Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1144 AM EDT Sat May 11 2019 Valid 12Z Tue May 14 2019 - 12Z Sat May 18 2019 15 UTC Update... A blend of the 00Z ECMWF/CMC and 06Z GFS was used as basis during days 3-5 (Tue-Thu) for this forecast update, with particular weight placed toward the ECMWF/CMC solutions. Models continue to slowly converge on the eventual track of a surface low along/off the Northeast U.S. coastline Tue-Wed, which shows some potential to produce accumulating snows across the higher elevations of interior northern New England. The ECMWF/CMC were more closely clustered with consensus on the track of this system while the GFS was a bit farther south. Solutions handle weakening southern stream shortwave energy crossing the Southern Plains Tue-Wed similarly, with differences mainly confined to the amplitude of the feature. Models show reasonably good consensus on an amplified upper trough reaching the West Coast Wed-Thu (a leading shortwave Wed followed by stronger height falls Thu). Solutions differ on the specifics of embedded smaller-scale features within the larger trough, favoring gradually increasing ensemble weighting. Stronger height falls crossing the Rockies by late next week should induce surface cyclogenesis across the High Plains by Fri night-Sat. Larger differences begin to emerge from Thu onward with the evolution/timing of a few northern stream shortwaves. The chaotic and somewhat blocked upper-level flow across Canada reduces predictability for smaller scale features. Thus, trended the forecast toward ECENS/NAEFS ensemble means during days 6-7 (Fri-Sat). Ryan Previous Discussion (issued at 0525 UTC)... Tonight's forecasts were derived from a blend of the latest deterministic 12z GFS /12z ECMWF blended with the 12z ECMWF/18z GEFS Ensemble Means days 3-7. Models are slowly converging on the latitude of the low developing off the New England coast Mon night-Tue. The 12z ECMWF/ECMWF Ensemble Mean/12z UKMET remain farther north than the GFS/18z GEFS Mean/12z Canadian. The forecasts used an intermediate solution to provide continuity and a consensus based solution. The models show an upper level low near the AZ/NM border with Mexico 12z Mon that slowly moves east and shears into an upper trough as it progresses across the Southern Plains/Lower Mississippi Valley through early next week. Well clustered forecasts lend the situation to a consensus-based forecast. Elsewhere, models have typical timing/amplitude differences with northern stream energy diving from the upper MS Valley into the Midwest/Great Lakes Tue night to Wed and crossing the central Appalachians/mid Atlantic/coastal waters Thu, with potential low pressure development off the coast Fri 17 May. There remains a large latitudinal spread as to where low pressure develops on the surface front. A blend of the 12z GFS/ECMWF models and their means was used until better agreement develops. Agreement amongst the models/ensembles supports an amplified upper trough reaching the West Coast by Wed 15 May, with a secondary trough reaching the west coast Thu 16 May, which then moves inland into the Great Basin and towards the Pacific northwest coast Fri 17 May. Models differ on the exact timing and phasing of the trough, with the ECMWF/ECMWF ensemble mean faster taking the upper trough across the Rockies on to the high Plains. Blending with the slower timing of the GEFS was used following historically slower motion of higher amplitude upper troughs. ...Weather Highlights/Threats... Expect showers over the New England early in the week as consolidation of low pressure off the northeastern coast occurs, with the system moving slowly. Some of the precipitation could fall in the form of snow over highest elevations of northern New Hampshire and western Maine as the core of the upper trough passes through. A cold front and frontal wave moving east across the Upper MS Valley, Upper Midwest/Great Lakes/Ohio Valley should produce showers during the early to middle portions of next week before the system moves into the northeast later in the week. The upper low tracking from the western U.S.-Mexico border into the southern Plains will be responsible for locally enhanced areas of showers/storms over the southern Plains and western Gulf coast in the early to middle portions of next week. Showers/storm also develop in the vicinity of a stationary front lingering over the Fl peninsula in the middle to latter portions of next week. The upper trough nearing the West Coast should bring increased clouds/precipitation to the Pacific Northwest and northern CA Wed 15 May, continuing inland into the Great Basin and northern Rockies Thu 16 May, and then the Plains on Fri 17 May and Sat 18 May. Short-range warmth over the Northwest will continue into the first half of next week, with highs of 10-15F above normal over the Interior Northwest and northern Rockies Tue, continuing in MT/WY Wed. The warm core moves east into the northern to central Plains Thu with cooling over the northwest. With the approaching trough, CA/OR/WA will see a reversal to cooler than normal temperatures in the middle to latter portions of next week. The system affecting the Mid Atlantic, Great Lakes, and northeast Tue will keep highs 10-15F or so below normal, continuing on Wed in northern Maine. Gradual moderation is expected as the trough moves offshore Thu 16 May. Petersen Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml