Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM EST Sun Jan 14 2024 Valid 12Z Wed Jan 17 2024 - 12Z Sun Jan 21 2024 ...Record-breaking cold could linger in the south-central U.S. midweek, with another Arctic airmass coming into the central U.S. late week... ...Overview... A deep and broad mean trough aloft will stay atop much of the lower 48 through late week, ahead of an upper ridge that tracks from the eastern Pacific into the western-central U.S. late week into the weekend. Progression of the upper ridge should displace the trough to the East Coast/western Atlantic by next Sunday. This pattern will allow for a couple of Arctic high pressure centers at the surface to support very cold temperatures with the first lingering in the eastern two-thirds of the country for midweek, ahead of another surge of cold air that pushes into the central U.S. and eventually the East late week into the weekend. At most locations this second round of cold temperatures should be less extreme than the the first. Snow and some lower elevation rain are forecast across the northern half or so of the West especially on Wednesday and lingering into Thursday, with a precipitation focus more along the West Coast by Friday into the weekend from moisture on the leading side of an approaching upper trough. In the East, possible low development along a surface frontal system reaching the area could produce some winter weather during Thursday-Saturday, but with continued significant uncertainty for details. ...Guidance/Predictability Assessment... Latest models and means continue to agree well for the large scale evolution through the period. A leading upper trough will depart from the East while two smaller scale shortwaves as of early Wednesday (one reaching the Northwest and the other extending from British Columbia into the Pacific) shear inland underneath an elongated southern Canada trough. This combination of energy will rotate through the eastern half of the lower 48 as a deep upper trough late in the week (with an upper low near New England by Saturday) as a rebuilding West Coast upper ridge moves inland. Ahead of the trough, surface details along the East Coast as of early Friday are still uncertain but consensus is steadily improving upon the idea of low pressure consolidating over the western Atlantic by later in the day and continuing east/northeast thereafter. Next weekend the guidance shows the upper ridge broadening as it reaches the central U.S., pushing the eastern trough to and beyond the East Coast, while mean troughing sets up along the West Coast and supports the arrival of a couple frontal systems. The primary guidance differences early in the period involve the low pressure system nearing the Pacific Northwest as of early Wednesday. Over the past couple days there has been steady improvement for the track forecast, with consensus narrowing on southern Vancouver Island or northwestern Washington. During the day Wednesday the GFS/GEFS become weaker than most other guidance due to the GFS cluster incorporating less British Columbia-Pacific shortwave energy. An intermediate solution tilted somewhat away from the GFS continues to be reasonable for this system. Along the East Coast on Friday, the past day's trends have yielded some loose clustering toward a broad area of one or more surface lows over or a little south of the central Appalachians/Mid-Atlantic eastward into the western Atlantic early in the day and then rapid consolidation offshore the Mid-Atlantic by later in the day. Guidance still diverges for exact track with meaningful differences as far as East Coast effects. GFS runs continue waffling with the new 12Z run west of 00Z/06Z runs, and the new 12Z ECMWF is close to it by early Saturday. CMC runs remain faster/eastward. The ensemble means become fairly east/progressive as well. As was the case yesterday, 00Z ECMWF-initialized machine learning models are emphatic with faster progression than the ensemble means. The full array of guidance favors staying closest to the ensemble means with only some operational input for detail enhancement. Over the West next weekend, guidance agrees with the general theme of one shortwave reaching the West Coast by Saturday and another following about a day behind, supporting two separate surface frontal systems. Differences in timing and amplitude have low predictability 6-7 days out in time so a model/mean blend is a good starting point to represent the larger scale consensus at the surface and aloft. The updated forecast utilized the 00Z-06Z GFS and 00Z ECMWF/UKMET/CMC for the first half of the period and then trended toward a blend of about half models/half ensemble means. This approach provided reasonable continuity for significant features. ...Weather/Hazards Highlights... Though the coldest air now phases into the short range period early this week, chilly temperatures remain across the Plains to East Coast Wednesday, around 10-25F below average. Low temperatures Wednesday morning could set records in the southern Plains/Lower Mississippi Valley. These areas will warm closer to normal Thursday, but another reinforcing surge of cold air is forecast to push into the north-central U.S. Thursday, well into the Plains/Mississippi Valley Friday, and into the east-central U.S. Saturday. Below normal temperatures by 15-30 degrees should be widespread, and this cold episode will still be notable even if not quite as extreme as the current/short range cold event over most areas. Locations near the East Coast could see the coldest daytime highs of the week on Saturday though. Meanwhile west of the Rockies, ridging aloft will promote above normal temperatures increasing in magnitude and coverage late week into the weekend. Best potential for plus 10F or greater anomalies will be for morning lows next weekend. Modestly above normal temperatures may extend into the northern Plains by next Sunday. A surface low pressure system and a shortwave aloft coming into the Northwest by Wednesday will help cause widespread precipitation across the northern half of California into the Pacific Northwest and east to the Rockies. This event will likely see snow confined to higher elevations than in the shorter term, but inland areas where cold air persists longer could still see some lower elevation winter weather issues, including potential for freezing rain in the northern Great Basin. Snow amounts could be heavy in higher elevation areas including the Cascades to northern/central Rockies. Meanwhile coastal areas of the Pacific may see occasional enhanced rainfall amounts for perhaps a nonzero potential for flooding concerns. On Wednesday coastal Oregon could be a focus for heavy rain while on Thursday coastal Washington seems more likely as moisture increases ahead of a Pacific warm front lifting northward. However, a lack of instability, guidance not showing any focused atmospheric river taking aim at the region, and continued model differences for amounts/placement of heaviest rain, still suggest that flash flood potential remains below the Marginal Risk threshold of 5 percent for the Days 4-5/Wednesday-Thursday Excessive Rainfall Outlooks. Additional rounds of precipitation for the West Coast states are possible late week through the weekend as upper troughing settles over the region. Some locally moderate/heavy totals are possible over favored terrain, depending on upper level and surface details that will be better resolved in coming days, and precipitation coverage should extend farther southeastward with time over the weekend. Farther east, expect continued periods of lake effect snow with persistence of cold air and breezy conditions into Saturday. Lake effect snow will likely taper off by Sunday as surface high pressure builds into the East. A frontal system with embedded low pressure reaching the East during the latter part of the week may produce some precipitation. This system could initially spread some light snow across the northern-central Plains by Thursday while some limited Gulf moisture comes into the east-central U.S., shifting to the East Coast Friday. The precipitation type in the Southeast should stay rain while generally from the Ohio Valley into the Appalachians and the northern Mid-Atlantic to Northeast should have snow. Areas in between are more uncertain with precipitation type while uncertainty also remains with the coverage and amounts of precipitation, which will continue to be refined in future forecasts. Currently the majority of guidance suggests that snowfall should be fairly light the latest probabilities for 0.25 inch liquid in the form of snow are no more than 10-30 percent or so during the Friday-Friday night time frame. Rausch/Tate 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