Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Thu Feb 16 2023 Valid 12Z Sun Feb 19 2023 - 12Z Thu Feb 23 2023 ...Overview... Into the first part of next week the guidance continues to show broadly cyclonic flow between an upper low generally just north of Hudson Bay and an upper ridge over the southern Gulf/northwestern Caribbean, while a closed upper low gradually drifts closer to northern Baja California. Then models/means maintain their recent signals for shortwave energy rounding the top of a strong eastern Pacific ridge early in the week to amplify into a deep upper trough near the West Coast by mid-late week. There is still some spread for specifics of individual shortwaves crossing the lower 48 during the first half of the period, affecting surface wave and front details, but there seems to be a little better clustering among latest runs in principle. Meanwhile differences for incoming West Coast trough energy are still affecting how quickly the southern stream Pacific upper low ultimately ejects eastward. Expect multiple days of significant snow from the favored terrain of the Pacific Northwest into the northern-central Rockies, while a much colder trend develops from the northern Plains into the West from Tuesday/Wednesday onward with wintry weather eventually becoming more possible at low elevations in the Northwest. Precipitation should expand and become heavier over portions of the eastern half of the country by around Tuesday, but with continued uncertainty for details. ...Guidance/Predictability Assessment... In order to derive a coherent forecast through the period, the updated blend employed an average of the 18Z GFS/12Z ECMWF during the first half of the period and then transitioned toward 40-60 percent total input of their respective ensemble means by days 6-7 Wednesday-Thursday with the remaining operational weight tilted slightly more to the ECMWF relative to the GFS. These solutions generally have the most in common for one area of low pressure progressing east-northeast from the central U.S. Sunday onward (while CMC runs by early next week develop a farther southeast wave that produces more QPF over Maine than other guidance) and a trailing wave crossing the northern tier early next week. The new 00Z GFS has become a northern extreme with this latter wave though. Meanwhile timing of the southern stream eastern Pacific upper low's ejection is still problematic. 12-18Z GFS/12Z ECMWF runs were fairly similar and within their ensemble mean trough, but the UKMET/CMC have been holding the upper low back and confidence was not great for the GFS/ECMWF timing. The new 00Z GFS/ECMWF have indeed nudged back a bit while a modest eastward adjustment in the UKMET yields a 00Z CMC/GFS compromise. As for the deepening West Coast trough, recent ECMWF/UKMET trends have been somewhat in the direction of the GFS that has been leading the guidance in terms of greatest trough amplitude. Interestingly, by the latter half of the period a somewhat greater proportion of GEFS members actually lie on the flatter side of the envelope relative to CMCens/ECens members. The GFS may still be a little quick as the energy reaches the Northwest though. A model/ensemble mean average seems reasonable in light of guidance spread at this time. One other note is that the guidance average appears to have backed off with the strength of surface low pressure as it arrives around Tuesday. There is still an impressive surface pressure gradient between Pacific Northwest low pressure and the Canada/northern Plains surface ridge. ...Weather/Hazard Highlights... Cold temperatures pushing into the northern half of the Plains Tuesday onward and significant cooling over the West Wednesday onward will become more pronounced with time. By next Thursday highs over the northern Plains and vicinity may be 20-35F below normal while highs over the Northwest into the Great Basin/California could be at least 10-20F below normal (which could set record cold highs at some locations). Even before this cold air arrives, northwest flow aloft from the weekend into the first part of next week will likely support rain and mountain snow over the Pacific Northwest and snow extending east/southeast into the northern-central Rockies. Snow could be fairly heavy over some areas of favored terrain by Sunday or Monday, persisting at least through midweek. The upper trough/surface low pressure reaching the Pacific Northwest by next Tuesday-Wednesday could lead to additional enhancement of precipitation over the Northwest at that time. The deep upper trough will support a pronounced decrease in snow levels, perhaps to very low elevations, and eventually support southward expansion of precipitation over the West. Even though guidance currently does not appear as strong with the low pressure reaching the Northwest, there may still be some areas of brisk to strong winds. This includes a pronounced gradient over the northern half of the Rockies/High Plains where northerly/easterly low level upslope flow could promote added snowfall. Snow potential may begin to extend farther eastward across the northern tier by midweek depending on how and when shortwave energy/surface low pressure emerge into the Plains. Over the eastern half of the country, there continues to be a general signal for some precipitation to develop in response to one or more shortwaves and surface lows/fronts, in addition to the upper low that may eventually eject from the eastern Pacific. Currently, enhanced precipitation appears most likely around the Lower Ohio and Tennessee Valleys starting Monday or Tuesday and continuing into Wednesday, with perhaps some wintry weather farther north. Recent guidance inconsistency lowers confidence in where the rain-snow line might be, but current consensus is suggesting best potential for wintry weather over the Great Lakes and Northeast. The succession of a couple northern tier systems will initially favor a broadening area of above normal temperatures over the eastern U.S. into the southern Plains. Passage of these systems will moderate high temperatures over northern areas by Tuesday-Thursday while well above normal readings settle over the southern tier with broad coverage of plus 10-20F anomalies for highs and perhaps up to plus 20-25F anomalies for lows over some areas. 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 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