Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Wed Jul 11 2018 Valid 12Z Sat Jul 14 2018 - 12Z Wed Jul 18 2018 ...Overview... Amplified flow should remain confined to the international border with Canada throughout the period. Broad longwave troughing will track from Alberta to the vicinity of Hudson Bay by Tuesday with the southern extension of height falls slipping across the Great Lakes and Ohio River valley. In its wake, renewed amplification is likely across western Canada into the Pacific Northwest by Day 6/7, July 17/18. Elsewhere, while an elongated upper trough lingers over the western Atlantic, an expansive 594-dm ridge is likely to unfold over the Desert Southwest while a weaker anticyclone is expected across the southeastern U.S. ...Guidance evaluation and preferences... Starting with the upper trough sitting over the western Atlantic, the National Hurricane Center has the region in a 50 percent threat for tropical cyclone development during the next 5 days. The 18Z GFS was most aggressive with this scenario depicting a couple of closed isobars surrounding the circulation. Eventually this elongated negative height anomaly lifts northeastward as heights build along the East Coast. Looking to the active pattern along the Canadian border, models are initially in decent agreement ejecting lower heights across the province of Alberta on Saturday. Issues with timing and overall placement become more numerous as the trough closes off and attains a rather formidable appearance. The 12Z ECMWF, and now the 00Z UKMET are well northeast of the recent GFS/CMC solutions. Multi-cycle comparisons have shown forecasts bouncing around a bit so confidence begins to diminish from Day 5/Monday onward. In spite of the model spread, it appears reasonable to believe a slow-moving baroclinic zone should be crossing the I-95 corridor by late Tuesday/early Wednesday next week. Where much of the forecast spread looms is with the next trough approaching western North America beginning early Tuesday. Most notably, the 12Z ECMWF and a fair share of its ensemble members trending significantly quicker with this feature. While perhaps a trend, it also has no support from all of the available 00Z guidance. Its downstream effects would favor a shortwave across western Canada where other models build heights. This appears to be a contentious region of the map so will continue to stay away from this outlying 12Z ECMWF suite in favor of its preceding cycle. There is a fairly unanimous signal in the guidance favoring a marked uptick in mid-level heights late in the weekend into next week. 594-dm heights are supported from the Desert Southwest into the Four Corners region with the GFS runs being the most aggressive. The medium range preference initially favored a combination of the 18Z/12Z GFS and previous 00Z ECMWF (10/0000Z) through Day 4/Sunday before gradually increasing ensemble influences into the picture. By the middle of next week, had little confidence in any one solution so took a multi-ensemble blend approach utilizing the 18Z GEFS/12Z NAEFS/00Z ECMWF ensemble means. ...Weather highlights/Threats... While many locations across the nation can expect temperatures near to above climatology, there are a few areas which will be on the cooler side. An active southwestern U.S. monsoonal circulation should keep these locations wet given daily thunderstorm chances with accompanying enhanced cloud cover. In a general sense, expected highs should be around 5 degrees below climatology. Meanwhile, Sunday should prove to be a rather cool day for July standards over the Northern Rockies into the adjacent Northern Plains as well as portions of Nebraska. 10 to 15 degree departures will keep highs in the 70s given cold advection in the wake of the trough passage. Regarding rainfall threats, as mentioned, the Four Corners region will remain wet due to monsoon considerations with activity gradually reaching the Central Great Basin and circulating into the Central Rockies. Farther east, widespread convection is anticipated along the slow moving front which will migrate from the middle of the nation toward the Eastern Seaboard from Day 4-7, July 15-18. There is also a signal for enhanced thunderstorm chances across the southeastern U.S. as high precipitable water air pools across the region. 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