Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Wed Feb 15 2023 Valid 12Z Sat Feb 18 2023 - 12Z Wed Feb 22 2023 ...Overview... While Pacific upper ridging along 140-155W longitude should support some degree of western U.S. troughing per teleconnections, guidance over the past day has been changing/diverging 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 and details within progressive and flat to broadly cyclonic flow between an upper low generally just north of Hudson Bay and an upper ridge over the southern Gulf/northwestern Caribbean. These issues affect specifics of precipitation that may develop over the eastern half of the country early-mid next week. There has been better stability in the forecast of rain/mountain snow in the Pacific Northwest and snow extending into the northern-central Rockies, possibly trending heavier during the first half of next week. ...Guidance/Predictability Assessment... Starting with yesterday's 00Z cycle, guidance has become more varied with the details of shortwave energy reaching the West by Sunday. ECMWF/CMC trends through the 12Z cycle had been flatter and more progressive but still with poor agreement and continuity for what becomes of the energy after Sunday (affecting surface low/front details east of the Rockies). On the other hand the 12Z UKMET actually pulled off some of the western energy and incorporated it into the upper low offshore California (adjusted away from in the new 00Z UKMET) while the new 00Z GFS incorporates some of this energy in less extreme form. These differences with western shortwave energy have broadened the spread for when the offshore upper low progresses eastward. Even before these latest issues arose, multi-day trends had been for a slower upper low. This led to some skepticism of the new 00Z GFS that has jumped ahead of even the 12Z/18Z runs that were already ahead of the 12Z ECMWF/CMC. However the new 00Z ECMWF has trended a little more amplified with western energy and now ejects the upper low about as fast as the 18Z GFS. Farther east, the 12Z ECMWF and 12Z/18Z GFS runs exhibited a notable southward trend for the mean surface front position by next Tuesday-Wednesday, while the faster ejection of the eastern Pacific upper low in the 00Z GFS/ECMWF reverses that trend. Meanwhile guidance is converging to some degree for what could be a potent upper trough and surface low pressure reaching the Northwest by next Wednesday. In principle GFS runs and the GEFS/ECMWF means have had a somewhat earlier lead on this feature than the ECMWF (which shifted to it in the 00Z/14 run), though GFS runs since the 12Z cycle have become more amplified with the upper trough versus other guidance. The 12Z CMC was not too far behind the GFS but has trended a little flatter in the new 00Z run, providing further support for reducing GFS influence for this feature late in the period. Combining preferences for above features led to following a composite of non-UKMET solutions for about the first half of the period, followed by transitioning to a model/ensemble mean mix that trended to the 12Z ECMWF/CMC/GFS for the model component given that the 12Z GFS was slowest of recent runs with the ejecting California low, albeit still faster than the 12Z CMC/ECMWF. ...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. 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. 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 over some areas. Expect modestly cool highs of 5-10F or so below normal across the South 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. 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