Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 30 2023 19:11:40 FOUS30 KWBC 301911 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 311 PM EDT Sat Sep 30 2023 Day 1 Valid 16Z Sat Sep 30 2023 - 12Z Sun Oct 01 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE GREAT BASIN, NEW MEXICO AND FAR WEST TEXAS, AND FLORIDA... ....16Z Update... The only change of significance this morning was to remove the Marginal Risk area across WI and the U.P. Ongoing rainfall across the U.P. has been weakening, while the entire area's soils are much drier than normal, per NASA Sport soil moisture imagery. Much of the rainfall that has already fallen in this area has been beneficial, owing to those dry soils. Later this evening into tonight, renewed shower activity is likely to develop, however the guidance has been decreasing on total rainfall amounts, with most of the CAMs suggesting just showers, with very little convection expected. Elsewhere the 3 Marginal Risk areas in FL, NM, and NV were largely unchanged. In FL, a stalled out front that locally resulted in 4 to 6 inches of rain in the Daytona Beach area has been drifting northward and weakening with daytime heating. Expect widespread convective coverage as usual again this afternoon, but no one spot is expected to see large amounts of rain as compared with the surrounding areas. The Marginal Risk remains in effect. Light to moderate rainfall is ongoing across NV, but is unlikely to result in any more than isolated flash flooding due to lower rainfall rates. Showers and thunderstorms are still expected to develop this afternoon and evening across NM, where the rainfall event has yet to begin. Wegman ....Previous Discussion... ....Great Basin/Nevada... A deep upper-trough will transport a plume of moisture (1-2 PWAT stndv) into the West today. A cold front associated with a deepening low pressure system will act as a focus for convection over the Great Basin, in particular. There's been some consolidation and better agreement between the hi-res and global guidance with respect to the location of the axis of heaviest precipitation. Some parts of central Nevada, southeastern Oregon and southwestern Idaho could see rainfall rates around 0.25"/hr with isolated higher amounts. Ample forcing over elevated terrain could lead to more efficient/enhanced rain rates. Between 1-3" of rainfall is expected through tonight. ....New Mexico/west Texas... Convection supported by a deep upper trough to the West and leading shortwave energy will support another localized flash flood threat over parts of eastern New Mexico and into west Texas. Decent instability (1000J/kg+), modest moisture flux and a favorable low level jet may produce bursts of heavy rainfall over our marginal risk area. Urban areas and dry washes are most vulnerable of flash flooding. ....Upper Midwest... An embedded shortwave within an omega ridge will continue driving convection through this afternoon. The heaviest rainfall may already be occurring over parts of eastern Minnesota and northwestern Wisconsin. That being said, impressive forcing, instability (1000J/Kg+) and moisture (2-3 PWAT stndv) could still generate excessive rainfall leading to isolated instances of flash flooding. ....Florida... A slow moving shortwave trough will allow for the continuation of scattered to isolated thunderstorms across the central/southern peninsula today. Some cells may produce localized heavy rainfall within the marginal risk area. Antecedent conditions will also contribute to the sensitivity to any isolated heavy rainfall threats. Kebede Day 2 Valid 12Z Sun Oct 01 2023 - 12Z Mon Oct 02 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF FLORIDA AND THE SOUTHERN HIGH PLAINS OF NEW MEXICO AND TEXAS... ....19Z Update... No significant changes were made to the inherited ERO areas for Day 2/Sunday. A slow moving front over FL will continue to support slow/nearly stationary cells developing along the Atlantic coast and drifting west/into the I-95 corridor again on Sunday from the Space Coast south to the Everglades. Portions of the Space Coast around Daytona Beach saw multiple inches of rain and localized flash flooding this morning, so any additional rainfall, particularly on the order of multiple inches, would likely result in additional flash flooding. As there's little agreement on where any higher totals would be, for now leaving at a Marginal until such time as there's better agreement on where