Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Wed Jan 18 2023 Valid 12Z Sat Jan 21 2023 - 12Z Wed Jan 25 2023 ...Heavy snow threat for the interior Northeast Sunday into Monday... ...Overview... Guidance 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... The 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. Recent guidance has increasingly agree to the notion that low pressure will track west of the Appalachians by Sunday and then deepen into a fairly strong low pressure system over New England by early Monday. The ECMWF/GFS and their ensemble mean solutions are reasonably well clustered, while recent CMC and UKMET tend to offer a farther southeast track off the coast. A favored composite of the latest GFS/ECMWF and ensemble means maintains good WPC product continuity with depiction of a stormy low to affect the Northeast for multiple days with a threat for enhanced winds and wrapping precipitation to include a threat for heavy snow ice for mainly interior areas. However, forecast spread meanwhile continues to not be stellar with respect to the upstream flow of energy/systems digging into the West and subsequent ejection downstream into the central and eastern U.S. into next week. The 06 UTC GFS seems an outlier run that showed much less digging of main energies into the West compared to other guidance including the 00 UTC GFS and ensembles. The pattern seems to favor at least decent digging to the lee of an amplified upstream upper ridge. The 12 UTC GFS has at least trended more amplified and alternate guidance still offer amplified solutions. Thought the 00 UTC ECMWF ensemble mean kept things on the best track continuity wise downstream from the West into the central and eastern U.S. into next week, albeit with more uncertainty than normal. Accordingly overall, the WPC medium range product suite was primarily derived from a composite blend of the 00 UTC GFS/ECMWF and GEFS/ECMWF ensemble means for this weekend before quickly switching to the 00 UTC ECMWF ensemble mean into next week. The ECMWF ensemble mean seems to offer stream flow amplitude and timing that seems the best continuity along with intermediate timings of embedded main systems in a period of quickly increasing forecast spread and uncertainty into these longer time frames. ...Weather/Hazard Highlights... While there is still a fair amount of uncertainty in forecast specifics, there continues to be 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 especially the Appalachians/interior Northeast, with 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 than the lead storm while still west of the Appalachians. Across the West, rain and mountain snow of mostly moderate amounts should extend from the Northwest through the northern and central Rockies with upper trough/low support and lower level upslope flow. 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/Schichtel 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