Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Oct 10 2023 00:16:00 FOUS30 KWBC 100015 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 815 PM EDT Mon Oct 09 2023 Day 1 Valid 16Z Mon Oct 09 2023 - 12Z Tue Oct 10 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley Day 2 Valid 12Z Tue Oct 10 2023 - 12Z Wed Oct 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTH TEXAS... ....2000Z Update... Some minor adjustments were made to the northern extent of the SLGT risk across South TX to align with the latest WPC forecast and ensemble-bias corrected QPF field. Elevated moisture stemming from the remnants of TS Max crossing through MX and the northeastern extent of the moisture associated with TS Lidia will allow for an efficient, warm rain process-driven heavy rain event across the outlined area. Despite the limited convective nature of the precip given the lack of overall instability, the well-above normal PWATs expected and overall QPF forecast will allow for localized flash flooding concerns within the urban corridors located along the RGV and TX Gulf coast. Some guidance is fairly bullish on the total QPF footprint with a majority of the precip falling within the 12 hr time frame from 00-12z Wednesday. In coordination with the local WFOs (CRP/BRO), will maintain the SLGT risk in place with only an adjustment on the northern periphery of the risk area based on the recent trends in guidance. Kleebauer ....South Texas... An amplifying trough moving across the southwestern U.S. and northwestern Mexico is expected to direct the remnant energy associated with Tropical Storm Max, followed quickly by energy associated with Tropical Storm Lidia, across northern Mexico.=20 Moisture associated with these systems in addition to increasing low level southerly flow ahead of a wave lifting through the northwestern Gulf of Mexico will support steadily rising PWs across the region on Tuesday. Guidance shows PWs climbing to 2-2.5 inches (2-3 standard deviations above normal) along a low level boundary lifting north along with the wave. This deep moisture, along with large-scale ascent afforded in part by right-entrance region upper jet forcing, will likely support the development of heavy precipitation across parts of the area. With plenty of differences in the details, guidance generally indicates two areas of heavy amounts -- one inland across southern Texas in the vicinity of the better mid-to-upper level forcing; and a second area near the coast and the developing surface wave.=20 Models disagree as to how far, if at all, this second area will develop and spread inland. While confidence in the details is limited, still felt there was enough of a model signal to maintain a Slight Risk across portions of the region, with the potential for excessive rain remaining a concern especially across urbanized areas and areas impacted by recent rains. Pereira Day 3 Valid 12Z Wed Oct 11 2023 - 12Z Thu Oct 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG PORTIONS OF THE CENTRAL GULF COAST... ....2000Z Update... The overall synoptic scale setup remains steadfast with only minor fluctuations in the overall height field and positioning of the warm front propagating north through the Gulf of Mexico. There is some higher confidence in the heaviest axis of rainfall likely further east within the last forecast SLGT risk issuance with the FL Panhandle and coastal AL being the main beneficiaries of the heaviest rainfall. This is backed up by ensemble probabilities within the GEFS/ECS where signals for 2" or greater total QPF are approaching the 30-40% territory along the Gulf coast into the western portion of the Panhandle. The limiting factor in the whole setup is still the overall instability axis and magnitude which will come down to the positioning of the frontal boundary in the Gulf as the main energy ejects northeast into the Gulf Coast. As of now, the greatest opportunity for higher impacts lies along a line from New Orleans-Mobile-Tallahassee and points south. The western edge out near New Orleans is the lowest confidence within the SLGT risk given the latest forecast probabilities, however the potential for higher impacts over Southeast LA is certainly within the mean error out at D3, so will keep continuity and monitor later trends for any future adjustments. Further north, the limited instability paradigm becomes more pronounced and the threat for widespread flash flooding concerns decreases pretty quickly, so opted to keep a tight gradient within the risk categories as flooding concerns should be mitigated from dry antecedent conditions and associated higher FFG's in place. Across the central plains and Mid Mississippi Valley, little deviation in the previous MRGL placement was needed as the heaviest QPF axis aligns perfectly within the narrow corridor of higher instability wrapping over the eastern and northern flank of our maturing lee-side low pressure. A tongue of 1000-1500 J/kg of MUCAPE is being forecast across western IL extending back through IA and eastern NE. Overall convective environment improves after 00z Thursday, and locally heavy rainfall within the ribbon of convective development is very much in the cards as PWATs rise precipitously to 1-2 deviations above normal within the core of expected development. Rates exceeding 2"/hr are possible during the setup, leading to a higher flash flood concern compared to a more traditional stratiform evolution. Kleebauer=20 ....Central to the eastern Gulf Coast... Low pressure moving into the northwestern Gulf of Mexico is forecast to track east with a warm front and corresponding pool of deep moisture lifting out ahead of it. Strong, deep southwesterly flow into the boundary will support PWs of 2-2.25+ inches spreading north into the region. In addition to the deep moisture, right-entrance region upper jet forcing will remain a factor as well -- supporting large-scale ascent and the development of organized precipitation within the region. While the overall setup is favorable for at least locally heavy amounts, confidence regarding their placement is less than average. The axis of heaviest amounts and the greater threat for flash flooding will be at least partially dependent on the timing of the wave and on the placement of the warm front. While warm rain processes will likely support efficient rain processes, limited instability north of the front will help to keep rain rates in check, limiting the flooding threat for those areas, especially given the dry antecedent conditions that cover much of the region. Heavier rain rates are expected along and south of the front. Given what is expected to become a fairly amplified wave, the front is expected to lift at least partially inland with heavy amounts along the coast. Therefore, a Slight Risk was introduced along the coast from southeast Louisiana to the Florida Panhandle. However, given the overall uncertainty regarding the timing of the wave and the placement of the warm front, significant adjustments may be required in future updates. ....Mid Mississippi Valley... Potentially heavy rain may develop across the region as an upper low develops and drops southeast across the central Rockies into the Plains. Amplifying southerly flow into a slow-moving boundary ahead of the low will produce increasing PWs into an area of strong lift supported by a favorable upper jet couplet. This may result in some localized heavy amounts centered across Iowa into northern Illinois beginning by late Wednesday and continuing into the overnight. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4Gvqg2JADeAxBtfV0C00Xyjk-7JcgLZci9R4Oi3zz4Pl= O-qF2Gc3h9sI7rrqrDZJgVqzyMjV04l7sm33dShFHR_lVs4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4Gvqg2JADeAxBtfV0C00Xyjk-7JcgLZci9R4Oi3zz4Pl= O-qF2Gc3h9sI7rrqrDZJgVqzyMjV04l7sm33dShF6QK9C0A$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4Gvqg2JADeAxBtfV0C00Xyjk-7JcgLZci9R4Oi3zz4Pl= O-qF2Gc3h9sI7rrqrDZJgVqzyMjV04l7sm33dShFVxWWaVc$=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) .