Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 AM EDT Wed Jun 27 2018 Valid 12Z Sat Jun 30 2018 - 12Z Wed Jul 04 2018 ...Overview and guidance evaluation/preferences... Guidance maintains similar ideas for the large scale pattern evolution through the period albeit with some embedded detail spread. Cyclonic flow across the Northern Tier will broaden during the weekend as an ejecting western shortwave progresses east and then northeast followed by weak upstream energy flowing around a strong eastern Pacific ridge. Then a Gulf of Alaska upper low will drop southeastward toward northern WA/southern BC. This feature combined with a retrogression of the Pacific ridge will lead to development of a mean trough near the West Coast next week. Flow on the western side of the trough may start to pull the overall trough axis a little offshore around midweek or so. This would be consistent with teleconnections relative to the positive anomaly center associated with the new position of the Pacific ridge. Meanwhile the center of the eastern U.S. upper ridge responsible for the heat wave over the eastern half of the country will track east-northeast from the Ohio Valley/Appalachians. Then upper ridging will likely build over the Plains/MS Valley/southern Rockies. Around the periphery of these ridges there may be some diffuse upper energy near the Gulf Coast and a western Atlantic weakness/upper low that may eventually approach the East Coast. Over the past day guidance has maintained fairly good continuity in principle for Northern Tier flow during the first half of the period. There are still persistent detail differences that are small enough in scale to have fairly low predictability, favoring a consensus blend at this time. Leading shortwave energy will push an upper MS Valley-central Plains front into the Great Lakes Sat-Mon. The front may decelerate for a time early next week as heights aloft become nearly parallel to the front. Toward the end of the period guidance starts to signal a hint of troughing aloft in the vicinity of eastern Canada/New England. Spread is consistent with detail uncertainty upstream, though teleconnections relative to the Pacific/West Coast pattern do favor eventual development of moderate troughing over eastern Canada/New England. Thus the surface front reaching the East could speed up a bit around midweek. Recent guidance trends for the Gulf of Alaska upper low have gone fairly strongly toward the ECMWF/ECMWF mean which have been on the faster side of the envelope over the past couple days. New 00Z solutions further this trend with the GFS/CMC now showing ejection of the upper low by day 7 Wed though not quite as fast as ECMWF runs. Guidance spread through the 18Z cycle recommended leaning more toward the ensemble means to downplay uncertain details by days 6-7 Tue-Wed. Diffuse nature of energy forecast over the western Atlantic and Gulf Coast lends itself to low predictability for specifics. There has been a consistent signal for Gulf Coast energy in some form over the past couple days, while the GFS/ECMWF have trended toward holding western Atlantic energy farther eastward. Above forecast considerations yielded a blend of operational model guidance (18Z GFS and 12Z ECMWF/UKMET/CMC) days 3-4 Sat-Sun followed by a steady trend toward more 18Z GEFS and 12Z NAEFS/ECMWF mean input such that ensembles contributed 70 percent to the forecast by day 7 Wed. ...Weather threats/highlights... Expect potential for some heavy/strong convection between the upper Great Lakes and central Plains early in the period, in association with a cold front initially near this area. Progression of the front will eventually bring showers/thunderstorms into the East but with any heavy activity tending to be more localized as the best dynamics lift into Canada after the weekend. The trailing part of the front will stall over the Plains and lift back as a warm front so there will be potential for additional rainfall over portions of the central U.S. after the more concentrated activity during the weekend. The other area of focus for heavy rainfall potential will be over the Gulf Coast/Florida, with the combination of upper level energy and surface troughing contributing to activity. In terms of highest temperature anomalies, the core of the heat wave affecting the eastern half of the U.S. should extend from the Midwest/Great Lakes into the Northeast. The greatest coverage of plus 10-20F above normal readings will likely exist Sat-Mon, followed by gradual moderation as the Plains front approaches. Best potential for record highs will be from the lower Great Lakes/upper Ohio Valley into the northern Mid Atlantic/Northeast Sat-Sun while broader coverage of record warm lows is possible Sat-Mon. The northern Rockies/High Plains will see below normal highs during the weekend followed by a trend toward near normal temperatures. The evolving pattern by late in the period will tend to support moderately below normal temperatures along the West Coast and over the Northwest toward midweek. Rausch WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation, and winter weather outlook probabilities 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