Extended Forecast Discussion NWS Weather Prediction Center College Park MD 237 AM EDT Thu Jul 12 2018 Valid 12Z Sun Jul 15 2018 - 12Z Thu Jul 19 2018 ...Overview... As an elongated upper trough slowly departs off the Eastern Seaboard, a sharpening trough will zipper across central Canada reaching the vicinity of Hudson Bay by Monday. Such height falls should gradually translate eastward possibly exiting the eastern U.S. by Day 7/July 19. In its wake, the synoptic pattern remains more nebulous in nature as guidance vary in which phase of the sinusoidal flow will be favored. It does appear some form of longwave trough should be positioned over western North America toward the end of the forecast. To the south of the active mid-latitude pattern focusing along the international border with Canada, a prominent 594-dm mid-level ridge should become fixed over the southwestern U.S. throughout the period. ...Guidance evaluation and preferences... There do not appear to be any significant issues with the handling of the slow moving trough lifting away from the Gulf Stream. This region is continuing to be monitored by the National Hurricane Center for tropical cyclone development with a 50 percent chance of such occurrence the next 5 days. The 12Z UKMET is the most bullish with cyclogenesis depicting a sub-1000 mb wave lifting toward the eastern tip of Nova Scotia by Monday evening. Meanwhile, the other region of only modest differences is the southwestern U.S. with a 2 sigma above climatology mid-level ridge setting up for the foreseeable future. The past few runs of the GFS are a bit more expansive with the 594-dm height contour relative to other guidance. A pronounced upper trough and the attendant height falls will slide eastward from central North America toward the Eastern Seaboard during the period. Relative to yesterday, the models have trended a bit slower with frontal progression with a position likely offshore of New England by Day 7/July 19. Ensemble spaghetti plots from the 12Z/00Z cycles would suggest the ECMWF members are slower as a whole although its pattern has looked a bit wonky upstream. So have not put tremendous stock in its solution suite. Plenty of run-to-run variability in the guidance though as this large-scale upper trough marches toward eastern North America by mid/late next week. The most contentious sector of the map is the northeastern Pacific eastward into western North America where the guidance are at odds with one another. Phase differences are immediately evident in operational and ensemble model comparisons. During the last three runs, the ECMWF ensembles continue to aggressively shove a trough into British Columbia. Investigating where this system comes from, it currently lurks near the far western Aleutians where the ECMWF sustains it as an open wave into northern British Columbia while the GFS shears it apart over the Yukon Territory. Suffice to say, this difference leads to significant phase differences on the map. The previous GFS runs and 12Z UKMET favor troughing into western Canada on Tuesday while the past 3 ECMWF cycles build heights ahead of another compact upper low. Still difficult to trust the ECMWF, particularly its operational run given how much it departs from the ensembles by the Day 4/5, July 16/17 timeframe. During the forecast, kept a 30 percent contribution of some variation of the 12Z ECMWF/ECMWF ensemble mean throughout the period. Did not want to completely discount it but the forecast was more heavily based on the 18Z/12Z GFS, 18Z GEFS/12Z NAEFS ensemble means, with some early minimal contributions from the 12Z CMC/UKMET. A complete ensemble approach was deemed necessary beyond Wednesday given a breakdown of the pattern. ...Weather highlights/Threats... While upcoming temperatures should remain rather close to climatology during the forecast period, there are a few exceptions to this rule. Sunday should prove to be a rather cool summer day across the Northern Rockies/Great Plains down into western Nebraska with highs 10 to 15 degrees below average. This would suggest highs in the 70s before rebounding by early next week as the cold advection ceases. Elsewhere, well above average readings should be commonplace across the Pacific Northwest with a building upper ridge. High temperatures will likely be 10 to 15 degrees above climatology from Sunday through Tuesday as readings soar into the 90s inland with 70s closer to the Pacific coast. Wet conditions should prevail over the Four Corners region up into the Central Rockies although coverage will be dependent on how expansive the 594-dm ridge becomes. Warm mid-level temperatures underneath this ridge should stunt more vertically developed convection. Farther east, an expansive region of showers and thunderstorms is expected in advance of the central North American trough. While the better focus should occur closer to the height falls, abundant instability would promote widespread convection within the vast warm sector. Rubin-Oster WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found at: http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 http://www.wpc.ncep.noaa.gov/heat_index.shtml