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FOUS30 KWBC 300024 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 723 PM EST Wed Nov 29 2023 Day 1 Valid 16Z Wed Nov 29 2023 - 12Z Thu Nov 30 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley Day 2 Valid 12Z Thu Nov 30 2023 - 12Z Fri Dec 01 2023 ...THERE IS A MARGINAL RISK AREA FROM THE CENTRAL GULF COAST TO THE MID-MISSISSIPPI/OHIO RIVER VALLEY... ...20Z Update... Primary change to the previous D2 issuance was trimming back on the northern extent of the MRGL risk across central and eastern MO into southern IL. Consensus on warm front approach late in the period will allow for some shallow convection to develop, but the lack of appreciable rates worthy of flash flood concerns will be a deterant with the latest 12z HREF 1"/hr probabilities totally void over the aforementioned area. Totals will be generally within the 0.75-1.5" territory, and likely over an extended period of time. Thus, brought the northern extent further south into southern MO where there's a slightly better risk for convection and locally higher rates. The trend continues to have a focal point of convection and flash flood concerns across east TX into the western half of LA where instability maximum for the period will be co-located within the best ascent from our passing shortwave trough to the north and surface convergence within the confines of a propagating cold front. Rates will be generally between 1-2"/hr with perhaps a few isolated storms capable of up to 2.5"/hr, mainly within the southwest LA coastal areas where instability is forecasted to be highest. The antecedent conditions present across the Lower Mississippi Valley will deter more widespread flood prospects and much of this rain will be beneficial. However, the prospect for supercell generation across east TX given the deep shear profiles and backing low-level wind field off the Gulf will yield some enhanced rainfall within the SPC Enhanced/Slight severe storm risks respectively. Later in the period, convegent signature along the leading edge of an advancing cold front will act as a focal point for a line of heavy thunderstorms to move eastward across the LA/TX border into the central portion of LA. CIPS analogs were very much in line with the current forecast with multiple analog from the 90s and 80s during stronger el ninos depicting a very similar setup with an outcomes well within the means of current guidance. Those events yielded more localized flash flood impacts with the primary concerns aligned with the urban settings around east TX into LA. This was also another reason to maintain the MRGL risk with less widespread focus and more localized impacts anticipated. Kleebauer ...Previous Discussion... Surface low over the plains will evolve with a developing cold front pushing eastward within the time frame. The latest guidance continues to show a swath of QPF of 1 to 3 inches lifting from the Gulf Coast to the Ohio River Valley. Moisture return from the Gulf will have the PW values upwards of 2 to 2.5 standard deviations above normal over a large portion of the Lower and Middle Mississippi Valley. One this deep moisture plume meets the greatest instability hourly rates will likely max out near 2 inches/hour, leading to local maximums near 4 or more inches during this period across Louisiana and the immediate vicinity. The Marginal Risk that was already in effect from the Central Gulf to southwest Indiana remains in effect. Minor adjustments were made along the eastern boundary to trim it back to the west reflecting the latest model trends and WPC forecast. Campbell Day 3 Valid 12Z Fri Dec 01 2023 - 12Z Sat Dec 02 2023 ...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL/EASTERN GULF COAST AND THE SOUTHEAST... ...20Z Update... Little to no change necessary from the previous forecast as the expected evolution of our shortwave trough and frontal alignment across the Southeast U.S has maintained continuity from previous guidance with only small mesoscale based details wavering from run-to-run. Convergence along an approaching cold front within the confines of a anomalously moist environment will generate locally heavy rainfall with flash flood concerns over any urbanized settings. The primary focus will likely be over southern AL into the FL Panhandle where the instability axis will be greatest and convective regimes could produce 1-2"/hr rainfall with locally up to 3"/hr near the coastal areas. A growing consensus of somewhere within the MRGL bounds could pick up 3-5" of rainfall over the course of the period, enough for a MRGL risk placement encompassing the forecast best axis of convective potential. Kleebauer ...Previous Discussion... The low pressure system will continue to advance east during this period with its warm front lifting trough the Deep South and Southeast. Convection is expected to be the most concentrated near the Gulf Coast and tracking northeast into southern Alabama and Georgia ahead of the warm front. With anomalous moisture continuing to feed northward into this boundary, enhanced rainfall can be expected. Most of the guidance has 1 to 2+ inches spreading from south-central Louisiana into central Georgia/northern Florida. There is suggestions that a narrow axis of 3 to 4 inches could setup within this flow but the exact location remains uncertain. It could span from far southwest Alabama to southwest Georgia per a couple of solutions, or further south across the Florida panhandle and inland. A Marginal Risk area was maintained from the previous Day 4 period as it reflected the part of the country were localized flash flooding problems could arise. Campbell Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt $$