Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Mon Oct 22 2018 Valid 12Z Thu Oct 25 2018 - 12Z Mon Oct 29 2018 ...Overview... Deep eastern troughing and a coastal storm will be the focus for the medium range period as the remnants of Hurricane Willa in the Pacific combine with northern stream energy to bring a widespread rainfall to much of the east later this week into this weekend. This will maintain a cool pattern for the central/eastern states and mild temperatures for the west. ...Guidance/Predictability Assessment and Preferences... The synoptic-scale pattern ahead will initially feature a sprawled out upper trough extending from the Canadian Maritimes back up toward northern Nunavut. The southern extension of these lower heights should gradually gain latitude in response to the next system taking shape across the central U.S. A phasing of streams continues to be advertised between a wave of low pressure traversing the northern Gulf of Mexico on Thursday/Friday (remnants of Willa) before the parent shortwave attains a negative tilt. Farther upstream, quasi-zonal flow sets up over the northeastern Pacific through Friday which favors a series of perturbations to track into the Pacific Northwest and British Columbia coasts. A broad positive height anomaly emerges for the weekend across the western U.S. while migratory shortwaves traverse the top of the ridge with a track from British Columbia/Alberta toward the north-central U.S. Many solutions support significant amplification with a couple of these waves as they move toward the Mississippi River Valley over the weekend. However, given the uncertainty with the tropically-infused nor'easter and a lack of upper level anomaly centers to guide the flow, opted to continue with a middle ground solution nearest the 00Z ECMWF ensemble mean. With the developing Gulf and western Atlantic system, the 00Z ECMWF/UKMET/Canadian lied farther west than the ensemble means and the recent GFS. Preferred a solution just west of the ensemble consensus given the likely amplified pattern and the tendency for the GFS to be too weak/east with east coast systems beyond ~100hrs. Recent FV3-GFS runs were closer to the ECMWF-led cluster but there remained significant timing differences as the low lifts up the coast. This also maintained continuity from the overnight forecast. By the end of the period, with so much spread in all directions as the low heads toward/into Canada and the secondary system rotates through the Ohio Valley, relied on the ensembles (mostly the ECMWF ensemble mean) to form a consensus with some artistic manipulations. ...Weather Highlights/Threats... Precipitation will focus along the central/eastern Gulf Coast and along the eastern seaboard with some modest precipitation over the Pacific Northwest as a couple frontal boundaries waver through WA/OR/ID. The eastern coastal cyclone will deepen over the Gulf Stream and spread comma-head precipitation back into the coastal plain as its tropical connection sustains itself through the period. Impacts will be driven by the proximity of the low to the coast. To the north, marginally cold air would support some wintry precipitation over the northeastern U.S. interior mountains (central/northern Appalachians). Back over the northwestern U.S., persistent onshore flow will allow for multiple days of orographically-driven precipitation. The usual focus should be over the upslope terrain of the Olympics and Washington Cascades. Below average temperatures are forecast from the Great Plains eastward given the persistent troughing. Some of the largest departures, on the order of 10 to 15 degrees, will congregate over much of the Eastern Seaboard given the extended period of cloud cover/precipitation as well as post-frontal cold advection. Meanwhile, milder weather will prevail over sections of the West Coast into the Intermountain West given the influence of Pacific air masses on the forecast and upper ridging moving into California, perhaps continuing into next week. Fracasso/Rubin-Oster WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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