Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Thu Jun 07 2018 Valid 12Z Sun Jun 10 2018 - 12Z Thu Jun 14 2018 ...Pattern overview... Guidance continues to suggest an evolution from an omega block appearance over Canada and about the northern half of the contiguous U.S. to a possible rex block configuration over Canada as northern U.S./southern Canada flow cuts through the initial ridge. At this time the precise details of this transition and the eventual shape of the pattern by the middle of next week are still fairly uncertain though. Meanwhile there is an improving signal toward a mean trough settling along or slightly inland from the West Coast by next Wed-Thu. A weakening mean ridge over the southern Plains early in the period will rebuild over the southern Rockies/High Plains and then possibly expand or shift eastward. The western periphery of an Atlantic/Caribbean ridge aloft will gradually build westward over the course of next week. ...Model guidance/predictability evaluation/preferences... Into about day 5 Tue a consensus blend (mostly operational guidance, including 06Z GFS/00Z ECMWF/00Z CMC/00Z UKMET, with a little ECMWF mean) best represents the most likely forecast scenario, as the core of a strong western North America trough heads into west-central Canada and energy east of Hudson Bay descends within a persistent mean trough. Within this eastern mean trough, the 06Z GFS is closer to the majority cluster over the past couple days versus the farther westward and southward 00Z GFS/UKMET which bring the center of a compact upper low close to the northern tip of Maine. By later in the period guidance has generally maintained the idea (which was fairly new 24-36 hours ago) that northern U.S. flow will continue eastward and ultimately lead to a low amplitude pattern across the northern states by next Wed-Thu while an ill-defined axis of lower heights prevail over southern Canada, south of a closed high. Confidence is low in the specifics though, favoring a blended approach. Upstream the previously chaotic model/ensemble plots show improved clustering today toward a trough settling near the West Coast late in the period. Overall the means best depicted this idea yesterday and had teleconnection support from surrounding anomaly centers in the D+8 means. However the deep/amplified ECMWF lies near the extreme side of the ensemble envelope so only a small minority of its forecast could be incorporated by late in the period. Farther southeast, an average of the GEFS/ECMWF means is reasonable for resolving differences in specifics of the southern Rockies/Plains ridge while the GFS/GEFS still appear to show premature development and progression of a tropical system over the northwestern Caribbean/Gulf of Mexico. The historically enthusiastic CMC as well as the NAVGEM seem to remain as a more realistic "extreme bound" that is much more suppressed than the GFS/GEFS. Thus GFS forecasts had to be excluded from the favored blend late in the period. The later part of the period trended toward more ensemble mean input, from the 00Z ECMWF mean and a split of 00Z GEFS/NAEFS. ...Weather highlights and hazards... A wavy frontal boundary extending from the East Coast through the Ohio Valley and into the northern Plains will provide a focus for showers/storms over multiple days, with some rainfall locally heavy. Convection may also be strong in some cases. Current SPC outlook indicates that parts of the northern Plains may have favorable ingredients for severe potential during the latter half of the weekend. The western part of this front will connect to the strong system emerging from the West and continued progression of the western front will likely bring further convection to the northern Plains into the MS Valley before possible weakening. The Southeast will see diurnally favored convection most days with activity enhanced by localized boundaries and possibly the front to the north. Departure of the strong weekend system will lead to a drier trend over the Northwest for the first half of next week. Then the arrival of upper troughing should bring some precipitation but with generally light amounts. The northern half of the Intermountain West and northern Rockies will see a day or two of highs at least 10F below normal early in the period as strong upper troughing passes over the area. At the same time warm sector highs/lows are likely to reach 10-20F above normal over and near the northern-central Plains Sun-Mon. Rebuilding of the upper ridge over the southern Rockies and vicinity will promote an expanding area of plus 5-15F anomalies for min/max readings over the West by midweek, eventually spreading into parts of the High Plains. Expect mean troughing aloft to keep the Northeast/Mid Atlantic on the cool side through at least early next week. Flattening of the mean pattern should lead to near-normal temperatures near the East Coast by next Wed-Thu. Rausch WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation, and winter weather outlook probabilities can be 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