Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 08 2023 08:22:21 FOUS30 KWBC 080820 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 320 AM EST Fri Dec 08 2023 Day 1 Valid 12Z Fri Dec 08 2023 - 12Z Sat Dec 09 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Chenard Day 2 Valid 12Z Sat Dec 09 2023 - 12Z Sun Dec 10 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE PACIFIC NORTHWEST... ....Pacific Northwest... An Atmospheric River will impact the Pacific Northwest Saturday, with IVT values upwards of 500-700 Kg/m/s forecast along the coast by Saturday evening before beginning to taper off Sunday morning. With flow relatively orthogonal to the coast some heavier rainfall totals are expected along the coastal ranges. Most model guidance has come back south a little bit with the core of the AR, but things are still generally on track from the previous forecast. Maximum rainfall totals are expected over northwest OR into western WA...with the global model consensus of 2-3" of rainfall. Once we get into more of the high res model time frame suspect we'll see QPF totals peaking more in the 3-5" range over this area..which will probably begin to cause some flooding impacts given the saturated soil conditions and above average streamflows over this region.=20 ....OH/TN Valley into the Southeast... A broad southwest-northeast axis of heavy rainfall is forecast later in the day Saturday through early Sunday from the the Ohio/Tennessee Valleys southwestward into the Mid-South and the Lower Mississippi Valley/central Gulf Coast as Gulf moisture returns northward ahead of an upper-trough/surface cold front pushing eastward. The upper forcing and surface front are pretty progressive off to the east, which will be a limiting factor for excessive rainfall amounts. With that said, there will likely be a swath where upper flow will be more parallel to the front, with storm orientation along the front allowing for at least some training. Where this occurs a slightly higher flash flood risk could evolve, although still some uncertainty on where this may end up. The better instability will be focused over the southwestern extent of the risk area. Meanwhile the mid level trough generally stays positively tilted through this period, which may tend to favor a slightly further south max QPF swath as well. Thus at the moment thinking the best alignment of higher instability (supporting more intense rainfall rates) and brief training along the front will be from northeast LA into central/northern MS, northern AL and central TN...with the 00z Gem Reg and UKMET best aligning with this thinking. Both of these models would support localized swaths of 2-3" of rainfall generally between 21z Sat and 06z Sun, which may begin to approach or locally exceed FFG. Still can not rule out a targeted Slight risk over this corridor as we see more high res model data...especially if 3"+ probabilities begin to increase. Either way the progressive nature of the event and dry soil conditions/low streamflows would keep this as a low end Slight risk at best...but something to continue to monitor. For now a broad Marginal risk was maintained. With time we may be able to trim some of the southern and northern extent of the Marginal risk...but the area was generally left unchanged to account for some lingering uncertainty in the expected max QPF swath. Chenard Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5cbCRf785dRdaZ2xelYj78VTdMkhp0tX1L1aUCLF7GWd= hGHvDrsQftwutG3rS7gZ_8U1IDx-pVaGR11crtlVSO7oEmk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5cbCRf785dRdaZ2xelYj78VTdMkhp0tX1L1aUCLF7GWd= hGHvDrsQftwutG3rS7gZ_8U1IDx-pVaGR11crtlV_5Qyx24$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5cbCRf785dRdaZ2xelYj78VTdMkhp0tX1L1aUCLF7GWd= hGHvDrsQftwutG3rS7gZ_8U1IDx-pVaGR11crtlVqSDKRe8$=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) .