Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Tue Feb 26 2019 Valid 12Z Fri Mar 01 2019 - 12Z Tue Mar 05 2019 ...Pattern Overview... Guidance has been consistent with the forecast mean flow from the multi-day mean perspective, showing broadly cyclonic flow over the Lower 48--just to the south of a general east-west axis of relatively low heights across southern Canada. This axis of low heights will contain one or more upper lows that would help to push very cold air into the central U.S. with the northern High Plains possibly seeing a couple days of exceptionally low anomalies of at least 30-40F below normal. At the same time flow from the Pacific will bring moisture into parts of the West (especially California into the central Rockies). Even with the agreeable mean flow, some embedded details have been difficult to resolve. In particular eastern U.S. details have varied with trends over the past day showing a weaker potential storm over the Great Lakes/Northeast this weekend but increased threat for areas of locally heavy rainfall over the Gulf Coast/Southeast during the period. ...Guidance Evaluation/Preferences... The combination of the short range Pacific Northwest upper low and initial southern Canada elongated upper low continue to provide forecast difficulties. The past two runs of the ECMWF/ECMWF mean (00Z/25 and 12Z/25) shifted more offshore with the western low and eastward with the Canada low. The result is minimal Great Lakes/Ohio Valley into New England surface development during the weekend with lowest pressures tending to stay offshore. Other solutions show varying degrees of faster ejection of the western low energy and farther west southern Canada low--leading to somewhat better defined low pressure than the ECMWF/ECMWF mean albeit with weaker depth than seen in some previous guidance (especially the GFS). Guidance behavior over the northern U.S./southern Canada over recent days does not lend a lot of confidence to any particular model system on its own. The one common theme in latest solutions is a less amplified overall trough aloft, leading to slower southward progression of the strong cold front dropping through the Plains. Meanwhile for the southern stream system heading into California this weekend, models/ensemble means have settled into the expected track fairly well but are remarkably persistent in their amount of timing spread and in some cases run-to-run variability. This favors maintaining a general compromise to help maintain stability in the forecast. Into the weekend a blend of the 18Z GFS and 12Z ECMWF/UKMET/CMC provides a reasonable intermediate starting point for this system while representing only 30 percent inclusion of the ECMWF for aforementioned issues to the north and east. New 00Z guidance thus far still supports leaning away from 12Z ECMWF specifics for the northern latitudes during the first half of the period. By the latter half of the period the 18Z GEFS/12Z ECMWF means provide a reasonable template for depicting the expected pattern though operational runs provide some degree of relative agreement, arguing for keeping as much as 50 percent of their ideas by day 6 Mon and 30 percent by day 7 Tue. In particular the operational runs are suggesting that another deep upper low center could track toward the Upper Great Lakes during the day next Tue. Farther west there is fair agreement on an upper ridge building into the West ahead of another Pacific system. Current means are similar after recent trends in the GEFS mean toward the ECMWF mean. The 12Z GFS was the one notably less probable solution by the end of next Tue as it was faster than other solutions to bring height falls to the West Coast. ...Weather Highlights/Hazards... After a brief period of drier weather over the West, expect the arrival of another Pacific system to bring moisture into California by around the start of the weekend with rain and higher elevation snow then spreading eastward into the central Rockies. Much of the precipitation with this system should depart by early next week. The Sierra Nevada range should see highest totals with other relative maxima over favored locations along the central California coast and the Great Basin/central Rockies. Meanwhile during the first half of the period low level upslope flow on the western side of cold high pressure over the Plains along with shearing shortwave energy should promote some snow along portions of the northern Rockies. From late this week into the weekend expect some snow to spread across the northern Plains and eventually into the Northeast, though recent weaker trends with surface low pressure have lowered potential snowfall amounts somewhat. Behind the system there should be a period of lake effect snow downwind from portions of the Great Lakes without ice cover. Expect some areas of rain to the south. Less amplified trends for flow aloft have increased the possibility that the front trailing from the weekend eastern U.S. system may hang up over Florida and the Gulf of Mexico, with embedded southern stream shortwaves helping to enhance rainfall along/north of the front. Some heavy rainfall may be possible over the central/eastern Gulf Coast region and northern Florida into the Southeast. With cold air pushing south/southeast with time there will be increasing potential that the northern fringe of precipitation across south-central latitudes could be in the form of snow by early next week. For temperatures the extremely cold anomalies over the central U.S. will continue to be the dominant story with highs possibly at least 30-40F below normal over the northern-central Plains from Sat onward. Especially by Sun there may be fairly broad coverage of locations nearing or reaching daily records for cold highs with some scattered daily record lows also possible. Some of this cold air will spread into the East by Sun-Tue with temperatures 10-20F below normal even reaching the East Coast. The Northwest will be chilly as well with some interior areas 10-20F below normal most days. Above normal temperatures should be confined to mostly morning lows over the southern half of the West and for some min/max readings over the South/East ahead of the initial Plains cold front. Bullet points from medium range hazards chart: https://www.wpc.ncep.noaa.gov/threats/threats.php Rausch WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation and winter weather outlook probabilities are found 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/wwd/pwpf_d47/pwpf_medr.php?day=4