Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Wed Jan 18 2023 Valid 12Z Sat Jan 21 2023 - 12Z Wed Jan 25 2023 ...Overview... Latest models and ensembles still maintain the theme of a positively tilted mean trough over the lower 48 while a strong ridge persists over the eastern Pacific and another ridge prevails southeast of Florida. Expect this pattern to produce highest precipitation totals over the Southeast and vicinity, but with meaningful totals of rain or snow also possibly extending farther west and north from the Plains into the Northeast with individual embedded systems. Locations from the Northwest U.S. east/southeast into the Rockies should see periods of rain and mountain snow while much of the West remains below normal for temperatures. There has been considerable guidance spread and variability for the systems of interest but latest runs are starting to show better clustering in some respects, providing hope for improved confidence sometime soon for sensible weather details. ...Guidance/Predictability Assessment... Recent forecast uncertainty has revolved around two primary features, a leading upper low emerging from the southern Rockies early Saturday (with questions over interaction with southern Canada/northern Plains energy to the north) and trailing shortwave energy dropping into the West during the weekend into early next week. Through the 00Z/17 cycle, GFS runs were generally on their own in depicting pronounced phasing over the central U.S. versus having the northern stream shortwave bypass the upper low. However the 12Z models unanimously switched to the phased GFS scenario and the latest 00Z runs are maintaining continuity with that idea. The end result is a trend toward at least some degree of low pressure tracking west of the Appalachians by around Sunday (though not as strong as some earlier GFS runs) and then fairly strong low pressure over or near New England by early Monday. The ECMWF/GFS and their means are fairly close while latest CMC runs have tended to be slow/south. The new 00Z UKMET is the most extreme with its farther southeast track off the coast. Even with all of the model and ensemble member spread and inconsistency before this recent convergence of solutions, ensemble means had been hinting at some degree of low pressure reaching the New England area for multiple days. For the next system dropping into the West, the best recent signal has been for an open wave to close off an upper low near the Four Corners (a little later/south versus a number of GFS runs) though the latest CMC/UKMET actually show an upper low track farther southwest. The most extreme side of the spread is still a question mark since teleconnections relative to the upstream ridge do not seem to favor strongly negative height anomalies over the Southwest, though perhaps sufficient progression to temper the multi-day mean could still leave the door open to that possibility. Meanwhile the 12Z ECMWF strayed to the fast side of the envelope with the ECMWF mean partway between the ECMWF and slower GFS/GEFS mean/CMC cluster. The new 00Z ECMWF is still on the east side of the spread, just with a somewhat deeper trend as its shortwave drops southward over the Rockies. By early next Wednesday the loose clustering of GFS/GEFS mean/CMC/ECens would have a low pressure system reaching somewhere between the Mississippi Valley and the Appalachians/Mid Atlantic/Great Lakes, versus the Southeast coast in the 12Z ECMWF. Even with its different details the new 00Z ECMWF ends up with its surface low near the 12Z ECMWF mean. Based on latest trends and clustering, the early part of the updated forecast followed an operational model composite early and then transitioned toward a combination of the 18Z GFS/12Z CMC and the 18Z GEFS/12Z ECMWF means to reflect the moderate majority scenario by late in the period. ...Weather/Hazard Highlights... While there is still a fair amount of uncertainty in forecast specifics, there has been some improvement in the broad theme of two primary systems and their general timing. The first should track from the Plains through the central/northern East Coast Saturday-Monday, bringing the potential for wintry weather from the central Plains into the Northeast and rain to the south. Expect the heaviest rain over the Southeast where Gulf moisture may interact with a leading warm front. Most guidance has been trending deeper for this storm near New England by Monday, so brisk to strong winds could be possible. There is still potential for a meaningful change in the most likely track so continue to monitor forecasts for this system. The next system may affect the central-eastern U.S. by Tuesday-Wednesday with a precipitation distribution that could have some similarities to the first system. However there are signals that this latter system could be deeper while still west of the Appalachians. Across the West, rain and mountain snow of mostly moderate amounts should extend from the Northwest/northern Rockies to the south/southeast. Southwestward extent of the precipitation will depend on exact track/amplitude of the weekend feature whose details remain unresolved. Most aspects of the temperature forecast have changed little over the past day. The northern three-fourths of the West and into parts of the High Plains still look chilly through the period with highs 5-20F below normal. An isolated location or two could see a record cold high. Locations across the eastern U.S. and northern Plains should see near to above normal highs with generally warmer anomalies (up to plus 10-20F over some areas) for lows. 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