Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 06 2024 20:30:40 FOUS30 KWBC 062030 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 330 PM EST Fri Dec 6 2024 Day 1 Valid 16Z Fri Dec 06 2024 - 12Z Sat Dec 07 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....Far Deep South Texas... The slow sagging frontal zone is nearly flat west to east across the Northern Gulf of Mexico and a weak surface to boundary layer cyclone has developed along the upstream edge near the southern tip of Texas. Enhanced deep layer moisture resides along and north of the boundary across this section of the front with total PWat values ranging from 1.5-1.75". Warm sea surface and proximity to the front allow for some weak instability in proximity to that low with values of 750-1000 J/kg available along/offshore. So there will remain some solid potential for thunderstorm activity across far south Texas. However, the vast majority of guidance continues to suggest best overall convergence and strongest updrafts will remain offshore or at the near coast. It is possible some weakening showers or rouge thunderstorm may make an appearance onshore in proximity to the urban corridor of the lower Rio Grande Valley, making the risk for flash flooding non-zero, but not rising to high enough probability/coverage to reach 5% threshold for delineating a Marginal Risk at this time. Gallina Day 2 Valid 12Z Sat Dec 07 2024 - 12Z Sun Dec 08 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....Middle to Upper TX Coast to the Southwest LA Coast... ....2030 UTC Update... Have removed the Marginal Risk area from the Day 2 ERO in this=20 region. While still a non-zero threat, given the latest model=20 trends (much lighter with the QPF, areal-average amounts less than=20 1" with heavier QPF and more appreciable deep- layer instability=20 remaining offshore), feel that ERO risk is below Marginal=20 thresholds (40km neighborhood prob of FFG exceedance less than 5%. Hurley Day 3 Valid 12Z Sun Dec 08 2024 - 12Z Mon Dec 09 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER MISSISSIPPI VALLEY AND CENTRAL GULF COAST... ....2030 UTC Update... Based on the relative fast timing with this system (peak low-mid level moisture transport/flux with weak, elevated instability within the anafrontal setup), per collaboration with the WFOs have dropped the Slight Risk in the Day 3 ERO for areas north of LCH=20 and LIX CWAs. Farther north, drier antecedent conditions (near to=20 below normal soil moisture percentiles) along with MUCAPE values under 500 J/Kg will inhibit the more intense short-term rainfall rates needed to fit the Slight Risk criteria. Therefore even with areas of 2-2.5" of rain expected over parts of eastern AR into northern MS, western TN, and northwest AL -- given the lack of instability the rainfall rates over these areas are expected to peak between 1.5-2.0" within a 6 hour period over isolated areas (FFGs in this region are currently ~ 3" in 6 hours). We did maintain the Slight Risk closer to the Gulf Coast, where more appreciable deep- layer instability and thus higher probability of more intense short-term rainfall rates are more probable. Hurley ....Previous discussion... Continued moisture flux transport via a moderately strong low-=20 level jet is expected to result in increasing coverage of flash=20 flooding on Sunday, as the bulk of the forcing and jet dynamics via the aforementioned strong shortwave trough will not arrive until=20 midday (along with maximized instability). Localized training is=20 expected to result in 2-4" totals, suggesting a lower-end Slight=20 Risk based on the probability spectrum (which ranges from 15-40%).=20 Given that most of the region is experiencing very dry antecedent=20 soil conditions (10th percentile or lower for much of MS/AR/LA, per NASA SPoRT-LIS 0-100 cm soil moisture), any flooding should be=20 relegated to flood-prone areas (with the greatest risk for any=20 localized significant flash flooding over west-central LA, where=20 antecedent soil conditions and streamflows are locally higher). Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4Bi5qKWBk8zWsIJPKYM2LLJlNuioD988SdlNLaT30V3k= NikNHOfWHffadel-91VZeZzjryXaGE_qy2DNIoSGo5lBcjQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4Bi5qKWBk8zWsIJPKYM2LLJlNuioD988SdlNLaT30V3k= NikNHOfWHffadel-91VZeZzjryXaGE_qy2DNIoSGq8UrgRM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4Bi5qKWBk8zWsIJPKYM2LLJlNuioD988SdlNLaT30V3k= NikNHOfWHffadel-91VZeZzjryXaGE_qy2DNIoSGjeJnRe0$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .