Extended Forecast Discussion NWS Weather Prediction Center College Park MD 313 PM EST Mon Feb 20 2023 Valid 12Z Thu Feb 23 2023 - 12Z Mon Feb 27 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... The large scale pattern will feature 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. Within this regime, a significant winter storm will be moving through the Northern Plains and Upper Midwest, and eventually into the Interior Northeast through Thursday. Heavy higher elevation snow will continue over a large portion of the West, focusing farther southward later in the week as an upper low drops southward near the coast, with precipitation rebounding 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 by around next Monday, 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 especially during Thursday-Friday, with above average max/min temperatures over the East and below average max/min temperatures for the West. ...Guidance/Predictability Assessment... A composite of operational model guidance represented consensus well for most aspects of the forecast during days 3-4 Thursday-Friday. There is good clustering for the system near the West Coast while the only issue arising for the system crossing the Great Lakes/Northeast is the exact detail of surface low pressure as it tracks offshore New England on Friday. By days 5-7 Saturday-Monday the GFS/GEFS runs have tended to stray a bit faster with the West Coast upper low versus most other guidance. The 00Z ECMWF mean suggests the corresponding ECMWF run could be a little too slow, leading to a preference about 70-75 percent toward the 00Z ECMWF/CMC relative to the GFS (with some ECMWF mean added late in the period). For the purposes of the updated forecast blend, GFS input used the 00Z run since it was a tad slower than the 06Z version. The new 12Z GFS has nudged a little slower and the 12Z CMC has adjusted faster, adding support for an intermediate timing. The new 12Z ECMWF has indeed trended faster and in fact is actually faster than the GFS by the time the system reaches the Plains next Monday. Behind this 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 expected to be in progress at the start of the period early Thursday should continue spreading widespread precipitation across the Northern Tier of the country through Thursday. This system will usher in significant heavy snowfall along and to the north of the surface low track from the Midwest through southern New England. Gusty winds and snow could lead to local blizzard conditions over the Midwest. Snow potential will increase for the Interior Northeast/New England Thursday into early Friday as well, with the combination of a leading wave and the Midwest low. The entire system will depart over the Atlantic on 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. There is still 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, with the experimental day 5 Excessive Rainfall Outlook expected to introduce a Marginal Risk area to reflect this potential. Timing of precipitation across the Southwest into the Plains has become somewhat more uncertain given shifting in the latest 12Z runs. Meanwhile, 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, especially during Thursday-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). Most of the West should be 10-20F below normal on Thursday followed by a slight moderation toward 5-15F below normal readings into the start of next week. Frontal passage will bring colder temperatures to the northern two-thirds of the East after Thursday, including some 10-20F below normal highs over New England Friday into Saturday. However the strong upper high over southern Florida/Cuba will keep the southern tier quite warm through the period with most days featuring some plus 10-20F anomalies, and possibly even a little warmer for morning lows. Daily records in both the cold and warm areas will likely be widespread on Thursday. Thereafter, expect the cold records to be mainly over/near the West Coast states while the warm records become more confined to the Southeast/Florida where monthly records cannot be ruled out. Rausch/Campbell 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