Extended Forecast Discussion NWS Weather Prediction Center College Park MD 328 PM EST Thu Feb 16 2023 Valid 12Z Sun Feb 19 2023 - 12Z Thu Feb 23 2023 ...Heavy snow likely at least across higher elevations of the West next week, perhaps spreading into lower elevations of the northern U.S. as well... ...Overview... Into the first part of next week, model 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. Individual shortwaves embedded within the main flow have been quite variable, which affects surface wave and front details as well as the precipitation forecasts in the central-eastern lower 48. By around midweek, shortwave energy looks to dive into the West and amplify positively tilted troughing there for colder temperatures across the West and especially into the north-central CONUS. Heavy snow will be likely across higher elevations in the West for the weekend and beyond, while wintry weather eventually becomes more possible at low elevations in the Northwest, with probabilities for notable snow amounts increasing in the north-central U.S. as well. ...Guidance/Predictability Assessment... For the earlier part of the forecast period days 3-5 (Sunday-Tuesday), model guidance issues involve a couple of shortwaves moving through the main flow in the central/eastern U.S. and with the eastward drift of the southern stream Pacific upper low going toward phasing of the . Regarding the latter, CMC runs along with many CMC ensemble members have persistently been on the western/slower side of the guidance envelope. GEFS members and the 00Z GFS tended to be on the faster side. The 06Z GFS and 00Z ECMWF and UKMET and the EC ensemble mean were more agreeable to favor those in the WPC model blend instead for the timing of this low feature tracking eastward and phasing. Overall the 00Z/06Z guidance cycle did end up a bit slower than the previous forecast, but some wobbles faster or slower are still possible. Meanwhile with the shortwaves tracking across parts of the central and eastern U.S., these smaller scale features often take well into the short range period to resolve, and unfortunately have fairly large impacts on the precipitation distribution and amounts in this situation. There has been a general trend downward in forecast precipitation across the Ohio/Tennessee Valleys compared to the previous couple of forecast cycles, as models seem to have more uncertainty in the placement of precipitation as well as less in general. By around Wednesday, models have been persistent in showing the strongest bout of shortwave energy to dive into the West after rounding an eastern Pacific ridge, with reinforcing energy by Thursday forming a positively tilted trough. Despite the increasing lead time this is one of the more persistent/agreeable aspects of the forecast, at least on the synoptic scale. At the surface this allows for a notable pressure gradient between lows in the Northwest and the Canada/northern Plains surface ridge, and then for low pressure to consolidate over the central High Plains and track toward the Midwest with snow behind. By the latter part of the medium range period a fairly even blend of the 06Z GFS/GEFS and 00Z EC/ECens was used for the WPC forecast. ...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. Some areas of brisk to strong winds are possible in the West, especially the Southwest. But a pronounced pressure gradient over the northern half of the Rockies/High Plains could promote added snowfall in northerly/easterly low level upslope flow. Snow potential is looking more likely to extend farther eastward across the northern tier by midweek. Though the details depend on how and when shortwave energy/surface low pressure emerge into the Plains, the current forecast shows a surface low tracking across the West to High Plains Tuesday-Wednesday and into the Midwest by Thursday, with snow on the backside across northern parts of the Plains into the Upper Midwest and Great Lakes region. 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. Additionally, potential for heavy rainfall farther south has decreased somewhat but remains uncertain. 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. Tate/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 and the Pacific Northwest, Tue-Wed, Feb 21-Feb 22. - Heavy rain across portions of the Middle and Lower Mississippi Valley, Central and Southern Appalachians, Tennessee Valley, and Ohio Valley, Tue-Wed, Feb 21-Feb 22. - Heavy snow across portions of the Northern and Central Great Basin, Northern and Central Rockies, and Pacific Northwest, Sun-Wed, Feb 19-Feb 22. - Heavy snow across portions of the Northern and Central Plains, Upper Mississippi Valley, Great Lakes, and Sierra Nevada, Wed-Thu, Feb 22-Feb 23. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - High winds across portions of the Southern Rockies, Southern Plains, and Southwest, Wed, Feb 22. - High winds across portions of the Central Rockies and Central Plains, Sun-Mon, Feb 19-Feb 20. - Much below normal temperatures across portions of the Northern and Central Plains, Northern and Central Great Basin, Northern and Central Rockies, and Pacific Northwest, Wed-Thu, Feb 22-Feb 23. - High winds across portions of the Aleutians, Mon-Tue, Feb 20-Feb 21. - High winds and heavy precipitation across portions of western Mainland Alaska, Wed-Thu, Feb 22-Feb 23. 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