with some deterministic going as high as 4-5" in the hardest hit areas. Maintained the SLGT given the higher FFG's and antecedent dry conditions in place, but flash flood threat is within the cards given the environment and synoptic scale forecast. Kleebauer ....Previous Discussion... A Slight Risk upgrade was introduced for portions of the central Plains with this update. An MCS will develop Sunday evening and persist through the night. Storms will slowly move southward through KS and into the OK and TX Panhandles Sunday night. A cold front tracking south down the Plains will clash with a northward moving LLJ and abundant Gulf moisture tracking north across TX and into the central Plains. This will be the focus for storms and the developing MCS. Once the storms develop Sunday afternoon, the opposing air masses will keep the front separating the two from moving very much, which will allow for training/repeating storms over the same areas into Sunday night across the Slight Risk area. Antecedent conditions in this area have been bone dry, so the soils in the area are likely to be hard and unlikely to absorb much heavy rainfall. This in turn should result in enhanced runoff from any stronger storms likely to develop in the area. The best chance of flash flooding will be in urban and flood-sensitive areas where strong and repeating storms produce prolonged heavy rainfall. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7nEfK91-ZkXUHlyWcPQBUYzI33cnf3kW66ZdIluf1D4K= Bj1x5BlRPY1pDycRnUZkq9qq_N27fm-RWgLGw1NR8b6ojls$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7nEfK91-ZkXUHlyWcPQBUYzI33cnf3kW66ZdIluf1D4K= Bj1x5BlRPY1pDycRnUZkq9qq_N27fm-RWgLGw1NRYoDBPsI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7nEfK91-ZkXUHlyWcPQBUYzI33cnf3kW66ZdIluf1D4K= Bj1x5BlRPY1pDycRnUZkq9qq_N27fm-RWgLGw1NR8xT94_o$=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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