Extended Forecast Discussion NWS Weather Prediction Center College Park MD 240 AM EDT Fri May 10 2019 Valid 12Z Mon May 13 2019 - 12Z Fri May 17 2019 Tonight's forecasts were derived from a blend of the latest deterministic 12/18z GFS /12z ECMWF equally weighted with the 12z ECMWF/18z GEFS Ensemble Means days 3-7. Models remains split on the latitude of the low developing off the Mid-Atlantic coastline Sun night-Mon. The 12z ECMWF/ECMWF Ensemble mean remain farther north than much of the other guidance. The 12z UKMET was in between the ECMWF and further south GFS forecasts Mon-Tue 14 May, and the forecast is closest to the 12z UKMET as an intermediate 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 the period. Minor timing differences with the low/wave led to a consensus-based forecast, with the GFS typically faster. Blending with the slower 12z ECMWF and ECMWF Ensemble Mean provided a desired middle ground. 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 Wed-Thu, with potential low pressure development off the coast Fri 17 May in the 12z ECMWF/12-18z GFS. The less amplified ECMWF Ensemble Mean and 18z GEFS mean highlight differences to be resolved. An intermediate solution was used until better agreement develops. Broad 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, but it is evident in each of the ECMWF/GFS/Canadian models and their means. A blend should smooth out differences. ...Weather Highlights/Threats... Expect showers over the northern mid Atlantic to New England as consolidation of low pressure off the northeastern coast occurs, with the system moving slowly. Confidence in specifics remains lower than desired in light of persistent model spread. 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 dropping southeast from the northern Plains may spread showers from the Upper MS Valley, Upper Midwest/Great Lakes/Ohio Valley during the early to middle portions of next 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 of next week. The upper trough nearing the West Coast should bring increased clouds/precipitation to the Pacific Northwest and CA Wed 15 May and continuing inland into the Great Basin and northern Rockies Thu 16 May. Short-range warmth over the Northwest will continue into the first half of next week, with highs of 10-20F above normal over the Interior Northwest and northern Rockies Mon-Tue. The warm core moves east into the Rockies and central Plains in the middle of the week 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 Mon-Tue will keep highs 10-15F or so below normal, with gradual moderation as the trough moves offshore Wed and 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