Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Oct 02 2023 15:56:29 FOUS30 KWBC 021556 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1155 AM EDT Mon Oct 02 2023 Day 1 Valid 16Z Mon Oct 02 2023 - 12Z Tue Oct 03 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN HIGH PLAINS... ....16z Update... ....Southern High Plains... Only a minor expansion of the northwest edge of the Slight Risk area was needed for this update, as the 12z HREF remains in good agreement concerning convective development across the region this afternoon. The HREF 40-km neighborhood probabilities for 2" exceedance generally range from 20-40% across the Slight Risk area, and probabilities for 3" exceedance are maximized at the southern end of the area (where FFGs are considerably higher). ....Florida... No changes needed for the Marginal Risk area, as the 12z HREF continues to indicate relatively high odds (up to 35%) for localized 5" exceedance over the course of the day, maximized near the relatively sensitive Miami metro. ....Texas Coast... The Marginal Risk area was adjusted a bit based on the new guidance, but this area also remains on track for localized 5" exceedance risk through the day (per 12z HREF 40-km neighborhood probabilities of 10-30%). Churchill ....Previous Discussion... ....Southern High Plains... An upper trough propagating through the West will trigger showers and thunderstorms across the High Plains today. The Southern High Plains, in particular, will face the threat of excessive rainfall and flash flooding. PWATs will be between 1-1.5" with decent low level southerly flow. The instability this afternoon will be over 1000J/Kg meaning storms that initiate could produce efficient rain rates of over 0.5"/hr over recently saturated soils. The Slight Risk area, in coordination with ABQ, was expanded northwestward since the last issuance to account for the especially vulnerable Hermits Peak Calf Canyon burn scar. The Slight's southwestern extent was maintained to account for a consistent GFS and HREF signal for another round of late night/early morning convection near Pecos and Monahans in west Texas. HREF probabilities are hitting on high confidence of over 3" in that area tonight. ....Florida... The Marginal Risk area from the last issuance was curtailed to encompass only southeastern Florida surrounding the eastern Keys and the Miami metro. A slow moving surface front is expected to continue its southward trajectory today. The main threat of flash flooding will be over the urban corridor where there's moderate confidence of HREF probabilities reaching over 3". ....Texas Coast... There's been a persistent signal for some heavy rain to occur along a convergence zone within southern Gulf Coast of Texas today. Ample instability around 1500-2000J/Kg and a modest low level jet will support a few intense rounds of convection this afternoon. HREF probabilities suggest moderate confidence of over 3" of rainfall. Urban areas such as Corpus Christi will be susceptible to flash flooding. Kebede Day 2 Valid 12Z Tue Oct 03 2023 - 12Z Wed Oct 04 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE GREAT PLAINS, THE GULF COAST OF TEXAS AND THE EAST COAST OF FLORIDA... ....Great Plains... A broad Marginal Risk of excessive rainfall is in place across the Great Plains and parts of the Upper Midwest on Tuesday as moisture streams out ahead of the deepening upper trough emerging over the High Plains. Showers and thunderstorms will focus around a cold front extending down across the Plains with the greatest chance for excessive rainfall occurring closest to the greatest instability from the Southern Plains up into northern portions of the Central Plains. Guidance has consistenly signalled this, but has lacked a strong signal for especially high rain rates to overcome relatively dry soils at this time. ....Texas Coast... Another Marginal Risk area was introduced for this issuance to account for another round of convection on Tuesday along a coastal convergence zone. Operational guidance has picked up a signal for this and the similarities with today's setup of low level convergence within an unstable environment and a modest low level jet support the possibility of excessive rainfall leading to isolated flash flooding. ....Florida... Recent rainfall has made much of Florida's Atlantic Coast susceptible to flash flooding, despite the operational guidance keeping the heaviest QPF well offshore. The confluence of these two factors should support a low confidence Marginal Risk area along the coast for now. If operational QPF trends continue on a downward trend then the Marginal Risk area might be removed altogether on the next issuance. Kebede Day 3 Valid 12Z Wed Oct 04 2023 - 12Z Thu Oct 05 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS... ....Southern Plains... There is reasonably high confidence in a heavy rainfall event unfolding over much of the Southern Plains of Oklahoma and north-central Texas on Wednesday. The operational models have consistently signaled 2-4" (locally higher) of 24 hour QPF for much of the current Slight Risk area centered over the Red River. The main threat of excessive rainfall will occur during the peak heating hours of the late afternoon and evening when convection is expected to initiate. The amplified upper trough swinging through the Great Plains and upper jet crossing will provide ample forcing of convection over Texas and Oklahoma. Sufficient instability of over 1000J/Kg will be in place with low level jet support to allow for higher rain rates. PWATs of 1.5-2" across the Texas Oklahoma border region will be approximately 3 standard deviations above normal. Further upgrades will be considered if confidence continues to grow. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-OPHQwa85tqhWu6bkbyGAOhMyIgjweAdS0fEtYJAEB--= yN5Cbx9rAjXf0B1PxTZ8_3PfociHC8fmd-VtmkFAxrKRo_Q$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-OPHQwa85tqhWu6bkbyGAOhMyIgjweAdS0fEtYJAEB--= yN5Cbx9rAjXf0B1PxTZ8_3PfociHC8fmd-VtmkFApqJIiec$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-OPHQwa85tqhWu6bkbyGAOhMyIgjweAdS0fEtYJAEB--= yN5Cbx9rAjXf0B1PxTZ8_3PfociHC8fmd-VtmkFAwY_mjBM$=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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