Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 09 2023 08:23:31 FOUS30 KWBC 090823 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 322 AM EST Sat Dec 09 2023 Day 1 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 from 18z Saturday into the overnight hours. Forecast IVT from the 00Z guidance supports peak values of 500-700 kg/m/s. With flow relatively orthogonal to the coast, some heavier rainfall totals are expected along the Coastal Ranges into the Cascades. Model guidance generally remains in good agreement, focusing the highest rainfall totals across northwest OR into western WA. A lack of instability should generally keep rainfall rates below 0.5"/hr, however HREF guidance indicates up to a couple hundred j/kg of CAPE offshore to right along the coast later today, which may allow for a few areas of embedded higher rates to make it onshore. HREF neighborhood probabilities of exceeding 0.5"/hr do get up towards 50%, suggesting that localized rates exceeding that threshold are probable. Most of the region will pick up 2-3" of rainfall, although localized 3-5" totals are likely in the more favored west to southwest facing slopes. Flooding impacts will be increased within the Slight Risk area given the saturated soil conditions and above average streamflows over this region from the atmospheric river event from earlier in the week.=20 ....OH/TN Valley into the Southeast... A Marginal risk was maintained for a broad swath from the central Gulf Coast northward into portions of the TN and OH valley. Convection ongoing at 12z this morning may pose a localized flash flood risk over portions of central AR with storm motions parallel to the convergence axis supporting some training activity. However by mid morning convection should align more southwest to northeast, generally allowing for less training and more of an eastward progression. By late morning into the afternoon hours a fairly substantial instability plume for the time of year will stretch from the Gulf Coast all the way into KY...with values exceeding 1000 j/kg with northern extent, and over 2000 j/kg further southwest. This will be plenty of instability to support convective development along the eastward moving cold front from LA into KY. In general this activity should stay progressive enough off to the east to limit the overall flash flood risk. HREF guidance shows high probabilities of rainfall exceeding 0.5"/hr, with lower probabilities of 1"/hr. This seems reasonable, with the instability supporting briefly intense rates, but the eastward motions limiting the duration of these higher rates. Most areas should see total rainfall of 1.5" or less as this system progresses off to the east...although do suspect that the 00z HREF is probably underdoing max QPF values to some extent over portions of MS/AL/TN where more robust instability should support localized 2"+ totals. Nonetheless even with these locally higher totals any flash flood risk should remain isolated in nature. One area to keep a closer eye is in the vicinity of the southern Appalachians...generally from northern GA into southeast TN and southwest NC. The 00z HREF shows 3"+ storm total neighborhood probabilities exceeding 40% across this area, with showery activity this morning and afternoon (with some upslope component in play as well), giving way to what should be a more intense convective line by tonight where hourly rainfall may push 1". This type of rain may begin to push us close to Slight risk levels...but with soil saturation and streamflows well below average, it seems like this rain probably won't be enough to cause anything more than isolated flood issues. Thus will keep the risk level at Marginal, but continue to monitor. Chenard Day 2 The Day 2 outlook will be updated by 0830Z. 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!77KApmb198YYjfOIfaZHWSh6jEBmkoLlN7YgmmSjwJsE= _14SUvJN-SNqhOKD1Q4-Wjd-aWWukgNTDt0faGpNPo-B6t0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!77KApmb198YYjfOIfaZHWSh6jEBmkoLlN7YgmmSjwJsE= _14SUvJN-SNqhOKD1Q4-Wjd-aWWukgNTDt0faGpNphrmqmw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!77KApmb198YYjfOIfaZHWSh6jEBmkoLlN7YgmmSjwJsE= _14SUvJN-SNqhOKD1Q4-Wjd-aWWukgNTDt0faGpNFusObjo$=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) .