Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 AM EST Sat Nov 10 2018 Valid 12Z Tue Nov 13 2018 - 12Z Sat Nov 17 2018 ...Overview... Most guidance maintains good continuity in principle. The majority cluster shows a brief flattening of the initially sharp/amplified upper level western ridge-eastern trough pattern before a somewhat broader/less amplified ridge-trough emerge late in the period after incoming Pacific energy passes through the western states. Within this evolution the first system of interest will be a rapidly strengthening storm tracking from the Mid-Atlantic into extreme eastern Canada Tue-Wed. The progressive Pacific energy will likely bring a weak front into the Northwest around Wed and then support an area of low pressure reaching near the upper Great Lakes late in the week. Upstream Pacific troughing aloft late in the period should push a front toward the Northwest by Fri-Sat. ...Guidance/Uncertainty Assessment... For the developing East Coast into Canada storm early in the period, important subtle detail differences remain but these are generally within typical guidance error for forecasts 3-4 days out in time. Starting with a consensus operational model blend--more 12Z GFS/FV3 GFS and 12Z ECMWF than 12Z UKMET/CMC--yielded a solution very close to continuity from the past couple cycles. Behind this system there is still a fair amount of uncertainty/relatively low confidence in the specifics of residual trailing shortwave energy that may drift eastward from the southern Plains/northern Mexico after separating from the rest of the overall trough that quickly crosses the eastern U.S. Taken as a whole the latest models/means seem to be signaling increased potential for some trailing shortwave energy to cross the southern latitudes but in weak enough form to keep precipitation coverage and totals on the modest side, while operational runs suggest much less coverage of light activity over the Southeast/Mid-Atlantic than seen in the ensemble means. For the shortwave energy reaching the Northwest around day 4 Wed the CMC continues to be the most amplified but the 00Z UKMET has come in with a rather strong solution as well. This relative strength versus other guidance persists downstream as the energy feeds into the mean trough. Strength of the initial western ridge should support leaning away from the stronger solutions at least near the West Coast. By the latter half of the period GFS runs thus far have been more eager than most other solutions to bring energy from the next eastern Pacific upper trough into western North America. This likely plays a part in the 18Z/00Z GFS runs in particular becoming fairly fast with the system tracking across the upper Great Lakes. The 12Z GFS/FV3 GFS were closer to consensus (which included the GEFS mean) for the Great Lakes system. Even with this consensus there is enough guidance spread and ECMWF inconsistency to temper confidence in a specific scenario for the time being. Along the West Coast leaning more toward the means by days 6-7 helped keep the forecast between the fast GFS and slower ECMWF. For days 3-4 Tue-Wed, the blend favored for the East Coast system represented consensus well elsewhere with CMC weight sufficiently low to downplay its stronger West Coast shortwave by day 4. Day 5 excluded the UKMET due to it becoming out of phase over the northern U.S./southern Canada while starting to incorporate 18Z GEFS/12Z ECMWF ensemble means. Days 6-7 started with 60-70 percent ensemble means with the 12Z FV3 GFS/ECMWF providing the rest of the input. ...Weather Highlights/Threats... The strengthening East Coast storm likely to reach Canada by Wed will bring significant precipitation to portions of the Northeast with highest totals most likely near the New England coast. Expect rain along and somewhat inland from the coast with any snow tending to be confined to western New England, the Great Lakes, and Appalachians. Somewhat lesser totals are likely along the cold front crossing the Southeast. Lake effect activity should persist at least into midweek. Parts of the Northeast may see a period of breezy to windy conditions after passage of the surface low. After the East Coast system departs, the contiguous U.S. should see fairly low coverage of meaningful precipitation. The Pacific Northwest will likely see two episodes of rain/higher elevation snow, the first around midweek and a second one toward Fri and/or Sat. The latter event should have somewhat higher amounts than the first. Low level flow from the Atlantic may bring some showers to the Florida peninsula while low confidence shortwave energy that could cross parts of the South may generate light rainfall over parts of the Southeast. Precipitation totals should be fairly modest with the system forecast to cross the upper Great Lakes around Thu-Fri. Much below normal temperatures are likely Tue-Wed from the southern/eastern Plains into most of the East, with decent coverage of highs 20F or more below normal and only slightly less extreme anomalies for morning lows. A few daily records may be possible within this area. Much of the eastern half of the lower 48 will likely remain below normal for the rest of the period but with some moderation compared to Tue-Wed. On the other hand above normal readings will prevail from most of the West into the northern High Plains. There may be localized plus 10-15F anomalies for min/max temperatures from Wed onward. 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