Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Wed Jun 06 2018 Valid 12Z Sat Jun 09 2018 - 12Z Wed Jun 13 2018 ...Pattern overview... Over the course of the period most guidance expects the amplified trough-ridge-trough pattern in place at the start of the weekend to evolve toward a more typical summertime pattern with progressive flow across the northern tier states. However the details of how this occurs are uncertain given significant model/ensemble changes over the past 12-24 hours. Meanwhile upper ridging anchored over the southern Plains as of Sat should weaken through the weekend, and then after passage of the vigorous upper trough through the West, rebuild with a center most likely positioned over the southern Rockies. Farther eastward an upper weakness over the northeastern Gulf of Mexico/Southeast U.S. will gradually become more ill-defined with time. ...Model guidance/predictability evaluation/preferences... From the large scale evolution perspective the greatest uncertainty arises by the latter half of the period, involving the strong trough energy emerging from the western U.S. and shape of the downstream ridge. Over the past 12-24 hours GFS/ECMWF runs and their ensemble means have changed considerably toward the idea of allowing some of the ejecting western U.S. flow to cut through the downstream ridge as an upper high closes off over or north/northwest of Hudson Bay. This trend is quite pronounced just going from the 00Z to 06Z cycle of the GEFS mean. The multi-day trend toward faster progression of the western trough fits this evolution. The 00Z CMC ensemble mean in particular and perhaps to some extent operational CMC appear too slow with the overall pattern given remaining guidance and trends, but they do provide some reservations about going completely toward the latest majority scenario without additional model runs to build some degree of confidence. As for some finer details... GFS runs appear somewhat over-developed with a wave expected over the lower Great Lakes as of early Sat so the forecast blend downplays that aspect of the GFS. Also downplayed from GFS guidance is a west of consensus track for a compact upper low that drops southward over eastern Canada. While it fits into multi-day trends, the 00Z ECMWF becomes faster than other solutions with northern Plains/upper MS Valley shortwave energy by early next week. Farther upstream model/ensemble solutions are still chaotic within a generally agreed upon band of low amplitude progressive flow. The combination of northern Pacific and northern Canada height anomaly centers in latest D+8 charts seem to teleconnect with a tendency toward modest troughing near the West Coast around or just after the end of the forecast period. Latest GEFS/ECMWF means best reflect this idea. Elsewhere, by the end of the period GFS runs center the southern Rockies ridge farther east than most other solutions. By day 7 Wed GFS/GEFS members are showing signals of low pressure/moisture emerging from the western Caribbean. This appears similar to their behavior leading up to Alberto, with premature development/moisture transport. GEFS members have already trended slower versus yesterday and the CMC--usually a good "extreme bound" given its historical proclivity toward tropical development--is still very suppressed through next Wed. Other solutions show minimal development as well. Thus at this time the probability of the GFS/GEFS scenario appears quite low. Based on the above considerations, the forecast started with various components of the 00Z ECMWF/00Z-06Z GFS/00Z UKMET with a little 00Z GEFS/ECMWF ensemble input early-mid period. The latter part of the forecast increased weight of the ensemble means while also incorporating some elements of continuity to temper recent changes in guidance for the evolution of the northern U.S./east-central Canada pattern. ...Weather highlights and hazards... The strong upper trough crossing the Northwest will bring an episode of active weather this weekend with highest precip totals likely to be over favored terrain in WA/OR and the northern Rockies. Some of the highest elevations could see a little snow from this event. The rest of the period could feature an occasional period of rain over extreme northern areas but with low confidence on existence/timing given the flat nature of mean flow aloft. The combination of shortwave energy aloft and wavy front from the north-central Plains to East Coast will promote areas of showers and storms/mesoscale convective complexes during the weekend, with some locally heavy rainfall. Over the East the convective threat will push southward along with the surface front, while the system emerging from the West should generate another period of convection from the northern Plains into mid-upper MS Valley, and perhaps farther eastward, from about Sun night onward. Expect diurnally favored convection over parts of the Southeast/FL with some enhancement from the weakness aloft. Expect the most extreme temperature anomalies to be associated with the strong upper trough passing through the West during the weekend. Readings should reach 10-20F above normal in the leading warm sector from the eastern slopes of the northern-central Rockies into the Plains. Some locations in the northern Plains may see record warm mins around Sun-Mon. Cold frontal passage will bring a day or two of minus 10-15F anomalies for highs from the northern half of the West coast into the northern Rockies. Rising heights over the West/southern Rockies ridging aloft should lead to above normal temperatures over much of the West by next Tue-Wed. Meanwhile the mean trough aloft over the Northeast and vicinity will likely keep highs near to below normal into early next week along the East Coast. 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