Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 PM EST Wed Feb 15 2023 Valid 12Z Sat Feb 18 2023 - 12Z Wed Feb 22 2023 ...Overview... The medium range period begins this weekend with flat to broadly cyclonic flow over much of the U.S. 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 across the eastern Pacific. While Pacific upper ridging along 140-155W longitude should support some degree of western U.S. troughing per teleconnections, guidance continues to change/diverge for some important details with how this will happen. As a result there has been increasing spread and run-to-run variability for other aspects of the forecast as well, including how quickly the upper low offshore California ultimately progresses eastward as well as details of the northern stream flow. These issues affect specifics of precipitation that may develop over the eastern half of the country early-mid next week. But forecasts of rain/mountain snow in the Pacific Northwest and snow extending into the northern-central Rockies remain on track, with heavy snow amounts likely in higher elevations during the first half of next week. ...Guidance/Predictability Assessment... Model guidance shows that multiple rounds of upper-level energy will trek around the northern fringes of the Pacific ridge and spill southward into the Southwest through the medium range period, but differences in the details continue to abound, which affects the southern stream Pacific closed low and when it may track eastward and phase with the main flow. Most 00/06Z deterministic models let the first round of energy go through the northern stream over the weekend and create some Northwest troughing without attempting to pull the southern stream low with it, though the 00Z GFS was the closest to doing so, with some phasing by Monday. Another round of energy by Monday-Tuesday may be more likely to coax the low eastward. In general the 00Z CMC and some members of its ensemble were among the slowest/farthest west solutions with the southern stream upper low, and in fact the 12Z CMC trended even slower. Meanwhile the GFS/GEFS was overall more fast/east and the 00Z EC was in between--though by day 7/Wednesday perhaps the deterministic run gets out of line. Overall preferred the middle ground solutions like the EC ensemble mean in terms of the track/timing of the low and its phasing. The most potent bout of shortwave energy looks to come into the Northwest by Wednesday, while the GFS runs in particular bring in the troughing more quickly Tuesday, leading to timing differences of the surface fronts as well. Meanwhile uncertain shortwave energy passing through the northern stream farther east across the country through the first half of next week creates differences in frontal positions as well as the details of enhanced QPF in the east-central U.S. with perhaps some wintry weather on the northern side. This will take more time to get resolved. Given the above considerations, the WPC forecast was based on a blend of the deterministic models favoring the 06Z GFS and 00Z ECMWF early on, gradually transitioning to a blend of more than half GEFS and EC ensemble means by the end of the period. ...Weather/Hazard Highlights... 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 become fairly heavy over some areas of favored terrain by Sunday or Monday, persisting through midweek. A strengthening storm system that may reach the Pacific Northwest by next Wednesday could lead to additional enhancement of precipitation over the Northwest at that time, with snow levels decreasing, while also producing strong winds. 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. Currently, enhanced precipitation appears most likely around the Lower Ohio and Tennessee Valleys starting Monday and continuing into Tuesday-Wednesday, with perhaps some wintry weather in the Upper Ohio Valley and northern Mid-Atlantic. However lack of confidence in specifics of these features (and in timing of the ejecting eastern Pacific upper low) currently makes it difficult to resolve precipitation coverage/amounts/timing as well as precipitation type. Expect modestly cool highs of 5-10F or so below normal across the on Saturday before temperatures rebound to above normal for the first half of next week. The rest of the East should see a brief period of above normal readings from Sunday onward, but with northern and then central areas trending cooler with a frontal passage by Tuesday-Wednesday. Much of the West should see a mix of moderately above/below normal temperatures before a cooler trend on Wednesday. At the same time the northern Plains and vicinity should trend noticeably colder toward the middle of next week, with some highs 10-20F below normal. 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 snow across portions of the Central Great Basin, the Northern Plains, the Northern Rockies, the Central Rockies, the Northern Great Basin, and the Pacific Northwest, Sun-Wed, Feb 19-Feb 22. - Heavy snow across portions of the Central Rockies, the Central Plains, and the Central Great Basin, Mon-Wed, Feb 20-Feb 22. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. 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