Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Nov 13 2024 15:53:53 FOUS30 KWBC 131553 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1053 AM EST Wed Nov 13 2024 Day 1 Valid 16Z Wed Nov 13 2024 - 12Z Thu Nov 14 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE DEEP SOUTH... ....16Z update... Across The lower Mississippi Valley, an axis of low level convergence extending northward from a surface low located just south of the Louisiana coast has resulted in training and=20 repeating of heavy rain this morning near I-10/I-49 in the vicinity of Lafayette. While some of the hires models (including the HRRR)=20 have been underdone with the localized higher rainfall totals=20 through 15Z this morning, there is good agreement that the flash=20 flood threat will shift northward through Mississippi during the=20 afternoon with the highest instability focusing potentially the=20 greatest rainfall totals across Louisiana and Mississippi into the=20 early overnight, prior to the rainfall shifting eastward. The=20 Marginal and Slight Risks were trimmed a bit on the west side to=20 account for the latest radar/short-term model trends but the outlook was otherwise left the same as earlier. For the West Coast, the maximum IVT axis was moving across northern California this morning with locally heavy rain embedded within. While the axis of IVT will weaken and shift south ahead of the cold front moving through the region today, weak instability in the=20 post-frontal environment will continue to support=20 showers/thunderstorms with high short-term rainfall rate potential from northern California into western Washington. Due to this=20 consideration and various burn scars across the region, the=20 Marginal Risk was left in place for this update. Otto ....previous discussion follows... ....Southeast U.S and Ohio Valley... A 1034mb surface ridge is currently analyzed over the Great Lakes and adjacent Ontario Province with an expected translation eastward over the next 24 hrs leading to an increasing return flow pattern over the Southeastern U.S. Current v-vector anomalies pinpoint the strongest return flow between sfc-850mb bisecting areas from LA into MS and Western AL leading to ample moisture being pulled poleward as far north as the Great Lakes. PWAT anomalies on the order of +2 to +3 deviations will be co-located with the strongest v-vector component leading to a broadening area of elevated moisture and accompanying theta-E's indicative of greater instability, especially the closer you are to the Gulf. Across the Plains, a potent shortwave trough will shift eastward with increasing mid and upper level ascent across areas downstream within the Low to Mid-Mississippi Valley area. The combination of the pattern evolution will aid in increased convective development and coverage, especially as we work through the late-morning time frame and beyond during the period. Heavy rain potential will be on the rise with the introduction of the thunderstorm risk with rainfall totals from the latest HREF blended mean now sufficiently into the 1.5"+ territory with associated CAMs signaling isolated totals >4" during the time frame of convective impact. This has ramped up probabilistic signals with the neighborhood probs now upwards of 40-70% across portions of South-Central LA through much of MS with the highest probs located over the MS Delta over to Jackson. Organized convective pattern is most likely to occur over this area thanks to the approach of a cold front upstream stemming from surface low development over the Southern Plains, moving northeast through the Mississippi Valley after nightfall. Hourly rates within the heaviest convection across the Deep South is likely to range from 1-2"/hr on average, but some non-zero prospects for 2.5-3"/hr persists within the CAMs and expressed through the probability fields on the 00z HREF output. Typically these signals would warrant a MRGL risk for excessive rainfall, but considering the soils across parts of LA and MS still recovering from the onslaught of heavy precip from a few days prior, the antecedent environment is a bit more conducive for flash flood prospects. In coordination with the local WFOs within LA and MS, a Slight Risk was added to encompass areas where heavier convection is most likely considering the pattern evolution, as well as covering the areas where top layer soil moisture anomalies lie above the 70th percentile as reported by the updated NASA SPoRT data. The remainder of the Southeast and points up into the Ohio Valley will maintain the previous MRGL risk forecast with only a minor expansion to the north and east to account for some of the trends of increasing heavy rainfall potential through Northern KY as the system remains fairly progressive in its forward propagation. Limited instability will also cap the risk for significant rainfall rates, but instances of >1"/hr are certainly plausible considering the prime mid-level evolution and approaching cold front providing surface based forcing. Totals will be closer to the 1-2" range for most, but 2-3" is within reason in the heaviest rain cores. ....Pacific Northwest... Atmospheric River from this evening will continue to unfold across the Pacific Northwest with the IVT pulse orienting further south through the morning and early afternoon with heavy rain impacting areas of Southwest OR through much of Northwestern CA with the heaviest rain focused at the immediate coast into the interior terrain aligning from the Klamath Mountains down towards the King Range south of Eureka. The forecast has been very consistent on the threat the past several days with little deviation in the expected outcome with totals ranging from 2-4" at the immediate coast from WA down through Northern CA with 3-6", local maxima of 8" during the 48-hr period of impact over places like the Olympic Range in WA state and the Klamath Range down in Southwestern OR. The premise of the MRGL risk was mainly for 3 or 6 hour FFG exceedance intervals, along with some 24 hour FFG potential, mainly located over the Olympics for the latter index. Hourly rates will be majority sub- FFG exceedance, however some 1-1.5"/hr rates during the height of the IVT surge Wednesday morning over the coastal mountain ranges could be sufficient for isolated flooding prospects before becoming more of an accumulative rainfall concern over time. The one area where the heavier precip is not anticipated but still a relative concern remains in the area located south of Mount Shasta and points west where remnant burn scars maintain locally lower FFG indices leading to a heightened threat for flash flooding where the scarring is located. This is the reason for the inland extent across Northern CA and to maintain consistent messaging with the local WFO for the threat. Kleebauer Day 2 Valid 12Z Thu Nov 14 2024 - 12Z Fri Nov 15 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Kleebauer Day 3 Valid 12Z Fri Nov 15 2024 - 12Z Sat Nov 16 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8VsI0I55aVXzMI0Yd9ga4nYb43QSrJXoYdRk4-uhdoky= KYKzsYCoMChKarB7SEB7SEVoB5SsNvK-PfnDBgUY_iZNWSc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8VsI0I55aVXzMI0Yd9ga4nYb43QSrJXoYdRk4-uhdoky= KYKzsYCoMChKarB7SEB7SEVoB5SsNvK-PfnDBgUYuOBfKm0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8VsI0I55aVXzMI0Yd9ga4nYb43QSrJXoYdRk4-uhdoky= KYKzsYCoMChKarB7SEB7SEVoB5SsNvK-PfnDBgUYDumMChQ$=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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