Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Sun Jun 10 2018 Valid 12Z Wed Jun 13 2018 - 12Z Sun Jun 17 2018 ...Pattern Overview... Latest guidance continues to refine ideas for flow evolution over Canada and contiguous U.S. The general consensus is for the strong upper high forecast to be northwest of Hudson Bay as of early Wed to collapse/merge with a ridge that develops and then progresses ahead of energy ejecting from the upper trough moving into western North America. Troughing should persist over the West through the end of the period given the strength/sharpness of the ridge over the eastern Pacific. At the same time an eastern U.S./Canada trough should amplify during the latter half of the week and then proceed into the western Atlantic during the weekend. Multi-day trends seem to suggest some weakening and/or eastward adjustment of mean ridge over the southern Plains. The overall pattern evolution may promote a flow of tropical eastern Pacific moisture into parts of the southwestern U.S./Rockies and eventually a separate area of moisture into the western half of the Gulf of Mexico. ...Model Guidance/Predictability Assessment... As has been the case in recent days, an operational model blend (more 06Z GFS/00Z ECMWF than 00Z UKMET/CMC) represented the most common themes of guidance well in the 3-5 day time frame. Then the ensemble means (00Z ECMWFens/NAEFS and 06Z GEFS) became more useful for their stability and teleconnection support, though some operational guidance matched up better in principle than in some previous cycles. One emerging trend in the guidance, reinforced in this cycle, is for a fairly progressive ejection of initial upper low energy initially near Vancouver Island and reaching inland by Thu. This trend, and possible incorporation of some upper low/trough energy to the north, is steadily increasing support for a fairly strong low pressure system tracking into south-central Canada. The ejection of leading energy is encouraged in part by an upstream shortwave/possible compact low that may reach the Northwest by around Sat and is much more defined than it was 24 hours ago. This second bundle of energy most likely drifts into the northern Intermountain West/Rockies by Sun with some troughing extending southwestward. This evolution compares well with teleconnection ideas relative to the upstream ridge's positive height anomaly center--some broadness to the northern part of the mean trough and only weak troughing extending over/through California. GFS/ECMWF runs are now fairly close to this idea as well, after the GFS trended away from a deeper CA upper low (now in the 00Z CMC) and ECMWF pulled the southern part of the trough westward. Exact details of the western trough will be critical for determining timing/axis of a potential surge of deep layer moisture from eastern Pacific T.S. Bud into parts of the southwestern U.S. The most agreeable focus in the GEFS/ECMWF ensembles is over southern Arizona late this week/early weekend with precipitable water values potentially reaching 4-6 standard deviations above normal and the 95th or greater climatological percentiles. Somewhat westward axis in the ECMWF scenario would lead to such anomalies extending into southern California. Some moisture may eventually continue northeastward along the surface front into the Rockies/Plains. Over/near the Gulf of Mexico, surface solutions still vary widely though a more agreeable signal is developing toward at least some shortwave energy/moisture reaching the western half of the Gulf by the latter half of the period. The ECMWF does not yet have a meaningful surface reflection but the past couple runs have noticeably increased the amount of shortwave energy over the western Gulf. Thus the depiction of a surface trough heading into the western Gulf continues to provide the most reasonable way to depict potential for some sensible weather effects later in the forecast. Confidence is fairly low with eastern U.S. trough details by the latter half of the period. Ensemble mean trends have been toward more progression, likely due in part to trends upstream, though the CMC and some recent ECMWF runs suggest energy could close off just beyond the East Coast. ...Weather Highlights/Hazards... There will be multiple areas of focus for enhanced rainfall potential during the period. One will be along an initially stalled frontal boundary over and east of the central Plains, with emerging western U.S. low pressure pushing it north as a warm front. While the Northwest will likely see just light-moderate activity with the strong upper trough moving into the region, expect precipitation to become heavier and more widespread late week/weekend from the Southwest into parts of the Rockies as the flow aloft may bring some moisture from T.S. Bud across the region. Confidence is not especially high with timing/axis specifics but compared to yesterday there is a strengthening signal toward some rainfall enhancement from the eastern Pacific tropical moisture. By late in the period some of this moisture could interact with the western front extending into the Plains and slower frontal motion could promote repeat activity/training at some locations. Also by the weekend there is increasing potential for some areas along the western half of the Gulf Coast to see increasing intensity of rainfall with low level Gulf inflow and upper level shortwave energy, even there is not a pronounced surface reflection. Farther east, the Southeast and Mid Atlantic will likely see showers and thunderstorms with a front providing added focus while a system southern Canada/Maine will likely produce an axis of enhanced rainfall over extreme northern New England during the latter half of the week. Well above average temperatures will progress from much of the West on Wed into the Plains and then MS Valley. Each day should feature an area of plus 10-20F anomalies. Upper trough moving into the West will bring a pronounced cooling trend, with clouds and rainfall possibly leading to much below normal max temperatures over parts of the Southwest/Great Basin/Rockies by the latter half of the period. 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