Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 PM EST Sat Jan 20 2024 Valid 12Z Tue Jan 23 2024 - 12Z Sat Jan 27 2024 ...Multi-day Excessive Rainfall and Flooding Threats for the Lower Mississippi/Tennessee Valleys and Gulf Coast states next week... ...Overview... Models and ensembles agree that the large scale pattern aloft next week will shift notably from this week's frigid pattern, as a ridge shifts from the central U.S. into the East and troughing becomes established in the western to 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 precipitation focusing shortwaves will work inland into the Pacific Northwest and dig into the western U.S. trough next week in unsettled/cooled flow, then propagate downstream to ride the western periphery of the downstream upper ridge. A significant rainfall event is looming as multiple days of heavy rain and runoff threats are expected amid the stalling upper flow mean pattern, with primary focus over the Lower Mississippi/Tennessee Valleys and Gulf Coast states. Ejecting shortwaves and favorable upper jet/frontal waves support will act to increase and focus moisture and instability return on the western periphery of a slow to dislodge eastern U.S. surface ridge. Expect widespread activity to also spread to the Midwest/Great Lakes and the Southeast/East through mid-later next week, with snow/ice potential on the north side of the precipitation shield. ...Guidance/Predictability Assessment... The latest suite of models and ensembles continue to show fairly good agreement through most of the period on the large scale pattern, but do show some variability in the 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-central U.S.., but with some lingering uncertainty on exact locations of the highest rainfall amounts. A general deterministic model blend worked well for the first half of the period. By Friday and especially next Saturday, there are some greater differences that arise regarding shortwave energy into the West which may finally kick the main trough eastward as strong upper ridging then builds along the coast. The last couple of runs of the GFS have consistently been fastest with this energy through the south-central Plains, while the ECMWF digs it deeper into the Southwest, resulting in longer maintainence of the Western mean trough. Given the late period timing, preferred a blend towards the mostly agreeable ensemble means which shows a somewhat middle ground solution at this point. The day 6-7 blend used 50 percent of the ensemble means with the deterministic GFS/ECMWF/CMC for a little added system definition. This maintained good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... Portions of the West can expect several periods of enhanced precipitation next week. An early week atmospheric river sinking southward through California will wane, but lingering moisture and emerging and favorable upper trough/jet support may allow for some local downpours/runoff issues from southern Arizona to southwest New Mexico. A WPC ERO Marginal Risk area is in place for the Day 4/Tuesday. Meanwhile, the Pacific Northwest/northern California and vicinity will see multiple periods of precipitation next week, seemingly with more emphasis into mid-later next week with stronger Pacific system approaches. Expect mainly moderate daily totals. Downstream, deeply increasing moisture ahead of the western U.S. upper troughing will increasingly spread rain into the south-central U.S. early next week. Heaviest rain totals are forecast to slowly spread through the Lower Mississippi/Tennessee Valleys and Gulf Coast states and Southeast by mid/later next week, and these will fuel growing flooding concerns given significant repeat/training potential. WPC Day 4/Tuesday and Day 5/Wednesday Excessive Rainfall Outlooks Slight Risk areas and surrounding Marginal Risk are maintained from Southeast Texas through the Lower Mississippi Valley then Tennessee Valley given slow pattern translation and overall favorable upper jet/flow support and sustained moisture return and repeat activity signature. Precipitation will also start spreading more earnestly across the Midwest/Great Lakes and the East into mid-later next week after a dry period, and given much warmer temperatures and snowmelt, areal and stream flooding may become a concern. There are some probabilities for freezing rain across parts of the Midwest. Wintry precip should then transition to rain as warm air spreads farther north. The Great Lakes/Appalachians to the interior Northeast seem most likely to also experience wintry weather with overall system and pattern evolution later next week with the last vestiges of the ambient cold airmass. Santorelli/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