Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Wed Jun 06 2018 Valid 12Z Sat Jun 09 2018 - 12Z Wed Jun 13 2018 ...Pattern overview... A northern stream omega block signature is noted in the guidance with the key player being a powerful upper trough approaching the Pacific Northwest early in the weekend. Characterized by 500-mb height anomalies around 2 sigma, an embedded closed low/strong band of height falls will slide eastward reaching the Northern Rockies by 11/1200Z. Ultimately this should suppress a mean upper ridge which will sprawled across the Desert Southwest eastward to the Southern Plains. Gradually this positive height anomaly will rebound by early next week with real estate generally covering the southwestern quarter of the country. To the north, the mentioned upper trough pushing through the Northern Rockies on Monday is expected to slow considerably and may even close off at some point near the Dakotas/south-central Canada border. All the while, this omega block set up will shift the pattern downstream with broad upper troughing across the northeastern U.S. gradually sliding offshore by the middle of next week. ...Model guidance/predictability evaluation/preferences... Initially focusing on the upper trough barreling through the Pacific Northwest, multi-day ensemble spaghetti plots have shown a faster trend. On Monday morning, the mean trough axis is roughly located over central Montana while three model cycles ago it was back toward the Pacific Northwest. Previous days offered random operational solutions which appeared more outlandish. However, through Day 5/June 11, the guidance has been fairly consistent which bolsters confidence across this region. The attendant height falls help erode the northern extent of the mean ridge over the southwestern to south-central U.S. with the 12Z CMC and 00Z/12Z UKMET solutions holding on to the ridge a bit longer. As the heights relax once the parent upper low moves toward the Saskatchewan/Manitoba border, a return of the semi-permanent ridge is likely during the Day 5-7, June 11-13, timeframe. It should generally re-assume a similar position with a 588-dm height contour stretching from the Desert Southwest toward the Southern/Central Plains. Considering the strong feature accelerating through the Northern Rockies on Monday, a great deal of uncertainty looms thereafter as solutions waver about multiple scenarios. The 00Z GFS has made a shift toward a more progressive solution after most guidance favor closing off the upper low somewhere across south-central Canada where a quasi-stationary motion is attained. Ensemble spaghetti plots vary considerably which promotes more emphasis on the global ensemble means. Farther downstream, the eastern extent of the omega block will drift eastward with the level of amplification up for debate. Some fairly amplified solutions are in the cards although trusting any one operational model would not be wise here. As an example, at 13/0000Z, the 00Z GFS keeps height falls restricted to New England while the preceding run shifts down to the lower mid-Atlantic region. However, at the very least, the ultimate synoptic configuration appears reasonable across the board. Regarding preferences, through Day 4/Sunday, was able to utilize a multi-operational model blend led by the 18Z/12Z GFS and 12Z ECMWF with minor contributions from the 12Z CMC/UKMET. Thereafter, gradually incorporated heavier weighting of the ensemble means with only a 20 percent influence from the 18Z/12Z GFS and 12Z ECMWF given significant uncertainties within the northern stream omega block. One region which remains stable is the re-establishment of the upper ridge across the southwestern U.S. with ensemble means in good agreement there. ...Weather highlights and hazards... While much of the country can expect above average temperatures during the period, there will be a few exceptions to this rule. In particular, expect numbers around 5 to 15 degrees below climatology this weekend from northern California and the Pacific Northwest eastward into the Central Great Basin/Upper Intermountain West. Many high temperatures will struggle to get out of the 50s with lows in the 30s across the Central Great Basin. Further east, deep troughing across the northeastern U.S. will favor slightly cooler temperatures, with similar impacts across Florida except driven more by extensive cloud cover and rainfall. Otherwise, the rest of the country will remain rather warm, especially over the middle of the country in advance of the barreling upper low. Primarily an issue this weekend, departures from climatology will range from 10 to 15 degrees across the Northern/Central Rockies to the adjacent Great Plains. Multiple mesoscale convective complexes are likely to develop north of the prevailing ridge in the form of "ridge rollers". Predictability of any such episode of course remains low but enhanced rainfall is expected anywhere from the Middle Mississippi Valley downstream to the Ohio Valley. There will also be enhanced rainfall across the Lower Great Lakes into the northeastern U.S. for much of the weekend, particularly along the west-east oriented frontal zone. The focus for convection will shift southward into early next week as the frontal zone moves south of 40N latitude. Farther west, enhanced onshore flow and seasonably strong dynamics will support widespread rainfall from the Pacific Northwest into the Upper Intermountain West. Local orographics should help focus higher amounts where upslope flow prevails. As the frontal zone shifts toward the middle of the country, expect renewed convection across the north-central states into next week. Rubin-Oster 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