Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1043 AM EST Tue Jan 22 2019 Valid 12Z Fri Jan 25 2019 - 12Z Tue Jan 29 2019 1530 UTC Update... Models showed good large scale agreement throughout the medium range period with dominant ridging off the west coast of North America and mean troughing across Hudson Bay extending south into the Great Lakes/Midwest. During days 3-5 (Fri-Sun) consensus among guidance was sufficient to base the forecast on a multi-model blend of the latest deterministic guidance. During days 6-7 (Mon-Tue), the most significant model differences continue with respect to the degree of phasing between northern/southern stream shortwave energy across the eastern U.S., and the implications for the track and timing of a potential low pressure system off the East Coast. The latest runs of the ECMWF and GFS showed very little in the way of phasing and thus a highly progressive system - a stark change from previous runs for the ECMWF in particular. The CMC, on the other hand, held onto a highly phased and amplified solution. A look at ensemble members does seem to suggest a solution at least somewhat less progressive than shown by the 00Z ECMWF/06Z GFS, with perhaps even a few more members joining this idea over the past couple ensemble cycles. Thus, the WPC forecast is not near as progressive as the ECMWF/GFS, and resembles something closer to the ECENS/GEFS ensemble means, which comprised the majority of the forecast blend during days 6-7. Ryan Previous Discussion (issued at 0658 UTC)... ...Overview... Latest models and ensembles maintain the theme of an amplified West Coast/eastern Pacific ridge and expansive central-eastern U.S. mean trough. Later in the period the D+8 multi-day means still suggest the ridge could retrograde just enough to allow for a little more southwestward elongation of shortwave energy over the West, per teleconnections relative to the ridge's corresponding positive height anomaly center. Another aspect that persists is spread/variability for some important embedded details--thus tempering confidence in the sensible weather forecast over some areas in spite of an overall mean pattern that has much higher predictability. ...Guidance Evaluation/Preferences... Based on the array of 12Z/18Z guidance the preferred blend started with an average of operational model solutions days 3-4 Fri-Sat followed by a transition to mostly 18Z GEFS/12Z ECMWF means late in the period as model details diverged from each other and the less volatile ensemble means. Forecast specifics still appear to be in flux to some degree with various differences/trends of note. Waviness expected to track across the northern Plains/Midwest early in the period has trended weaker over the past day in response to a slightly more sheared appearance of shortwave energy, with a general trend toward the faster side of prior spread which was represented by the ECMWF. On the other hand the upper low initially over Hudson Bay has trended farther south in latest model runs. This brings a stronger surface reflection/frontal system into the Great Lakes during the weekend. Note that the new 00Z CMC has corrected from its 12Z run that had brought the upper low as far south as the Ohio Valley. Farther west and south there is loose agreement that shortwave energy should drop southward across the west-central U.S. into the southern Plains/Mexico by around Sat and then continue around the southern/eastern periphery of the mean trough. Differences in shortwave detail as well as how flow to the north may interact with this energy lead to a variety of possible outcomes for potential Gulf/western Atlantic low pressure during days 5-7 Sun-Tue. For timing and track the GEFS/ECMWF ensemble means have provided the most consistent starting point for this possible development. Better guidance agreement in future runs may help to guide deeper adjustments if needed. During the latter half of the period there is decent agreement that the combination of Pacific energy rounding the ridge and some Canadian flow will support northern U.S. low pressure and a trailing cold surge that will again focus over the central and eventually eastern states but also bring somewhat more cooling to the Rockies/Interior West than preceding fronts. Prefer a model/mean blend that tilts more to the means late in the forecast. ...Weather Highlights/Hazards... Locations from the northern Plains into the Great Lakes may see a series of snow events associated with surface waves tracking over or just south of this area. Confidence is not yet particularly high for pinpointing exact intensity/coverage of snow due to guidance spread and run-run variability/trends. Over the Great Lakes there will likely be periods of pure lake effect snow as well as some lake enhancement with the aforementioned systems. Difficulties in resolving important details aloft complicate the forecast across the South and East from the weekend into next week. Best consensus would have the Gulf Coast region receiving at least some rainfall during the weekend. Any moisture extending farther north could meet enough cold air to produce snow at times. Moisture may then extend along the East Coast depending on western Atlantic low pressure evolution. Best potential for some snowfall exists over the Northeast. Over the Rockies expect two primary episodes of locally enhanced snow progressing from north to south, one late this week and another from late weekend into early next week. The Midwest/Upper Mississippi Valley should continue to be the most prominent focus for coldest temperature anomalies during the period with some readings at least 20F below normal Fri-Sat and then again by next Tue. The rest of the eastern half of the country should see near to below normal temperatures. On the other hand expect the West Coast states to remain warmer than average with plus 10-15F anomalies in some cases. The northern Rockies/High Plains will likely be on the warm side as well Fri-Sun, including much above normal morning lows during the weekend. The Rockies and vicinity should see a pronounced cooling trend early next week. Rausch WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation and winter weather outlook probabilities are found 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/wwd/pwpf_d47/pwpf_medr.php?day=4