heavier rainfall totals may fall...always a challenge in FL. Day 2/Sun remains the quietest of the 3 short-term days as far as rainfall totals into NM/TX. Any upgrades will be highly contingent on over-performance this afternoon in this area, as a lack of rainfall overall should preclude any more than isolated flash flooding. Wegman ....Previous Discussion... ....Southern High Plains of New Mexico/Texas... The deep diffluent pattern over the Great Plains will continue to generate convection in the Southern Plains on Sunday. Instability around 1000J/Kg and PWATs of over 1" could support some instances o flash flooding Sunday afternoon and evening. The global models appear to be well clustered over the Southern High Plains as well, however the past couple of GFS runs appear to suggest that the heaviest rainfall will occur on the southern edge of the marginal risk area so an upgrade is not necessary at this time. Ensemble mean exceedance probabilities of 1" in 24 hours including the 00z GEFS are well clustered over the northeastern New Mexico and the Texas panhandle. ....Florida... Shortwave energy present across the state of Florida will support isolated flash flooding concerns on Sunday. Scattered to isolated thunderstorms will continue to develop along a quasi-stationary front draped across the southern peninsula. There will be plenty of instability in place so there's always the risk of some cells over performing current forecasts with efficient rainfall rates that exceed flash flood guidance. Kebede Day 3 Valid 12Z Mon Oct 02 2023 - 12Z Tue Oct 03 2023 ,...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN HIGH PLAINS OF NEW MEXICO AND TEXAS... ....19Z Update... For Day 3/Mon there is a much better signal for heavy rain in all of the guidance along the NM/TX border. The Marginal Risk in this area was maintained, as overall rainfall totals have come down a little bit, and the entirety of the NM/TX border making up the eastern end of NM is in play for locally heavier rainfall totals. Thus, the area is in a higher-end Marginal, with a decent chance that come better agreement and CAMs guidance, that a portion of this area may need to be upgraded to a Slight. For now however, have opted to leave as a Marginal due to the relatively diffuse signal in this area. ....Southeast Florida... The third straight day of rainfall along the Atlantic Coast, particularly from West Palm Beach southward through Miami, necessitated introducing a Marginal for the I-95 corridor. The same very slow moving front should continue to make southward progress, but by afternoon the abundance of moisture and instability should allow for renewed convective development in this area. ....Northeast WY through ND... A Marginal Risk area was considered here along a stalled front, but antecedent dry conditions and poor overall signal left this area for now risk-free. However, the area will continue to be monitored. Wegman ....Previous Discussion... ....Southern High Plains... The Southern High Plains will be under the influence of deep diffluent flow from a potent upper trough propagating through the West on Monday. This will be coupled with the arrival of a strong upper jet aloft and favorable low level jet dynamics. Cape values in the 1000-1500J/Kg range and moisture anomalies well over 2 standard deviations (1-1.5in") should support thunderstorm activity with excessive rainfall potential. The latest guidance has certainly trended wetter and is well clustered over the Southern High Plains region for the location of heaviest rainfall potential. The ensemble mean excellence probabilities are in good agreement for 1 and 2" within the drawn marginal risk area. Mitigating factors for an upgrade at this time include relatively dry soils and high flash flood guidance. Potential severe thunderstorms could overcome the mitigating factors by producing higher rain rates, but there is still some uncertainty around the severe weather threat at this time. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5Lm1DNawJs3dNCGNUfQBWVZ7f4r_3qImQaMvC4qWnnUP= trPV7ztUvne0t2ae_HHo5oh_BXfd9RVax4xT_ajAGdF7cc8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5Lm1DNawJs3dNCGNUfQBWVZ7f4r_3qImQaMvC4qWnnUP= trPV7ztUvne0t2ae_HHo5oh_BXfd9RVax4xT_ajA-SdZOrs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5Lm1DNawJs3dNCGNUfQBWVZ7f4r_3qImQaMvC4qWnnUP= trPV7ztUvne0t2ae_HHo5oh_BXfd9RVax4xT_ajAhEUdOZI$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .