Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sat Jan 14 2023 Valid 12Z Tue Jan 17 2023 - 12Z Sat Jan 21 2023 ...Active pattern continues for the West and other parts of the lower 48 but with central/southern California trending drier mid-late week... ...Overview and Guidance/Predictability Assessment... The models and ensembles continue to show an initial pattern featuring a sequence of Pacific systems tracking into the West and then northeastward beyond the Rockies followed by a transition toward a building eastern Pacific upper ridge that would lead to mean upper troughing over the interior lower 48, albeit with some uncertainty over the character of the trough. This evolution will likely begin a period of drier weather to central-southern California after Monday and then the rest of the state after another system brings some rain/mountain snow to the Pacific Northwest/northern California around midweek. The Climate Prediction Center's 8-14 day outlook continues to suggest that the drier pattern for California will extend beyond the medium range period. The dominant system of interest next week will cross the West with rain and mountain snow during the first half of next week and should spread a broad area of significant weather across portions of the central-eastern U.S. Wednesday-Friday. Guidance now appears fairly agreeable and stable with the Atlantic system reaching just east of Maine by early Tuesday and the system reaching the Great Lakes at the same time and opening up thereafter. There are still significant uncertainties with the system crossing the West on Tuesday and continuing northeastward the rest of the week. GFS runs have been particularly inconsistent over the past 2-3 days, earlier being among the deepest with the system over the central U.S. and then straying to the weak/suppressed/progressive side in most recent runs, only to have the 00Z run finally come back closer to the majority cluster. Some of the issues in the GFS and even other models have involved fine-scale (thus having low predictability) details of shortwave energy entering the West during the first half of the week. Preference based on 12Z/18Z data was to maintain an evolution closer to the ECMWF/CMC and their means, with the 12Z GFS/GEFS mean at least somewhat closer to that idea than the 18Z runs. Behind this system, models and individual ensemble members remain varied for how incoming trough energy will amplify over the western half of the country, with 12Z/18Z guidance generally reverting back to a slower/sharper trough like 24 hours ago versus the somewhat faster/broader trough seen in some of yesterday's 00Z cycle. A model/mean blend accounts for some but not all of this adjustment, in light of recent oscillations. Latest GFS runs are on the sharp/amplified side of the spread for a shortwave that may round the top of the eastern Pacific ridge by next Saturday. Overall guidance preferences led to the first half of the forecast starting with an operational model composite, including less than typical weight of the GFS (12Z run for what was included). Then the blend adjusted toward an even weight of models/means (including the 12Z run for GEFS input along with the 12Z ECMWF mean) by day 7 Saturday. ...Weather/Hazard Highlights... The system moving through the West on Tuesday should bring rain and higher elevation snow across the Four Corners states. Then expect this system to bring a variety of weather to the central and eastern U.S. mid-late week. This includes the potential for heavy rainfall over and near the lower half of the Mississippi Valley, Tennessee Valley, and lower Ohio Valley. The upper dynamics/low level Gulf inflow and guidance signals appear sufficiently favorable to depict a Slight Risk area on the Day 5 experimental Excessive Rainfall Outlook valid 12Z Wednesday-12Z Thursday. Some locally moderate to heavy rainfall may extend farther through the East thereafter but with lower confidence in details. Meaningful wintry weather will be possible in the cold sector, currently most likely along an axis from the central Plains through the Great Lakes and Northeast. Details will depend on the storm's exact track and strength which are yet to be resolved, so some adjustments to the forecast will be possible. With the evolving pattern change later in the period, the next system to reach the West Coast will bring its brief period of locally moderate to heavy rainfall/mountain snow to the Pacific Northwest and northern California around midweek with lighter totals then extending into the Rockies. The South may see a rebound in rainfall by next Saturday as a front lingering over the Gulf lifts back northward ahead of an approaching cold front. The forecast pattern should maintain highs 5-15F below normal over the southern two-thirds or so of the West through the period, with readings close to or slightly above normal over the far north. Very warm temperatures over the eastern half of the country should erode from west to east mid-late week with system/frontal passage. Ahead of this system, expect a broad area of lows 15-25F above normal and highs 10-20F above normal. Most daily records should be for warm lows, while the best chance for record highs still appears to be over southeastern Texas and lower Mississippi Valley on Tuesday. Rausch 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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