Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1202 PM EST Thu Jan 09 2020 Valid 12Z Sun Jan 12 2020 - 12Z Thu Jan 16 2020 ...Significant late week/weekend system affecting the eastern U.S. to cross the Northeast on Sunday... 17 UTC Update... A blend of the 00Z ECMWF/CMC/06Z GFS was used as a forecast starting point initially on days 3-4 (Sun-Mon). Starting on day 5 (Tue) and onward, the GFS was not preferred as it was quite a bit faster to move a broad upper-level trough from the Pacific Northwest eastward across the CONUS northern tier. Given an expected anomalous upper ridge building northward across the north central Pacific toward the Aleutians, teleconnections would favor a slower and more amplified system across the western U.S. Tue-Wed, as shown by the ECMWF/CMC. Additional major differences were noted by days 6-7 (Wed-Thu) with the GFS flattening the subtropical ridge across the Southeast quite a bit more than the ECMWF and the ensemble consensus. Teleconnections associated with the aforementioned setup across the north central Pacific favor a persistent/stronger southeastern U.S. ridge, as shown by the ECMWF. This results in colder air associated with a strong Canadian surface high expected to nose into the north central U.S. week remaining a bit more confined to the northern Rockies/Upper Midwest (whereas the GFS would spread the cold air much farther south/east by the middle of next week). Thus, during days 5-7, weight was gradually increased for ECENS and NAEFS ensemble means, with some continued use of the 00Z ECMWF/CMC given their favored solutions over the GFS. Overall, changes to forecast continuity during this update were relatively minor. Ryan Previous Discussion (issued at 0700 UTC)... ...Overview/Guidance Assessment... Latest guidance is still suggesting that during next week the large scale pattern may begin to evolve away from the amplified configuration consisting of a western U.S. trough and strong east-central Pacific/western Atlantic ridges. This will occur as the initial east-central Pacific ridge collapses in favor a ridge building into the Bering Sea and mainland Alaska while the western Atlantic ridge flattens while its center gravitates to near Cuba. The gradual trend toward flatter flow across the lower 48 will tend to keep best potential for heaviest eastern U.S. precipitation totals over the Southeast near a persistent frontal boundary while a series of fast-moving Pacific systems will continue to focus the greatest western U.S. activity over the Pacific Northwest/northwest California and northern Rockies. Specifics of flow aloft become uncertain by the end of the period next Thu but moisture should start sagging southward toward central California. Low heights heights aloft crossing areas near the western-central U.S.-Canadian border (likely anchored by an upper low crossing southern Canada) will play a role in the much below normal temperatures forecast over the northern Plains and extending into the northern parts of the West to a lesser extreme. Much above normal temperatures will persist over the eastern U.S. but with decreasing coverage by next Thu. Models/ensembles are finally developing a more robust consensus for the evolution of the system crossing the Northeast on Sun. This is due to the previously fast/sheared GFS adjusting back to established consensus from the past day or so indicating an occluded system over the eastern half of the Great Lakes region as of 12Z Sun. The 18Z GFS was still a bit fast but close enough to include as a minority input for the blend but the new 00Z GFS and GEFS mean have completed the trend to the majority cluster. Farther upstream the guidance depicts a rapidly moving series of Pacific shortwaves feeding into and then ejecting from the western U.S. mean trough aloft. There has been decent clustering for identifying each feature as it comes into the West Coast but models/ensembles have had a difficult timing resolving how each impulse will reflect at the surface to the east of the Rockies. Thus confidence remains lower than average for specifics over the central/eastern U.S. during the first half of next week. By late in the period a more coherent signal seems to arise, toward low pressure crossing southeast Canada and/or New England (albeit with some timing spread) and a trailing front reaching the southern/eastern states. By next Wed-Thu the main focus over the West will be a fairly strong storm system forecast to approach the West Coast. Currently the majority cluster includes the GFS/GEFS mean/CMC/CMC mean/ECMWF mean which all suggest that the past couple ECMWF runs may be too fast to bring the system inland. Looking at larger-scale flow, teleconnections relative to the upstream ridge's positive height anomaly center suggest that recent GFS runs may be pulling the core of downstream troughing too far westward in the D+8 means. This is likely to be a greater issue after the end of the medium range period so a minority component of the GFS could still be used as a blend component into next Thu. While some GFS/CMC runs may have been a bit extreme, other solutions have been trending toward their general idea of an upper low over southwest Canada tracking close enough to the U.S. border to bring fairly low heights across far northern parts of the western/central U.S. during the period. The updated forecast consisted of an operational model consensus early in the period and then transitioned toward increasing GEFS/ECMWF mean weight, reaching 60 percent by day 7 Thu. Due to fast timing of the incoming Pacific system late in the period, the forecast removed the 12Z ECMWF by Thu while maintaining some GFS/CMC input. ...Weather Highlights/Threats... The system expected to track across the Northeast on Sun will bring a potential for meaningful winter weather to northern New England and some light-moderate rainfall just to the south. The front trailing from this system will settle over the Southeast while the farthest southwest part of the front should lift from the Gulf of Mexico into the southern U.S. as a warm front. This front and modest Gulf inflow should promote periods of moderate to locally heavy rainfall over parts of the South, with some repeat activity possibly helping to enhance totals at some locations. However confidence is fairly low for specifics of shortwaves aloft/surface waves so it will take additional time to resolve coverage and magnitude of heaviest rainfall. Meanwhile at least one area of light to moderate precipitation (snow over northern areas) should spread across northern areas of the central/eastern U.S. but again with lower than average confidence in the details. The progression of systems affecting the Northwest should provide the best rain/mountain snow focus over favored terrain in western Oregon and far northwest California with meaningful precipitation also over Washington and across the northern Rockies. Central latitudes of the Interior West/Rockies should see lighter activity. Moisture may begin to push a little farther south toward central California around or just after the end of the period as favored by teleconnections relative to the upper ridge building into the Bering Sea/Alaska. With cold air pushing southward from Canada into northern parts of the West, there will be increased potential for snow at very low elevations (possibly near or at sea-level) by mid-late week. This is reflected in the Winter Weather Outlook probabilities valid in that time frame. The storm system which most guidance has approaching the West Coast Wed-Thu will have to be monitored closely for the possibility of significant inland/low elevation snow depending on track and strength of the cold air push to the north. Expanding influence of cold air across the northern tier will bring readings 25-40F below normal over parts of the northern Plains for multiple days next week while areas over and west of the northern Rockies may be as cold as 15-30F below normal. It is close to the climatologically coldest time of year so it may take more extreme anomalies than these to reach daily record values at many locations. Warmest anomalies over the East will likely be for morning lows with fairly broad coverage of readings 20-30F above normal. Highs of 10-20F above normal will be common with pockets of warmer anomalies. Arrival of a cold front should begin a cooling trend over the South and East by next Thu. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml