Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM 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 a mean ridge that tracks from the eastern Pacific eastward into the western and central U.S. late week into the weekend and displaces the trough east. This pattern will allow for a couple of Arctic high pressure centers at the surface to support very cold temperatures 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, a possibly developing low pressure/frontal system could produce some winter weather for Thursday-Saturday, but with considerable uncertainty for details. ...Guidance/Predictability Assessment... Latest models/means agree well on the large scale evolution during the period, with some reasonable consensus on some details early in the period but less so by late week into the weekend. As the period begins early Wednesday, shortwave energy enhancing the broad trough with its focus around the Appalachians or so will support a surface low over Atlantic Canada with finally some better agreement in placement/strength in the incoming 00Z suite of guidance. This shortwave pivots quickly east into the Atlantic. In the West, models show two shortwaves early Wednesday with one atop the Northwest and one in British Columbia. These support a reasonably agreeable fairly compact but vigorous surface low near Vancouver Island/northern Washington. These shortwaves look to get absorbed into the backside of the broad trough into Thursday. Models continue to show the upper ridge marching into the West Coast Thursday and amplifying as it tracks east Friday, pushing farther into the central U.S. over the weekend. This has been very consistent for multiple model cycles. Northwest flow between the ridge and trough brings in cold surface high pressure to the High Plains with just some typical differences in exact placement and strength. Ahead of this surface high is where the most impactful model differences are, namely low pressure/frontal systems in the East late week into the weekend. Models seem to be having trouble defining a low center within this broad area of low pressure in the eastern third of the U.S. into the western Atlantic around Friday, but there is some signal for a surface low tracking northeast through the Atlantic east of the Mid-Atlantic and Northeast late Friday into Saturday. The 18Z GFS was on the fast side with the low track while the 12Z GFS was slow; ensemble means and the ECMWF and CMC position seemed to be the best middle ground at this time, but with relatively low confidence. Upstream of the ridge, troughing in the eastern Pacific possibly affecting the West Coast by the weekend has some typical model differences for the late medium range period. The WPC forecast was based on a blend of the deterministic guidance favoring the 18Z GFS and 12Z ECMWF early in the medium range period. Given the increasing model differences particularly with the eastern system, blended in some GEFS and EC ensemble means by Day 5 and increased the ensemble proportion to 40 percent by the late period, which maintained some strength of systems but minimized too many individual model details. ...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, so still notable even though not quite as cold as the current/short range cold event. Meanwhile west of the Rockies, ridging aloft will promote above normal temperatures increasing in magnitude and coverage late week into the weekend. 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. 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 preclude us from introducing any Marginal Risks (which are for a 5 percent or greater chance of flooding) in the Excessive Rainfall Outlooks at this time. Additional rounds of precipitation for the West Coast states are possible late week. Farther east, expect continued periods of lake effect snow with persistence of cold air and breezy conditions. Then a front and possible low pressure system 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 into 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. 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