Extended Forecast Discussion NWS Weather Prediction Center College Park MD 217 AM EST Tue Feb 21 2023 Valid 12Z Fri Feb 24 2023 - 12Z Tue Feb 28 2023 ...Major winter storm to track along the Northern Tier of the U.S. this week... ...Record cold possible in the West while record warmth is possible in the Southeast... ...Overview... There will be an amplified mean trough aloft near the West Coast and strong progressive flow downstream, between an upper low north or northeast of Hudson Bay and an upper ridge over southern Florida or Cuba. A significant winter storm will be winding down as the system tracks offshore the Northeast. For the west, heavy higher elevation snow will persist and spread southward as temperatures drop along the Coast. Precipitation will rebound again over the Pacific Northwest/Northern Rockies next weekend/early next week as another system approaches. As the West Coast upper low emerges into the Plains early in the coming week, expect rain and thunderstorms to become widespread from the Southern/Central Plains through and perhaps beyond the Mississippi Valley with some severe weather possible. Mixed precipitation including ice could be a threat along the rain/snow transition zone draped across portions of the Midwest/Great Lakes. Meanwhile the amplified pattern will lead to significant temperature extremes on both sides of the spectrum Friday, with above average max/min temperatures over the East and below average max/min temperatures for the West. ...Guidance/Predictability Assessment... The latest suite of guidance maintains a general consensus with the amplified flow and the varying features, therefore a composite of the ECWMF, CMC, UKMET and GFS. Like it's previous runs, the GFS and the GEFS mean continue to run faster than the rest of the guidance while the ECWMF lags behind. Behind this leading system, spread looks fairly typical for the upper trough expected to amplify toward the West Coast by next Sunday-Monday. Meanwhile guidance shows some differences for a northern stream trough (or multiple shortwaves) progressing from the northern Plains through the Northeast during the weekend. Associated low pressure should cross the Great Lakes/southeastern Canada before consolidating/deepening over the Atlantic. The 12Z GFS appears to be on the slow/deep side of the spread for its main shortwave. Preferences for the ejecting West Coast upper low worked well for the other features of interest. ...Weather/Hazard Highlights... A winter storm will already have impacted the Great Lakes region and parts of the Northeast prior to the the start of the forecast and will linger over the Northeast Friday. Then by late week into the weekend, precipitation chances will increase again for the east-central U.S., with the heaviest amounts possible over Kentucky/Tennessee east into the Appalachians. Model timing differences may lead to some uncertainty in the latitude of this precipitation axis. Lighter precipitation, mostly snow, may spread across the Great Lakes/Northeast. Widespread heavy snow chances and very cold temperatures will continue over much of the West through Friday as an energetic upper-level trough deepens over the region and a couple of frontal systems pass through. The focus for heavier snow chances will generally shift southward over California and the Interior West through Friday as an upper low descends along the coast. Multiple feet of snow are likely in some Western higher elevations throughout the week. Additionally, some very strong and gusty winds will accompany the southern of the two frontal systems passing through southern California, the Southwest, and into the adjacent Southern High Plains around midweek. There may be a period of time late this week when flow ahead of the upper low may focus enhanced moisture into favored terrain along the southern California coast. Given the potential for at least a couple of inches of rainfall with the potential for runoff, an experimental Day 4 and Day 5 Marginal Risk for excessive rainfall was raised. Precipitation looks likely to return to the Pacific Northwest/Northern Rockies by next weekend/early next week. The amplified pattern will support a pronounced temperature contrast across the country Friday when Northern Rockies to High Plains locations may see readings 20-40F below normal at the same time the East sees temperatures 20-30F or so above normal (through Friday morning). Frontal passage will bring colder temperatures leading to 10-20F below normal highs over New England Friday into Saturday. However the strong upper high over southern Florida/Cub Campbell/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 Hazards: - Heavy precipitation across portions of California, Thu-Sat, Feb 23-Feb 25. - Heavy snow across portions of the Pacific Northwest and the Northern Great Basin, Sat-Sun, Feb 25-Feb 26. - Heavy snow across portions of California and the Pacific Northwest, Sun-Mon, Feb 26-Feb 27. - Heavy snow across portions of the Central Great Basin, California, and the Southwest, Thu-Fri, Feb 23-Feb 24 and Mon, Feb 27. - Heavy snow across portions of the Northeast, the Great Lakes, and the Upper Mississippi Valley, Thu, Feb 23. - Flooding occurring or imminent across portions of the Lower Mississippi Valley and the Ohio Valley. - Much below normal temperatures across portions of the Central Plains, the Central Great Basin, the Northern Plains, the Northern Rockies, the Central Rockies, the Middle Mississippi Valley, California, the Northern Great Basin, the Upper Mississippi Valley, and the Pacific Northwest, Thu-Sat, Feb 23-Feb 25. - Much below normal temperatures across portions of the Mid-Atlantic, the Northeast, and the Great Lakes, Fri-Sat, Feb 24-Feb 25. 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