Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Nov 11 2024 20:56:24 FOUS30 KWBC 112056 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 356 PM EST Mon Nov 11 2024 Day 1 Valid 16Z Mon Nov 11 2024 - 12Z Tue Nov 12 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....16Z update... A non-zero threat for flash flooding will remain across the central Gulf Coast through early Tuesday morning. GPS data from early this morning showed precipitable water values at or just above 2 inches in the western FL Panhandle and low level convergence near a boundary/front in the vicinity of the Gulf Coast will support locally heavy rainfall in the offshore waters which could migrate to coastal sections through tonight. While moisture is anomalously high, instability is forecast to remain limited for inland locations while ML/MUCAPE up to a few hundred J/kg remains restricted to coastal sections through Tuesday morning. The biggest change from overnight and the 12Z hires guidance is a shift toward the east with the highest QPF but otherwise the forecast reasoning in the previous discussion remains on track. A non- zero threat appears to exist from far southeastern LA to the western FL Panhandle, along coastal locations where slow moving/repeating cells could produce locally excessive rainfall. Otto ....previous discussion follows... A non-zero chance for flash flooding exists within the far Southeast Parishes of LA and along the immediate AL/MS coastal plain as lingering moisture on the northern periphery of the remnant low from Rafael will persist within the area mentioned above. Frontal progression to the northwest will slowly sink southeast with some CAMs initiating a period of stronger boundary layer convergence that would capable of north-south alignment of heavier rainfall situated over the Southeast-most Parishes in LA. The factor that will likely limit the threat will be the lower end rates between 1-2"/hr max that fall well-below the current FFG markers located over the proposed impact area. Totals of 2-4" with locally as high as 6" are depicted within a few of the CAMs, but it's not a full agreement in the suite neighborhood probabilities dropping off precipitously for accumulations >3". Decided to retain the current nil ERO nationally, but wanted to point out the low- end probability (<5%) for isolated flood concerns in the zone encompassing the above Parishes to the immediate coastline across both AL/MS. Kleebauer Day 2 Valid 12Z Tue Nov 12 2024 - 12Z Wed Nov 13 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS WESTERN WASHINGTON AND NORTHWEST OREGON... ....2030z Update... No changes needed based on the new 12z data. Inherited Marginal Risk area was maintained, and the excessive rainfall threat remains below Marginal Risk criteria elsewhere (though is non-zero along the central Gulf Coast, where localized 1"+ totals are possible). Churchill ....Previous Discussion... The next Atmospheric River will impact the PAC Northwest with an IVT pulse between 500-800 kg/ms signaling a modest moisture advection regime that historically leads to marginal flash flooding concerns along the immediate WA/OR coasts, the terrain of the Olympics and along the foothills of the Cascades where the orographic enhancement pattern is defined. PWAT anomalies between 1-1.5 deviations above normal will enter the region by the second half of the period with the heaviest precip likely coinciding with the initial moisture surge into the coastal plain. Some modest instability will be focused along the immediate coast as defined by the some of the hi- res guidance and mean SBCAPE output within the ensemble blend. Rates will be generally ~0.5"/hr within the heavier bands, but some 0.75-1.25"/hr rates are plausible within the Olympics in Northwest WA state, as well the immediate coast of WA down into Northwest OR. This is the primary axis of the IVT pulse with a distinct west to southwest orientation of the mean flow. This initial surge will be more of a primer for the following day as the pattern spills into Wednesday, but some isolated flash flood concerns are plausible within the initial surge of the AR. The previously inherited MRGL risk was maintained with only a minor adjustment a touch further south in Oregon to reflect the latest trends in the mean QPF depicting >2" along the OR coast. Totals will be generally between 1-3" with isolated to 4" into the Olympic Peninsula, much of the precip falling within the 00-12z window on Wednesday. Kleebauer Day 3 Valid 12Z Wed Nov 13 2024 - 12Z Thu Nov 14 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS WESTERN WASHINGTON AND OREGON, NORTHWEST CALIFORNIA, AND OVER THE SOUTHEASTERN U.S... ....2030z Update... No changes needed for this update, as both of the inherited