Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Tue Nov 13 2018 Valid 12Z Fri Nov 16 2018 - 12Z Tue Nov 20 2018 ...Overview... Latest guidance is fairly consistent with the overall pattern of a ridge building into western North America while broad mean troughing prevails downstream over the rest of the continent. There continues to be some day-to-day trending for the character of the western ridge as well as the shortwave energy to its east. Farther upstream there is general agreement that flow within a Pacific trough near 150W as of early day 3 Fri will split, with northern energy going around the northern side of the ridge while the rest forms a closed low that may track toward California. However there is a fair amount of spread and variability with the exact track/timing of this feature. In a minority of guidance there is the possibility that flow farther upstream could begin to have some influence on the West Coast pattern by the end of the period. ...Guidance/Uncertainty Assessment... Early in the period the most prominent system should be tracking northeastward from near the Mid-Atlantic coast. Over recent days guidance has had considerable difficulty in resolving the timing of the upper low/trough energy and specifics of surface low pressure. The model/ensemble envelope has narrowed considerably over the past 24 hours to provide some increase of forecast confidence, with an emphasis on a 12Z ECMWF/18Z GFS average providing a reasonable intermediate solution. Latest UKMET runs have been on the fast side of the spread (faster than the GFS/GEFS mean) while CMC runs have supported the slower ECMWF. Over the western half of the country there are still some evolving trends. The most notable from the past 24 hours involve a more westward elongation of shortwave energy dropping out of southwestern Canada in response to the more closed or nearly closed nature of the upper ridge building into the Northwest. The 12Z ECMWF was the most extreme in closing off the high enough to pull some of the shortwave energy all the way beyond the California coast. Remaining guidance plus the occasional historical tendency for the ECMWF to overdo retrogression of energy under upper highs recommend leaning away from this aspect of the 12Z ECMWF. Changes aloft lead to a stronger push of cooler southward across the Intermountain West. Well offshore the West Coast the recent/latest operational model runs have fairly well latched onto the idea of separating trough energy forming a lower latitude closed low that may track toward California. However there is still a lot of spread for the precise track and timing of this low. At least into day 6 Mon the 12Z-18Z GFS/12Z FV3 GFS and 12Z ECMWF/CMC provided a fairly robust cluster that reflects the expected pattern better than the less defined ensemble means. While not the case in recent ECMWF runs (addressed next), remaining models have tended to exhibit slower trends over recent days and the new 00Z CMC/UKMET suggest the upper low could be slower yet. Meanwhile both the ECMWF and ECMWF mean are significantly faster to bring upstream trough energy into the picture leading to greater height falls over the Northwest late in the period. With multi-day trends having favored stronger and more persistent Northwest ridging, would prefer to lean more toward the GFS/GEFS versus the ECMWF/ECMWF mean. The new 00Z GFS/GEFS/CMC solutions further suggest a lower probability for the 12Z ECMWF/ECMWF mean scenario. There is decent consensus showing strong high pressure tracking from western Canada into the central/eastern U.S., behind a cold front that progresses south and east from the northern tier states. Typically low predictability for shortwave details 6-7 days out in time lowers confidence for frontal/surface low pressure details near the Canadian border by next Mon-Tue. The forecast blend based on guidance through the 18Z cycle started with an operational model blend for days 3-4 Fri-Sat with greater emphasis on the 12-18Z GFS/12Z ECMWF relative to the CMC/UKMET. Then with time the initial blend transitioned toward a greater proportion of GFS/FV3 GFS and GEFS relative to the ECMWF/ECMWF mean. ...Weather Highlights/Threats... The system tracking off the Northeast coast on Fri will likely produce some meaningful wintry precipitation over parts of the Northeast. Best potential exists over interior locations while closer to the coast there will be greater sensitivity to the exact track of the surface low. A fairly weak system crossing Great Lakes early in the period will bring some light precipitation to the region. Then trailing cold low level flow will support one or more episodes of lake effect snow. Early next week a front and possible embedded waviness may bring some precipitation to parts of the Great Lakes region but confidence in specifics is low at this time. Over/near the northern-central Rockies and High Plains expect a period of snow from late week into the weekend in association with shortwave energy aloft and enhancement from low level upslope flow. Greatest accumulations with this activity should be over northern areas. Aside from the seemingly less likely ECMWF scenario, recent drier trends suggest the West Coast states should see little to no precipitation for most of the period. Parts of the southeastern half of Texas may see periods of rain by the latter half of the period with some assistance from a persistent surface trough along the Mexico/southern Texas coast. Highest totals should be over extreme southern locations. Strong high pressure dropping into the Plains will bring cold temperatures of at least 10-20F below normal for highs into many parts of the Plains/Mississippi Valley during the weekend. Anomalies should be a little less extreme over the South and East by the first part of next week but some areas will likely still be 10-15F or more below normal. Expect the West Coast states to see above normal temperatures through at least Sun with some localized anomalies exceeding plus 10F for highs. Warmth should continue to some extent early next week. 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