Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sat Jan 19 2019 Valid 12Z Tue Jan 22 2019 - 12Z Sat Jan 26 2019 ...Overview... Latest guidance is persistent and agreeable regarding the expected pattern evolution toward a very amplified eastern Pacific/West Coast ridge and east-central North American trough. Establishment of this pattern will promote a southward push of cold Canadian air into the northern U.S. by late this week with the upper half of the Mississippi Valley and vicinity the most likely to see the lowest temperatures. Teleconnections relative to corresponding height anomaly centers support the expected axis of cold air over the east-central U.S. along with some precipitation chances along the East Coast, while the West Coast should be dry near the upper ridge axis. Within the consensus mean pattern aloft there will continue to be significant medium to smaller scale differences that will affect sensible weather and take additional time to resolve. ...Guidance Evaluation/Preferences... Models/ensembles continue to show significant differences and trends for various aspects of the forecast, highlighting the sensitivity of shortwave specifics and stream interaction. Early in the period models/means are improving their clustering for low pressure tracking from the central Plains through the Great Lakes and into eastern Canada. Solutions appear to be converging toward an intermediate timing somewhat closer to earlier GFS runs versus the ECMWF that had been much slower. Meanwhile the past day has seen an increasing signal toward trailing shortwave energy hanging back enough to produce what could be a strong frontal wave affecting the eastern states mid-late week. In spite of this improving signal for wave existence, model/ensemble spread is still dramatic and shows little clustering. For example model forecasts valid 12Z Thu range between the east-central Gulf Coast (12Z ECMWF) to the northern Mid-Atlantic (new 00Z GFS/CMC). Longitude of this wave is another issue beyond timing. Overall an intermediate solution appeared best in light of the full array of guidance through the 12Z/18Z cycles. Behind this system there will likely be multiple shortwaves passing through/around the northern part of the eastern Pacific and progressing into North America while flow around the mean upper low over/near Hudson Bay should extend southward into the northern U.S.. For the most part shortwave details are sufficiently small in scale to yield low confidence in specifics, and will be prone to run-run/day-day variability. Thus expect worse than average continuity for associated waves/frontal systems. In the Thu-Sat time frame the primary model issue of note is that the 12Z ECMWF shows greater westward extent of upper troughing over the West than most guidance--achieving extreme to outlier status depending on height contour and location. Guidance evaluation led to a blend preference incorporating the 18Z GFS and 12Z ECMWF/UKMET/CMC early in the period. Rapidly lowering confidence in specifics after Wed favored a corresponding increase of 18Z GEFS/12Z ECMWF mean input so that the means provided 80 percent of the mass field ingredients by days 6-7 Fri-Sat. ...Weather Highlights/Hazards... During Tue-Wed Plains low pressure tracking through the Great Lakes and into eastern Canada will spread snow from the north-central Plains and Upper Mississippi Valley through the central-northern Great Lakes into New England. The trailing front should begin to interact with Gulf moisture by Tue night-Wed with expanding areas of locally moderate/heavy warm sector rainfall. Confidence is increasing that a significant wave may form along the trailing front and spread a broad shield of precipitation--some heavy--across the eastern states at least through Thu. However track and timing of this wave remain very uncertain and this tempers confidence in pinpointing location of heaviest rain/snow as well as precipitation types. Currently the best potential for highest total precipitation is within an area from the east-central Gulf Coast/Tennessee Valley/southern Appalachians northeastward into southern New England. Best snow/wintry mix potential will lie to the northwest of that area. After passage of this system, one or more waves/fronts may produce areas of light-moderate snow over some areas east of the Rockies and cold flow over the mostly ice-free Great Lakes should promote episodes of lake effect snow. The forecast looks fairly consistent for the focused band of shortwave energy and moisture that will bring a period of enhanced snow and low elevation rain to locations from the Pacific Northwest into the Northern/Central Rockies Tue-Wed night. Late week into the weekend additional shortwave energy within northwesterly flow aloft along with a persistent stationary front at the surface may promote some snow along the northern half of the Rockies. Portions of the East will see a very cold start to the period on Tue (some lows 20F or more below normal over the southeastern Great Lakes/northern Appalachians) before a brief warm-up ahead of the approaching front/wave, with some readings up to 10-20F or so above normal. Cold air dropping into the east-central U.S. later in the period may bring temperatures down to at least 10-20F below normal near the Mississippi Valley. Continue to expect moderately above normal temperatures over the West Coast states after midweek as the upper ridge axis settles nearby. Rausch WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes 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 https://www.wpc.ncep.noaa.gov/heat_index.shtml