Marginal Risk areas remain on track with the latest 12z data. Introduction of a Slight Risk area for portions of the Southeast CONUS remains possible with future updates (should solutions like the 00z ECMWF or 12z UKMET pan out), but disagreement among model solutions will preclude an upgrade at this time. Churchill ....Previous Discussion... ....Pacific Northwest... Atmospheric River from the D2 period continues through much of the D3 time frame with a waning signal towards Wednesday night into Thursday. IVT core will come ashore between Astoria down through the King Range of Southwestern OR with a signal for an additional 2-4" of rainfall with locally as high as 5.5" within the orographic enhancement in-of the Klamath Mountains. Rates will maintain a steady- state of 0.25-0.5" over a span of several hours, but the primary core of the IVT pulse will likely ramp up rates >0.5"/hr with some 1-1.5"/hr rates plausible across those elevated coastal areas of OR and Northwestern CA. Rainfall will continue up into WA state with the heaviest rain confined to the Olympics with an additional 2-4" likely occurring northwest of Olympia. Two day totals over the coastal areas of WA/OR will settle between 3-6" with a max of 8" confined to both the Klamath-Siskiyou and Olympic Ranges thanks to some orographic enhancement providing localized QPF maxima that is customary from AR orientations extending from southwesterly flow. Additional flash flood opportunities will exist over portions of Northwest CA to areas south of Mount Shasta where remnant burn scars will act as an area of heightened concern with local FFG indices much lower than surrounding areas. Totals will be between 2-4" in this area with the max of 5" occurring over the Northwest tip of CA north of Eureka where heavier rainfall longevity will enhance totals locally. A MRGL risk was maintained for the threat with much of Western WA/OR encompassed, as well as Northwest CA with an extension inland to south of Mount Shasta to cover for the burn scar areas that could see >1.5" of rainfall on Wednesday. ....Southeast U.S... A sprawling surface ridge east of the Mississippi will quickly migrate eastward allowing for a strong return flow pattern across the Southeastern U.S along the the western flank of the ridge. Lingering moisture stemming from the remnants of Rafael will fester in the Northern Gulf before being ushered poleward around that clockwise circulation from the high pressure controlling the pattern across the Mississippi Valley and points east. PWATs will elevate up close to +2 deviations lending credence to a suitable airmass for maintaining convective activity. Further west, a shortwave trough will migrate into the Central Plains with surface low pressure forming downstream across the Southern Plains/Mid- Mississippi Valley creating a strengthening convergence pattern along a cold front that develop on the tail end of the low. Progressive flow within the setup will alleviate some of the concerns of lingering precip that will develop from the Lower Mississippi Valley to points north and east. The jury is still out on the expected intensity of the precip that develops, but considering the convective prospects given the theta-E advection pattern anticipated along with a source of surface based convergence along and ahead of the forecasted cold front, some stronger cores capable of rates >1-2"/hr and totals breaching 3" will be possible, as noted by some of the current deterministic. Ensemble bias corrected QPF output indicates some 2-3" amounts across parts of the Deep South signaling the threat. UFVS First Guess Field has a MRGL risk encompassing parts of LA and MS where the heavy rainfall from recent days has really dropped the FFG indices across parts of the area. The antecedent conditions over Southwest and Central LA will be privy to a heightened flash flood threat leading a MRGL risk extending back into those areas. The MRGL encompasses much of the Lower Mississippi Valley over into AL extending up into TN, coinciding with the forecasted meridional extent of the theta-E progression which would lead to convective precipitation modes. Will assess as we get closer, but this is the proxy for where the expected heavier rainfall threats could occur. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-BPcQzQMiyqi8VUTow8qk0XLIE_O0A4WjuAGTmmamGOl= k3e27rJ8a33-y9TSsqEBvIhfHQbngWait-EoJIA8s-8rizY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-BPcQzQMiyqi8VUTow8qk0XLIE_O0A4WjuAGTmmamGOl= k3e27rJ8a33-y9TSsqEBvIhfHQbngWait-EoJIA8UuxmEDI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-BPcQzQMiyqi8VUTow8qk0XLIE_O0A4WjuAGTmmamGOl= k3e27rJ8a33-y9TSsqEBvIhfHQbngWait-EoJIA8XYC_DuQ$=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) .