Extended Forecast Discussion NWS Weather Prediction Center College Park MD 210 AM EST Sun Jan 21 2024 Valid 12Z Wed Jan 24 2024 - 12Z Sun Jan 28 2024 ...Multi-day Excessive Rainfall/Flooding Threat from Southeast Texas to the Lower Mississippi/Tennessee Valleys and Gulf Coast states this week... ...Overview... Guidance still overall agrees that the large scale pattern aloft this week will shift notably from the ongoing frigid pattern, as a ridge shifts from the central U.S. into the East Coast and troughing becomes established and works slowly from the the western to the central U.S.. This will channel moisture return from the Gulf of Mexico, with warming temperatures in the eastern half of the country. A series of deepened Pacific systems will periodically focus lead flow/precipitation inland into the Pacific Northwest before digging into a mean western U.S. trough in unsettled/cooled flow. Meanwhile downstream, a significant rainfall event remains looming as multiple days of heavy rain and runoff threats are expected amid the stalling upper flow mean pattern, with primary focus from southeast Texas through the Lower Mississippi/Tennessee Valleys and Gulf Coast states. Favorable upper jet/frontal waves support will act to increase and focus moisture and instability return as ejecting impulses ride/lift on the western periphery of a slow to dislodge eastern U.S. ridge. Expect widespread activity to also spread to the Midwest/Great Lakes and the Southeast/East through later week, with snow/ice potential on the north side of the precipitation shield. ...Guidance/Predictability Assessment... Recent models and ensembles continue to show fairly good agreement for this regular workweek on the large scale pattern and offer similar weather hazard potential, but do show some variability in the local details, which may take until the short range to fully resolve. Regardless, there is excellent agreement that an impactful and multi-day rainfall event should unfold across The South, but with some lingering uncertainty on exact locations of the highest rainfall amounts. A general guidance blend seemed to provide a good forecast basis Wednesday into Friday. By later Friday and next weekend, greater differences arise regarding shortwave energy into the West which may finally kick the main trough eastward as strong upper ridging then builds through the West Coast. Recent GFS and to a lesser extend GEFS runs have been fastest to progress this energy downstream through the central U.S., while recent ECMWF runs tended to dig energies more deeply toward the Southwest, resulting in longer maintainence of the Western mean trough. Given upstream higher amplitude preference from Alaska to the eastern Pacific, leaned toward a compromise to slower evolution side of the full envelope of solutions for the CONUS, probably overall closest to the ECMWF ensemble mean. the latest 00 UTC model cycle shows the GFS trending slower and the ECMWF trending faster, now both better in line with the ECMWF ensemble mean. ...Weather/Hazards Highlights... The Pacific Northwest/northern California will see periods of moderate rains into mid-later week with multiple Pacific system approaches. Digging energies into a mean upper trough over the West in this period should allow for precipitation to work inland to the north-central Intermountain West/Rockies to include some enhanced terrain/mountain snows. Upper ridge building up/inland through the West Coast heading into the weekend should increasingly limit activity with a more northward to offshore focus. Downstream, increasing moisture ahead of the western U.S. upper troughing will increasingly spread rain into the south-central U.S. early week. Heaviest rain totals remain forecast to slowly spread through Texas to the Lower Mississippi/Tennessee Valleys and Gulf Coast states and Southeast into mid/later week, and these will fuel growing flooding concerns given significant repeat/training potential. WPC Day 4/Wednesday and Day 5/Thursday Excessive Rainfall Outlooks have Slight Risk and surrounding Marginal Risk areas from Southeast Texas to the Lower Mississippi/Tennessee Valleys and Gulf Coast states given slow pattern translation with favorable upper support and deep moisture return and repeat activity signature. Precipitation will also start spreading more earnestly across the Midwest/Great Lakes and the East into mid-later week after a dry period, and given much warmer temperatures and snowmelt, areal and stream flooding may become a concern. There is also quite a bit of uncertainty with later week indiviual lifting frontal waves within the larger scale flow and the extent and focus of potential coastal waves next weekend to monitor. Meanwhile, there remains some probabilities for freezing rain across parts of the Midwest/Great Lakes into mid-later week. Wintry precip should transition to rain as warm air spreads farther north. Lingering cooled air from the Great Lakes/Appalachians to the northern Northeast seem most likely to see wintry weather later week and into the next weekend with additional but still uncertain frontal/coastal waves. Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw