Extended Forecast Discussion NWS Weather Prediction Center College Park MD 109 AM EDT Sun May 12 2019 Valid 12Z Wed May 15 2019 - 12Z Sun May 19 2019 Tonight's forecasts were derived from a blend of the latest deterministic 12z ECMWF blended with the 12z ECMWF/18z GEFS Ensemble Means days 3-7, with less weighting on the 12z deterministic Canadian global model. 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/12z Canadian global remain farther north than the GFS/18z GEFS Mean Wed. The forecasts gave more weighting to the majority cluster, and less to the operational 18z gfs, now the southern most solution. This system moves across the Canadian maritimes and adjacent coastal waters Thu. Elsewhere, models have typical timing/amplitude differences with northern stream energy crossing the Midwest/Great Lakes 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 Canadian/ECMWF models and the 18z GEFS/12z ECMWF Ensemble 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. The 18z GFS was excluded from the blend as the upstream east Pacific Ocean low was half a day behind the pack of the other models/means Thu 16 May to 12z Fri 17 May. Height falls crossing the Rockies on to the Plains by late next week should induce surface cyclogenesis across the northern Plains Fri night-Sat, drifting slow east northeast towards the upper MS Valley Sun 19 May. Good clustering among the 12z ECMWF/18z GEFS Mean/12z ECMWF Ensemble Mean led to a consensus of these forecasts. The 00z GFS clusters well with this group of forecasts low positions as well. ...Weather Highlights/Threats... Expect showers over the New England mid week as low pressure is slow to move off the northeast coast. 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. Showers/storms also develop in the vicinity of a stationary front lingering over the southern 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. Higher elevation snows develop as colder air aloft moves into the northwest US, starting in northern CA/western OR Thu and then spreading into the higher elevations of the Northern Rockies and northern Great Basin next weekend. With low pressure moving slowly east across the Plains next weekend, return moist southerly flow provides fuel to enhance shower/thunderstorm activity, with potential for locally heavy rain to develop over portions of central Texas. Temperature-wise, highs of 10-15 F above normal are forecast over MT/WY/western Dakotas 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 northeast Wed will keep highs 10-15F or so below normal over northern New England. Gradual moderation is expected as the trough moves offshore Thu 16 May, with slightly above temperature Fri into the weekend. Temperatures several degrees above normal develop over the southeast late in the week and continue through the weekend. 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