Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Fri Jan 19 2024 Valid 12Z Mon Jan 22 2024 - 12Z Fri Jan 26 2024 ...Excessive rain and flooding potential to increase from the south-central to Eastern U.S. next week... ...Heavy precipitation for California/West... ...Overview... Guidance continues to indicate 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 mean troughing becomes established in the western to central U.S.. This will promote warm and moist inflow likely from both the Pacific and the Gulf, with warming temperatures in the eastern half of the country. As shortwaves track through the western trough, multiple rounds of precipitation are likely in the West into next week, with heavy rain into California in particular. Multiple days of heavy rain and potential for flash flooding are expected across the south-central U.S. next week amid the stalling upper flow pattern, with ejecting shortwaves and favorable upper jet support acting to develop frontal waves 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... There remains above average forecast confidence on the overall synoptic pattern next week amidst good agreement in the deterministic guidance and ensembles on a fairly blocky pattern with troughing over the West, and ridging across the East. There is plenty of variability still in the smaller scale details of individual shortwaves/systems, which has implications for exact QPF amounts across the South and into the Eastern U.S.. The updated forecast for today used a blend of the deterministic guidance through day 5, increasing to about 50% ensemble means late period to help smooth out some of the more uncertain flow details. This approach maintains a very consistent forecast with the previous WPC shift. ...Weather/Hazards Highlights... The West Coast states can expect rounds of precipitation next week. Early week, an atmospheric river is forecast to bring moisture to California and sink south gradually, while a potent shortwave and the left exit region of the jet stream will provide upper-level dynamical support. These should combine with some instability sneaking onshore, leading to periods of high rain rates likely reaching over half an inch per hour. A Slight Risk area was held for the Day 4/ERO Monday given this setup, with focus on southern California. The Sierra Nevada will receive heavy snow and rain and snow are expected farther east into the Intermountain West and the Southwest where a Marginal Risk area has been denoted. Meanwhile, the Pacific Northwest will see periodic precipitation next week and perhaps with more emphasis late next week with stronger system approach. Deeply increasing moisture ahead of the western U.S. upper troughing will increasingly spread rain into the south-central U.S. early next week. Rain totals are forecast to quickly ramp up on Monday and last through midweek for the southern Plains into the Lower Mississippi Valley/Gulf Coast, and these heavy totals could produce flash flooding concerns. A Slight Risk remains for Day 4/Monday for southeast Texas into western Louisiana where the best potential for instability to produce high rain rates and with global models and ensemble probabilities supporting upwards to local 3-6" amounts, with a Marginal Risk extending farther north in Oklahoma and over the Lower Mississippi Valley. On Day 5/Tuesday, a Slight and a surrounding Marginal Risk area have been introduced for the Lower Mississippi Valley given slow pattern translation and continued very favorable upper and moist lower atmospheric support, albeit with a slightly more broadly defined guidance focus. There is some increasing probabilities for freezing rain across parts of the lower/mid-Mississippi Valley into the Midwest, especially parts of the Ozarks on Monday. Wintry precip should then transition to rain as warm air spreads farther north. The Great Lakes to northern Northeast seem most likely to see persistent wintry weather with overall system/pattern evolution. Precipitation should start spreading more earnestly across the Midwest/Great Lakes and the Southeast/East into mid-later next week after a dry period, and given much warmer temperatures and snowmelt, areal and stream flooding may be a concern. 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